Sexual Reproduction: Meiosis, Germ Cells, and ... - U-Cursos

OVERVIEW OF SEXUAL REPRODUCTION 1271between the cells of the germ line (or germ cells), which include gametes andtheir specified diploid precursor cells, and the somatic cells, which form the restof the body and ultimately leave no progeny (see Figure 21–3A). In a sense, thesomatic cells exist only to help the germ-line cells survive, develop, and transmittheir DNA to the next generation.Meiosis Creates Genetic DiversitySexually reproducing organisms inherit two full sets of chromosomes, one fromeach parent. Each set contains autosomes, which are common to all members ofthe species, and sex chromosomes, which are differently distributed according tothe sex of the individual. Therefore, each diploid nucleus contains two closelysimilar versions of each autosome, plus a set of sex chromosomes appropriate tothe sex of the individual. The two copies of each autosome, one from the motherand one from the father, are called homologous chromosomes, or homologs,and in most cells they maintain a separate existence as independent chromosomes.During meiosis, however, each chromosome must communicate with itsunique homologous partner by physically pairing and undergoing geneticrecombination. This communication is essential to enable the homologs to segregateaccurately into different daughter cells during meiosis.A crucial feature of meiosis is that it generates haploid cells that are geneticallydifferent from one another and from the two haploid cells that formed theorganism in the first place. The genetic differences arise by two mechanisms.First, an individual gamete contains either the maternal or paternal version ofeach chromosome; because the choice of maternal or paternal occurs independentlyand randomly for each pair of homologs, the original maternal and paternalchromosomes are reshuffled into novel combinations in the haploid cells.Second, although the maternal and paternal versions of each chromosome havesimilar DNA sequences, they are not identical, and they undergo genetic recombinationduring meiosis—a process called crossing-over (discussed in Chapter 5)to produce novel hybrid versions of each chromosome; thus, each chromosomein a gamete contains a unique mixture of genetic information from both parents.We discuss these two mechanisms in more detail later (see Figure 21–13).Sexual Reproduction Gives Organisms a Competitive AdvantageThe machinery of sexual reproduction is elaborate, and the resources spent on itare large (Figure 21–4). What are its benefits, and why did it evolve? Sexually reproducingindividuals produce varied offspring, whose varied genotypes are at leastas likely to represent a change for the worse as a change for the better. Why, then,should they have a competitive advantage over individuals that breed true, by anasexual process? This question continues to perplex evolutionary biologists.One advantage of sexual reproduction seems to be that the reshuffling ofgenes helps a species to survive in an unpredictably variable environment. If twoparents produce many offspring with a wide variety of gene combinations, thechance that at least one of their progeny will have the combination of featuresnecessary for survival in a changing environment is increased. Indeed, a populationof budding yeast genetically engineered so that it cannot undergo meioticgenetic recombination and therefore cannot reproduce sexually adapts muchless well over time to harsh environmental conditions than does the wild-typepopulation, which can reproduce sexually.Another advantage of sexual reproduction seems to be that it can help eliminatedeleterious genes from a population: females generally mate with thefittest males, so that the least fit males leave no progeny and serve only as a sortof genetic trashcan. This stringent selection among males means that “good”genes are transmitted and “bad” genes are lost from the population more efficientlythan they would otherwise be. As a result, members of the sexually reproducingpopulation are expected to have much higher average fitness than membersof an equivalent population that reproduces asexually.Figure 21–4 A peacock displaying hiselaborate tail. This extravagant plumageserves to attract females for the purposeof sexual reproduction. It has evolvedbecause only the fittest and mosthandsome males leave progeny.(Courtesy of Cyril Laubscher.)