CHAPTER 14 1 Mutation and Genetic Change SECTION Genes in Action KEY IDEAS As you read this section, keep these questions in mind: What is the origin of genetic differences among organisms? What kinds of mutations are possible? What are the possible effects of mutations? How can genetic change occur on a larger scale? What Are Mutations? In genetics, a mutation is a change in the structure or amount of genetic material in an organism. Most genetic differences between organisms began with a mutation. Most mutations occur when DNA or chromosomes are damaged. Mutagens, such as radiation and some chemicals, can make mutations more likely. What Kinds of Mutation Are Possible? There are three main ways that DNA can change. In a point mutation, a single nucleotide in a DNA molecule changes. In an insertion mutation, extra nucleotides are added to a DNA molecule. In a deletion mutation, nucleotides are removed from a DNA molecule. READING TOOLBOX Summarize After you read this section, write a short summary of the information in each figure. If you have trouble, work with a partner or a small group. READING CHECK 1. Define What is a mutagen? EFFECTS OF POINT MUTATIONS No mutation Silent mutation Original DNA strand A T G C C A T C G Point mutation A T G C C T T C G Original reading frame Same reading frame Original amino acids Point mutation Same reading frame Different amino acids Met Pro Ser Missense mutation Met Gln Ser Same amino acids Met Pro Ser A point mutation is a silent mutation if it does not affect the sequence of amino acids the gene codes for. A T G C A A T C G 2. Describe A gene has a nonsense mutation. Will the protein it produces be longer, shorter, or the same length as the protein produced by the normal gene? (Hint: What does a stop codon do?) A point mutation is a missense mutation if it changes one of the amino acids in the sequence. A point mutation is a nonsense mutation if it changes a codon to a stop codon. Interactive Reader 145 Genes in Action
SECTION 1 Mutation and Genetic Change continued EFFECTS OF INSERTIONS AND DELETIONS 3. Apply Concepts Suppose three nucleotides are inserted into a gene. Will this insertion mutation cause a frameshift mutation? Explain your answer. Original DNA strand A Original reading frame Original amino acids Met No mutation T G C C A T C G Pro Ser Remember that the genetic code is read in words of three letters each (codons). Insertions or deletions can change the reading frame by changing the groupings of nucleotides that are read during translation. Insertion mutation Different reading frame Different amino acids Frameshift mutation A T G G C C A T C G Met Ala Ile This insertion mutation has caused a frameshift mutation. It has changed the reading frame of the DNA sequence. As a result, the DNA codes for a different set of amino acids. What Are Chromosomal Mutations? In some cases, mutations can affect an entire chromosome. Most of these chromosomal mutations occur during crossing-over in meiosis. A B C D E Original chromosome Gene 4. Compare How is a chromosomal duplication different from a chromosomal translocation? A B D E In a chromosomal deletion, a piece of a chromosome is lost. A B A B C D E In a chromosomal duplication, a piece of a chromosome remains attached to its homologous chromosome after meiosis. The chromosome then carries both alleles for all the genes on that piece. A B D C E In a chromosomal inversion, a piece of a chromosome reattaches to its original chromosome, but in the opposite direction. A B I C D E In a chromosomal translocation, a piece of a chromosome ends up on a completely different, nonhomologous chromosome. What Are the Effects of Genetic Change? Some mutations are harmful to an organism. Some mutations are helpful. However, most mutations are neither harmful nor helpful. Not all mutations can be passed on to offspring. Only mutations in germ cells can be passed on to offspring. The table on the next page gives some examples of human diseases that are caused by inherited mutations. Interactive Reader 146 Genes in Action
SECTION 1 Mutation and Genetic Change continued Disorder Dominant or recessive? Effect of mutant allele Sickle cell anemia recessive The protein that carries oxygen in the blood is defective. Tay-Sachs disease recessive in most cases An enzyme in nerve cells is defective. Cystic fibrosis recessive An enzyme in cells that secrete proteins is defective. Hemophilia A recessive (sex-linked) A protein that helps blood clot is defective. Physical symptoms poor blood circulation; organ damage nervous system damage; early death mucus buildup in certain organs; shortened life span lack of formation of blood clots; can cause severe bleeding from minor injuries 5. Identify Give two examples of recessive genetic disorders and one example of a dominant genetic disorder. Huntington disease dominant A protein in brain cells is abnormal. brain damage; shortened life span CANCER-CAUSING MUTATIONS Mutations in somatic cells may change the cells functions. Mutations in genes that control the normal growth or division of cells can cause cancer. This occurs when mutations cause normal somatic cells to start growing and dividing abnormally. What Is Large-Scale Genetic Change? Large-scale genetic change can occur when entire chromosomes or sets of chromosomes are copied or sorted incorrectly during meiosis. Normally, during meiosis, pairs of chromosomes separate in a process called disjunction. As a result, each gamete contains one copy of each chromosome. During nondisjunction, a pair of chromosomes does not separate properly. As a result, a gamete can have more than one copy of a chromosome. If the gamete fertilizes another gamete, the resulting zygote will have an extra chromosome. Another kind of large-scale genetic change happens through nondisjunction of all chromosomes. This produces a cell with multiple sets of chromosomes. This condition of polyploidy is common in plants. READING CHECK 6. Define What is nondisjunction? Interactive Reader 147 Genes in Action
Section 1 Review SECTION VOCABULARY mutation a change in the structure or amount of the genetic material of an organism nondisjunction the failure of homologous chromosomes to separate during meiosis I or the failure of sister chromatids to separate during mitosis or meiosis II polyploidy an abnormal condition of having more than two sets of chromosomes 1. Describe How are nondisjunction and polyploidy related? 2. Identify What is the origin of almost all genetic differences between organisms? 3. Describe Explain the difference between point mutations, insertion mutations, and deletion mutations. 4. Compare How is a missense mutation different from a nonsense mutation? How are they similar? 5. Apply Concepts Skin cancer can occur if the DNA in skin cells is mutated by ultraviolet radiation in sunlight. Can the mutation that causes skin cancer be passed on to offspring? Explain your answer. 6. List Describe three types of chromosomal mutations. Interactive Reader 148 Genes in Action
CHAPTER 14 SECTION 2 KEY IDEAS Genes in Action Regulating Gene Expression As you read this section, keep these questions in mind: Can the process of gene expression be controlled? What is a common form of gene regulation in prokaryotes? How does gene regulation in eukaryotes differ from gene regulation in prokaryotes? Why are proteins so important and versatile? What Is Gene Regulation? Every somatic cell in your body has the same chromosomes and, therefore, the same genes. However, your somatic cells are not all the same because not all genes are expressed in all cells all the time. Cells have complex systems that regulate, or determine, which genes are expressed in a given cell at a given time. The particular genes that are expressed in a cell determine the proteins that the cell produces. This affects the cell s structure and function. READING TOOLBOX Define As you read this section, underline words you don t understand. When you figure out what they mean, write the words and their definitions in your notebook. How Are Genes Regulated in Prokaryotes? Most gene regulation in prokaryotes depends on operons. An example of such regulation is shown below. An Example of Gene Regulation in Prokaryotes: The lac Operon A gene called a repressor gene codes for a repressor protein. The repressor protein can bind to either the operator site of the DNA or to lactose. When there is no lactose around, the repressor protein binds to the operator site of the DNA. This prevents transcription of the DNA. An operon is a group of genes with related functions and the regions of DNA that regulate them. These genes code for proteins that help break down lactose. 1. Define What is an operon? 2. Infer Would the lac operon work if the repressor protein could not bind to lactose? Explain your answer. When lactose is present, the repressor protein binds to the lactose. It leaves the operator site of the DNA. When the repressor protein leaves the operator site, the genes can be transcribed. The proteins that break down lactose are produced. Interactive Reader 149 Genes in Action
SECTION 2 Regulating Gene Expression continued Background Recall that transcription is the process in which the information in a gene is translated into an mrna molecule. Translation is the process in which the information in an mrna molecule is converted into a polypeptide. READING CHECK 3. Define What is a transcription factor? How Are Genes Regulated in Eukaryotes? There are several differences between gene regulation in prokaryotic cells and eukaryotic cells: More proteins are involved in gene regulation in eukaryotic cells than in prokaryotic cells. Transcription and translation in eukaryotic cells can be regulated separately, because they are separated by the nuclear membrane. Operons are very rare in eukaryotic cells. Much of the DNA in a eukaryotic cell will never be transcribed or translated into proteins. Gene regulation in eukaryotic cells can happen before transcription, after transcription, or after translation. GENE REGULATION BEFORE TRANSCRIPTION Most gene regulation in eukaryotes happens before transcription. The proteins that are involved in this kind of regulation are called transcription factors. The figure below shows an example of gene regulation by transcription factors. An Example of Gene Regulation in Eukaryotes This transcription factor is an activator. It can bind both to an enhancer site on the DNA and to an RNA polymerase. This RNA polymerase can bind to the promoter site on the DNA. This transcription factor can bind to the RNA polymerase and to the coding region of the gene. It helps to make sure the RNA polymerase binds to the correct region of the gene. 4. Describe In the figure, what allows transcription to begin? When the activator binds to both the enhancer site and the RNA polymerase, transcription can begin. Many of the genes in eukaryotes contain introns. Introns are segments of the genetic code that will not be translated into amino acids. Portions of the gene that will be translated into amino acids are called exons. Interactive Reader 150 Genes in Action
SECTION 2 Regulating Gene Expression continued GENE REGULATION AFTER TRANSCRIPTION The mrna that forms during transcription contains both introns and exons. During a process called RNA splicing, the introns are removed from the mrna strand. The remaining exons are spliced, or joined, together to form the mrna strand that will be translated into a protein. 5. Explain Why is RNA splicing necessary? RNA Splicing Exon Transcription Intron mrna Introns removed and exons spliced together mrna leaves nucleus Translation mrna 6. Describe Where does RNA splicing occur? GENE REGULATION AFTER TRANSLATION Proteins do not always go straight into action after they are formed. Some undergo chemical changes that alter their shape, stability, and reactions with other molecules. During the process of protein sorting, specific proteins are directed to places in the cell where they are needed. What Roles Do Proteins Have in Cells? A protein s sequence of amino acids determines its structure. The structure, in turn, determines the protein s function. Parts of a protein that have specific chemical structures and functions are called domains. Proteins play key roles in gene expression. Some help to make mrna, trna, and rrna. Others serve as regulatory proteins, such as transcription factors. Proteins are also important in other ways. They help to control the shape and activity of a cell. In fact, proteins do most of the work that keeps a cell functioning. Because proteins have so many different functions, they have a huge variety of structures. READING CHECK 7. Define What is protein sorting? Interactive Reader 151 Genes in Action
Section 2 Review SECTION VOCABULARY domain in a protein, a functional unit that has a distinctive pattern of structural folding exon one of several nonadjacent nucleotide sequences that are part of one gene and that are transcribed, joined together, and then translated intron a nucleotide sequence that is part of a gene and that is transcribed from DNA into mrna but not translated into amino acids operon a unit of adjacent genes that consists of functionally related structural genes and their associated regulatory genes; common in prokaryotes and phages transcription factor an enzyme that is needed to begin and/or continue genetic transcription 1. Compare What is the difference between an intron and an exon? 2. Identify What controls most gene regulation in prokaryotes? 3. List Give three differences between gene regulation in eukaryotes and gene regulation in prokaryotes. 4. Identify Fill in the blank spaces in the table to describe ways that genes are regulated in eukaryotic cells. When regulation occurs Example and description transcription factors determine when a gene is transcribed After transcription, but before translation After translation 5. Describe Give two ways that proteins are important to cells. Interactive Reader 152 Genes in Action
CHAPTER 14 3 Genome Interactions SECTION Genes in Action KEY IDEAS As you read this section, keep these questions in mind: What can we learn by comparing genomes? Can genetic material be stored and transferred by mechanisms other than chromosomes? What are the roles of genes in the development of multicellular organisms? What Can We Learn from Genomes? Remember that a genome is all the DNA that an organism or species has in one set of its chromosomes. Genomes can vary in size from about 400 genes in some microbes to more than 100,000 genes in some plants. Human genomes contain about 30,000 genes. By comparing the genomes of different organisms, scientists can learn how different species are related to one another. For example, scientists have learned that humans have about 81% of our genes in common with dogs, but only about 16% in common with slime molds. This indicates that humans and dogs are more closely related than humans and slime molds. Is All DNA Found in Chromosomes? Not all DNA in a cell is part of a gene, or even part of a chromosome. For example, mitochondria and chloroplasts contain DNA. Mobile genetic elements (MGEs) are units of DNA or RNA that can move among locations in a genome. They exist outside chromosomes. MGEs can transfer genetic material between individuals and species. The table below describes some examples of MGEs. Type of MGE Plasmid Transposon Virus Description small, circular piece of DNA; can be transferred between bacterial cells set of genes that move randomly between chromosomes; also called jumping genes small, nonliving particle consisting of DNA or RNA inside a protein coating READING TOOLBOX Summarize As you read this section, underline the main ideas. When you finish reading, write an outline of the section using the underlined ideas. Describe Which species do you think humans share the most genes with? Which species do you think we share the fewest genes with? Talk about your ideas with a small group. Then, use the resources in your library or on the World Wide Web to find out if you are correct. 1. Compare What is the difference between a plasmid and a transposon? Interactive Reader 153 Genes in Action
SECTION 3 Genome Interactions continued READING CHECK 2. Describe What produces the different structures and functions of different cells in multicellular eukaryotes? How Do Genes Affect Growth and Aging? In multicellular eukaryotes, different cells in different parts of the body have different functions. As the organism develops from a zygote, different genes are expressed in different cells. The genes that are expressed in a cell affect the structure and function of the cell. During the process of cell differentiation, new cells are modified and specialized as they multiply to form an organism. Homeotic genes regulate cell differentiation. Mutations in these genes can cause physical deformities. Genetic regulation of development is similar in all animals. For example, a set of homeotic genes called hox occurs in all animals with a head and a tail end. 3. Describe What do hox genes control? Hox genes are found in most animals that have a head and a tail end. These genes control the locations at which different body parts develop. Mutations in hox genes can cause physical deformities, such as a leg developing in place of an antenna. In multicellular organisms, two kinds of proteins regulate the cell cycle: CDK and cyclin. Without one or both of these proteins, cells may develop too slowly or too quickly. For example, errors in CDK or cyclin proteins can cause cancer. Most cells in multicellular organisms are genetically programmed to stop functioning and fall apart if they are damaged or get too old. This process of programmed cell death is called apoptosis. Interactive Reader 154 Genes in Action
Section 3 Review SECTION VOCABULARY apoptosis in multicellular organisms, a genetically controlled process that leads to the death of a cell; programmed cell death cell differentiation the process by which a cell becomes specialized for a specific structure or function during multicellular development genome the complete genetic material contained in an individual or species plasmid a genetic structure that can replicate independently of the main chromosome(s) of a cell; usually, a circular DNA molecule in bacteria (prokaryotes) transposon a genetic sequence that is randomly moved, in a functional unit, to new places in a genome 1. Compare What is the difference between a genome and a gene? 2. Infer A scientist is studying three different species. The scientist concludes that species A is more closely related to species B than to species C. How might the scientist have come to this conclusion? 3. Identify Give three examples of MGEs. 4. Describe What can happen to an organism if its hox genes are mutated? 5. Apply Concepts Most cells undergo apoptosis if their DNA is damaged. How can this be beneficial to an organism? 6. Identify Give two groups of proteins that help to regulate the cell cycle. Interactive Reader 155 Genes in Action