Reading Essentials for Biology Glencoe (794133), страница 31
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Themutation could result in a protein that does not work correctlyand the offspring may not survive. Sometimes a mutation is helpful. The mutation may produce a new trait. Maybe the mutationresults in the ability to see farther or run faster. The offspringmay survive better in its environment. It can pass the new trait onto its own offspring. Later you will learn how mutations played arole in evolution.Mutations in body cells are usually caused by outside forces.Radiation from the sun, X rays, or radioactive materials can changethe DNA of skin, muscle, or bone cells.
Since these cells are notsex cells, the changes in the DNA are not passed on to offspring.However, the organism may be harmed by the mutation. Whenmutated cells divide they pass on the mutation. For example,damage to a stomach cell may cause it to lose its ability to makeREADING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.MutationsNameDateClassSection11.3Genetic Changes, continuedthe acid needed to digest food.
When that cell divides, the newcells will have the same mutation.Sometimes a mutation in a body cell affects the way the celldivides. This may cause the cells to grow and divide too quickly,producing cancer. Too much exposure to ultraviolet radiation insunlight mutates skin cells, causing skin cancer.2. Infer Why is a mutationin a body cell not passedon to offspring?________________________Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.What is happening at the DNA level?Recall how information flows from DNA to mRNA to protein.If the DNA is mutated, what will happen to the mRNA? It willtake the changed information into the cytoplasm and tRNA willbring the wrong amino acids to the rRNA. The protein that iscreated will be based on the mutated information.Mutations occur in two different ways at the DNA level. Thefirst type of mutation is called a point mutation. A point mutationhappens when there is a change in a single base pair in DNA.If the DNA sequence should be A-A-G U-U-U-G-G-C but isA-A-G-U-U-U-A-G-C then the amino acid chain is made withserine instead of glycine.
A point mutation is a little like a letterchange in a sentence.THE DOG BIT THE CATTHE DOG BIT THE CARAs you can see, it changes the meaning of the whole sentence.A frameshift mutation involves more than a single codon. Ithappens when a nitrogenous base is deleted or added. The illustration on the right shows what happens when a base is deleted.It changes everything that follows it. IfNormalwe use our sentences as an example, wemRNA A U G A A Gcan see what the result might be.THE DOG BIT THE CATProteinMetLys(correct)THE DOB ITT HEC ATPoint(deleted base (G))mutationTHE DOC GBI TTH ECA TmRNA A U G A A G(added base (C))ProteinMetLysAs you can see, a frameshift mutationoccurs when a single nitrogenous baseis added or deleted from the DNAsequence.
It shifts the reading of theFrameshiftmutationcodon by one base. A frameshift mutamRNA A U G A A Gtion is usually more harmful to anorganism than a point mutation.ProteinMetLysREADING ESSENTIALS________________________________________________Identify Details Point toeach type of mutation as youread about it.U U U G G C G C A U U G U A APheGlyAlaLeuStopReplace G with AU U U A G C G C A U U G U A APheSerAlaLeuStopDeletion of UU...U U G G C G C A U U G U A ALeuAlaHisCys...Chapter 11125NameDateClassSection11.3Genetic Changes, continuedChromosomal Alterations3. Are chromosomal mutations more common inplants or animals?________________________________________________________________________Changes occur to chromosomes as well as the DNA sequenceon the chromosomes. Sometimes parts of the chromosomes breakoff during mitosis or meiosis.
The pieces may join to the wrongchromosomes, join backwards, or join in the wrong places.Occasionally the broken pieces get lost. These structure changesin chromosomes are called chromosomal mutations.Chromosomal mutations can happen in any organism, but theyare especially common in plants. As you remember from an earlier chapter, mutations affect the way genes are distributed duringmeiosis. Some of the gametes have too many chromosomes; someof the gametes don’t have enough chromosomes. Few chromosomal mutations are passed on because the fertilized egg usuallydies. If the organism does develop, it is often not able to reproduce, so the mutation is not passed on.4. What is a mutagen?________________________________________________________________________Some mutations seem to just happen.
They are mistakes in basepairing during DNA replication. These mutations are said to bespontaneous. Many mutations are caused by environmental factors.Any outside agent that can cause a change in DNA is called amutagen (MYEW tuh jun). Mutagens include radiation, chemicals, and high temperatures.Some mutagens cause DNA to break apart. This can change thesequence of the bases. A base may disappear, or two bases mayfuse together. Other mutagens cause one base to be substitutedfor another.Can DNA be repaired?5. Conclude In what wayscould you limit yourexposure to mutagens?________________________________________________________________________126Chapter 11As you can see from observing the world around you, the geneticcode is usually passed on accurately.
But mistakes or mutations canoccur. Because of this, repair mechanisms are present in organisms. Cells contain enzymes that check the DNA sequence. If theenzymes find an incorrect sequence of nucleotides, they replace itwith the correct sequence. The repair mechanisms usually workvery well. But the more an organism is exposed to a mutagen, themore likely it is that a mistake will not be corrected. For this reason it is best to limit exposure to mutagens.READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Causes of MutationsNameDateClassSection11.3Genetic Changes, continued◗ After You ReadMini Glossarychromosomal mutation: mutation that occurswhen parts of the chromosomes break offduring mitosis or meiosis and join to thewrong chromosome, or join backwards or inthe wrong place on the chromosomeframeshift mutation: mutation that occurs whena single nitrogenous base is added ordeleted from the DNA sequence; causes ashift in the reading of codons by one basemutagen (MYEW tuh jun): any outside agentthat can cause a change in DNA; includeshigh temperatures, radiation, or chemicalsmutation: any change in a DNA sequencepoint mutation: a change in a single base pairin DNA1.
Read the key terms and definitions in the Mini Glossary above. Why is a frameshift mutation usually more harmful than a point mutation? Write your answer on the lines below.____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.2.
Use the partially completed outline below to help you review what you have read. Fill in theblanks where information is missing.I. Mutations occurA. In reproductive cellsB. IncellsII. Types of MutationsA.mutationsB.mutationsC.mutationsIII. Causes of MutationsA. Just happens = SpontaneousB. Environmental factor =Visit the Glencoe Science Web site at science.glencoe.com tofind your biology book and learn more about genetic changes.READING ESSENTIALSChapter 11127NameDateClassSection12.1 Mendelian Inheritance ofHuman TraitsSC.F.2.4.2 The student knows that every cell contains a “blueprint” coded in DNA molecules that specify how proteinsare assembled to regulate cells.◗ Before You ReadCertain genetic traits run in families.
Skim the Read to Learn section and highlight two importantfacts about how diseases are passed from generation to generation. Then write the facts you highlighted on the lines below.____________________________________________________________________________________________________________________________________________________________________________________________________________________________________◗ Read to LearnCreate an Outline Using theheadings, make an outline ofthe information in this section.1. What is a pedigree?________________________________________________Have you ever seen a family tree? What does a family treeshow? It shows the relationships among family members.
It showshow grandparents, parents, children, aunts, uncles, cousins, andsiblings are related.Geneticists are scientists who study how inherited or genetictraits pass from one generation to the next. Sometimes they usewhat is called a pedigree to study a family’s genetic puzzle. Apedigree is a visual diagram of genetic inheritance used bygeneticists to map genetic traits. A pedigree uses a set of symbolsto identify males and females in a certain family that carry thegenetic trait being studied.Why is a pedigree important?Pedigree SymbolsMaleParentsFemaleSiblingsAffectedmaleKnownheterozygotesfor recessivealleleAffectedfemaleMating128Chapter 12DeathPerhaps a family has members who suffer from a rare geneticillness. By studying the pedigree, the geneticist will be able to seeif it is likely that the trait will be passed on to children or grandchildren.
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