How Does a Plant Remember Winter Part 2

C G T G C A G T A DNA rand Transcription NUCLEUS C U C n Codo G Ribosome Polypeptide tRNA chain Lysine Cod on C A CYTOPLASM mRNA A Co do n G U Visual Summary Figure 14-7 Overview of Transcription and Translation UA C A A G UU U DNA encodes the information for an organism’s traits. mRNA transcribes the genes, and then tRNA builds the proteins. These proteins produce the organism’s traits. Look at Figure 14-6. Like an assembly line worker who attaches one part to another, the ribosome helps form a covalent bond, called a peptide bond between the first and second amino acids. In this example, they are methionine and phenylalanine. At the same time, the bond holding the first tRNA molecule to its amino acid is broken. That tRNA then moves into a third site, from which it exits the ribosome. The ribosome then moves to the next codon, where tRNA brings in another amino acid. The polypeptide chain continues to grow until the ribosome reaches a “stop” codon on the mRNA molecule. Then it releases both the newly synthesized polypeptide and the mRNA molecule. The Roles of tRNA and rRNA in Translation The three major forms of RNA—mRNA, tRNA, and rRNA—are each involved in the process of translation. The mRNA molecule carries the coded message that directs the process. tRNA molecules deliver the amino acids, enabling the ribosome to “read” the mRNA’s message and to get translation just right. Ribosomes themselves are composed of roughly 80 proteins and three or four different rRNA molecules. These rRNA molecules hold ribosomal proteins in place and carry out the chemical reactions that join amino acids together. As you can see in Figure 14-7, RNA molecules not only carry the genetic code, they also play a key role in translating it.  READING CHECK Summarize How do the three types of RNA work together at a ribosome to synthesize a polypeptide? A U G UU C A A A mRNA Translation READING TOOL Use Figure 14-7 to explain the relationship among mRNA, tRNA, and ribosomes. Molecular Genetics Figure 14-8 Ada Yonath In 2009, Ada Yonath was awarded the Nobel Prize in Chemistry for mapping the structure of ribosomes. Gregor Mendel might have been surprised to learn that most genes contain nothing more than instructions for assembling proteins. He might have asked what proteins could possibly have to do with the color of a flower, the shape of a leaf, or the gender of a newborn baby. The answer is that proteins have everything to do with these traits. 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 proteins that regulate patterns of tissue growth in a leaf. Yet another may trigger the female or male pattern of development in an embryo. In short, proteins are microscopic tools, each specifically designed to build or operate a component of a living cell. After scientists learned that genes were made of DNA, a series of other discoveries soon followed. When they explained the genetic code, a new scientific field called molecular biology had been established. Molecular biologists seek to understand living organisms by studying them at the molecular level, using molecules like DNA and RNA. Molecular biology provides a way to understand the links between genes and the characteristics they influence. Scientists such as Ada Yonath, shown in Figure 14-8, continue to make advancements in this field. One of the most interesting discoveries of molecular biology is the near-universal nature of the genetic code. Although some organisms show slight variations in the amino acids assigned to particular codons, the code is always read three bases at a time, always in the same direction, and is always translated on ribosomes composed of RNA and protein. Despite their enormous diversity in form and function, living organisms display remarkable unity at life’s most basic level, the molecular biology of the gene.  LESSON 14.2 Review KEY QUESTIONS 1. How does the cell interpret the genetic code? 2. How do ribosomes use mRNA and tRNA to assemble proteins? 3. How are proteins and genes related? CRITICAL THINKING 4. Synthesize Information Why can the same tRNA and rRNA molecules be used to synthesize a wide variety of polypeptides? 5. Infer How does the genetic code show a shared history among all organisms? 6. Construct an Explanation How do proteins determine the traits of an organism? LESSON Gene Regulation andExpression 14.3 KEY QUESTIONS • How are prokaryotic genes regulated? • How are genes regulated in eukaryotic cells? • What controls the development of cells and tissues in multicellular organisms? Think of a library filled with how-to books. Would you ever need to consult all of these books at the same time? Of course not. If you wanted to know how to fix a leaky faucet, you’d open the book about plumbing but not the one on carpentry. Now picture a bacterium such as E. coli with a genetic library of more than 4000 genes. How does it pick exactly the right gene to put into action? Prokaryotic Gene Regulation To conserve energy and resources, cells control which genes they express. For example, it would be wasteful for a bacterium to produce the enzymes to make a molecule it could get from its environment instead. By regulating gene expression, bacteria can respond to changes in their environment, such as the presence or absence of nutrients. How do they do this? DNA-binding proteins in prokaryotes regulate genes by controlling transcription. Some of these regulatory proteins switch genes on, while others turn genesoff. How does an organism know when to turn a gene on or off? E.coli, shown in Figure 14-9, provides us with a clear example. A total of 4288 genes code for proteins in E. coli. Among them is a cluster of 3 genes that must be turned on together before the bacterium can break apart lactose, a type of sugar. Lactose provides food for the bacterium. Because the three genes are “operated” together, the three lactose genes in E. coli are called the lac operon. An operon is a group of genes that are regulated together. HS-LS3-1: Ask questions to clarify relationships about the role of DNA and chromosomes in coding the instructions for characteristic traits passed from parents to offspring. VOCABULARY operon operator differentiation homeotic gene homeobox gene Hox gene READING TOOL Identify the main ideas and details for each heading in this lesson. Record your observations in the table in your Biology Foundations Workbook. Figure 14-9 Escherichia coli Even “simple“ organisms like E. coli have thousands ofgenes to regulate (falsecolored SEM 3600x). Figure 14-10 Gene Regulation in Prokaryotes The lac genes in E. coli are turned off by lac repressors and turned on in the presence of lactose. Promoter RNA polymerase binds here Operator Repressor Lac Repressor binds here gene P O Codes for repressor protein Lac genes Active repressor The Lac Operon Why must E. coli be able to switch the lac genes on and off? Lactose is a compound made up of two sugars: galactose and glucose. To use lactose for food, the bacterium must transport lactose across its cell membrane and then break the bond between glucose and galactose. These tasks are performed by proteins coded for by the genes of the lac operon. Remarkably, the bacterium almost seems to “know” when the products of the lac operon genes are needed, and when they’re not needed. For example, if the bacterium grows in a medium where lactose is the only food source, the genes must be transcribed to produce the proteins. However, if the environment changes to another food source, such as glucose, then the genes are not transcribed. How does the cell turn the expression of these genes on and off, and in such a useful way? Promoters and Operators The secret to the control When lactose is not present, the repressor protein binds to the operating region. This blocks RNA polymerase from transcribing the lac genes. P O of the lac operon lies on one side of the operon’s three genes, where there are two regulatory regions. The first is a promoter (P), which is a site where RNA polymerase can bind to begin transcription. The other region is called the operator (O). The O site is where a DNA-binding protein known as the lac repressor can bind to DNA. The Lac Repressor Blocks Transcription When lactose is not present, the lac repressor binds to the O region and RNA polymerase cannot reach the lac genes to begin transcription, as shown in Figure 14-10. In effect, the binding of the repressor protein switches the operon “off” by preventing the transcription of its genes. RNA polymerase When lactose is present, it binds to the repressor. This causes the release of the repressor, which then moves away from the operating region. Transcription can now take place. Lactose utilization enzymes Translation mRNA P Lactose O Inactive repressor Lactose Turns the Operon “On” Besides its DNA binding site, the lac repressor protein has a binding site for lactose itself. When lactose is in the medium, some of it diffuses into the cell and attaches to the lac repressor. This changes the shape of the repressor protein in a way that causes it to fall off the operator. Now, with the repressor no longer bound to the O site, RNA polymerase can bind to the promoter and transcribe the genes of the operon. As a result, in the presence of lactose, the operon is automatically switched on. Messenger RNA transcribed from the lac genes is then translated into proteins that enable lactose to cross the cell membrane and to be broken down for use as an energy source by the cell. Many other prokaryotic genes are switched on or off by similar mechanisms.  READING CHECK Review What is the function of the operator in the lac operon? Eukaryotic Gene Regulation The general principles of gene expression in prokaryotes also apply to eukaryotes, although the regulation of many eukaryotic genes is much more complex. Figure 14-11 shows several features of a typical eukaryotic gene. One of these is the TATA box, a short region of DNA, about 25 or 30 base pairs before the start of a gene, containing the sequence TATATA or TATAAA. The TATA box binds a protein that helps position RNA polymerase by marking a point just before the beginning of a gene. Figure 14-11 Eukaryotic Gene Regulation Many eukaryotic genes include a region called the TATA box that helps position RNA polymerase. RNA polymerase Transcription factors DNA Transcription Factors DNA-binding proteins known as transcription factors play an important part in regulating gene expression. Some transcription factors open up tightly packed chromatin to help attract RNA polymerase. Others block access to certain genes, much like prokaryotic repressor proteins. Sometimes multiple factors must bind to DNA sequences known as enhancers before a gene can be transcribed. By binding DNA sequences in the regulatory regions of eukaryotic genes, transcription factors control gene expression. A transcription factor can activate scores of genes at once, thereby dramatically affecting patterns of gene expression. Steroid hormones, for example, are chemical messengers that enter cells and bind to receptor proteins. These hormone–receptor complexes then act as transcription factors that bind to DNA and activate multiple genes. Eukaryotic gene expression can also be regulated by many other factors, including the exit of mRNA molecules from the nucleus, the stability of mRNA, and even the breakdown of a gene’s protein products. TATA box Enhancer Gene Transcription factors form binding sites for RNA polymerase. RNA polymerase Direction of transcription DNA TATA box Gene READING TOOL Construct a flowchart to show how transcription factors affect gene regulation in eukaryotes. Cell Specialization Why is gene regulation in eukaryotes more complex than in prokaryotes? Think about the way in which genes are expressed in a multicellular organism. The genes that code for liver enzymes, for example, are not expressed in nerve cells. Keratin, an important protein in skin cells, is not produced in blood cells. Cell differentiation requires genetic specialization, yet most of the cells in a multicellular organism carry the same DNA in their nucleus. Complex gene regulation in eukaryotes makes it possible for cells to be differentiated and specialized. Gene regulation also allows multicellular organisms to reproduce. A new organism generally begins as a single cell. Complex changes in gene expression allow the cell to develop into a functioning multicelluar organism.  INTERACTIVITY Investigate prokaryotic and eukaryotic gene regulation. Fruit fly chromosome Fruit fly embryo Mouse chromosomes Mouse embryo Adult fruit fly Adult mouse Figure 14-12 Hox Genes and Body Development In fruit flies, a series of Hox genes along a chromosome determine the basic body structure. Mice have similar genes on four different chromosomes. The colored areas on the fly and mouse show the approximate body areas affected by genes of the corresponding colors. Interpret Visuals What section of the bodies of flies and mice is coded by the genes shown in blue? BUILD VOCABULARY Word Origins The word part homeo comes from the Latin and Greek part homio, meaning “similar to” or “the same kind.” Homeobox genes are a group of similar genes that regulate specific structures. Genetic Control of Development Regulating gene expression is especially important during the earliest stages of development. The activation of genes in different parts of an embryo cause cells to differentiate. The process of differentiation gives rise to specialized tissues and organs. Homeotic Genes American biologist Edward B. Lewis was the first to show that a specific group of genes controls the identities of body parts in the embryo of the common fruit fly. Change one of these genes, and a body part like an antenna might actually be changed into a leg. From Lewis’s work it became clear that a set of master control genes, known as homeotic genes, regulates organs that develop in specific parts of the body. Molecular studies of homeotic genes show that they share a very similar 180-base DNA sequence, which was given the name homeobox. Homeobox genes code for transcription factors that activate other genes that are important in cell development and differentiation. In flies, a group of homeobox genes known as Hox genes are located side by side in a single cluster. As shown in Figure 14-12, Hoxgenes determine the identities of each segment of a fly’s body. They are arranged in the exact order in which they are expressed, from anterior to posterior. A mutation in one of these genes can completely change the organs that develop in specific parts of thebody. Remarkably, clusters of Hox genes exist in the DNA of other animals, including humans. These genes are arranged in the same way—from head to tail. The function of Hox genes in humans seems to be almost the same as it is in fruit flies: They tell the cells of the body how to differentiate as the body grows. This means that nearly all animals, from flies to mammals, share the same basic tools for building the different parts of the body. Common patterns of genetic control exist because all these genes have descended from the genes of common ancestors. Master control genes are like switches that trigger particular patterns of development and differentiation in cells and tissues. The details can vary from one organism to another, but the switches are nearly identical. Recent studies have shown that the very same Hox gene that triggers the development of hands and feet is also active in the fins of certainfish.  INTERACTIVITY Investigate the science behind the genetically modified mosquito being developed to fight Zika in the United States. Epigenetics Cells have another way to regulate gene expression. Recall that nuclear DNA is coiled around protein clusters, called nucleosomes, to form chromatin. In places where chromatin is tightly packed, transcription factors cannot access their DNA binding sites, and gene expression is blocked. By contrast, in regions where chromatin is opened up, gene expression is enhanced, as shown in Figure 14-13. Chromatin becomes more compact. Figure 14-13 Effect of Chemical Marks Enzymes attach methyl groups. Other enzymes remove methyl groups and attach acetyl groups. Chromatin opens up for transcription. Cells can regulate the state of chromatin by enzymes that attach chemical groups to DNA and to histone proteins. Specifically, the attachment of large numbers of methyl groups (—CH3) will cause chromatin to condense, shutting down gene expression. Conversely, the attachment of acetyl groups (—CO-CH3) will open chromatin up for transcription and gene expression. These chemical marks on chromatin are said to be epigenetic, meaning they are above the level of the genome. Epigenetic marks do not change DNA base sequences. Instead, they influence patterns of gene expression over long periods of time. During development, for example, the marks help to determine which genes are expressed in a liver cell and which are expressed in muscle or skincells.  READING CHECK Summarize How can cells regulate gene expression? The presence or absence of chemical marks determines how compact chromatin is in a given region of DNA. For transcription to occur, the chromatin must be less compact. CASE STUDY Figure 14-14 Epigenetics A caterpillar, pupa, and butterfly all have exactly the same genes. Epigenetic mechanisms control which genes are on and which are off during each stage of the insect‘s life cycle.  INTERACTIVITY Figure 14-15 Environmental Influences on Gene Expression The coloring of the coat of a Himalayan rabbit is one example of the effect of the environment on gene expression.  Predict What would happen if a cold pack were strapped to a Himalayan rabbit’s back while its fur was growing in? Raised at temperatures above 30ºC Environmental Influences As you have seen, cell differentiation is controlled at least in part by the regulation of gene expression. Conditions in an organism’s environment play a role too. In prokaryotes and eukaryotes, environmental factors such as temperature, salinity, and nutrient availability can regulate gene expression. For example, the lac operon in E. coli is switched on only when lactose is the sole food source in the bacteria’s environment. The environment can often influence how and when epigenetic marks are attached to chromatin. One well-known example involves Dutch children born near the end of World War II. When these children reached adulthood, they had much higher rates of obesity, diabetes, and cardiovascular disease than their parents and ancestors. Why? Because epigenetic marks had been attached to these individuals‘ DNA as they developed inside their malnourished mothers. Insufficient nourishment during the first three months of development changed the way genes were expressed in the growing fetus. As a result of these epigenetic changes, the body was altered so that it was better suited for an environment where food was scarce. However, life gradually returned to normal after the war, and food because plentiful again. This had a profoundly negative, lifelong impact on individuals whose gene expression was epigenetically altered to cope with food shortages. Environmental factors can also directly affect the expression of other genes. A dramatic example involves Himalayan rabbits carrying a particular gene for fur color. In a rabbit raised at temperatures between 15°C and 25°C, the gene produces black color in the fur on the animal’s nose, ears, and on the tips of its feet, as shown in Figure 14-15. The gene is not fully active at temperatures above 30°C, so only the coldest portions of the rabbit’s body allow the gene to function. If a rabbit is raised at temperatures above 30°C, the gene is completely inactive, and the entire rabbit is white.  LESSON 14.3 Review KEY QUESTIONS 1. What controls transcription in prokaryotes? 2. What are three mechanisms by which transcription factors regulate eukaryotic gene expression? 3. Why are master control genes almost universal and common to different organisms? CRITICAL THINKING 4. Construct an Explanation How does the lac operon help E. coli conserve energy and other resources? 5. Engage in Argument You are studying the genes of fruit flies. Your partner claims that each gene matches to a specific body part, such as the head, wings, and tail. How would you argue for or against this claim? 14.4 LESSON Mutations KEY QUESTIONS • In what ways do mutations change genetic information? • How do mutations affect genes? A mutation causes dwarfism in Bantam chickens. As you’ve seen, the sequence of bases in DNA are like the letters of a coded message. But what would happen if a few of those letters changed accidentally, altering the message? Could the cell still understand its meaning? Think about what might happen if someone changed at random a few lines of code in a computer program. Would the program be affected, and in ways that you would notice? Types of Mutations Now and then cells make mistakes in copying their own DNA. An incorrect base is inserted or a base is skipped during transcription. These variations are called mutations, from the Latin word mutare, meaning “to change.” Mutations are heritable changes in genetic information. Mutations can involve changes in the sequence of nucleotides in DNA or changes in the number or structure of chromosomes. Point Mutations Mutations that change a single base pair are known as point mutations because they occur at a single point in the DNA sequence. Point mutations usually involve a substitution, in which one base is changed to a different base. Substitutions usually affect no more than a single amino acid, and sometimes have no effect at all. For example, if a mutation changed one codon of mRNA from CCC to CCA, the codon would still specify the amino acid proline. Mutations like this are known as silent mutations, since they don’t affect amino acid sequence. But changing CCC to ACC might be more significant, since it would replace proline with the amino acid threonine. Mutations that change the amino acid specified by a codon are called missense mutations. HS-LS3-1: Ask questions to clarify relationships about the role of DNA and chromosomes in coding the instructions for characteristic traits passed from parents to offspring. HS-LS3-2: Make and defend a claim based on evidence that inheritable genetic variations may result from: (1)new genetic combinations through meiosis, (2) viable errors occurring during replication, and/or (3) mutations caused by environmental factors. VOCABULARY mutation point mutation frameshift mutation mutagen polyploidy READING TOOL As you read, find a brief description of each type of genetic mutation. Fill in the table in your Biology Foundations Workbook with the description and the possible effects. Sometimes a point mutation can have severe effects on gene expression. For example, if a mutation changed an mRNA codon from AAA to UAA, it would result in a stop codon instead of lysine. This is known as a nonsense mutation because it would cause translation to stop before the protein was finished. Depending on the position of the stop codon, it could result in the production of a defective protein. READING TOOL As you read about the different types of mutations in this section, create a twocolumn table in which you list each type of mutation and the effect it has on genetic information. Insertions and Deletions Mutations in which one or many bases are inserted or removed from the DNA sequence are called insertions and deletions. As shown in Figure 14-16, the significance of these changes can be dramatic. Remember that the genetic code is read three bases at a time. If a nucleotide is added or deleted, the bases are still read in groups of three, but now those groupings change in every codon that follows the mutation.  Insertions and deletions are also called frameshift mutations because they shift the “reading frame” of the genetic message. By shifting the reading frame, frameshift mutations can change every amino acid that follows the point of the mutation. They can alter a protein so much that it is unable to perform its normal functions. INTERACTIVITY Learn about the different types of point mutations and their effects. Figure 14-16 Substitution Point Mutations Point mutations involve a change in a single base pair. This change can involve the switch (substitution), loss (deletion), or gain (insertion) of a base pair. Point mutations can be classified based on how they affect the polypeptide for which the gene codes. T C T AC G T A TG G A AA AUG C A U A C C U UU Met His Before Mutations (normal) T A C G C A T G G A A A A UG C G U A C C U U U Met Arg Thr Thr Phe Insertion TA C A G C A T G G A A A AU G U C G U A C C U U U Phe Met Ser Tyr Leu Deletion G T A C C A T G G A A A .. A U G G U A C C U U U .. Met Val Pro ? ABC D E AB C D E F D E F A C B D Inversion AC Original Chromosome A B C F Deletion AB C D E F A B B C G H D E F Duplication D E F Translocation A B C A B G H C Chromosomal Mutations Chromosomal mutations involve changes in the number or structure of chromosomes. These mutations can change the location of genes on chromosomes and can even change the number of copies of some genes. There are four types of chromosomal mutations: deletion, duplication, inversion, and translocation. In Figure 14-17, the top chromosome in each example is the original chromosome. The chromosome underneath it is the chromosome that results from the mutation. Deletion involves the loss of all or part of a chromosome; duplication produces an extra copy of all or part of a chromosome; and inversion reverses the direction of parts of a chromosome. Translocation occurs when part of one chromosome breaks off and attaches to another.  READING CHECK Describe What is a frameshift mutation? Effects of Mutations Genetic material can be altered by natural events or by artificial means. The resulting mutations may or may not affect an organism. And some mutations that affect individual organisms can also affect a species or even an entire ecosystem. Many mutations are produced by errors in genetic processes. For example, some point mutations are caused by errors during DNA replication. The cellular machinery that replicates DNA inserts an incorrect base roughly once in every 10 million bases. But small changes in genes can gradually accumulate over time.  Modeling Lab Open-ended Inquiry The Effect of Mutations Problem How can mutations affect a protein? In this lab, you will be exploring different types of mutations and the possible effects they can have on a protein. You can find this lab in your digital course. E F D E F D E F Figure 14-17 Chromosomal Mutations Mutations that change the structure or number of chromosomes are called chromosomal mutations.  Analyze Diagrams What is the difference between inversion and translocation?  INTERACTIVITY Investigate different types of mutations and their effects on organisms.  ANIMATION Figure 14-18 Damage to DNA leads to mutation The ultraviolet light in sunlight can cause mutations in skin cells, resulting in skin cancer. Sunscreen and protective clothing help shield beachgoers from harmful UV radiation. BUILD VOCABULARY Root Words The root word gen means “producing.” A mutagen is something that produces a mutation. VIRTUAL LAB Identify which type of mutation can lead to a white-eyed fruit fly. Mutagens Some mutations arise from mutagens, chemical or physical agents in the environment. Chemical mutagens include certain pesticides, a few natural plant alkaloids, tobacco smoke, and environmental pollutants. Physical mutagens include some forms of electromagnetic radiation, such as ultraviolet light, shown in Figure 14-18, and X-rays. If these agents interact with DNA, they can produce mutations at high rates. Cells can sometimes repair the damage; but when they cannot, the DNA base sequence changes permanently. Some compounds interfere with base-pairing, increasing the error rate of DNA replication. Other mutagens weaken the DNA strand, causing breaks and inversions that produce chromosomal mutations. Harmful and Helpful Mutations As you’ve already seen, some mutations don’t even change the amino acid specified by a codon, while others may significantly alter a complete protein or even an entire chromosome. The effects of mutations on genes vary widely. Some have little or no effect, some produce beneficial variations, and some negatively disrupt gene function. Many if not most mutations are neutral; they have little or no effect on the expression of genes or the function of the proteins for which they code. Whether a mutation is negative or beneficial depends on how its DNA changes relative to the organism’s situation. Mutations are often thought of as negative, since they can disrupt the normal function of genes. However, without mutations, organisms could not evolve. Mutations are the source of genetic variability in a species. Harmful Effects Some of the most harmful mutations are those that dramatically change protein structure or gene activity. The defective proteins produced by these mutations can disrupt normal biological activities, and result in genetic disorders. Some cancers, for example, are the product of mutations that cause the uncontrolled growth of cells. A human genetic disorder called xeroderma pigmentosum is the result of a mutation that inactivates a protein that normally repairs DNA damaged by ultraviolet (UV) light. Individuals with this mutation have to avoid sunlight as much as possible because their skin cells can become cancerous as a result of UV damage to DNA. Figure 14-19 Effects of Mutations Polyploidy is a condition that is generally lethal in animals. But in plants, like these limes (left), polyploidy often produces stronger plants and larger fruits. A mutation damaged the immune system of the nude mouse (bottom), making it valuable for scientific research. Helpful Effects Some of the variation produced by mutations can be highly advantageous to an organism or species. Mutations often produce proteins with new or altered functions that can be useful to organisms in different or changing environments. For example, over the past 20years, mutations in the mosquito genome have made many African mosquitoes resistant to the chemical pesticides once used to control them. This may be bad news for humans, but it is highly beneficial to the insects themselves. Beneficial mutations occur in humans, too, including ones that increase bone strength and density, making fractures less likely. Some mutations increase resistance to HIV, the virus that causes AIDS. Plant and animal breeders often make use of “good” 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, like the one shown in Figure 14-19, are often larger and stronger than diploid plants. Important crop plants—including bananas, limes, and strawberries—have been produced this way. Polyploidy also occurs naturally in citrus plants, often through spontaneous mutations.  LESSON 14.4 Review KEY QUESTIONS 1. Describe the ways mutations can affect DNA and chromosomes. 2. What are the possible ways that a mutation may affect an organism? CRITICAL THINKING 3. Construct Explanations The effects of a mutation are not always visible. Choose a species and explain how you would determine whether a mutation has occurred. How might you determine what type of mutation it is? 4. Evaluate Claims A science student claims that a substitution mutation is less likely to affect gene function than an insertion or deletion mutation. Use logical reasoning to evaluate this claim. 5. Synthesize Information What are three possible ways that a mutation could change DNA, yet have no measurable effect on the organism? 6. Construct an Explanation After many years of normal function, a cell begins producing different proteins than it produced before. Propose two different explanations for this change.