Unit 4: The Cell CycleHaving learned how cells are structured and operate in the previous unit, this unit continues to delve into the lives of cells. Here, we will explore how cells regulate their life cycle, specifically how and when reproduce. Our study of the cell cycle helps to lay the foundation for our next unit on metabolism as we examine photosynthesis and cellular respiration. In contrast, the processes we study in this unit center around DNA and its sister molecule RNA. As such we will spend considerable time studying the structure and function of these nucleic acids. Understanding the significance of a cell's ploidy state and number will be essential to this unit as we study how somatic cells replicate themselves through mitosis and cytokinesis, while gametes reproduce through meiosis. Moving forward we will examine the types of mutations that can occur in cells as they replicate their DNA or use their DNA to build proteins and how these mutations can impact cells. This leads us to a study of cancer, cell replication gone wild.
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The Cell Cycle
Cell Cycle
As cells grow and live they move through what is called the cell cycle. Most of a cell's life is spent growing and preparing for cell division as can be seen in the illustration to the right. The cell cycle can be divided into 2 major parts, Interphase and M Phase. Interphase is literally defined as the "time between divisions", but that hardly does Interphase justice as so much is accomplished during this stage of the cell cycle including processes such as photosynthesis, cellular respiration, protein synthesis and DNA synthesis. The actual division of the cell occurs during M phase which consists of mitosis and cytokinesis. It is important to note that this only applies to somatic or body cells; sex cells have a slightly different cell cycle and do not under go mitosis, but rather meiosis. |
During interphase the cell grows in size, replicates its DNA and organelles and performs photosynthesis and/or cellular respiration depending on the cell type. Interphase can be divided into 3 stages: G1, S and G2. G1 or Gap 1 phase is a period of growth for the cell. During the G1 phase, the cell synthesizes new proteins and new organelles while growing in size and carrying out its basic functions. G2 phase is also a time for growth, but the cell must also prepare for mitosis. The organelles required for mitosis, such as the centrioles and spindle fibers, are synthesized during G2 and many of the cellular organelles like ribosomes are replicated in preparation for the division of the cell, insuring both daughter cells will have adequate numbers of organelles. S phase (short for synthesis) is when the cell replicates is DNA. This will be discussed in greater detail later on in the unit. Important metabolic processes like photosynthesis and cellular respiration occur throughout Interphase, especially during the G1 and G2 phases.
M phase is the portion of the cell cycle in which the cell divides. Mitosis refers specifically to the division of the somatic cell's nucleus during which the genetic material of the parent cell is evenly distributed between the daughter cells. Mitosis is immediately followed by cytokinesis (literally the division of the cytoplasm) where the cell actually divides. Mitosis and cytokinesis together complete cell division. Because mitosis evenly divides up the genetic material of the parent cell between the daughter cells it is a prerequistie for asexual reproduction. During asexual reproduction a single cell divides to form two new daughter cells that are each identical to the original parent cell. Sexual reproduction occurs when two sex cells (gametes) merge to form a new, unique cell known as a zygote. Unlike asexual reproduction, sexual reproduction is preceded by a process called meiosis that divides the nucleus of the parent cell twice and does not evenly distribute genetic material.
M phase is the portion of the cell cycle in which the cell divides. Mitosis refers specifically to the division of the somatic cell's nucleus during which the genetic material of the parent cell is evenly distributed between the daughter cells. Mitosis is immediately followed by cytokinesis (literally the division of the cytoplasm) where the cell actually divides. Mitosis and cytokinesis together complete cell division. Because mitosis evenly divides up the genetic material of the parent cell between the daughter cells it is a prerequistie for asexual reproduction. During asexual reproduction a single cell divides to form two new daughter cells that are each identical to the original parent cell. Sexual reproduction occurs when two sex cells (gametes) merge to form a new, unique cell known as a zygote. Unlike asexual reproduction, sexual reproduction is preceded by a process called meiosis that divides the nucleus of the parent cell twice and does not evenly distribute genetic material.
Limits to Cell Size
At this point, you may be asking yourself: why does a cell need to divide at all? Why can't the cell just become infinitely large? Isn't size an advantage? Does size really matter? The truth is a cell can only become so large; this is true a for a 2 major reasons:
Information Overload
All cells have DNA- this is one of the basic tenets of biology. DNA contains all of the information a cell needs: everything from day to day processes to protein production to cell division. This information is required by different organelles within the cell so as the cell expands in size, there is greater and greater demand for this information. Unfortunately, the amount of DNA available to a cell is locked at the cell's birth, the DNA cannot become larger as the cell grows. Eventually, the DNA will become overloaded and unable to provide information to the cell: there will simply be too many organelles trying to access the cell's information in the cell's DNA at the same time. We can compare this to a library in a big city: as the city's population expands, more people demand books, but the library is only so large and only has so many books. If the city expands in size too much, the library will be unable to keep up with the demand for books.
Surface Area to Volume Ratio
The other major reason cell's cannot grow too large has to do with resource exchange and consumption. As you know, cells require nutrients like oxygen and glucose to survive. These resources are imported into the cell across the cell membrane as waste products (like carbon dioxide) are exported and generally move through the cell vai diffusion meaning their speed is constant and relatively slow. This means that the cell's rate of resource exchange is determined by its surface area. On the other hand, the cell's rate of resource consumption (and by extension its rate of waste generation) is determined by its volume. As cells become larger, their surface area to volume ratio decreases; if this ratio decreases too much, the cell runs into a problem- it is consuming resources faster than it can bring them in and generating wastes faster than it can get them out of the cell. Therefore, cells must maintain a small size by dividing. This ensures the cells are able keep a large surface area to volume ratio and exchange resources faster than they can consume them.
At this point, you may be asking yourself: why does a cell need to divide at all? Why can't the cell just become infinitely large? Isn't size an advantage? Does size really matter? The truth is a cell can only become so large; this is true a for a 2 major reasons:
Information Overload
All cells have DNA- this is one of the basic tenets of biology. DNA contains all of the information a cell needs: everything from day to day processes to protein production to cell division. This information is required by different organelles within the cell so as the cell expands in size, there is greater and greater demand for this information. Unfortunately, the amount of DNA available to a cell is locked at the cell's birth, the DNA cannot become larger as the cell grows. Eventually, the DNA will become overloaded and unable to provide information to the cell: there will simply be too many organelles trying to access the cell's information in the cell's DNA at the same time. We can compare this to a library in a big city: as the city's population expands, more people demand books, but the library is only so large and only has so many books. If the city expands in size too much, the library will be unable to keep up with the demand for books.
Surface Area to Volume Ratio
The other major reason cell's cannot grow too large has to do with resource exchange and consumption. As you know, cells require nutrients like oxygen and glucose to survive. These resources are imported into the cell across the cell membrane as waste products (like carbon dioxide) are exported and generally move through the cell vai diffusion meaning their speed is constant and relatively slow. This means that the cell's rate of resource exchange is determined by its surface area. On the other hand, the cell's rate of resource consumption (and by extension its rate of waste generation) is determined by its volume. As cells become larger, their surface area to volume ratio decreases; if this ratio decreases too much, the cell runs into a problem- it is consuming resources faster than it can bring them in and generating wastes faster than it can get them out of the cell. Therefore, cells must maintain a small size by dividing. This ensures the cells are able keep a large surface area to volume ratio and exchange resources faster than they can consume them.
DNA and DNA Replication
The History of DNA
It was not always known that DNA was the storage form of genetic material or what the structure of DNA was. It took the work of many brilliant scientists and careful experimentation to identify DNA as the molecule of heridity in living organisms and even more research to unlock DNA's structure. In fact, most scientists originally hypothesized that protein (not DNA) was responsible for storing genetic information in organisms. This idea was later disproven.
Griffith contributed one of the earliest findings about DNA and discovered that genetic material could be transferred between organisms. This signfied that there had to be a physical molecule that carried instructions for life, now the questions was which one? Early on, scientists narrowed their search down to DNA and protein as the molecule that stores and transfers genetic material. Hershey and Chase proved that it was DNA (not protein) that stored this information. Finally, Watson and Crick discovered DNA's unqiue double helix structure, but their work would not have been possible with the research of Rosalind Franklin, who took pictures of DNA molecules using X-ray crystallography. The work of each of these scientists is described in greater detail below.
It was not always known that DNA was the storage form of genetic material or what the structure of DNA was. It took the work of many brilliant scientists and careful experimentation to identify DNA as the molecule of heridity in living organisms and even more research to unlock DNA's structure. In fact, most scientists originally hypothesized that protein (not DNA) was responsible for storing genetic information in organisms. This idea was later disproven.
Griffith contributed one of the earliest findings about DNA and discovered that genetic material could be transferred between organisms. This signfied that there had to be a physical molecule that carried instructions for life, now the questions was which one? Early on, scientists narrowed their search down to DNA and protein as the molecule that stores and transfers genetic material. Hershey and Chase proved that it was DNA (not protein) that stored this information. Finally, Watson and Crick discovered DNA's unqiue double helix structure, but their work would not have been possible with the research of Rosalind Franklin, who took pictures of DNA molecules using X-ray crystallography. The work of each of these scientists is described in greater detail below.
The Griffith Experiment
Frederick Griffith was the first to demonstrate that genetic material could be exchanged between organisms. In his experiment, two types of bacteria were cultivated: R strain and an S strain. In this context, the R stands for rough: the R strain bacteria lacked a capsule and had exposed antigens (antigens are specific structures on the surface of cells that are used for identification). The S stands for smooth: the S strain bacteria have a capsule giving them a smooth texture. The S strain bacteria are more difficult for immune cells to detect because their capsule hides their antigens. This increases the virulence or disease causing ability of the S strain bacteria. |
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When injected into mice the virulent S strain caused a fatal case of pneumonia, but the R strain were harmless because the mouse's immunse system was able to identify and destroy the R strain bacteria. In another trial, Griffith heat-killed the S strain and mixed the now dead S strain bacteria with the benign R strain and injected the mixture into a mouse. Based on the previous results, Griffith expected the mouse to live, but instead the mouse died. When he examined the dead mouse, he found it was infected with living S strain bacteria. Somehow the virulence factor had transferred from the S to R strain through a process Griffith called transformation.
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Oswald Avery built off Griffith's work, adding various enzymes to the S and R strain mixture. The enzymes would destroy different molecules believed to aid in transformation. Only when enzymes that destroyed DNA were supplied to the mixture did transformation stop. This meant DNA was the transforming material and likely storage form of genetic material!
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Hershey-Chase Experiment
To further examine DNA, Alfred Hershey and Martha Chase performed their own experiment. To conduct their research they used bacteriophages, viruses that specifically target bacteria. Bacteriophages, like all viruses, are composed of a protein shell and DNA (some viruses, called retroviruses, can use RNA for genetic material as well) and infect a host cell, using the cell to produce new viruses. Hershey and Chase radioactively tagged the bacteriophages: one set had their DNA tagged and the other set had their protein coat tagged. As they monitored the bacteriophages and bacteria, they found that it was the viral DNA, not viral protein, that was injected into the |
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host. Hershey and Chase were able to do this by utilizing a centrifuge- a device that rapidly spins samples, separating the components according to their density. After a sample is run through a centrifuge and spun around rapidly, the solid portions form a dense pellet while the less dense liquid portions remain in the supernatant. The sample (of bacteriophage and bacteria) contained cells, virus particles and the liquid growth medium. Hershey and Chase found radioactive DNA in the pellet, but protein remained in the supernatant. These results demonstrated that it was the DNA that entered the bacteria cells, which were located in the pellet, while the protein remained outside the cell, in the less dense extracellular fluid. This confirmed the work of Griffith and Avery and proved DNA was the molecule of heredity.
Watson, Crick and Rosalind Franklin
Now it was known that DNA was the molecule of heredity, but the structure of DNA remained a mystery. In the 1950's Rosalind Franklin attempted to solve this mystery using X-rays. Using a technique called X-ray crystallography, Franklin photographed DNA. At first, her results did not make sense, the DNA appeared to have a X pattern (see the image to the right). But Watson and Crick, also studying DNA's structure, used Franklin's results to determine that DNA has a double helix shape, much like a spiral staircase. The X shape pattern was just a consequence of a three dimensional image being represented into two dimensions. |
DNA Structure
As you already know, DNA is a type of nucleic acid, one of the 4 types of macromolecules. As with any nucleic acid, DNA is composed of 5 elements: carbon, hydrogen, oxygen, phosphorous and nitrogen. Each DNA molecule is composed of monomer subunits called nucleotides and each nucleotide is composed of a phosphate, a pentose (5-carbon sugar) and a nitrogenous base. In DNA, the sugar is deoxyribose (hence the name Deoxyribose Nucleic Acid) and there are four possible nitrogenous bases: adenine (A), guanine (G), thymine (T) and cytosine (C). Adenine and guanine have a double ring structure in their nitrogenous bases and are referred to as purines. Cytosine and thymine have only a single ring in their nitrogenous bases and are known as pyrimidines. The structure of purines and pyrimidines have important consequences for the structure of DNA as a whole.
As you already know, DNA is a type of nucleic acid, one of the 4 types of macromolecules. As with any nucleic acid, DNA is composed of 5 elements: carbon, hydrogen, oxygen, phosphorous and nitrogen. Each DNA molecule is composed of monomer subunits called nucleotides and each nucleotide is composed of a phosphate, a pentose (5-carbon sugar) and a nitrogenous base. In DNA, the sugar is deoxyribose (hence the name Deoxyribose Nucleic Acid) and there are four possible nitrogenous bases: adenine (A), guanine (G), thymine (T) and cytosine (C). Adenine and guanine have a double ring structure in their nitrogenous bases and are referred to as purines. Cytosine and thymine have only a single ring in their nitrogenous bases and are known as pyrimidines. The structure of purines and pyrimidines have important consequences for the structure of DNA as a whole.
DNA contains two phosphate-sugar backbones and is considered a double stranded molecule, with each strand running anti-parallel: the strands run in opposite directions. The head of a strand of DNA molecule is denoted with a 5' and the tail is denoted with a 3'. These numbers denote to which carbon in the pentose sugar the nucleotides are attached. Free nucleotides can be added to the 3’ carbon, but not the 5’ which has important implications for DNA replication. Because DNA is antiparallel, the 5' end of one strand will always be opposite of the 3' end of the other strand. The strands themselves (which are made
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of sugar and phosphate) are held together by hydrogen bonds that form between the bases (A, T, C and G). But the bases do not pair randomly, they are very specific and observe what are called complementary base pair rules: A (a purine) always binds with T (a pyrimidine) and G (purine) always binds with C (a pyrimidine). Purines always bind with pyrimidines for spacial reasons and are held together by hydrogen bonds: 3 hydrogen bonds hold G and C together, 2 hydrogen bonds hold A and T together. An easy way to remember which bases pair up is the expression All Terrain Golf Cart.
This leads to what is known as Chargaff's rule, which states that the %A in a DNA molecule = % T and the %G = %C. For example, imagine that in a DNA molecule, 28% of the nitrogenous bases were adenine. This would mean that another 28% of the nitrogenous bases would be thymine because for every A there has to be a matching T. Additonally, we would know that guanine and cytosine would each comprise of 22% of the DNA molecule. The reason G and C both comprise the same percentage of the DNA is because guanine always matches with thymine; they exist in a one to one ratio. So to calculate what percentage of the molecule was G and C we would simply subtract 56% (28% adenine + 28% thymine) from 100% (the total DNA molecule) which is 44%. This means that 44% of the total number of nucleotides in the DNA molecule are guanine and cytosine. Now we can divide 44 by 2 since G and C making up the same percentage of nucleotides in a DNA molecule.
This leads to what is known as Chargaff's rule, which states that the %A in a DNA molecule = % T and the %G = %C. For example, imagine that in a DNA molecule, 28% of the nitrogenous bases were adenine. This would mean that another 28% of the nitrogenous bases would be thymine because for every A there has to be a matching T. Additonally, we would know that guanine and cytosine would each comprise of 22% of the DNA molecule. The reason G and C both comprise the same percentage of the DNA is because guanine always matches with thymine; they exist in a one to one ratio. So to calculate what percentage of the molecule was G and C we would simply subtract 56% (28% adenine + 28% thymine) from 100% (the total DNA molecule) which is 44%. This means that 44% of the total number of nucleotides in the DNA molecule are guanine and cytosine. Now we can divide 44 by 2 since G and C making up the same percentage of nucleotides in a DNA molecule.
Condensing and Storing DNA
As a completed molecule, DNA is extremely long (about 6 feet in humans) and must therefore be condensed. In eukaryotic cells, DNA primarily exists as chromatin where DNA is condensed by wrapping around specialized proteins called histones. Chromatin refers to any structure that contains DNA and proteins (specifically histones). The DNA wrapped around the histone proteins form a unit called a nucleosome which resembles a small bead. A single nucleosome will contain a spherical core of 8 histone proteins with DNA wrapped around multiple times which condenses the DNA molecule. Nucleosomes combine to form chromatin as the mass of DNA and proteins continues to condense. By carefully packaging DNA like this, DNA can fit inside the nucleus where it can be housed and protected from harm. Without such extensive packaging mechanisms, DNA would be unable to fit in the cell let alone the nucleus of a eukaryotic cell. The chromatin can be further condensed during cell division to form chromosomes. Humans have 46 chromosomes arranged into 23 homologous pairs. In each homologous pair, there will be a maternal and paternal chromsome that each code for the same genes, but may have different alleles or versions of the gene present. Because there are two copies of each chromosome in most animals, the organism is said to be diploid. Finally, each replicated chromosome is composed of 2 identical sister chromatids, which will be divided up during mitosis and meiosis. |
During Interphase, DNA exists as chromatin and appears a large, unorganized mass of DNA and proteins within the nucleus. During M Phase the chromatin condenses to form chromosomes which are discrete units, we can actually count individual chromosomes.
DNA in Prokaryotes
In prokaryotic organisms like bacteria, DNA exists not only as chromosomes and chromatin but also plasmids. Plasmids are short, circular pieces of DNA that can reproduce independently and often carry genes that help the bacteria survive. For example, plasmids often carry genes related to antibiotic resistance, allowing for the rapid appearance of this trait. |
Plasmids can be exchanged between bacteria cells through a process called conjugation. In conjugation a specialized cilia called a pilus is used to transfer the plasmid from one bacteria to another. The donor bacteria uses its pilus to grab onto the recipient bacteria and pull the two cells together. The pilus will form a bridge between the two cells to allow for the gene transfer. The plasmid in the donor cell is then replicated and sent through the pilus into the recipient cell, thereby transfering the genes from one bacteria to another. During this process, the newly replicated plasmid is temporarily linear, allowing it to fit through the narrow passage provided by the plus Later, after the plasmid has been transferred to the recipient cell, it will reform into its circular shape. In this manner, bacteria can convey important survival traits, like antibiotic resistance onto other cells, rapidly accelerating evolution.
Of course, conjugation is not the only means of genetic transfer available to bacteria. Recall that in Griffith's experiment, the genes for capsule development were transferred from the dead S (smooth) strain bacteria to the living R (rough) strain bacteria. Because the virulent S strain bacteria were dead, some other means of genetic transfer had to have occurred other than conjugation. In this case the process was transformation. In bacterial transformation, living bacteria cells collect fragments of DNA from the environment and integrate the foreign DNA into their genomes. Because Griffith heat killed the S strain bacteria, many of the S train bacteria would have lysed, their contents spilling out into the environment, including their DNA. The living R strain collected the dead S strain DNA, including the genes that coded for capsule development. These genes were integrated into the genome of the R strain bacteria and so the R strain bacteria began to develop capsules and ultimately become S strain.
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The final major mechanism for genetic transfer available to bacteria is called transduction. In this process a virus inserts foreign DNA into its host's genome. In nature, viruses use this process to replicate themselves and transduction is not limited to only bacteria, but rather all cells including our own. In fact, it is estimated that between 5-8% of our genome comes from retroviruses. While transduction is a natural process, humans have used it for their own purposes especially genetic modification to create genetically modified organisms (GMOs) and in gene therapy to repair or replace damaged genes that cause disease.
DNA vs RNA
DNA and RNA are not only both nucleic acids, but are in fact considered sister molecules. As such, it is not suprising DNA and RNA have a huge number of similarities like how they can both store and transmit genetic information, contain a sugar-phosphate backbone (1 for RNA, 2 for DNA) and are built from nucleotides. Of course, DNA and RNA also have some key differences including 3 major structural differences. The first structural difference is that RNA contains the 5-carbon sugar ribose as opposed to the deoxyribose sugar found in DNA. Both sugars are nearly identical expect deoxyribose is missing an oxygen compare to ribose (hence the name). Second, DNA is a double stranded molecule, but RNA is single stranded. As a result, RNA is a less stable molecule. This is at least part of the reason DNA was chosen by evolution to be the molecule for storing genetic information, even though early cells |
originally used RNA as a storage form for their genetic material. The third and final structral difference between DNA and RNA is that while both nucleic acids have 4 possible nitrogenous bases, RNA does not have thymine (T), but instead uses uracil (U). Uracil fulfills all the same functions as thymine and can even pair with adenine.
These three structural differences result in a difference of function between RNA and DNA. Unlike DNA, RNA is not used to store genetic information in the cell (at least not permanently), but it does support DNA in a number of ways such as aiding in DNA replication and protein synthesis. To better serve its variety of functions, RNA does not exist as a single type of molecule but rather as three: rRNA, mRNA and tRNA.
Types of RNA
There are a variety of different types of RNA available to the cell and each plays a vital role. One form of RNA you should already be familiar with is rRNA. rRNA or ribosomal RNA is used to produce ribosomes the organelle responsible for carrying out protein production. mRNA or messenger RNA is used during transcription, the first step of protein synthesis. Messenger RNA carries the message of DNA to the ribosome, we will investigate this further in protein synthesis. And finally, tRNA or transfer RNA is used during translation, the second step of protein synthesis. tRNA carries amino acids to the ribosome and again we will investigate this further later. RNA also plays a pivotal role in DNA replication: as we shall see, without RNA, the process could never begin.
These three structural differences result in a difference of function between RNA and DNA. Unlike DNA, RNA is not used to store genetic information in the cell (at least not permanently), but it does support DNA in a number of ways such as aiding in DNA replication and protein synthesis. To better serve its variety of functions, RNA does not exist as a single type of molecule but rather as three: rRNA, mRNA and tRNA.
Types of RNA
There are a variety of different types of RNA available to the cell and each plays a vital role. One form of RNA you should already be familiar with is rRNA. rRNA or ribosomal RNA is used to produce ribosomes the organelle responsible for carrying out protein production. mRNA or messenger RNA is used during transcription, the first step of protein synthesis. Messenger RNA carries the message of DNA to the ribosome, we will investigate this further in protein synthesis. And finally, tRNA or transfer RNA is used during translation, the second step of protein synthesis. tRNA carries amino acids to the ribosome and again we will investigate this further later. RNA also plays a pivotal role in DNA replication: as we shall see, without RNA, the process could never begin.
DNA Replication
DNA replication (also called DNA synthesis) takes place during the S phase of the cell cycle. DNA synthesis is crucial to the cell division; every cell needs a copy of the organism's DNA and so that DNA must be replicated prior to the cell division. This means that prior to cell division in somatic cells, the parent cell will have 2 complete copies of the DNA, one for each daughter cell following cell division. For a long time, it was unknown how exactly DNA was replicated. Some believed DNA followed a conservative model in which the original molecule would remain in tact and serve as a guide or template for the construction of a completely new DNA molecule. Others argued for a disruptive model, in which each of the two resulting DNA molecules would be part new and part old in a piece meal or random fashion. Ultimately, it was the semiconservative model that proved correct. Under this model, the two strands of the original DNA molecule were split apart, allowing each syrand to serve as a template. New nucleotides are added to the template strands using complementary base pair rules, resulting in the completion of 2 new DNA molecules that were each one half old (the template strand) and one half new (the freshly synthesized complementary strand).
Meselson-Stahl Experiment
The reason we now that the semiconservative model is the correct one is because of Messelson-Stahl Experiment. In this experiment, Meselson and Stahl, culitivated E. coli bacteria in various isotopes of nitrogen. Because DNA contains nitrogen, the isotopes would be integrated into newly replicated DNA molecules to track how they formed. The bacteria were originally grown in a "heavy" Nitrogen-15 medium so all of their DNA would contain 15N. The bacteria were then transfered to a medium with only 14N, a lighter isotope. After a single round of replication, all of the DNA molecules were of an intermediate weight. |
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This ruled out the conservative model which predicted that half of the molecules would contain only 14N and the other half would contain 15N. Now only the dispersive and semiconservative models remained; a another round of DNA replication in 14N medium was required. Under the dispersive model, it was predicted that all DNA molecules would be of the same weight and even lighter than the previous generation; no intermediate weight DNA should remain after the second round of replication. This was not the case as half of the DNA molecules were of the intermediate weight (14N and 15N together) and the other half were light weight (only 14N). These results were consistent with the semiconservative model.
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DNA replication is a complicated process, but we will try to stick to the basics. To begin the process, the DNA molecule must be unlocked and unraveled. This is accomplished by an enzyme called DNA helicase, which breaks the hydrogen bonds between the nitrogenous bases while topoisomerase keeps the DNA molecule stable. Without topoisomerase, the DNA molecule that was not unwound would become strained and could be damaged. A replication fork forms as DNA helicase moves up the DNA molecule, leaving behind two single strands of DNA.
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These single strands serve as a template to build a complementary strand of DNA using complementary base pairing. One strand, known as the leading strand, can be formed continuously while the other, known as the lagging strand, is formed slowly as a series of small segments (Okazaki fragments) that are later stitched together by DNA ligase. The Okazaki fragments of the lagging strand can be seen in the image to the left. Without DNA ligase, the new DNA molecules would contain small nicks which could result in errors during protein synthesis and other complications. The leading strand of one replication fork is actually the lagging strand of the other replication fork within the same replication bubble. This is because DNA can only be synthesized in the 5' to 3' direction resulting in one end of the strand being synthesized continuously as the replication bubble opens while the other end must constantly work backwards.
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DNA polymerase is the enzyme that adds complementary bases to the template strands to build the new DNA. DNA polymerase is very efficient, but has three major drawbacks:
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Why 5' to 3' direction?
There are three primary parts to a nucleotide: a phosphate, a nitrogenous base (A, T, C or G in DNA) and a pentose or 5-carbon sugar (deoxyribose in DNA). Within that sugar molecule each carbon atom can is labeled with a number to designate it: 1', 2', 3', 4' or 5'. This is where the terms 3' and 5' come from, they refer to specific carbon atoms in the sugar molecule. The 3' carbon atom is particularly important because DNA polymerase can only add new nucleotides to the -OH (hydroxyl group) located on that atom. Now the phosphate, attached to the 5' carbon of another nucleotide, is bonded to the hydroxyl group of the 3' carbon. In this way, new DNA is synthesized in a 5' to 3' direction as seen in the illustration to the right. |
Telomeres and Telomerase
Each time the DNA molecule is replicated it becomes a little smaller. This is because the RNA primer on the lagging strand near the end of the DNA molecule can never be replaced by DNA nucleotides. When the primer is removed there will be nothing for the DNA polymerase to attach DNA nucleotides to which means the very tip of the lagging DNA strand cannot be replicated. This means that every time DNA synthesis occurs, important information can be lost. To solve this problem, cells add telomeres to the ends of the DNA molecule. Telomeres are repetitive sequences of DNA that do not code for anything, but protect the rest of the molecule. Telomerase is an enzyme that adds the telomeres to the molecule and is very active in stem cells and other cells that replicate frequently, but not in adult cells. It has been theorized that telomerase activity is linked to old age: tortoises are able to live 150 years because they have high rates of telomerase activity in their cells to protect their DNA from degradation. At the moment this is still just a hypothesis, but scientists are hopeful that if proven true, human life expectancy could be augmented through telomerase supplements. |
Of course, there is a reason adult cells typically lack telomerase activity in humans: our cells are meant to have a predetermined life span. This preventative measure helps to regulate cellular replication by limiting the number of times a DNA molecule can be successfully replicated and minimize the risk of cancer. As you might expect, there is an adnormally high rate of telomerase activity in cancer cells, which allows the cancerous cells to replicate endlessly. In essence, telomeres slowly degrade with each division of the cell and thereby control how many times a cell can divide essentially determing the lifespan of an organism. Telomerase replenishes telomeres allowing indefinite cellular division as seen in stem cells and even the cells of long lived tortoises.
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Protein Synthesis
As you know, proteins are one of the most important molecules for a cell to have. As such, cells are constantly generating new proteins through the process of protein synthesis. Protein synthesis is highly regulated, the cell has to be careful to build proteins in the correct manner or there can be serious consequences. The instructions for building all the proteins requried by the cell are located in the DNA of the cell. This actually presents eukaryotic cells with a bit of a problem because the DNA is located in the nucleus, but protein synthesis occurs at the ribosomes in the cytoplasm or on the rough ER. To circumvent this problem, the cell makes use of mRNA and divides the process of protein synthesis into two major phases: transcription and translation.
Transcription
The word transcription literally means "to write down." During transcription, the cell takes the instructions of how to build a protein from DNA and "writes down" the instructions using a molecule called mRNA. Transcription serves two key purposes: it prevents the DNA from having to the leave the safety of the nucleus (remember, ribosomes are found in the cytoplasm) and allows multiple ribosomes to work proteins at the same time. If ribosomes used DNA directly, only a single protein could be produced at a time since there is only only copy of DNA per cell. But by generating multiple mRNA transcripts from the single DNA molecule, multiple ribosomes can undergo protein synthesis simultaneously, greatly increasing the rate at which proteins are produced.
The word transcription literally means "to write down." During transcription, the cell takes the instructions of how to build a protein from DNA and "writes down" the instructions using a molecule called mRNA. Transcription serves two key purposes: it prevents the DNA from having to the leave the safety of the nucleus (remember, ribosomes are found in the cytoplasm) and allows multiple ribosomes to work proteins at the same time. If ribosomes used DNA directly, only a single protein could be produced at a time since there is only only copy of DNA per cell. But by generating multiple mRNA transcripts from the single DNA molecule, multiple ribosomes can undergo protein synthesis simultaneously, greatly increasing the rate at which proteins are produced.
During transcription, the instructions to build a protein (from DNA) are written down as an mRNA molecule. The DNA is unwound by an enzyme called RNA polymerase which simulatenously constructs the mRNA transcript. The RNA polymerase does not just attach any where to the DNA, it looks for special regions called promoters which signal where to begin transcription. Proteins called trascription factors bind to the promoter region and help guide RNA polymerase. The promoters usually contain the nucleotide sequences T-A-T-A, earning them the name TATA box. RNA polymerase "reads" the DNA strand and adds RNA nucleotides in a complementary fashion to build the mRNA transcript. When the mRNA molecule is complete, the DNA reforms.
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mRNA processing
Before transcription can officially end, the mRNA transcript must be processed before it leaves the nucleus. Each mRNA will contain exons and introns. Exons are regions of genetic material that will be expressed during proteins synthesis but introns are non-coding "junk" regions. The introns are removed from the mRNA and the remaining exons are spliced together. Finally a cap and a poly-A tail (a long series of adenosine nucleotides) are added to the transcript to prevent degradation as the mRNA travels through the cytoplasm. The mRNA transcript exits the nucleus and heads to the ribosomes for the next step of protein synthesis, translation. It should be noted that the transcription described above applies only to eukaryotic cells. Prokaryotic cells undergo the same process, but it occurs in the cytoplasm since there is no nucleus and mRNA processing does not occur. |
Translation
Transcription is the second half of protein synthesis and occurs at the ribosomes. It does not matter if those ribosomes are free in the cytoplasm or attached to the endoplasmic reticulum- protein synthesis will proceed in the same manner. Each ribosome has two parts: a large and small subunit. These subunits combine together and "read" the mRNA transcript, translating the genetic code into a protein. You will also notice that there are 3 sites within a ribosome: E-site, P-site and A-site. These sites help to direct and control polypeptide formation and will be explained in greater detail later. But you should note that their names help explain their functions: The "E" stands for exit, the "P" stands for polypeptide and the "A" stands for amino acid. |
You should recall that proteins are macromolecules and therefore polymers. As with any polymer, proteins are made of smaller parts called monomers. The monomer subunit of proteins are amino acids and there are 20 different amino acids used to build proteins. During translation, amino acids are added to a growing polypeptide (protein) in a specific order determined by the mRNA transcript. mRNA transcripts contain many codons, three letter nucleotide sequences that code for a specific amino acid. The order of the codons in the mRNA transcript determines the order of the amino acids in the polypeptide and the order of the amino acids also determines the function of the protein. We can use a codon wheel to the right to determine which amino acid is coded for by each possible codon combination. With a codon wheel, the first letter is the one at the center of the wheel and you work your way outward.
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As the ribosome reads the mRNA transcript, it scans for the start codon AUG in a process called Initiation. AUG codes for methionine and signals the ribosome to begin translation and start synthesizing the protein. When the ribosome identifies a codon, it adds the corresponding amino acid by using tRNA. tRNA (transfer RNA) molecules contain a single amino acid and an identifying three nucleotide sequence called the anti-codon. The anti-codon will match up to the codon using complementary base pairing and the amino acid on the tRNA will be transferred to the developing protein molecule. The polymerization of the amino acid sequence is known as Elongation.
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Every ribosome has three sites: an amino site (A), a polypeptide site (P) and an exit site (E). New amino acids are introduced at the A site, while the growing polypeptide rests in the P site. The polypeptide then attaches to the amino acid in the A site so that the whole amino acid is sitting in the A site, still attached to the tRNA molecule. The P site now contains an "empty" tRNA molecule with no amino acid. The ribosome then shifts down to the next codon, moving the empty tRNA to the E site and the tRNA with the polypeptide to the P site. The empty tRNA exits the ribosome from the E site (hence the name) and the A site is free to accept a new tRNA molecule, allowing the whole process to repeat.
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Elongation will continue until the ribosome encounters a stop codon (UGA, UAA or UAG) at which point Termination begins and the formation of the protein ceases. The stop codons do not code for an amino acid, but cause protein synthesis to end immediately. The polypeptide is released from the ribosome and the ribosome itself dissociates into its large and small subunits. The amino acid sequence will continue to fold and attain its correct three dimensional , tertiary structure following Termination.
Codon Redundancy
If you do some simple math you will quickly realize there are many more possible codons than amino acids. Because there are four possible nucleotides and each codon is composed of three nucleotides there are a total of 64 codons compared to just 23 amino acids. This concept is known as codon redundancy, multiple codons code for the same amino acid. This is a defense mechanism designed to limit the effectiveness of mutations- changing a single nucleotide in a codon does not necessarily translate to a different amino acid being incorporated. For example, the codon GCG codes for the amino acid alanine. But imagine a substitution mutation occurred and the codon read GCA- this codon will still code for alanine and the mutation would have no effect on the protein or cell. In this case a mutation has indeed altered the codon, but it will have no impact on protein synthesis or the cell. |
Of course codon redundancy does not always work: If CAC were rewritten as CAA the amino acid would change from Histidine to Glutamine. Insertion and deletion mutations are particularly problematic because they cause a frameshift (all nucleotides shift their position following the deletion or insertion) and alter all codes downstream of the mutation. If you examine the example frameshift mutation to the right you can see just how damaging insertions and deletions can be. Now, its not just one codon that could be altered but all of them because the ribosome's reading frame has been altered.
Mutations
A mutation occurs whenever the DNA or RNA of an organism is altered. There are a number of ways in which mutations can form: environmental damage is probably the most obvious but DNA can also be mutated during replication as mentioned previously. There are two major categories of mutations: point mutations and frameshift mutations. In point mutations, only a single nucleotide is altered which means that at most a single codon and resulting amino acid will be |
altered. Point mutations typically take the form of a base substitution in which the correct nucleotide is replaced with an incorrect one. Because of this the term "base substitution" can realistically be used interchangeably with "point mutation" because they only affect a single nucleotide.
Point mutations can be further classified by their impact on the cell. For example, consider again the mutation of GCG to GCA by a base subsitution. Because of codon redundancy, both codons code for the same amino acid: alanine. Since the amino acid was unaffected by the mutaton, the base substitution is said to be a silent mutation and the organism is unaffected. In other words, in a silent mutation DNA/RNA is altered but the mutation is never actually expressed- it remains silent. However, consider the conversion of GCG to GGG. Now the alanine
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amino acid is replaced with glycine. Because the incorrect amino acid was introduced to the protein a missense mutation is said to occur. Finally, a nonsense mutation occurs whenever a codon that codes for an amino acid is covnerted to a stop codon, prematurely ending the protein's construction and terminating protein synthesis.
The other major type of mutation is the frameshift mutation, which differs starkly with the point mutations we have seen so far. In a frameshift mutation, a nucleotide is either added (this is known as an insertion) or removed (this is known as a deletion) from sequence. Because the order of nucleotides in the mRNA trancript ultimately determines the order of the amino acids in the developing protein, a
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frameshift mutation is especially dangerous to the cell. Insertions and deletions completely alter the order or "reading frame" in which the nucleotides are read by the ribosomes. Unlike point mutations which can only affect a single codon, a frameshift mutation has the ability to alter every codon in the sequence down stream of the actual mutation.
Mitosis
Mitosis is a process that occurs during the M phase of the cell cycle in somatic cells. Mitosis evenly distributes parental chromosomes and is therefore closely associated with asexual reproduction in which one cell gives rise to 2 identical daughter cells. Mitosis only refers to the division of the nucleus and is immediately followed by cytokinesis, the process that officially divides the cell. Mitosis ensures that each daughter cell receives a complete copy of the parent cell's genome and is divided into 4 major phases: Prophase, Metaphase, Anaphase and Telophase (PMAT). To be clear, mitosis is a continuous process and the four phases simply denote predicatable patterns as the cell moves
through the process. Somatic cells |
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are considered diploid, they contain two versions of each chromosome, one from the mother (maternal chromosome) and one from the father (paternal chromosome) of the organism. Human somatic cells contain a total of 46 chromosomes arranged into 23 homologous pairs. Cells that are diploid are denoted with the label 2n, since they contain two sets of each chromosome.
Ploidy Number
A cell's ploidy number refers to the number of different sets of chromosomes in a cell. For example, our somatic cells (such as skin, blood and muscle cells) have 23 unique chromosomes, but there are two versions of each- a maternal chromosome and paternal chromosome. This means human somatic cells are diploid- there are two versions of each chromosome for a total of 46. These versions exist in pairs, referred to as homologous chromosomes. Each chromosome within a homologous pair codes for the same genes and are in fact different versions of the same chromosome (hence the name homologous). In contrast, human gametes (sperm and egg cells) have a single version of each of the 23 chromosomes meaning they are haploid. Therefore human gamete cells a total of 23 chromosomes. There are other ploidy numbers that cells can express that are not observed in human cells. For instance many plants, including bananas are triploid, with three versions of each chromosome present in each cell. The diagram on the left illustrates this using 3 different types of chromosomes (red, purple and yellow). |
When discussing policy number, biologists typically use the variable n. In this context n refers to the number of unique chromosomes present in a cell. For example, in human cell there are 23 unique chromosomes and so n = 23. When our cells are in a diploid state, the term 2n is used because there are two copies of each chromosomes present (2n = 46 total chromosomes). In this way, human somatic cells are labeled 2n and our gametes are labeled n since they are haploid and contain only a single copy of each chromosome.
Chromosome Structure
In humans there are 46 chromosomes arranged into 23 homologous pairs. One chromosome of each homologous pair comes from the mother (called a maternal chromosome) and one comes from the father (paternal chromosome). Homologous chromosomes are similar but not identical. Each carries the same genes in the same order, but the alleles for each trait may not be the same. Following DNA synthesis each chromosome consists of two identical sister chromatids held together by a centromere. The centromere is a reinforced region that not only holds the sister chromatids together put also serves as the site where the spindle fibers will attach during mitosis and meiosis. Every chromosome can be identified by the presence of a centromere: one centromere = 1 chromosome. This is why the policy number of the cell |
does not change following S phase and DNA replication; it does not matter if the chromosome is linear or has the X shape it is still counted as a chromosome if it has a centromere.
Prophase
During prophase the DNA condenses from chromatin into discrete packages called chromosomes. The nuclear envelope dissolves, exposing the chromosomes to the cytoplasm. The centrioles migrate to the poles of the cell and begin to generate the finger-like spindle fibers. Recall that the centrioles and spindle fibers are composed of microtubules and the spindle fibers can be lengthened or shortened by adding or removing microtubules. Prophase is typically the longest phase of mitosis. During prophase the cell is still considered diploid. |
Metaphase
In between prophase and metaphase, each chromosome (which consists of two identical sister chromatids thanks to DNA replication) is attached to the spindle fibers. During metaphase the chromosomes are positioned at the equator of the cell using the spindle fibers to physically move the chromosomes into position. Metaphase is generally the shortest phase of mitosis. During metaphase the cell is still considered diploid. |
Anaphase
During anaphase the chromosomes are pulled part, separating the sister chromatids to ensure that each daughter cell will have the same genetic material. This is accomplished by removing tubulin subunits from the spindle fibers through depolymerization, causing them to retract and pull the chromsome apart toward oppostive ends of the cell. Once the sister chromatids separate, they are considered independent chromosomes since each will inherit a centromere. Because of this the cell is actually considered tetraploid following the division of the sister chromatids since it now technically has 4 copies of each chromosomes until the cytoplasm divides during cytokinesis. This will be remedied once the cell divides, restoring the correct ploidy number. |
Telophase
The spindle fibers successfully move the sister chromatids (now considered chromosomes) to the poles of the cell and detach. Two new nuclear envelopes form around the chromosomes and mitosis is complete. Each nucleus is considered diploid and contains two complete, identical sets of the chromosomes although the cell as whole is tetraploid (2n + 2n = 4n). Cytokinesis will immediately follow telophase, formally dividing the parental cell into two, identical daughter cells and restoring the correct ploidy state to the cells. |
Cytokinesis
Cytokinesis is defined literally as the division of the cell, but cytokinesis does not occur in the same way in all cell types. In animal cells, the cell membrane pinches inward through the use of a contractile ring. This forms a cleavage furrow and ultimately divides the parent cell into two daughter cells. But in plant cells the process is different. A contractile ring cannot be utilized because of the rigid cell wall; the cell wall is not flexible enough to contract inward. Instead a cell plate develops between the two new nuclei, eventually developing into two cell membranes. Finally, a cell wall develops from the cell plate in between the two cell membranes, completing the process. |
Meiosis
Meiosis is the division of sex cells in which genetic material is not distributed evenly and is actually a two division process. Meiosis is vital to sexual reproduction, generating the gametes (sex cells) that will fuse during sexual reproduction to form a unique zygote (fertilized egg), and seeks to increase genetic variability. The greater variation within a population, the more likely it is for that population to survive. As you know, somatic cells contain two sets of each chromosome and are referred to as diploid (2n). Sex cells however only have one version of each chromosome and are known as haploid (n). This is why there is a double division of the cell during meiosis, a diploid cell (remember the parent cell technically becomes tetraploid prior to cell division because of DNA replication) must become haploid.
Without the double division of meiosis and subsequent halving of genetic material, then the amount of genetic information would double with each generation. Gametes fuse during fertilization and pool there chromosomes: human gametes contain 23 total chromosomes each so following fertilization the zygote has a total of 46 chromosomes arranged into 23 homologous pairs. But imagine we did not halve our DNA during gamete formation: then our gametes would have 46 chromosomes each. Fertilization would lead to a new organism with 92 chromosomes. If the process continued, the next generation would have 184 chromosomes, then 368 and so on. Obviously this would be a problem for the cell as that would simply too much DNA. |
Meiosis follows the same basic pattern as mitosis, there is a prophase, metaphase anaphase, telophase and finally cytokinesis but these each occur twice during meiosis. Therefore meiosis can actually be divided into two stages: meiosis I and meiosis II.
Meiosis I
Meiosis I begins with a single, diploid parent cell, but ends with two daughter cells, each with half the genetic material of the parent making them haploid. The major difference between meiosis I and mitosis is the formation of a tetrad from homologous chromosomes pairs during Prophase I as well as an event known as crossing over that increases genetic variability. Prophase I Prophase I begins with the chromatin condensing into chromosomes just as it did in mitosis. Each chromosome consists of two identical sister chromatids. However, the chromosomes do not remain on their own as in mitosis. Instead, homologous chromosomes pair up and bind together through a process called synapsis. This results in a structue called a tetrad named for the fact that it contains a total of 4 chromatids (2 sister chromatids per chromosome). Within the tetrad the non-sister chromatids (these are chromatids belonging to different chromsomes) overlap and exchange small portions of genetic code, through the process of crossing over. This will be important later and helps to increase the genetic diversity of sex cells produced at the end of meiosis. This process is depicted in the diagram to the right. |
Meiosis I continued
Meiosis continues largely in the same fashion as mitosis, the major difference being the presence of tetrads as opposed to individual chromosomes. The tetrads are attached to the spindle fibers and moved to the center of the cell. The spindle fibers then contract and pull the homologous chromosomes apart. The chromosomes arrive a the poles of the cell and are enveloped in new nuclear envelopes. However, the chromosomes at the end of meiosis I still have two sister chromatids; further division is necessary. It should be noted that these sister chromatids are no longer identical because of crossing over. |
Meiosis II
Meiosis II begins with the two cells produced from meiosis I and finishes with four daughter cells, each with a single copy/version of each chromosome, making them haploid. The main goal of meiosis II is to separate the sister chromatids of each chromosome. And when meiosis II is complete all four daughter cells will be unique from each other and their original parent cell thanks to crossing over. Because meiosis forms sex cells and fuels sexual reproduction, genetic variation is a positive advantage. The more genetically diverse a species can become, the more flexible and adaptable the species is, which increases its survivability. This is very different from mitosis which helps to generate two identical daughter cells, a perfect example of asexual reproduction.
Meiosis II begins with the two cells produced from meiosis I and finishes with four daughter cells, each with a single copy/version of each chromosome, making them haploid. The main goal of meiosis II is to separate the sister chromatids of each chromosome. And when meiosis II is complete all four daughter cells will be unique from each other and their original parent cell thanks to crossing over. Because meiosis forms sex cells and fuels sexual reproduction, genetic variation is a positive advantage. The more genetically diverse a species can become, the more flexible and adaptable the species is, which increases its survivability. This is very different from mitosis which helps to generate two identical daughter cells, a perfect example of asexual reproduction.
The End Result: Evolution and Sexual Selection
As you can see, mitosis and meiosis share a great deal in common, but have a few important differences. Mitosis occurs in somatic cells and distributes DNA equally, which means that the parent cell and the daughter cells are identical to each other. In other words mitosis enables asexual reproduction. In addition, under mitosis the number of copies or versions of each chromosome (the ploidy number) does not change from generation to generation; parent and daughter cells have two copies of each chromosome, a paternal and maternal version, making them diploid. Asexual reproduction has its advantages and purpose for organisms: it allows for rapid growth and repair. |
On the other hand, meiosis is used to generate sex cells like sperm and eggs and is closely tied to sexual reproduction. During meiosis the ploidy number of the cell halves: the parent cell is diploid, but the double division that occurs in meiosis generates 4 haploid daughter cells. In addition, at the conclusion of meiosis all four daughter cells are genetically unique from the parent cell and each each other. This is because of crossing over. In crossing over, homologous chromosomes pair up and form a tetrad (bivalent is an older term). As you know, the sister chromatids of each chromosome are identical, but the non-sister chromatids are unique from one another. Crossing over exchanges portion of the nonsister chromatids within a homologous pair, thereby ensuring that all 4 chromatids and the four resulting gametes are unique from one other. The question is why?
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The golden rule of ecology is that diversity is key to survival. The more genetically diverse a group of organisms is, the more adaptable that group becomes. Life is unpredictable and so species must be ready to adapt to changes that occur in the environment. If all individuals were identical, as would be the case if all organisms reproduced using mitosis and asexual reproduction, there would be no diversity. All organisms would have the same advantages...and the same weaknesses. In an asexually reproducing world, all organisms in an ecosystem could be wiped out by a microbe because all the organisms were all susceptible to that microbe.
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We don't live in an asexually reproducing world because sexual reproduction carries the advantage of diversity. Meiosis and sexual reproduction ensure that individuals are unique, some may be susceptible to a bacterial strain, but others will be resistant. Consider the peppered moth of England during the Industrial Revolution. Most of the moths were colored white to blend in the the tree bark and lichen of the forests they lived in, but some were black because of natural variations. Meiosis ensured that different phenotypes (physical traits) were present in that population and so when the ecosystem began to change, the moths were able to change with it. That is the advantage of meiosis and the reason for crossing over.
Nondisjunction
Meiosis is not a perfect process and occasionally errors can occur that have dire consequences for organisms. One such error is called nondisjunction. Nondisjunction can occur in either meiosis I or II and results from failures during anaphase. In meiosis I this means that the homologous chromosomes failed to separate. Nondisjunction can also occur during meiosis II if the sister chromatids fail to separate. Either way it happens, nondisjunction means some gametes receive an extra chromosome and some are missing a chromosome. When a gamete with an extra chromosome (n+1) merges with a healthy gamete |
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(n) a condition called trisomy (literally, three chromosomes) occurs. The organism now has three copies of a particular chromosome instead of the normal, healthy two. A common form of this is in humans is trisomy 21 or Down syndrome. As the name implies, a person with down syndrome has an extra 21st chromosome. When an organism has a single copy of a chromosome, the condition is known as monosomy (literally 1 chromosome). An example of a monosomy on humans would be Turner syndrome where a person has a single X chromosome and no other sex chromosome. Recall that in humans there are two sex chromosomes: X and Y, where XX = female and XY = male.
There are a greater variety of similar diseases and disorders than stem from nondisjunction, some are fatal while others have dramatic impact on the individual. It is important to understand that nondisjunction does not make the entire organism triploid, but rather a single chromosome will have three different versions within the cell; the other chromosomes will have the normal two versions of each chromosome.
Karyotype
When diagnosing genetic disorders like trisomies and monosomies, karyotypes are very helpful. They provide a complete map of a organism's genome, depicting each chromosome in an organized manner. The karyotype below is taken from a normal human, while the karyotype to right shows the genetic make up of a human with Turner Syndrome. Notice that there are two copies of every chromosome except the 23rd- the sex chromosome. There is only a single copy/version of the 23rd chromosome, this is an example of a monosomy- Turner Syndrome. |
Spermatogenesis vs. Oogenesis
There are two major types of meiosis in animals: spermatogenesis and oogenesis. Spermatogenesis generates sperm cells and follows meiosis exactly as we have described it so far. Males use spermatogenesis to produce millions and millions of sperm cells; males play a number games, they prefer quantity over quality in regards to their sex cells. Oogenesis forms eggs cells and undergoes meiosis in a slightly altered manner. In oogenesis, one large egg cell is formed along with three polar bodies. The polar bodies are very small compared to the large egg cell and are primarily used to inherit genetic material, ensuring the egg cell is haploid following oogenesis. The female invests resources very heavily into a single egg cell; she prefers quality cells over quantity. The polar bodies will actually die off and dissolve very quickly leaving only the large, haploid egg cell. This is why females typically have far fewer eggs than males have sperm. |
Cancer
The cell cycle is closely regulated by a number of regulatory proteins that control when the cell is able to divide. One such class of proteins are called cyclins, which as the name suggests instruct the cell on when to divide. But when the cellular controls of cell division are lost or damaged as a result of the accumulation of too many mutations to a cell's DNA, a condition called cancer begins. Cancer is the uncontrolled growth an division of cells and do not respond to regulatory proteins or signals that control cell division. Cancer cells tend to be smaller than
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normal, healthy cells because they keep going through cellular division with little time for the cancerous cells to grow and restore their size during interphase phase. G1 phase is generally reduced the most because S phase and G2 are still directly vital to cellular division as they each provide the genetic material and organelles necessary for division respectively. As they grow and divide, cancer cells form a mass of cells called a tumor. Tumors block nerve endings and steal resources from normal healthy cells, but not all tumors are the same: benign tumors do not invade other tissues while malignant tumors colonize the body, infesting and destroying healthy tissues. Cancer can be caused by a number of genetic and environmental factors and is a very complicated
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disease. Treatment can take a variety of forms including chemotherapy which uses toxic chemicals to kill the cancerous cells directly, radiation therapy in which radiation is used to damaged the DNA of cancerous cells, thereby stopping them from proliferating and surgery meant to outright remove the cancer cells. All of these techniques can be effective at killing the cancer cells, but they also take a major toll in the body. Chemotherapy and radiation therapy do not selective target cancer cells, all cells in the patients can be damaged. Essentially, you are hoping to kill the cancer before you kill yourself. Obviously that is a little bit of an exageration as new techniques are constantly being developed to minimize the harm to the patient and maximize the effectiveness of the treatment.