Biology Ch 13


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Biology Ch 13

  1. 1. AN INTERVIEW WITH basis of inheritance, the structures that carried A big and useful surprise in this gene- the genes. Decades later, the master regulatory discovery enterprise has been how many genesTerry l. Orr-Weaver genes that set up the animal body plan in em- have been conserved" through evolutionaryHow the daughter cells resulting from cell bryonic development were discovered in history and are still verysimiJar in organisms asdivision end up with equal numbers of chro- Drosoph.ila. These genes determine, for exam- distantly related as fruit flies and humans. Itsmosomes is the focus of much of Terry Orr- ple, where the flys head is and where its legs very hard to figure out directly what humanWeavers research; she also studies how cells are. Then, to the amazement of biologists. it genes do. But when we discover a new gene incontrol the DNA replication that precedes turned out that exactly the same genes control Drosophila, invariably it turns out thai theres acell division. Her research group has identi· how the human body plan is set up! $0 similar gene in humans that does the samefied a number of proteins involved in these Drosophila has a really rich heritage. When I thing. So Drosophila turns out to be an evenprocesses, A professor of biology at MIT and started working on Drosophila, it had a 70- better model organism than we had guessed.the first woman to become a member of the year-long history of use in genetics, prOVidingWhitehead Institute for Biomedical Research, an incredible array of knowledge and methods. Besides genetics, what other approachesDr. Orr-Weaver has an undergraduate de- and methods do you use?gree in chemistry from UC San Diego and a How do geneticists approach biological A combination ofgenetics, biochemistry, and cellPh.D. in biological chemistry from Harvard. questions? biology turns out to be incredibly powerful forShe is a member of the U.S. National Acad- Geneticists want to discover the genes that are our research. For instance, with the microscope,emy of SCiences and a past president of the involved in a biological process and then figure a tool ofcell biology. we can literally watch theGenetics Society of America. out what the genes do. First. you decide what chromosomes as they undergo mitosis or meio- process youre interested in-in our case, how sis.Ifwe have a mutation causing a defect in oneAfter receiving your Ph.D., you switched chromosomes gel partitioned during cell divi- ofthose processes, we can look directly at howfrom using single-celled yeast as your modll sion. Next, you figure out what you would see the chromosomes behave in mutant cells. And inorganism to thl multicelled fruit fly if you had a mutant in which that process was our research on DNA replication, we can look di-Drosophila melanogaster. Why? perturbed-how would you recognize it? Then rectly at proteins that attach to the DNA during[ was excited by the possibility of working at you generate mutations, usually with the help DNA replication. In this lab, we try to do thingsthe interface of two fields. At that time, the cell of high-energy radiation or a chemical, hoping as directly as possible!cycle field was exploding from work with sin- that one or more of the resulting mutants are Heres an example from our work on DNAgle cells-mainly yeasts and mammalian cells affected in the process of interest. vt1at youre replication in Drosophila. Using a genetic ap-grown in culture-and biochemical experi- doing by making a mutation is generating a proach. we discovered a mutant that couldntments using extracts. Meanwhile, develop- disease state in your organism. vt1en youve carry out DNA replication-although such amental biology was entering a new era with found a mutant youre interested in. you can mutant can live and grow for a while by usingthe discovery of pattern formation genes. [ re- find out what gene has been made defective, proteins its mother stockpiled in the egg. Italized there was a key question at the interface and you know that that gene has to play an im- looked like the gene affected might code for aof these two fields that was being ignored: If an portant role in the process youre interested in. really important protein. But we wanted to findorganism starts as a single cell and ends up as a Now that we have the genome sequences of out if it was a protein that played a direct role inmulticellular entity, how are pattern and cell many organisms, including the fruit fly and the DNA replication, and we couldnt tell that fromdivisions coordinated? And I realized there human, you might think that all their genes are the genetics alone. However, by labeling thehad to be intrinsic regulation from the cell cy- known-so whats to discover? But it turned out normal version of the protein with a fluores-cle components but also extrinsic develop- that we didnt know the functions of many of the cent tag, we could use the microscope to seemental control feeding into that. This was genes that showed up in genome sequences. where the protein was located in cells, and weclearly going to be an important area of study. Even in an organism as simple and well-studied saw that the protein got on the DNA right at a as yeast, at the publication of the genome se- place where DNA synthesis starts. That estab-In choosing a multicellular model organism, quence we didnt know what 70% ofthe genes did. lished that this protein was directly involved.why use Drosophila? And we still dont have a clue about halfoftheA lot of fundamental discoveries in biology human genes. So having genome sequences pro- How is meiosis different from mitosis?have been made with fruit nils. For example, it vides a foundation, but we must still find out Let me first review mitosis. After a cell dupli-was research with DroS<Jph.i1a that established what all the genes do, and we geneticists feel cates its DNA, mitosis ensures that each dupli-in 1916 that chromosomes were the physical the best way to go about this is 10 use genetics. cated chromosome gets partitioned to the,%
  2. 2. daughter cells so that you end up with two brings the homologous chromosomes to- In fact, errors in meiosis are the leading causecells that have exactly the same DNA content gether. Given the relatively gigantic volume of of menial retardation in the United States.and chromosome number. Thats what hap- the nucleus and the huge mass of chromatin in Some scientists argue that, in evolutionarypens in normal cell division. a eukaryotic cell, how do the right chromo- terms, the human species is able to cope with But what about making a sperm or an egg? somes find each other? Thats the number one such a high rate of meiotic errors because mostIn fertilization, a sperm and egg are going to mystery about meiosis. And [would say the of the pregnancies are lost very early,fuse and give rise to a new progeny. [fthe or- second big mystery is why humans are so un-ganism is diploid-that is, has two similar believably bad at carrying out meiosis. Whal do you like besl about research?copies of each chromosome, as nilS and hu- Whats so great about research is that you getmans do-and the progeny is going to be What makes you say thaB Are we worse to unwrap presents all the time! ifhen youdiploid, then youve got to make sperm and than fruit flies? find an interesting mutant, its like a beautifullyeggs that have only one copy of each chromo- Were about a thousand times worse. Here are wrapped present. And then you unwrap it, andsome, so that when those sperm and egg come some amazing statistics: Twenty percent of it can be a big surprise to learn which gene istogether you restore the right chromosome recognized pregnancies end in spontaneous affected and what its protein product is. Some-number. And to produce sperm and eggs with miscarriage, and out of those at least half are times you unwrap it and its beautiful and sat-only half the diploid chromosome number, you due to a mistake during meiosis. And for every isfying and completely makes sense; otherneed a special kind of cell division-meiosis. pregnancy that proceeds far enough to be rec- times you dont at first understand what theThere has to be a way to bring together the ognized, there have been many that ended gift is, and then you have to figure that out, [two similar copies of each chromosome and without being recognized. So about 10-20% of want to tell students how wonderful it is get-then pull them apart, with each going to a dif- the time meiosis doesnt work properly in hu- ting these presents to unwrap. But they alsoferent daughter cell. [n meiosis, there is an mans. In fruit flies, meiosis seems to occur in- need to realize that it might take three years toextra round of chromosome partitioning accurately only 0.01-0.05% of the time. Even get the ribbon and all the tape off. It can takewhere what we call the homologous pairs of mice, which are mammals like ourselves, do some real patience to unwrap the present!chromosomes-the chromosome that initially much better than humans. So I would call thiscame from dad and the similar chromosome the second big mystery regarding meiosis: In your view, how important is il forthat came from mom-get separated from Why are humans so bad at it? scientists to reach out beyond the scientificeach other. To sum up, in mitosis youre sepa- community?rating identical copies of each chromosome, Tell us more about what happens when I think scientists do have an obligation to edu-whereas in meiosis you have an extra round of meiosis doesnt work correctly. cate the public. Its especially critical for peo-division stuck in there, where the copy ofeach Most of the time the result is early death of the ple to understand the importance of basicchromosome from dad and the copy from embryo. In humans, if any chromosome other research and the study of model organisms,mom get separated from each other. than X or Y is present in only one copy or Because if Ill shortchange basic research, in more than two copies, the embryo dies very the long run Ill wont be able to benefit fromWhat imporlanl questions about meiosis early. The only exceptions are for chromo- medical applications. The reason weve donestill need to be answeredl somes number 13, 18, and 21: When oneof so well in this country is our willingness to in-What we dont understand at all is how the these is present in three copies, the embyo usu- vest in basic research science.chromosomes of a homologous pair find each ally dies at an early stage, but it may survive. The challenge is figuring out how to convey aother. That pairing is unique to meiosis; it Even in the case of Down syndrome, a rela- message like this in an era ofSQundbites. Thedoesnt happen in mitosis. The two strands of tively common condition where a person has media and the public want to hear that a piece ofthe DNA dont come apart, so although the three copies of chromosome 21, only a minor- research is directly going to cure a disease. Sohomologous chromosomes have very similar ity of fetuses survive to term, And when the in- how can we get a message across that requires anDNA sequences, its not base-pairing that dividual does survive, there are serious effects, explanation more complex than a catchphrase? There are many stories Ill can tell where discov- eries in basic research have led to very important applications, but these stories take at lea;,t a couple of paragraphs to communicate. Also, theres a real need for scientists to try to build a bridge between science and the public. Its especially important to educate our rep- resentatives in government. A coalition of sev- eral biology societies, including the Genetics Society of America, has a Joint Steering Com- mittee for Public Policy, which has been ter- rific in educating members of Congress and their staffs. They bring in scientists to explain their research and why investing in research is important. Maybe thats a good starting point. Learn about an experiment by Terry Orr- Weaver in Inquiry Figure 16,22 on page 322, Jane Reece and Terry Orr-Weaver 247
  3. 3. Mei"",,-, anSexual ifeCycles .... Figure 13.1 What accounts for family resemblance? KEY CONCEPTS13.1 Offspring acquire genes from parents by Genetics is the scientific study of heredity and hereditary inheriting chromosomes variation. In this unit, you will learn about genetics at multiple13.2 Fertilization and meiosis alternate in sexual life levels, from organisms to cells to molecules. On the practical cycles side, you will see how genetics continues to revolutionize med-13.3 Meiosis reduces the number of chromosome icineand agriculture, and you will be asked to consider some so- sets from diploid to haploid cial and ethical questions raised by our ability to manipulate13.4 Genetic variation produced in sexual life cycles DNA, the genetic material. At the end of the unit, you will be contributes to evolution able to stand back and consider the whole genome, an organ- isms entire complement of DNA. The rapid accumulation of the genome sequences of many species, including our own, has taught us a great deal about evolution on the molecular level- in other words, evolution of the genome itself. In fact, genetic methods and discoveries are catalyzing progress in all areas of biology, from cell biology to physiology, developmental biology, ost people who send out birth announcementsM mention the sex of the baby, but they dont feel the need to specify that their offspring is a human be-ing! One of the characteristics of life is the ability of organ- behavior, and even ecology. We begin our study of genetics in this chapter by examin- ing how chromosomes pass from parents to offspring in sexu- ally reproducing organisms. The processes of meiosis (aisms to reproduce their own kind-elephants produce little special type of cell division) and fertilization (the fusion ofelephants, and oak trees generate oak saplings. Exceptions sperm and egg) maintain a species chromosome count duringto this rule show up only as sensational but highly suspect the sexual life cycle. We will describe the cellular mechanics ofstories in tabloid newspapers. meiosis and how this process differs from mitosis. Finally, we Another rule often taken for granted is that offspring resem- will consider how both meiosis and fertilization contribute toble their parents more than they do Wlrelated individuals. If you genetic variation, such as the variation obvious in the familyexamine the famUy members shown in Figure 13.1-Sissy shown in Figure 13.1.Spacek and Jack Fisk with daughters Madison and SchuylerFisk-you can pick out some similar features among them. The ~7;::~~g~~ire genestransmission oftraits from one generation to the next is called in-heritance, or heredity (from the Latin heres, heir). However, sonsand daughters are not identical copies ofeither parent or oftheirsiblings. Along with inherited similarity, there is also variation. from parents by inheritingFarmers have exploited the principles of heredity and variation chromosomesfor thousands of years, breeding plants and animals for desiredtraits. But what are the biological mechanisms leading to the Family friends may tell you that you have your mothers freck-hereditary similarity and variation that we call a "family resem- les or your fathers eyes. However, parents do not, in any lit-blance"? This question eluded biologists until the development of eral sense, give their children freckles, eyes, hair, or any othergenetics in the 20th century. traits. What, then, is actually inherited?248
  4. 4. Inheritance of GenesParents endow their offspring with coded information in theform of hereditary units called genes. The genes we inheritfrom our mothers and fathers are our genetic link to our par-ents, and they account for family resemblances such as sharedeye color or freckles. OUf genes program the specific traitsthat emerge as we develop from fertilized eggs into adults. The genetic program is written in the language of DNA,the polymer of four different nuc1eotides you learned aboutin Chapters 1 and 5. Inherited information is passed on inthe form of each genes specific sequence of DNA nu-deotides, much as printed information is communicated inthe form of meaningful sequences of letters. In both cases,the language is symbolic. Just as your brain translates theword apple into a mental image of the fruit, cells translategenes into freckles and other features. Most genes program (a) Hydra (b) Redwoodscells to synthesize specific enzymes and other proteins, ... Figure 13.2 Asexual reproduction in two multicellularwhose cumulative action produces an organisms inherited organisms. (a) This relatively simple animal. a hydra. reproduces bytraits. The programming of these traits in the form of DNA budding. The bud. a localized mass of mitotically dividing cells. develops into a small hydra. whICh detaches from the parent (LM). (b) Each tree inis one of the unifying themes of biology. this circle of redwoods grew from a single parent tree. whose stump is in The transmission of hereditary traits has its molecular ba- the center of the circle.sis in the precise replication of DNA, which produces copiesofgenes that can be passed along from parents to offspring. In capable of reproducing asexually (Figure 13.2). Because theanimals and plants, reproductive cells called gametes are the cells of the offspring are derived by mitosis in the parent, thevehicles that transmit genes from one generation to the next. ~chip off the old block" is usually genetically identical to itsDuring fertilization, male and female gametes (sperm and parent. An individual that reproduces asexually gives rise to aeggs) unite, thereby passing on genes of both parents to their done, a group ofgenetically identical individuals. Genetic dif-offspring. ferences occasionally arise in asexually reproducing organ- Except for small amounts of DNA in mitochondria and isms as a result of changes in the DNA called mutations,chloroplasts, the DNA of a eukaryotic cell is packaged into which we will discuss in Chapter 17.chromosomes within the nucleus. Every living species has a In sexual reproduction, two parents give rise to offspringcharacteristic number of chromosomes. For example, hu- that have unique combinations ofgenes inherited from the momans have 46 chromosomes in almost all of their cells. Each parents. In contrast to a clone, offspring ofsexual reproductionchromosome consists of a single long DNA molecule elab- vary genetically from their siblings and both parents: They areorately coiled in association with various proteins. One variations on a common theme of family resemblance, not ex-chromosome includes several hundred to a few thousand act replicas. Genetic variation like that shown in Figure 13.1 isgenes, each of which is a specific sequence of nuc1eotides an important consequence of sexual reproduction. Whatwithin the DNA molecule. A genes specific location along mechanisms generate this genetic variation? The key is the be-the length of a chromosome is called the genes locus (from havior of chromosomes during the sexual life cycle. nthe Latin, meaning "place ; plural, loci). Our genetic en-dowment consists of the genes carried on the chromosomes CONCEPT CHECK 13.1we inherited from our parents. I. How are the traits of parents (such as hair color) transmitted to their offspring?Comparison of Asexual 2. Explain how asexually reproducing organisms pro-and Sexual Reproduction duce offspring that are genetically identical to eachOnly organisms that reproduce asexually produce offspring other and to their parents.that are exact copies ofthemselves. In asexual reproduction, 3. •i,!lfU1jM A horticulturalist breeds orchids, tryinga single individual is the sole parent and passes copies of all its to obtain a plant with a unique combination of desir-genes to its offspring. For example, single-celled eukaryotic able traits. After many years, she finally succeeds. Toorganisms can reproduce asexually by mitotic cell division, in produce more plants like this one, should she breed itwhich DNA is copied and allocated equally to two daughter or clone it? Why?cells. The genomes of the offspring are virtually exact copies For suggested answers, see Appendix A.ofthe parents genome. Some multicellular organisms are also (HAPTE~ THIRTEEN Meiosis and Sexual Life Cycles 249
  5. 5. r;:~~~I~::i~~~nd 13.3 • meiosis Preparing a karyotype alternate in sexual life cycles APPLICATION A karyotype is a display of condensed chromo· somes arranged in pairs. Karyotyping can be used to screen forA life cycle is the generation-ta-generation sequence of abnormal numbers of chromosomes or defective chromosomesstages in the reproductive history of an organism, from con- associated with certain congenital disorders, such as Down syndrome.ception to production of its own offspring. In this section, weuse humans as an example to track the behavior of chromo-somes through sexual life cycles. We begin by considering thechromosome count in human somatic cells and gametes; wewill then explore how the behavior of chromosomes relates tothe human life cycle and other types of sexual life cycles.Sets of Chromosomes in Human CellsIn humans, each somatic cell-any cell other than those in-volved in gamete formation-has 46 chromosomes. Duringmitosis, the chromosomes become condensed enough to bevisible in a light microscope. Because chromosomes differ insize, in the positions of their centromeres, and in the patternof colored bands produced by certain stains, they can be dis~tinguished from one another by microscopic examinationwhen sufficiently condensed. TECH Nt QUE Karyotypes are prepared from isolated somatic cells, which are treated with a drug to stimulate mitosis and then Careful examination of a micrograph of the 46 human grown in culture for several days. Cells arrested in metaphase arechromosomes from a single cell in mitosis reveals that there stained and then viewed with a microscope equipped with a digi-are two chromosomes of each of 23 types. This becomes tal camera. A photograph of the chromosomes is displayed on a computer monitor, and the images of the chromosomes aredear when images of the chromosomes are arranged in arranged Into pairs according to size and shape.pairs, starting with the longest chromosomes. The resulting Pair of homologousordered display is called a karyotype (Figure 13.3). The two replicated chromosomeschromosomes composing a pair have the same length, cen- ~tromere position, and staining pattern: These are called Centromerehomologous chromosomes, or homologs. Both chromo-somes of each pair carry genes controlling the same inher- [illk . , .ited characters. For example, if a gene for eye color is I ,, Ofsituated at a particular locus on a certain chromosome, then •• ~ • • " .- the homolog of that chromosome will also have a gene spec- ,,, Ii" • , .. .. H ,ifying eye color at the equivalent locus. The two distinct chromosomes referred to as X and Yarean important exception to the general pattern of homolo- T Sister chromatids Metaphase -" , ii ,,~ I ; #i l( ,gous chromosomes in human somatic cells. Human females chromosome · , . ·, E •• .have a homologous pair of X chromosomes (XX), but maleshave one Xand one Ychromosome (XY). Only small parts of ", II ii , ~ " " "the X and Yare homologous. Most of the genes carried on 0 H II !Ithe X chromosome do not have counterparts on the tiny Y, " D•and the Y chromosome has genes lacking on the X. Becausethey determine an individuals sex, the X and Y chromo- ".• • ., " " II • • " .. H •somes are called sex chromosomes. The other chromo- RESULTS This karyotype shows the chromosomes fromsomes are called autosomcs. a normal human male. The size of the chromosome, position of The occurrence of homologous pairs of chromosomes in the centromere, and pattern of stained bands help identify spe-each human somatic cell is a consequence ofour sexual origins. cific chromosomes. Although difficult to discern in the karyotype, each metaphase chromosome consists of two closely attachedWe inherit one chromosome of each pair from each parent sister chromatids (see the diagram of a pair of homologousThus, the 46 chromosomes in our somatic cells are achlally two replicated chromosomes).sets of 23 chromosomes-a maternal set (from our mother)250 UNIT THREE Genetics
  6. 6. and a paternal set (from our father). The number of chromo- Behavior of Chromosome Setssomes in a single set is represented by n. Any cell with two in the Human Life Cyclechromosome sets is called a diploid cell and has a diploidnumber of chromosomes, abbreviated 2n. For humans, the The human life cycle begins when a haploid sperm from thediploid number is 46 (2n = 46), the number of chromosomes father fuses with a haploid egg from the mother. This unionin our somatic cells. In a cell in which DNA synthesis has oc- of gametes, culminating in fusion of their nuclei, is calledcurred, all the chromosomes are replicated, and therefore each fertilization. The resulting fertilized egg, or zygote, isconsists oftwo identical sister chromatids, associated closely at diploid because it contains two haploid sets of chromosomesthe centromere and along the arms. Figure 13.4 helps clarify bearing genes representing the maternal and paternal familythe various terms that we use in describing replicated chromo- lines. As a human develops into a sexually mature adult, mitosissomes in a diploid ceiL Study this figure so that you understand of the zygote and its descendants generates all the somatic ceUsthe differences between homologous chromosomes, sister of the body. Both chromosome sets in the zygote and all thechromatids, nonsister chromatids, and chromosome sets. genes they carry are passed with precision to the somatic cells. Unlike somatic cells, gametes (sperm and eggs) contain a The only cells of the human body not produced by mitosissingle chromosome set. Such cells are called haploid cells, are the gametes, which deelop from specialized reUs calledand each has a haploid number of chromosomes (n). For hu- germ cells in the gonads-ovaries in females and testes inmans, the haploid number is 23 (n = 23). The set of 23 con- males (Figure 13.5). Imagine what would happen if humansists of the 22 autosomes plus a single sex chromosome. An gametes were made by mitosis: They would be diploid likeunfertilized egg contains an X chromosome, but a sperm may the somatic cells. At the next round offertilization, when twocontain an X or a Y chromosome. gametes fused, the normal chromosome number of 46 would Note that each sexually reproducing species has a charac- double to 92, and each subsequent generation would double theteristic diploid number and haploid number. For example, the Key Haploid gametes (n " 23)fruit ny, DrosoplJila melanogaster, has a diploid number of 8 Haploid (n) Egg (n)and a haploid number of 4, while dogs have a diploid number DiplOId (2n)of78 and a haploid numberof39. Now that you have learned the concepts ofdiploid and hap-loid numbers of chromosomes, lets consider chromosomebehavior during sexual life cycles. We11 use the human life cy-cle as an example. Koy • Maternal set of 2n =6 chromosomes (n = 3) { Paternal set of • chromosomes (n = 3)Two sister chromatidsof one replicated Diploidchromosome zygote Centromere (2n = 46) Mitosis and developmentTwo nonSlSter ~""::.l,, .., ,L-_7_palf of homologousduomalldsm !!I chromosomesa homologous pair (one from each set) Mulllcellular diploid adults (2n = 46)• Figure 13.4 Desaibing chromosomes. A cell with a dipbdnumber of 6 (2n = 6) is depicted here followang chromosome replicatJOn • Figure 13.5 The human life cycle. In each generation, theand condensa1JOfl. Each of the SIX replicated chromosomes conSISts of number of chromosome sets doubles at fertilization. but IS halvedtwo SiSler chromatids assoaated dosefy along their lengths. Each dunnq meIOSIS. For humans, the number of chromosomes in a haplOIdhomologous pair IS composed of one chromosome from the maternal cell is 23. COOSlSllnq of ooe set (n = 23); the number of chromosomesset (~ and one from the paternal set (blue). Each set is made up of In the diploid lY90te and all somatIC cells ansmg from It is 46,thr~ chromosomes IrI thIS example. NonSlSter chromatids are any two Consrstlnq of two sets (2n = 46).chromatids In a pa... of homologous chromosomes that a~ not SISter ThIS figure IntrodllCe5 a color code that wia be IlSfC:f for other life"""""""n What is the haploid number of this celf? ~ a. . chromosomes haploid Of diploid? ~set~ of cydes later In thIS book. The teal arrows highlight haploid stages of a tife~, and the betge arrows highlight dIploid stages. CHAHII THIRTUN Meiosis il.nd Sexua.l Life Cycles 251
  7. 7. number of chromosomes yet again. This does not happen, Plants and some spe<ies of algae exhibit a second type oflifehowever, because in sexually reproducing organisms, the gam- cycle called alternation of generations. This type includesetes are formed by a modified type of cell division called both diploid and haploid stages that are multicellular. The mul-meiosis. This type ofcell division reduces the number ofsets of ticellular diploid stage is called the sporophyte. Meiosis in thechromosomes from two to one in the gametes, counterbalanc- sporophyte produces haploid cells called spores. Unlike a gam-ing the doubling that occurs at fertilization. In animals, meiosis ete, a haploid spore doesnt fuse with another cell but dividesoccurs only in the ovaries or testes. As a result of meiosis, each mitotically, generating a multicellular haploid stage called thehuman sperm and egg is haploid (n = 23). Fertilization restores gametophyte. Cells of the gametophyte give rise to gametes bythe diploid condition by combining two haploid sets of chro- mitosis. Fusion of two haploid gametes at fertilization resultsmosomes, and the human life cycle is repeated, generation af- in a diploid zygote, which develops into the next sporophyteter generation (see Figure 13.5). You will learn more about the generation. Therefore, in this type ofHfe cycle, the sporophyteproduction of sperm and eggs in Chapter 46. generation produces a gametophyte as its offspring, and the In general, the steps of the human life cycle are typical of gametophyte generation produces the next sporophyte gener-many sexually reproducing animals. Indeed, the processes ation (Figure 13.6b). Clearly, the term alternation ofof fertilization and meiosis are the unique trademarks of generations is a fitting name for this type of life cycle.sexual reproduction, in plants as well as animals. Fertiliza- A third type oflife cycle occurs in most fungi and some pro-tion and meiosis alternate in sexual life cycles, maintaining tists, including some algae. After gametes fuse and form aa constant number of chromosomes in each species from diploid zygote, meiosis occurs without a multicellular diploidone generation to the next. offspring developing. Meiosis produces not gametes but hap- loid cells that then divide by mitosis and give rise to either uni-The Variety of Sexual Life Cycles cellular descendants or a haploid multicellular adult organism.Although the alternation of meiosis and fertilization is com- Subsequently, the haploid organism carries out further mi-mon to all organisms that reproduce sexually, the timing of toses, producing the cells that develop into gametes. The onlythese two events in the life cycle varies, depending on the diploid stage found in these species is the single-celled zygotespecies. These variations can be grouped into three main (Figure 13.6c).types of life cycles. In the type that occurs in humans and most Note that either haploid or diploid cells can divide by mito-other animals, gametes are the only haploid cells. Meiosis oc- sis, depending on the type oftife cycle. Only diploid cells, how-curs in germ cells during the production of gametes, which ever, can undergo meiosis because haploid cells have a singleundergo no further cell division prior to fertilization. After set of chromosomes that cannot be further reduced. Thoughfertilization, the diploid zygote divides by mitosis, producing the three types ofsexual life cycles differ in the timing of meio-a multicellular organism that is diploid (Figure 13.6a). sis and fertilization, they share a fundamental result: geneticK.y Haploid (n) Haploid unicellular or• Diploid (2n) multICellular organism Gametes n o n n FERTILIZATION n Gametes 20DiPIOid~~-::~::;::::::multicellular Diploidorganism multicellular J.~ Mitosis organism / ~ Zygote (sporophyte)(a) Animals (b) Plants and some algae (c) Most fungi and some protists.. Figure 13.6 Three types of sexual life cycles. The common feature of all three cycles is the alternation ofmeiosis and fertilization, key events that contribute to genetic variation among offspring. The cycles differ in the limingof these two key events.252 UNIT THREE Genetics
  8. 8. variation among offspring. A closer look at meiosis will revealthe sources of this variation. CONCEPT CHECK 13.Z I. How does the karyotype of a human female differ io diploid PA Homologous pair of chromosomes ~ ~ from that of a human male? 2. How does the alternation of meiosis and fertilization in the life cycles of sexually reproducing organisms maintain the normal chromosome count for each ) species? 1J Chromosomes 3. Each sperm of a pea plant contains seven chromo- replicate somes. What are the haploid and diploid numbers for Homologous pair of replicated chromosomes peas? 4. • i,llfu1iM A certain eukaryote lives as a unicellu- lar organism, but during environmental stress, its cells / produce gametes. The gametes fuse, and the resulting zygote undergoes meiosis, generating new single cells. What type of organism could this be? Si"" m chrom~ ) Diploid cell with replicated For suggested answers, see Appendix A. chromosomesr:·~~::~s·r::~~es the number ; 0 X of chromosome sets from diploid to haploid X Homolog,", chromosomes separate Haploid cells with replicated chromosomesMany of the steps of meiosis closely resemble correspondingsteps in mitosis. Meiosis, like mitosis, is preceded by the repli-cation of chromosomes. However, this single replication is 8 Sister chromatids DP" c~followed by not one but two consecutive cell divisions, calledmeiosis I and meiosis II. These two divisions result in fourdaughter cells (rather than the two daughter cells of mitosis),each with only half as many chromosomes as the parent cell. Haploid cells with unrepllCated chromosomesThe Stages of MeiosisThe overview of meiosis in Figure 13.7 shows that both mem- ... Figure 13.7 Overview of meiosis: how meiosis reducesbers of a single homologous pair of chromosomes in a diploid chromosome number. After the chromosomes replicate in interphase. the diploid cell divides twice. yielding four haploidcell are replicated and that the copies are then sorted into four daughter cells This overview tracks just one pair of homologoushaploid daughter cells. Recall that sister chromatids are mO chromosomes, which for the sake of simplicity are drawn in thecopies of one chromosome, closely associated all along their condensed state throughout (they would not normally be condensed during interphase), The red chromosome was inherited from thelengths; this association is called sister chromatid cohesion. To- female parent, the blue chromosome from the male parent.gether, the sister chromatids make up one replicated chromo- ••l.f.WIII Redraw the cells in this figure using a simple DNAsome (see Figure 13.4). In contrast, the two chromosomes of a double helix (0 represent each DNA moleculehomologous pair are individual chromosomes that were inher-ited from different parents. Homologs appear alike in themicroscope. but they may have different versions of genes, Figure 13.8, on the next rno pages, describes in detail thecalled alleles, at corresponding loci (for example, an allele for stages of the two divisions of meiosis for an animal cell whosefreckles on one chromosome and an allele for the absence of diploid number is 6. Meiosis halves the total number of chro-freckles at the same locus on the homolog). Homologs are not mosomes in a very specific way, reducing the number of setsassociated with each other except during meiosis, as you will from two to one. with each daughter cell receiving one set ofsoon see. chromosomes. Study Figure 13.8 thoroughly before going on. CHAPTE~ THI~HEN Meiosis and Sexual Life Cycles 253
  9. 9. • FIguro 13.1 @:j[;mmim·TheMeioticDivisionofanAnimaICell ..........MEIOSiS I: Separates homologous chromosomes" Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis SISter chromatids Centrosome remdlll attached (WIth centnole pair)Sister ChlilSlTlilla Centromerechromattds (WIth kinetochore) Metaphase p"~ Homologous"omologoo,chromosomes Fragments of nuclear chromosomes separate envelope Each pair of homologous Two haploid cellsReplkated homologous Microtubule chromosomes separates form; each chromosomechromosomes (red and blue) attached to still consists of twopair and exchange segments; kinetochore sister chromatids2n " 6 in this example Chromosomes line up by homologous pairs Prophase I • Centrosome movement, spindle Anaphase I Telophase I and • Chromosomes begin to formation, and nuclear envelope • Breakdown 01 proteins Cytokinesis condense, and homologs loosely b/eakdown occur as in mitosis. responsible for sister chromatid • At the beginning of telophase I, pair along their lengths. aligned • In late prophase I(after the stage cohesion along chromatid arms each half of the cell has a gene by gene. shown), microtubules from one allows homologs to separate. complete haploid set of replicated chromosomes. Each • Crossing over (the exchange of pole or the other attach to the • The homologs move toward corresponding seglTl(!nts of DNA two kinetochores, protein opposite poles. guided by the chromosome is composed of two mole<ules by nonsisler chromatids) structures at the centromeres of spindle apparatus. sister chromatids; one or both is completed while homologs ale the two homologs. The chromatids include regions of • Sister chromatid cohesion persists nonsister chromatid DNA. in synapsis, held tightly together homologous pairs then move at the centromere. causing by proteins along their lengths toward the metaphase plate. chromatids to move as a unit • Cytokinesis (division of the (before the stage shown). toward the same pole. cytoplasm) usually occurs • Synapsis ends in mid·prophase. Metaphase I simultaneously with telophase I. and the chromosomes in each forming two haploid daughter ceI~ • Pairs of homologous pair move apart slightly, as chromoso~ are now arranged • In animal cells. a cleavage furrow shown above. on the metaphase plate, with IOlms.. (In plant cells. a cell plate one chromosome in each pair IOlms..) • Each homologous pair has one 01 more chiasmata, points where facing each pole. • In some spe<ies. chromoso~ Gossing over has occuued and • Bolh dwomatids of all! tonoIog are deconr:lense aoo the nudear the homologs are still associated attadled 10 kinetochore envtlope re-forms.. due 10 cohesion betweef1 sister iOOouDJIes frrm all! jXIIe; those of • No replication occurs between dllomalids (sister chromatid the other hlmolog are attathed 10 meiosis Iand meiosis II. cohesion). iOOoniluIes frrm the oppo5ite pole. 254 UNIT TUff Genetics
  10. 10. .................. MEIOSIS II: Separates sister chromatids ..... Prophase 11 Metaphase n-l,.--1111 Anaphase 11 Telophase II and Cytokinesis During another round of (ell division, the sister dlromatids finally separate: four haploid daughter cells result., containing unrepficated chromosomes SISler chromatids Haploid daughtef cells separate fOfml"9Prophase II Metaphase II Anaphase II Telophase II• Aspindle apparatus forms. • The chromosomes are positioned • Breakdown of proteins holding and Cytokinesis• In late prophase II (nOI shown on the metaphase plate as in the sister chromatids togethel at • Nuclei form, the chlOmosomes here), chromosomes, each still mitosis. the centromere allows the begin de(Ondensing, and composed of two chromatids • Because of crossing over in chromatids to separate, The cytokinesis occurs. asso<iated <lIthe centromere, meiosis I, the two sister chromatids move toward • The meiotic division of one move toward the metaphase II chromatids of each chromosome opposite poles as individual parent cell produces four plale. are not genetically identical. chromosomes. daughter cells, each with a • The kinetochores of sisler haploid set of (unreplicatedl chromatids afe attached to chlOmosomes. microtubules extlnding from • Each of the fOUl daughter cells is opposite poles, geflelically distinct hom the olhel daughter cells and flOm the parent cell. BioFlix Vlsn the Study Area at www.masteringbio.(om fOf the 8IoAix 3-D Ammation on """" (HAHUI THIRTUH Meiosis and Sexual Life C)"des 255
  11. 11. MITOSIS MEIOSIS Parent cell Chiasma (site of crossing over) MEIOSIS I (before chromosome replication) Prophase Chromosome .Ai =- t ,LcOh-,-,m-,,-,-m-,-( Prophase I Homologous chromosome replication replication pair held together by ReplICated chromosome chiasma and sister (two sister chromatids) 2n= 6 chromatid cohesion Chromosomes Chromosomes line Metaphase line up Individu- up by homologous Metaphase I ally at the pairs at the metaphase plate metaphase plate Anaphase Sister chromatids Homologs Anaphase I Telophase separate during separate Telophase I anaphase during -- .....-- anaphase I; Haploid sister n=3 ® " ..... - 20 @ ..... 20 chromatids remain attached at centromere Sister chroma- MEIOSIS II Daughter cells tids of mitosis separate o dunng Daughter cells of meiosis II anaphase II SUMMARY Property MitOSIS MeIOSIS DNA Occurs during interphase before mitosis begins Occurs during interphase before meiosis I begins replication Number of One, including prophase, metaphase, Two, each including prophase, metaphase, anaphase, and divisions anaphase, and telophase telophase Synapsis of Does not occur Occurs during prophase I along with crOSSing over homologous between nonsister chromatids; resulting chiasmata chromosomes hold pairs together due to sister chromatid cohesion Number of Two, each diploid (2n) and genetically Four, each haploid (n), containing half as many chromosomes daughter cells identical to the parent cell as the parent cell; genetically different from the parent and genetic cell and from each other composition Role in the Enables multicellular adult to arise from Produces gametes; reduces number of chromosomes by half animal body zygote; produces cells for growth, repair, and, and introduces genetic variability among the gametes in some species, asexual reproduction... Figure 13.9 A comparison of mitosis and meiosis in diploid cells.••I;t-WI Could any other combinations of chromosomes be generated during meiosis II from the specific cellsshown in telophase P Explain. (Hint: Draw fhe cells as they would appear in metaphase II.)256 UNIT THREE Genetics
  12. 12. AComparison of Mitosis and MeiosisFigure 13.9 summarizes the key differences between meiosis What prevents the separation of sisterand mitosis in diploid cells. Basically, meiosis reduces the chromatids at anaphase I of meiosis?number of chromosome sets from two (diploid) to one (hap- EXPERIMENT Yoshinori Watanabe and colleagues knew that[oid), whereas mitosis conserves the number of chromosome during anaphase I. the protein shugoshin is present only aroundsets. Therefore, meiosis produces cells that differ genetically centromeres, They wondered whether it protects cohesins therefrom their parent cell and from each other, whereas mitosis from degradation in meiosis I. ensuring that chromatids stay to-produces daughter cells that are genetically identical to their gether while homologs separate. To test this hypothesis. they used a species of yeast in which meiosis produces haploid spores linedparent cell and to each other. up in a specific order inSide a spore case To follow the movement Three events unique to meiosis occur during meiosis I: of chromosomes. they fluorescent/y labeled a region near the cen- tromere of both chromatids in one homolog, leaving the other ho- 1. Synapsis and crossing over. During prophase I, repli- molog unlabeled. They then disabled the gene coding for cated homologs pair up and be<ome physically connected shugoshin and compared this yeast strain (shugoshin ) with nor- along their lengths by a zipper-like protein structure, the mal yeast cells (shugoshin +), The researchers expected that the two labeled chromosomes ariSing from the labeled chromatids in nor- synaptonemal complex; this process is called synapsis. mal cells would end up in separate spores at one end of the spore Genetic rearrangement between nonsister chromatids, case. They further predicted that if shugoshin does protect co- known as crossing over, is completed during this stage. hesins from cleavage at the centromere at anaphase I, then the la- beled chromosomes in shugoshin- cells would separate randomly Following disassembly of the synaptonemal complex in in meiosis II, sometimes ending up in the same spore. late prophase, the hvo homologs pull apart slightly but re- main connected by at least one X-shaped region called a Shugoshin+ (normal) Shugoshin- chiasma (plural, chiasmata). A chiasma is the physical Spore case Fluorescent label ~tL manifestation of crossing over; it appears as a cross be- cause sister chromatid cohesion still holds the mo origi- nal sister chromatids together, even in regions where one Metaphase I -- of them is now part of the other homolog. Synapsis and crossing over normally do not occur during mitosis. -)(-- t Anaphase I -1/ ~.-- 2, Homologs on the metaphase plate, At metaphase I of meiosis, chromosomes are positioned on the metaphase plate as pairs of homologs, rather than individual chro- k t Metaphase II II l. mosomes, as in metaphase of mitosis. 3. Separation of homologs. At anaphase I of meiosis, the -I -• .... t Anaphase II ,II ""- OR ,II ""- replicated chromosomes ofeach homologous pair move toward opposite poles, but the sister chromatids of each t ~ replicated chromosome remain attached. In anaphase of Mature spores mitosis, by contrast, sister chromatids separate. Spore Two of three possible arrange- How do sister chromatids stay together through meiosis I ments of labeled chromosomesbut separate from each other in meiosis II and mitosis? Sisterchromatids are attached along their lengths by protein com- # 100 RESULTS In shug-plexes called cohesins. In mitosis, this attachment lasts until the oshin +cells, the two labeled 80end of metaphase, when enzymes cleave the cohesins, freeing chromosomes ended up in ~ 60the sister chromatids to move to opposite poles of the cell. In different spores in almost all ~ 40meiosis, sister chromatid cohesion is released in two steps. In cases, In shugoshin- cells. &. 20 they were in the same spore ~ 0 .t,2~::-;;cmetaphase I, homologs are held together by cohesion between in about half the cases. Shugoshin+ Shugoshin-sister chromatid arms in regions where DNA has been ex- CONCLUSION The researchers concluded that shugoshin pro·changed. At anaphase I, cohesins are cleaved along the arms, tects cohesins at the centromere at anaphase I. thus maintainingallowing homologs to separate. At anaphase II, cohesins are the attachment between sister chromatids and ensuring that theycleaved at the centromeres, allowing chromatids to separate. separate properly during meiosis II, Figure 13.10 shows one of a series of experiments carried SOURCE T. S, KitaJIII<l, S. A. KiJWa>hlma, and Y. Watanabe, Theout by Yoshinori Watanabe and colleagues at the University of conserved konetochore proteon shugoshin protects centromeric coheSIon duringTokyo. They knew that similar proteins were present in co- meIOsis. N~lure 427510-517 aOO4)hesin complexes during mitosis and meiosis, and they won-dered what was responsible for preventing cohesin cleavage at Nmu". Draw a graph showing what you expect happened to the chromatids of the unlabeled chromosome in both strains of cells,the centromere while it was occurring along sister chromatid CHAPTER THIRTEEN Meiosis and Sexual Life Cycles 257
  13. 13. arms at the end of metaphase I. They found a protein they Origins of Genetic Variation Among Offspringnamed shugoshin (Japanese for "guardian spirit") that protectscohesins from cleavage at the centromere during meiosis I. In species that reproduce sexually, the behavior of chromo-Shugoshin is similar to a fruit fly protein identified lOyears ear~ somes during meiosis and fertilization is responsible for mostlier by Terry Orr-Weaver, this units interviewee. of the variation that arises each generation. Lets examine three mechanisms that contribute to the genetic variation Meiosis I is called the reductional division because it arising from sexual reproduction: independent assortment ofhalves the number of chromosome sets per cell-a reduc·tion from two sets (the diploid state) to one set (the haploid chromosomes, crossing over, and random fertilization.state). During the second meiotic division, meiosis II (some-times called the equational division), the sister chromatids Independent Assortment of Chromosomesseparate, producing haploid daughter cells. The mechanism One aspect ofsexual reproduction that generates genetic vari-for separating sister chromatids is virtually identical in ation is the random orientation of homologous pairs of chro-meiosis II and mitosis. The molecular basis of chromosome mosomes at metaphase of meiosis L At metaphase I, thebehavior during meiosis continues to be a focus of intense homologous pairs, each consisting of one maternal and oneresearch interest. paternal chromosome, are situated on the metaphase plate. (Note that the terms maternal and paternal refer, respectively, CONCEPT CHECI( 13.3 to the mother and father of the individual whose cells are un· I. How are the chromosomes in a cell at metaphase of dergoing meiosis.) Each pair may orient with either its mater· mitosis similar to and different from the chromo- nal or paternal homolog closer to a given pole-its orientation somes in a cell at metaphase of meiosis II? is as random as the flip of a coin. Thus, there is a 50% chance 2. -M"I. Given that the synaptonemal complex disappears by the end of prophase, how would the that a particular daughter cell of meiosis I will get the mater- nal chromosome of a certain homologous pair and a 50% two homologs be associated if crossing over did not chance that it will get the paternal chromosome. occur? What effect might this ultimately have on Because each homologous pair of chromosomes is posi- gamete formation? tioned independently ofthe other pairs at metaphase I, the first For suggested answers, see Appendi~ A. meiotic division results in each pair sorting its maternal and paternal homologs into daughter cells independently of every other pair. This is called independent assortment. Each daugh· ter cell represents one outcome of all possible combina· tions of maternal and paternal chromosomes. As shown in~:~:t~;v~~i~~on produced Figure 13.11, the number of combinations possible for daughter cells formed by meiosis of a diploid cell with two homologous pairs of chromosomes is four (two possible in sexual life cycles contributes arrangements for the first pair times two possible arrange- to evolution ments for the second pair). Note that only two of the fourHow do we account for the genetic varia-tion illustrated in Figure 13.1? As you will Possibility 1 Possibility 2learn in more detail in later chapters, mu-tations are the original source of genetic Two equally probable arrangements ofdiversity. These changes in an organisms chromosomes atDNA create the different versions of metaphase Igenes known as alleles. Once these differ~ences arise, reshuffling of the alleles dur-ing sexual reproduction produces thevariation that results in each member of aspecies having its own unique combina- Metaphase IItion of traits. Daughter.... Figure 13.11 The independent cellsassortment of homologouschromosomes in meiosis. Combination 3 Combination 4258 UNIT THREE Genetics
  14. 14. combinations of daughter cells shown in the figure would result ing meiosis II further increases the number of genetic types offrom meiosis of a single diploid cell, because a single parent cell daughter cells that can result from meiosis.would have one or the other possible chromosomal arrangement You will learn more about crossing over in Chapter 15. Theat metaphase I, but not both. However, the population of daugh- important point for now is that crossing over, by combiningter cells resulting from meiosis of a large number of diploid cells DNA inherited from two parents into asinglechromosome, iscontains aU four types in approximatelyequal numbers. In the case an important source of genetic variation in sexual life cycles.of n = 3, eight combinations of chromosomes are possible fordaughter cells. More generally, the number of possible combina- Random Fertilization ntions when chromosomes sort independently during meiosis is 2 , The random nature of fertilization adds to the genetic vari·where n is the haploid number of the organism. ation arising from meiosis. In humans, each male and fe- In the case of humans (n = 23), the number of possible 23 male gamete represents one of about 8.4 million (2 )combinations of maternal and paternal chromosomes in the possible chromosome combinations due to independent as-resulting gametes is 223 , or about 8.4 million. Each gamete that sortment. The fusion of a male gamete with a female gameteyou produce in your lifetime contains one of roughly 8.4 mil- during fertilization will produce a zygote with any of aboutlion possible combinations of chromosomes. 70 trillion (2 23 x 223 ) diploid combinations. If we factor inCrossing Over the variation brought about by crossing over, the number of possibilities is truly astronomical. It may sound trite, butAs a consequence of the independent assortment of chro- you really are unique.mosomes during meiosis, each of us produces a collection ofgametes differing greatly in their combinations of the chro-mosomes we inherited from our two parents. Figure 13.11 Nonsist~r chromatids Prophas~ Isuggests that each individual chromosome in a gamete is ex- h~ld tog~ther of m~iosisclusively maternal or paternal in origin. In fact, this is not during synapsisthe case, because crossing over produces recombinantchromosomes, individual chromosomes that carry genes(DNA) derived from two different parents (Figure 13.12). ---;:0-/ Pair of o In crossingtover and prophas~ synapsis homologsIn meiosis in humans, an average of one to three crossover occur; then homologsevents occur per chromosome pair, depending on the size of mov~ apart slightly. Chiasma.the chromosomes and the position oftheir centromeres. site of f.) Chiasmata and Crossing over begins very early in prophase I, as homolo- crossing atlachm~nts betweengous chromosomes pair loosely along their lengths. Each gene over sister chromatids holdon one homolog is aligned precisely with the corresponding homologs together; they move to thegene on the other homolog. In a single crossover event, spe- Centromere metaphase I plate,cific proteins orchestrate an exchange of corresponding seg-ments of two nonsister chromatids-one maternal and one IEM o Breakdown ofchroma· holding sister prot~inspaternal chromatid ofa homologous pair. In this way, crossing Anaphase [ tid arms togetherover produces chromosomes with new combinations of ma- allows homologs withternal and paternal alleles (see Figure 13. I2). recombinant chroma- tids to separate, In humans and most other organisms studied so far, cross-ing over also plays an essential role in the lining up ofhomolo-gous chromosomes during metaphase I. As seen in Figure 13.8,a chiasma forms as the result of a crossover occurring while Anaphase IIsister chromatid cohesion is present along the arms. Chias-mata hold homologs together as the spindle forms for the firstmeiotic division. During anaphase I, the release of cohesionalong sister chromatid arms allows homologs to separate.During anaphase II, the release ofsister chromatid cohesion atthe centromeres allows the sister chromatids to separate. Daughter cells At metaphase II, chromosomes that contain one or more re-combinantchromatids can be oriented in two alternative, non-equivalent ways with respect to other chromosomes, because Recombinant chromosomestheir sister chromatids are no longer identical. The differentpossible arrangements of nonidentical sister chromatids dur- ... Figure 13.12 The results of crossing over during meiosis. CHAPTE~ THIRTEEN Meiosis and Sexual Life Cycles 259
  15. 15. The Evolutionary Significance of Genetic Although Darwin realized that heritable variation is whatVariation Within Populations makes evolution possible, he could not explain why offspring resemble-but are not identical to-their parents. Ironically,Now that youve learned how new combinations of genes Gregor Mendel, a contemporary ofDarwin, published a theoryarise among offspring in a sexually reproducing popula- of inheritance that helps explain genetic variation, but his dis-tion, lets see how the genetic variation in a population re- coveries had no impact on biologists until 1900, more than 15lates to evolution. Darwin recognized that a population years after Darwin (1809-1882) and Mendel (l822-1884) hadevolves through the differential reproductive success of its died. In the next chapter, you will learn how Mendel discoveredvariant members. On average, those individuals best the basic rules governing the inheritance of specific traits.suited to the local environment leave the most offspring,thus transmitting their genes. This natural selection re·suits in the accumulation of those genetic variations fa- CONCEPT CHECK 13.4vored by the environment. As the environment changes, 1. What is the original source of all the different anelesthe population may survive if, in each generation, at least of a gene?some of its members can cope effectively with the new 2. The diploid number for fruit flies is 8, while that forconditions. Different combinations of alleles may work grasshoppers is 46. If no crossing over took place,better than those that previously prevailed. Mutations are would the genetic variation among offspring from athe original source of different alleles, which are then given pair of parents be greater in fruit flies ormixed and matched during meiosis. In this chapter, we grasshoppers? Explain.have seen how sexual reproduction greatly increases the 3. -Q@i1 IM Under what circumstances wouldgenetic variation present in a population. In fact, the abil- crossing over during meiosis not contribute to geneticity of sexual reproduction to generate genetic variation is variation among daughter cells?one of the most commonly proposed explanations for the For $ugg~sted an$w~rs, see Appendix A.persistence of sexual reproduction.-W ]f.- Go to the Study Area at www.masteringbio.(om for BioFlix of two sets of23-one set from each parent. In human diploid 3-D Animations. MP3 Tuto~. Videos, Pr3ctke Tests. an eBook, and mor~_ cells, there are 22 homologous pairs of autosomes, each with a maternal and a paternal homolog. The 23rd pair, the sex chromosomes, determines whether the person is female (XX) or male (XY). SUMMARY OF KEY CONCEPTS .. Behavior of Chromosome Sets in the Human Life Cycle At sexual maturity, ovaries and testes (the gonads) produce.i.III1I_ 13.1 haploid gametes by meiosis, each gamete containing a SingleOffspring acquire genes from parents by inheriting set of 23 chromosomes (n == 23). During fertilization, an egg and sperm unite, forming a diploid (2n = 46) single-chromosomes (pp. 248-249) celled zygote, which develops into a multicellular organism.. Inheritance of Genes Each gene in an organisms DNA exists by mitosis. at a specific locus on a certain chromosome. We inherit one set .. The Variety of Sexual life Cycles Sexual life cycles differ of chromosomes from our mother and one set from our father. in the timing of meiosis relative to fertilization and in the.. Comparison of Asexual and Sexual Reproduction In point(s) of the cycle at which a multicellular organism is asexual reproduction, a single parent produces genetically produced by mitosis. identical offspring by mitosis. Sexual reproduction combines sets of genes from two different parents. forming genetically diverse offspring. .ill"I-13.3 Meiosis reduces the number of chromosome sets from Actiity Asexual and Sexual Life Cycles diploid 10 haploid (pp. 253-258) .. The Stages of Meiosis The two cell divisions of meiosis pro-.i.IIIII-13.2 duce four haploid daughter cells. The number of chromosome sets is reduced from two (diploid) to one (haploid) duringFertilization and meiosis alternate in sexual life cycles meiosis I, the reductional division.(pp.250-253) .. A Comparison of Mitosis and Meiosis Meiosis is distin-.. Sets of Chromosomes in Human Cells Normal human so- guished from mitosis by three events of meiosis I, as shown matic cells are diploid. They have 46 chromosomes made up on the next page:260 UNIT THREE Genetics