Page 116 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

108 Chapter 4 The Chromosome Theory of Inheritance

meiotic division. In some species, however, the chromo- parental cell present at the beginning of this division. For
somes simply stay condensed. Most importantly, no S this reason, meiosis II is termed an equational division.
phase exists during the interphase between meiosis I and
meiosis II; that is, the chromosomes do not replicate during
meiotic interphase. The relatively brief interphase between Mistakes in Meiosis Produce
meiosis I and meiosis II is known as interkinesis. Defective Gametes

Segregational errors during either meiotic division can
During Meiosis II, Sister Chromatids lead to aberrations, such as trisomies, in the next genera-
Separate to Produce Haploid Gametes tion. If, for example, the homologs of a chromosome pair
do not segregate during meiosis I (a mistake known as non-
The second meiotic division (meiosis II) proceeds in a fash- disjunction), they may travel together to the same pole and
ion very similar to that of mitosis, but because the number eventually become part of the same gamete. Such an error
of chromosomes in each dividing nucleus has already been may at fertilization result in any one of a large variety of
reduced by half, the resulting daughter cells are haploid. possible trisomies. Most autosomal trisomies in humans,
The same process occurs in each of the two daughter cells as we already mentioned, are lethal in utero; one exception
generated by meiosis I, producing four haploid cells at the is trisomy 21, the genetic basis of Down syndrome. Like
end of this second meiotic round (see Fig. 4.15, meiosis II). trisomy 21, extra sex chromosomes may also be viable but
cause a variety of mental and physical abnormalities, such
as those seen in Klinefelter syndrome (see Table 4.1).
Prophase II: The chromosomes condense
If the chromosomes decondensed during the preceding in-
terphase, they recondense during prophase II. At the end Meiosis Contributes to Genetic Diversity
of prophase II, the nuclear envelope breaks down, and the
spindle apparatus re-forms. The wider the assortment of different gene combinations
among members of a species, the greater the chance that at
least some individuals will carry combinations of alleles
Metaphase II: Chromosomes align that allow survival in a changing environment. Two aspects
at the metaphase plate of meiosis contribute to genetic diversity in a population.
The kinetochores of sister chromatids attach to microtubule First, because only chance governs which paternal or ma-
fibers emanating from opposite poles of the spindle appa- ternal homologs migrate to the two poles during the first
ratus, just as in mitotic metaphase. Nonetheless, two sig- meiotic division, different gametes carry a different mix of
nificant features of metaphase II distinguish it from maternal and paternal chromosomes. Figure 4.17a shows
mitosis. First, the number of chromosomes is one-half that how different patterns of homolog migration produce dif-
in mitotic metaphase of the same species. Second, in most ferent mixes of parental chromosomes in the gametes. The
chromosomes, the two sister chromatids are no longer amount of potential variation generated by this random
strictly identical because of the recombination through independent assortment increases with the number of
crossing-over that occurred during meiosis I. The sister chromosomes. In Ascaris, for example, where n = 2
chromatids still contain the same genes, but they may carry (the chromosome complement shown in Fig. 4.17a),
different combinations of alleles. the random assortment of homologs could produce only
2
2 , or four types of gametes. In a human being, however,
23
where n = 23, this mechanism alone could generate 2 , or
Anaphase II: Sister chromatids move more than 8 million genetically different kinds of gametes.
to opposite spindle poles A second feature of meiosis, the reshuffling of genetic
Just as in mitosis, severing of the connection between sister information through crossing-over during prophase I,
centromeres allows the sister chromatids to move toward ensures an even greater amount of genetic diversity in gam-
opposite spindle poles during anaphase II. etes. Because crossing-over recombines maternally and pa-
ternally derived genes, each chromosome in each different
gamete could consist of different combinations of maternal
Telophase II: Nuclear membranes and paternal alleles (Fig. 4.17b).
re-form, and cytokinesis follows Of course, sexual reproduction adds yet another means
Membranes form around each of four daughter nuclei in of producing genetic diversity. At fertilization, any one of a
telophase II, and cytokinesis places each nucleus in a sep- vast number of genetically diverse sperm can fertilize an egg
arate cell. The result is four haploid gametes. Note that with its own distinctive genetic constitution. It is thus not
at the end of meiosis II, each daughter cell (that is, each very surprising that, with the exception of identical twins,
gamete) has the same number of chromosomes as the the 6 billion people in the world are each genetically unique.