Ch 12

Editor's Notes

• #5: Like many stories in science, the discovery of the molecular nature of the gene began with an investigator who was actually looking for something else. In 1928, British scientist Frederick Griffith was trying to figure out how bacteria make people sick. More specifically, Griffith wanted to learn how certain types of bacteria produce a serious lung disease known as pneumonia.
• #6: Griffith's Experiments Griffith injected mice with four different samples of bacteria. When injected separately, neither heat-killed, disease-causing bacteria nor live, harmless bacteria killed the mice. The two types injected together, however, caused fatal pneumonia. From this experiment, biologists inferred that genetic information could be transferred from one bacterium to another.  Inferring  After heating the disease-causing bacteria, why did Griffith test whether material from the bacterial culture would produce new colonies in a petri dish?
• #7: Transformation is the process in which one strain of bacteria is changed by a gene or genes from another strain of bacteria
• #8: In 1944, Avery and MacLeod, and McCarty purified the transforming principle of Fred Griffith's earlier experiment with pneumococcus bacteria. By a process of elimination, these scientists showed that DNA was the factor that caused genetic transformation. They made an extract, or juice, from the heat-killed bacteria. They then carefully treated the extract with enzymes that destroyed proteins, lipids, carbohydrates, and other molecules, including the nucleic acid RNA. Transformation still occurred. Obviously, since these molecules had been destroyed, they were not responsible for the transformation. Avery and the other scientists repeated the experiment, this time using enzymes that would break down DNA. When they destroyed the nucleic acid DNA in the extract, transformation did not occur. There was just one possible conclusion. DNA was the transforming factor.
• #10: They collaborated in studying viruses, nonliving particles smaller than a cell that can infect living organisms. Bacteriophage: virus that infects bacteria
• #11: They collaborated in studying viruses, nonliving particles smaller than a cell that can infect living organisms. Bacteriophage: virus that infects bacteria
• #12:  Hershey and Chase reasoned that if they could determine which part of the virus—the protein coat or the DNA core—entered the infected cell, they would learn whether genes were made of protein or DNA. To do this, they grew viruses in cultures containing radioactive isotopes of phosphorus-32 (32P) and sulfur-35 (35S). This was a clever strategy because proteins contain almost no phosphorus and DNA contains no sulfur. The radioactive substances could be used as markers. If 35S was found in the bacteria, it would mean that the viruses' protein had been injected into the bacteria. If 32P was found in the bacteria, then it was the DNA that had been injected. Click the article at right for more information on radioisotopes.
• #16: Nucleotide: monomer of nucleic acids made up of a 5-carbon sugar, a phosphate group, and a nitrogenous base
• #17: There are four kinds of nitrogenous bases in DNA. Two of the nitrogenous bases, adenine (AD-uh-neen) and guanine (GWAH-neen), belong to a group of compounds known as purines. The remaining two bases, cytosine (SY-tuh-zeen) and thymine (THY-meen), are known as pyrimidines. Purines have two rings in their structures, whereas pyrimidines have one ring.
• #20: She used a technique called X-ray diffraction to get information about the structure of the DNA molecule. Franklin purified a large amount of DNA and then stretched the DNA fibers in a thin glass tube so that most of the strands were parallel. Then she aimed a powerful X-ray beam at the concentrated DNA samples, and recorded the scattering pattern of the X-rays on film. Franklin worked hard to make better and better patterns from DNA until the patterns became clear. The result of her work is the X-ray pattern shown below. By itself, Franklin's X-ray pattern does not reveal the structure of DNA, but it does carry some very important clues. The X-shaped pattern in the photograph in the image below shows that the strands in DNA are twisted around each other like the coils of a spring, a shape known as a helix. The angle of the X suggests that there are two strands in the structure. Other clues suggest that the nitrogenous bases are near the center of the molecule.
• #21: At the same time that Franklin was continuing her research, Francis Crick, a British physicist, and James Watson, an American biologist, were trying to understand the structure of DNA by building three-dimensional models of the molecule. Their models were made of cardboard and wire. They twisted and stretched the models in various ways, but their best efforts did nothing to explain DNA's properties. Then, early in 1953, Watson was shown a copy of Franklin's remarkable X-ray pattern. The effect was immediate. In his book The Double Helix, Watson wrote: “The instant I saw the picture my mouth fell open and my pulse began to race.”
• #22: When Watson and Crick evaluated their DNA model, they realized that the double helix accounted for many of the features in Franklin's X-ray pattern but did not explain what forces held the two strands together. They then discovered that hydrogen bonds could form between certain nitrogenous bases and provide just enough force to hold the two strands together. hydrogen bonds can form only between certain base pairs—adenine and thymine, and guanine and cytosine. Once they saw this, they realized that this principle, called base pairing, explained Chargaff's rules.
• #23: Watson and Crick Griffith Avery Hershey & Chase Rosalind Franklin
• #24: DNA is present in such large amounts in many tissues that it's easy to extract and analyze. Where are the genes that Mendel first described a century and a half ago?
• #26: Eukaryotic DNA is a bit more complicated. Many eukaryotes have as much as 1000 times the amount of DNA as prokaryotes. This DNA is not found free in the cytoplasm. Eukaryotic DNA is generally located in the cell nucleus in the form of a number of chromosomes. The number of chromosomes varies widely from one species to the next. For example, diploid human cells have 46 chromosomes, Drosophila cells have 8, and giant sequoia tree cells have 22.
• #27: DNA molecules are surprisingly long. The chromosome of the prokaryote E. coli, which can live in the human colon (large intestine), contains 4,639,221 base pairs. The length of such a DNA molecule is roughly 1.6 mm, which doesn't sound like much until you think about the small size of a bacterium. To fit inside a typical bacterium, the DNA molecule must be folded into a space only one one-thousandth of its length. To get a rough idea of what this means, think of a large school backpack. Then, imagine trying to pack a 300-meter length of rope into the backpack! The DNA must be dramatically folded to fit within the cell.
• #28: The DNA in eukaryotic cells is packed even more tightly. A human cell contains almost 1000 times as many base pairs of DNA as a bacterium. The nucleus of a human cell contains more than 1 meter of DNA. How is so much DNA folded into tiny chromosomes? The answer can be found in the composition of eukaryotic chromosomes. Eukaryotic chromosomes contain both DNA and protein, tightly packed together to form a substance called chromatin. Chromatin consists of DNA that is tightly coiled around proteins called histones. Chromatin is granular material visible within the nucleus; consists of DNA tightly coiled around proteins Histones are protein molecule around which DNA is tightly coiled in chromatin Together, the DNA and histone molecules form a beadlike structure called a nucleosome. Nucleosomes pack with one another to form a thick fiber, which is shortened by a system of loops and coils. Eukaryotic Chromosome Structure During most of the cell cycle, these fibers are dispersed in the nucleus so that individual chromosomes are not visible. During mitosis, however, the fibers of each individual chromosome are drawn together, forming the tightly packed chromosomes you can see through a light microscope in dividing cells. The tight packing of nucleosomes may help separate chromosomes during mitosis. There is also some evidence that changes in chromatin structure and histone-DNA binding are associated with changes in gene activity and expression. What do nucleosomes do? Nucleosomes seem to be able to fold enormous lengths of DNA into the tiny space available in the cell nucleus. This is such an important function that the histone proteins themselves have changed very little during evolution—probably because mistakes in DNA folding could harm a cell's ability to reproduce.
• #31: Before a cell divides, it duplicates its DNA in a copying process called replication. Replication is the copying process by which a cell duplicates its DNA This process ensures that each resulting cell will have a complete set of DNA molecules.  
• #32: Strands are separated Helicase unwinds the DNA double helix. The point where the DNA is separated into single strands and new DNA will be synthesized is known as the Replication Fork. DNA replication takes place simultaneously at each fork. The mechanism of replication is identical at each fork.
• #33: Single Strand Binding Proteins (SSBs) quickly coat the newly exposed single strand. Why? They bind loosely to the DNA and are displaced when the polymerase enzymes begin synthesizing the new DNA strands. Now that they are separated, the new DNA strands can act as templates for the production of two new complimentary DNA strands SSBs coat the strand to prevent the single strands from snapping back together (Reannealing). It maintains them as separate strands. Remember that the double helix consists of two antiparallel DNA strands with complimentary 5prime and 3prime strands running in opposite directions. Polymerase enzymes can synthesis nucleic acids strands only in the 5prime to 3prime direction hooking the 5prime phosphate group of an incoming nucleotide onto the 3prime hydroxyl group at the end of the growing nucleic acid chain. Because the chain grows by extension off the 3prime hydroxyl group, strand synthesis is said to proceed in a 5prime to 3 prime direction. Even when the strands are separated however, DNA Polymerase cannot simply begin copying the DNA. DNA polymerase can only extend the nucleic acid chain but cannot start one from scratch. So how do we “fix this problem??
• #34: To give the DNA polymerase a place to start, an RNA polymerase called ? (Primase) first copies a short stretch of the DNA strand. - Primase synthesizes RNA primers. This creates a complimentary RNA segment up to 60 nucleotides long that is called a primer. Now DNA polymerase can copy the DNA strand. Primers are necessary because DNA polymerase can only extend the nucleic acid chain but cannot start one from scratch.
• #36: The DNA polymerase starts at the 3prime end of the RNA primer, and using the original DNA strand as a guide begins to synthesize the new complimentary DNA strand. - Each of the two new strands gets synthesized in the 5prime to 3 prime direction - Two polymerase enzymes are required – one for each parental DNA strand. Due to the antiparallel nature of the DNA strands, however, the polymerase enzymes on the two strands start to move in opposite directions. One polymerase can remain on it’s DNA template and copy the DNA in one continuous strand. However, the other polymerase can only copy a short stretch of DNA before it runs into the primer of the previously sequenced fragment. It is therefore forces to repeatedly release the DNA strand and slide further upstream to begin extension from another RNA primer. What helps hold the DNA polymerase onto the DNA as the DNA moves through the replication machinery. (Sliding clamp). The sliding clamp makes the polymerase processive. The continually synthesized strand is known as the leading strand and the strand that is synthesized in short pieces is the lagging strand. The short pieces separated by RNA primers in the lagging strand are called Okazaki Fragments.
• #37: Before the lagging strand DNA exits the replication factory, its RNA primers must be removed and the Okazaki fragments must be joined together to create a continuous DNA strand. Next, the sequence gap created by the RNase H is then filled in by DNA polymerase which extends the 3prime end of the neighboring Okazaki fragment. The First step is the removal of the RNA primer. RNase H which recognizes RNA-DNA hybrid helices degrades the RNA by hydrolyzing its phosphodiester bonds. Finally the Okazaki fragments are joined together by DNA ligase which hooks together the 3prime end of one fragment to the 5prime phosphate group of the neighboring fragment in an ATP or NAD dependant reaction (This means that it requires energy).
• #39: DNA replication is carried out by a series of enzymes. These enzymes “unzip” a molecule of DNA. The unzipping occurs when the hydrogen bonds between the base pairs are broken and the two strands of the molecule unwind. Each strand serves as a template for the attachment of complementary bases. DNA Replication DNA replication involves a host of enzymes and regulatory molecules. You may recall that enzymes are highly specific. For this reason, they are often named for the reactions they catalyze. The principal enzyme involved in DNA replication is called DNA polymerase (PAHL-ih-mur-ayz) because it joins individual nucleotides to produce a DNA molecule, which is, of course, a polymer. DNA polymerase also “proofreads” each new DNA strand, helping to maximize the odds that each molecule is a perfect copy of the original DNA.
• #42: Strands are separated Helicase unwinds the DNA double helix. The point where the DNA is separated into single strands and new DNA will be synthesized is known as the Replication Fork. DNA replication takes place simultaneously at each fork. The mechanism of replication is identical at each fork.
• #43: Single Strand Binding Proteins (SSBs) quickly coat the newly exposed single strand. Why? They bind loosely to the DNA and are displaced when the polymerase enzymes begin synthesizing the new DNA strands. Now that they are separated, the new DNA strands can act as templates for the production of two new complimentary DNA strands SSBs coat the strand to prevent the single strands from snapping back together (Reannealing). It maintains them as separate strands. Remember that the double helix consists of two antiparallel DNA strands with complimentary 5prime and 3prime strands running in opposite directions. Polymerase enzymes can synthesis nucleic acids strands only in the 5prime to 3prime direction hooking the 5prime phosphate group of an incoming nucleotide onto the 3prime hydroxyl group at the end of the growing nucleic acid chain. Because the chain grows by extension off the 3prime hydroxyl group, strand synthesis is said to proceed in a 5prime to 3 prime direction. Even when the strands are separated however, DNA Polymerase cannot simply begin copying the DNA. DNA polymerase can only extend the nucleic acid chain but cannot start one from scratch. So how do we “fix this problem??
• #44: To give the DNA polymerase a place to start, an RNA polymerase called ? (Primase) first copies a short stretch of the DNA strand. - Primase synthesizes RNA primers. This creates a complimentary RNA segment up to 60 nucleotides long that is called a primer. Now DNA polymerase can copy the DNA strand. Primers are necessary because DNA polymerase can only extend the nucleic acid chain but cannot start one from scratch.
• #45: The DNA polymerase starts at the 3prime end of the RNA primer, and using the original DNA strand as a guide begins to synthesize the new complimentary DNA strand. - Each of the two new strands gets synthesized in the 5prime to 3 prime direction - Two polymerase enzymes are required – one for each parental DNA strand. Due to the antiparallel nature of the DNA strands, however, the polymerase enzymes on the two strands start to move in opposite directions. One polymerase can remain on it’s DNA template and copy the DNA in one continuous strand. However, the other polymerase can only copy a short stretch of DNA before it runs into the primer of the previously sequenced fragment. It is therefore forces to repeatedly release the DNA strand and slide further upstream to begin extension from another RNA primer. What helps hold the DNA polymerase onto the DNA as the DNA moves through the replication machinery. (Sliding clamp). The sliding clamp makes the polymerase processive.
• #46: The continually synthesized strand is known as the leading strand and the strand that is synthesized in short pieces is the lagging strand. The short pieces separated by RNA primers in the lagging strand are called Okazaki Fragments.
• #47: Before the lagging strand DNA exits the replication factory, its RNA primers must be removed and the Okazaki fragments must be joined together to create a continuous DNA strand. Next, the sequence gap created by the RNase H is then filled in by DNA polymerase which extends the 3prime end of the neighboring Okazaki fragment. The First step is the removal of the RNA primer. RNase H which recognizes RNA-DNA hybrid helices degrades the RNA by hydrolyzing its phosphodiester bonds.
• #48: Finally the Okazaki fragments are joined together by DNA ligase which hooks together the 3prime end of one fragment to the 5prime phosphate group of the neighboring fragment in an ATP or NAD dependant reaction (This means that it requires energy).
• #50: SSBs coat the strand to prevent the single strands from snapping back together (Reannealing). It maintains them as separate strands. Remember that the double helix consists of two antiparallel DNA strands with complimentary 5prime and 3prime strands running in opposite directions. Polymerase enzymes can synthesis nucleic acids strands only in the 5prime to 3prime direction hooking the 5prime phosphate group of an incoming nucleotide onto the 3prime hydroxyl group at the end of the growing nucleic acid chain. Because the chain grows by extension off the 3prime hydroxyl group, strand synthesis is said to proceed in a 5prime to 3 prime direction. Even when the strands are separated however, DNA Polymerase cannot simply begin copying the DNA. DNA polymerase can only extend the nucleic acid chain but cannot start one from scratch. So how do we “fix this problem?? To give the DNA polymerase a place to start, an RNA polymerase called ? (Primase) first copies a short stretch of the DNA strand. - Primase synthesizes RNA primers. This creates a complimentary RNA segment up to 60 nucleotides long that is called a primer. Now DNA polymerase can copy the DNA strand. The DNA polymerase starts at the 3prime end of the RNA primer, and using the original DNA strand as a guide begins to synthesize the new complimentary DNA strand. - Each of the two new strands gets synthesized in the 5prime to 3 prime direction - Two polymerase enzymes are required – one for each parental DNA strand. Due to the antiparallel nature of the DNA strands, however, the polymerase enzymes on the two strands start to move in opposite directions. One polymerase can remain on it’s DNA template and copy the DNA in one continuous strand. However, the other polymerase can only copy a short stretch of DNA before it runs into the primer of the previously sequenced fragment. It is therefore forces to repeatedly release the DNA strand and slide further upstream to begin extension from another RNA primer. What helps hold the DNA polymerase onto the DNA as the DNA moves through the replication machinery. (Sliding clamp). The sliding clamp makes the polymerase processive. The continually synthesized strand is known as the leading strand and the strand that is synthesized in short pieces is the lagging strand. The short pieces separated by RNA primers in the lagging strand are called Okazaki Fragments. Before the lagging strand DNA exits the replication factory, its RNA primers must be removed and the Okazaki fragments must be joined together to create a continuous DNA strand. The First step is the removal of the RNA primer. RNase H which recognizes RNA-DNA hybrid helices degrades the RNA by hydrolyzing its phosphodiester bonds. Next, the sequence gap created by the RNase H is then filled in by DNA polymerase which extends the 3prime end of the neighboring Okazaki fragment. Finally the Okazaki fragments are joined together by DNA ligase which hooks together the 3prime end of one fragment to the 5prime phosphate group of the neighboring fragment in an ATP or NAD dependant reaction (This means that it requires energy).
• #51: The major types of proteins, which must work together during the replication of DNA, are illustrated, showing their positions. When DNA replicates, many different proteins work together to accomplish the following steps:   The two parent strands are unwound with the help of DNA helicases. Single stranded DNA binding proteins attach to the unwound strands, preventing them from winding back together. The strands are held in position, binding easily to DNA polymerase, which catalyzes the elongation of the leading and lagging strands. (DNA polymerase also checks the accuracy of its own work!). While the DNA polymerase on the leading strand can operate in a continuous fashion, RNA primer is needed repeatedly on the lagging strand to facilitate synthesis of Okazaki fragments. DNA primase, which is one of several polypeptides bound together in a group called primosomes, helps to build the primer. Finally, each new Okazaki fragment is attached to the completed portion of the lagging strand in a reaction catalyzed by DNA ligase.
• #52: Tetramer - A tetramer is a protein with four subunits (tetrameric).
• #53: DNA carries the information for making all of the cell's proteins. These pro­teins implement all of the functions of a living organism and determine the organism'­s characteristics. When the cell reproduces, it has pass all of this information on to the daughter cells. Before a cell can reproduce, it must first replicate, or make a copy of, its DNA. Different types of cells replicated their DNA at different rates. Some cells constantly divide, like those in your hair and fingernails and bone marrow cells. Other cells go through several rounds of cell division and stop (including specialized cells, like those in your brain, muscle and heart). Finally, some cells stop dividing, but can be induced to divide to repair injury (such as skin cells and liver cells). In cells that do not constantly divide, the cues for DNA replication/cell division come in the form of chemicals. These chemicals can come from other parts of the body (hormones) or from the environment.
• #61: The double helix structure explains how DNA can be copied, but it does not explain how a gene works. In molecular terms, genes are coded DNA instructions that control the production of proteins within the cell. The first step in decoding these genetic messages is to copy part of the nucleotide sequence from DNA into RNA, or ribonucleic acid. These RNA molecules contain coded information for making proteins.
• #62: You can think of an RNA molecule as a disposable copy of a segment of DNA. In many cases, an RNA molecule is a working copy of a single gene. The ability to copy a single DNA sequence into RNA makes it possible for a single gene to produce hundreds or even thousands of RNA molecules.
• #64: RNA molecules have many functions, but in the majority of cells most RNA molecules are involved in just one job—protein synthesis. The assembly of amino acids into proteins is controlled by RNA.
• #65: Most genes contain instructions for assembling amino acids into proteins. The RNA molecules that carry copies of these instructions are known as messenger RNA (mRNA) because they serve as “messengers” from DNA to the rest of the cell. Proteins are assembled on ribosomes, shown in the model below. Ribosomes are made up of several dozen proteins, as well as a form of RNA known as ribosomal RNA (rRNA). During the construction of a protein, a third type of RNA molecule transfers each amino acid to the ribosome as it is specified by coded messages in mRNA. These RNA molecules are known as transfer RNA (tRNA).
• #66: RNA molecules are produced by copying part of the nucleotide sequence of DNA into a complementary sequence in RNA, a process called transcription. Transcription requires an enzyme known as RNA polymerase that is similar to DNA polymerase.
• #67: During transcription, RNA polymerase binds to DNA and separates the DNA strands. RNA polymerase then uses one strand of DNA as a template from which nucleotides are assembled into a strand of RNA.
• #69: How does RNA polymerase “know” where to start and stop making an RNA copy of DNA? The answer to this question begins with the observation that RNA polymerase doesn't bind to DNA just anywhere. The enzyme will bind only to regions of DNA known as promoters, which have specific base sequences. In effect, promoters are signals in DNA that indicate to the enzyme where to bind to make RNA. Similar signals in DNA cause transcription to stop when the new RNA molecule is completed.
• #71: Like a writer's first draft, many RNA molecules require a bit of editing before they are ready to go into action. Remember that an RNA molecule is produced by copying DNA. Surprisingly, the DNA of eukaryotic genes contains sequences of nucleotides, called introns, that are not involved in coding for proteins. The DNA sequences that code for proteins are called exons because they are “expressed” in the synthesis of proteins. When RNA molecules are formed, both the introns and the exons are copied from the DNA. However, the introns are cut out of RNA molecules while they are still in the nucleus. The remaining exons are then spliced back together to form the final mRNA as shown in the figure below.
• #75: T
• #76: The genetic code is read three letters at a time, so that each “word” of the coded message is three bases long. Each three-letter “word” in mRNA is known as a codon, as shown in the figure below. A codon consists of three consecutive nucleotides that specify a single amino acid that is to be added to the polypeptide.
• #79: Codon A codon is a group of three nucleotides on messenger RNA that specify a particular amino acid.
• #81: Because there are four different bases, there are 64 possible three-base codons (4 × 4 × 4 = 64). The figure at right shows all 64 possible codons of the genetic code. As you can see, some amino acids can be specified by more than one codon. For example, six different codons specify the amino acid leucine, and six others specify arginine. There is also one codon, AUG, that can either specify methionine or serve as the initiation, or “start,” codon for protein synthesis. Notice also that there are three “stop” codons that do not code for any amino acid. Stop codons act like the period at the end of a sentence; they signify the end of a polypeptide, which consists of many amino acids.
• #82: There is also one codon, AUG, that can either specify methionine or serve as the initiation, or “start,” codon for protein synthesis. Notice also that there are three “stop” codons that do not code for any amino acid. Stop codons act like the period at the end of a sentence; they signify the end of a polypeptide, which consists of many amino acids.
• #83: The sequence of nucleotide bases in an mRNA molecule serves as instructions for the order in which amino acids should be joined together to produce a polypeptide. However, anyone who has tried to assemble a complex toy knows that instructions generally don't do the job themselves. They need something to read them and put them to use. In the cell, that “something” is a tiny factory called the ribosome.
• #85: The decoding of an mRNA message into a polypeptide chain (protein) is known as translation. Translation takes place on ribosomes.   During translation, the cell uses information from messenger RNA to produce proteins. A. Messenger RNA is transcribed in the nucleus, then enters the cytoplasm and attaches to a ribosome. B. Translation begins at AUG, the start codon. Each tRNA has an anticodon whose bases are complimentary to a codon on the mRNA strand. The ribosome positions the start codon to attract its anticodon, which is part of the tRNA that binds methionine. The ribosome also binds the next codon and its anticodon.
• #86: C.  Like an assembly line worker who attaches one part to another, the ribosome forms a peptide bond between the first and second amino acids, methionine and phenylalanine. At the same time, the ribosome breaks the bond that had held the first tRNA molecule to its amino acid and releases the tRNA molecule. The ribosome then moves to the third codon, where a tRNA molecule brings it the amino acid specified by the third codon. D. The polypeptide chain continues to grow until the ribosome reaches a stop codon on the mRNA molecule. When the ribosome reaches a stop codon, it releases the newly formed polypeptide and the mRNA molecule, completing the process of translation.
• #89: Gregor Mendel might have been surprised to learn that most genes contain nothing more than instructions for assembling proteins, as shown in the figure at right. He might have asked what proteins could possibly have to do with the color of a flower, the shape of a leaf, a human blood type, or the sex of a newborn baby. Assembling Proteins The answer is that proteins have everything to do with these things. Remember that many proteins are enzymes, which catalyze and regulate chemical reactions. A gene that codes for an enzyme to produce pigment can control the color of a flower. Another gene produces an enzyme specialized for the production of red blood cell surface antigen. This molecule determines your blood type. Genes for certain proteins can regulate the rate and pattern of growth throughout an organism, controlling its size and shape. In short, proteins are microscopic tools, each specifically designed to build or operate a component of a living cell.
• #92: Now and then cells make mistakes in copying their own DNA, inserting an incorrect base or even skipping a base as the new strand is put together. These mistakes are called mutations, from a Latin word meaning “to change.”   Mutations are changes in the genetic material.
• #93: Gene mutations involving changes in one or a few nucleotides are known as point mutations, because they occur at a single point in the DNA sequence. Point mutations include substitutions, in which one base is changed to another, as well as insertions and deletions, in which a base is inserted or removed from the DNA sequence. Substitutions usually affect no more than a single amino acid.
• #99: The effects of insertions or deletions can be much more dramatic. Remember that the genetic code is read in three-base codons. If a nucleotide is added or deleted, the bases are still read in groups of three, but now those groupings are shifted for every codon that follows, as shown in the figure at right. Changes like these are called frameshift mutations because they shift the “reading frame” of the genetic message. By shifting the reading frame, frameshift mutations may change every amino acid that follows the point of the mutation. Frameshift mutations can alter a protein so much that it is unable to perform its normal functions.
• #105: Chromosomal mutations involve changes in the number or structure of chromosomes. Such mutations may change the locations of genes on chromosomes, and may even change the number of copies of some genes. Deletions involve the loss of all or part of a chromosome, while duplications produce extra copies of parts of a chromosome. Inversions reverse the direction of parts of chromosomes, and translocations occur when part of one chromosome breaks off and attaches to another.
• #107: In contrast, beneficial mutations may produce proteins with new or altered activities that can be useful in changing environments. One such mutation produces resistance to HIV, the virus that causes AIDS. Plant and animal breeders often take advantage of such beneficial mutations. For example, when a complete set of chromosomes fails to separate during meiosis, the gametes that result may produce triploid (3N) or tetraploid (4N) organisms. The condition in which an organism has extra sets of chromosomes is called polyploidy. Polyploid plants are often larger and stronger than diploid plants. Important crop plants have been produced in this way, including bananas and many citrus fruits.
• #109: The lac genes are turned off by repressors and turned on by the presence of lactose.