Answers to AllQuestions and ProblemsChapter 11.1 In a few sentences, what were Mendel’s key ideas aboutinheritance?ANS: Mendel postulated transmissible factors—genes—toexplain the inheritance of traits. He discovered thatgenes exist in different forms, which we now call alleles.Each organism carries two copies of each gene. Duringreproduction, one of the gene copies is randomly incorporated into each gamete. When the male and femalegametes unite at fertilization, the gene copy number isrestored to two. Different alleles may coexist in an organism. During the production of gametes, they separatefrom each other without having been altered bycoexistence.ANS: There are 3 141 423 nucleotides in the gene’s coding sequence. Its polypeptide product will contain 141amino acids.1.7 The template strand of a gene being transcribed is CTTGCCAGT. What will be the sequence of the RNA madefrom this template?ANS: GAACGGUCT1.8 What is the difference between transcription andtranslation?ANS: Transcription is the production of an RNA chain using aDNA chain as a template. Translation is the productionof a chain of amino acids—that is, a polypeptide—usingan RNA chain as a template.1.2 Both DNA and RNA are composed of nucleotides. Whatmolecules combine to form a nucleotide?1.9 RNA is synthesized using DNA as a template. Is DNAever synthesized using RNA as a template? Explain.ANS: Each nucleotide consists of a sugar, a nitrogen-containingbase, and a phosphate.ANS: Sometimes, DNA is synthesized from RNA in a processcalled reverse transcription. This process plays an important role in the life cycles of some viruses.1.3 Which bases are present in DNA? Which bases are present in RNA? Which sugars are present in each of thesenucleic acids?ANS: The bases present in DNA are adenine, thymine, guanine, and cytosine; the bases present in RNA are adenine,uracil, guanine, and cytosine. The sugar in DNA isdeoxyribose; the sugar in RNA is ribose.1.4 What is a genome?ANS: A genome is the set of all the DNA molecules that arecharacteristic of an organism. Each DNA moleculeforms one chromosome in a cell of the organism.1.5 The sequence of a strand of DNA is ATTGCCGTC. Ifthis strand serves as the template for DNA synthesis,what will be the sequence of the newly synthesizedstrand?ANS: TAACGGCAG1.6 A gene contains 141 codons. How many nucleotides arepresent in the gene’s coding sequence? How many aminoacids are expected to be present in the polypeptideencoded by this gene?1.10 The gene for a-globin is present in all vertebrate species.Over millions of years, the DNA sequence of this genehas changed in the lineage of each species. Consequently,the amino acid sequence of a-globin has also changed inthese lineages. Among the 141 amino acid positions inthis polypeptide, human a-globin differs from sharka-globin in 79 positions; it differs from carp a-globin in68 and from cow a-globin in 17. Do these data suggestan evolutionary phylogeny for these vertebrate species?ANS: The human and cow a-globins are least different; therefore, on the assumption that differences in a-globinreflect the degree of phylogenetic relationship, thehuman and the cow are the most closely related organisms among those mentioned. The next closest “relative”of humans is the carp, and the most distant relative is theshark.1.11 Sickle-cell anemia is caused by a mutation in one of thecodons in the gene for b-globin; because of this mutation,the sixth amino acid in the b-globin polypeptide is avaline instead of a glutamic acid. A less severe type of anemia is caused by a mutation that changes this same codonWC-1ONLINE AnswerstoOddNumberedQuestionsandProblems.indd 18/14/2015 6:42:39 PM
2-WC Answers to All Questions and Problemsto one specifying lysine as the sixth amino acid in theb-globin polypeptide. What word is used to describe thetwo mutant forms of this gene? Do you think that an individual carrying these two mutant forms of the b-globingene would suffer from anemia? Explain.ANS: The two mutant forms of the b-globin gene are properlydescribed as alleles. Because neither of the mutant allelescan specify a “normal” polypeptide, an individual whocarries each of them would probably suffer fromanemia.1.12 Hemophilia is an inherited disorder in which the bloodclotting mechanism is defective. Because of this defect,people with hemophilia may die from cuts or bruises,especially if internal organs such as the liver, lungs, orkidneys have been damaged. One method of treatmentinvolves injecting a blood-clotting factor that has beenpurified from blood donations. This factor is a proteinencoded by a human gene. Suggest a way in which modern genetic technology could be used to produce thisfactor on an industrial scale. Is there a way in which theinborn error of hemophilia could be corrected by humangene therapy?ANS: The gene for the human clotting factor could be isolatedfrom the human genome and transferred into bacteria,which could then be grown in vats to produce largeamounts of the gene’s protein product. This product couldbe isolated from the bacteria, purified, and then injectedinto patients to treat hemophilia. Another approach wouldbe to transfer a normal copy of the clotting factor geneinto the cells of people who have hemophilia. If expressedproperly, the transferred normal gene might be able tocompensate for the mutant allele these people naturallycarry. For this approach to succeed, the normal clottingfactor gene would have to be transferred into the cells thatproduce clotting factor, or into their precursors.Chapter 22.1 Carbohydrates and proteins are linear polymers. Whattypes of molecules combine to form these polymers?ANS: Sugars combine to form carbohydrates; amino acidscombine to form proteins.2.2 All cells are surrounded by a membrane; some cells aresurrounded by a wall. What are the differences betweencell membranes and cell walls?ANS: Cell membranes are made of lipids and proteins; theyhave a fluid structure. Cell walls are made of more rigidmaterials such as cellulose.Eukaryotic cells usually possess a well-developed internal system of membranes and they also have membranebounded subcellular organelles such as mitochondriaand chloroplasts; prokaryotic cells do not typically havea system of internal membranes (although some do), nordo they possess membrane-bounded organelles.2.4 Distinguish between the haploid and diploid states.What types of cells are haploid? What types of cells arediploid?ANS: In the haploid state, each chromosome is representedonce; in the diploid state, each chromosome is represented twice. Among multicellular eukaryotes, gametes are haploid and somatic cells are diploid.2.5 Compare the sizes and structures of prokaryotic andeukaryotic chromosomes.ANS: Prokaryotic chromosomes are typically (but not always)smaller than eukaryotic chromosomes; in addition, prokaryotic chromosomes are circular, whereas eukaryoticchromosomes are linear. For example, the circular chromosome of E. coli, a prokaryote, is about 1.4 mm in circumference. By contrast, a linear human chromosomemay be 10–30 cm long. Prokaryotic chromosomes alsohave a comparatively simple composition: DNA, someRNA, and some protein. Eukaryotic chromosomes aremore complex: DNA, some RNA, and a lot of protein.2.6 With a focus on the chromosomes, what are the keyevents during interphase and M phase in the eukaryoticcell cycle?ANS: During interphase, the chromosomes duplicate. DuringM phase (mitosis), the duplicated chromosomes, eachconsisting of two identical sister chromatids, condense(a feature of prophase), migrate to the equatorial plane ofthe cell (a feature of metaphase), and then split so thattheir constituent sister chromatids are separated into different daughter cells (a feature of anaphase); this lastprocess is called sister chromatid disjunction.2.7 Which typically lasts longer, interphase or M phase? Canyou explain why one of these phases lasts longer than theother?ANS: Interphase typically lasts longer than M phase. Duringinterphase, DNA must be synthesized to replicate all thechromosomes. Other materials must also be synthesizedto prepare for the upcoming cell division.2.8 In what way do the microtubule organizing centers ofplant and animal cells differ?2.3 What are the principal differences between prokaryoticand eukaryotic cells?ANS: The microtubule organizing centers of animal cells havedistinct centrosomes, whereas the microtubule organizing centers of plant cells do not.ANS: In a eukaryotic cell, the many chromosomes are contained within a membrane-bounded structure called thenucleus; the chromosomes of prokaryotic cells are notcontained within a special subcellular compartment.2.9 Match the stages of mitosis with the events they encompass: Stages: (1) anaphase, (2) metaphase, (3) prophase,and (4) telophase. Events: (a) reformation of the nucleolus, (b) disappearance of the nuclear membrane,ONLINE AnswerstoOddNumberedQuestionsandProblems.indd 28/14/2015 6:42:39 PM
Answers to All Questions and Problems WC-3(c) condensation of the chromosomes, (d) formation ofthe mitotic spindle, (e) movement of chromosomes tothe equatorial plane, (f) movement of chromosomes tothe poles, (g) decondensation of the chromosomes, (h)splitting of the centromere, and (i) attachment of microtubules to the kinetochore.ANS: (1) Anaphase: (f), (h); (2) metaphase: (e), (i); (3) prophase:(b), (c), (d); (4) telophase: (a), (g).2.10 Arrange the following events in the correct temporalsequence during eukaryotic cell division, starting withthe earliest: (a) condensation of the chromosomes,(b) movement of chromosomes to the poles, (c) duplication of the chromosomes, (d) formation of the nuclearmembrane, (e) attachment of microtubules to the kinetochores, and (f) migration of centrosomes to positions onopposite sides of the nucleus.ANS: (c), (f), (a), (e), (b), (d).2.11 In human beings, the gene for b-globin is located on chromosome 11, and the gene for a-globin, which is anothercomponent of the hemoglobin protein, is located on chromosome 16. Would these two chromosomes be expected topair with each other during meiosis? Explain your answer.ANS: Chromosomes 11 and 16 would not be expected to pairwith each other during meiosis; these chromosomes areheterologues, not homologues.2.12 A sperm cell from the fruit fly Drosophila melanogastercontains four chromosomes. How many chromosomeswould be present in a spermatogonial cell about to entermeiosis? How many chromatids would be present in aspermatogonial cell at metaphase I of meiosis? Howmany would be present at metaphase II?ANS: There are eight chromosomes in a Drosophila spermatogonial cell about to enter meiosis. There are 16 chromatids in a Drosophilia spermatogonial cell at metaphase I ofmeiosis. There are eight chromatids in a Drosophilia cellat metaphase II of meiosis.2.13 Does crossing over occur before or after chromosomeduplication in cells going through meiosis?ANS: Crossing over occurs after chromosomes have duplicatedin cells going through meiosis.2.14 What visible characteristics of chromosomes indicatethat they have undergone crossing over during meiosis?ANS: The chiasmata, which are visible late in prophase I ofmeiosis, indicate that chromosomes have crossed over.2.15 During meiosis, when does chromosome disjunctionoccur? When does chromatid disjunction occur?How many are present in the male gametophyte? Arethese nuclei haploid or diploid?ANS: Leaf tissue is diploid. The female gametophyte containseight identical haploid nuclei. The male gametophytecontains three identical haploid nuclei.2.17 From the information given in Table 2.1 in this chapter,is there a relationship between genome size (measured inbase pairs of DNA) and gene number? Explain.ANS: Among eukaryotes, there does not seem to be a clearrelationship between genome size and gene number. Forexample, humans, with 3.2 billion base pairs of genomicDNA, have about 20,500 genes, and Arabidopsis plants,with about 150 million base pairs of genomic DNA, haveroughly the same number of genes as humans. However,among prokaryotes, gene number is rather tightly correlated with genome size, probably because there is solittle nongenic DNA.2.18 Are the synergid cells in an Arabidopsis female gametophyte genetically identical to the egg cell nestled betweenthem?ANS: Yes.2.19 A cell of the bacterium Escherichia coli, a prokaryote, contains one chromosome with about 4.6 million base pairsof DNA comprising 4288 protein-encoding genes. A cellof the yeast Saccharomyces cerevisiae, a eukaryote, containsabout 12 million base pairs of DNA comprising 6268genes, and this DNA is distributed over 16 distinct chromosomes. Are you surprised that the chromosome of aprokaryote is larger than some of the chromosomes of aeukaryote? Explain your answer.ANS: It is a bit surprising that yeast chromosomes are, on average, smaller than E. coli chromosomes because, as a rule,eukaryotic chromosomes are larger than prokaryoticchromosomes. Yeast is an exception because its genome—not quite three times the size of the E. coli genome—isdistributed over 16 separate chromosomes.2.20 Given the way that chromosomes behave during meiosis,is there any advantage for an organism to have an evennumber of chromosome pairs (such as Drosophila does),as opposed to an odd number of chromosome pairs (suchas human beings do)?ANS: No, there is no advantage associated with an even number of chromosomes. As long as the chromosomes comein pairs, they will be able to synapse during prophase Iand then disjoin during anaphase I to distribute thegenetic material properly to the two daughter cells.ANS: Chromosome disjunction occurs during anaphase I.Chromatid disjunction occurs during anaphase II.2.21 In flowering plants, two nuclei from the pollen grain participate in the events of fertilization. With which nucleifrom the female gametophyte do these nuclei combine?What tissues are formed from the fertilization events?2.16 In Arabidopsis, is leaf tissue haploid or dipl
WC-1 Answers to All Questions and Problems Chapter 1 1.1 In a few sentences, what were Mendel’s key ideas about inheritance? ANS: Mendel postulated transmissible factors—genes—to explain the inheritance of traits. He discovered that genes exist in different forms, which we now call alleles.