Page 117 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

4.5 Gametogenesis 109

Figure 4.17 How meiosis contributes to genetic diversity. essential concepts
(a) The variation resulting from the independent assortment of
nonhomologous chromosomes increases with the number of • In meiosis, chromosomes replicate once (before meiosis I),
chromosomes in the genome. (b) Crossing-over between
homologous chromosomes ensures that each gamete is unique. but the nucleus divides twice (meiosis I and II).
(a) Independent assortment (b) Recombination • During metaphase I, homologous chromosomes connect
Orientation I Orientation II to opposite spindle poles. The independent alignment of
each pair of homologs ensures the independent
A B A b C d assortment of genes carried on different chromosomes.
Metaphase I
a b a B c D • Crossing-over during the first meiotic division maintains
Prophase I the connection between homologous chromosomes until
Meiosis I anaphase I and contributes to the genetic diversity of
A B A b gametes.
Telophase I • Sister chromatids separate from each other during
a b a B meiosis II so that gametes have only one copy of each
chromosome.
Metaphase I • Fertilization—the union of egg and sperm—restores the
diploid number of chromosomes (2n) to the zygote.
A B A b
• Errors during meiosis may produce gametes with
Metaphase II missing or extra chromosomes, which often is lethal
a b a B to offspring.
C c d
Meiosis II C d c
A D D
A
B b Metaphase II
B b
A A
a 4.5 Gametogenesis
b Telophase II a B
b B
a a learning objectives
1. Compare the processes of oogenesis and
B
b A
A b d spermatogenesis in humans.
B A A C c 2. Distinguish between the sex chromosome complements
d
B
a Gametes of human female and male germ-line cells at different
b a
b a C D stages of gametogenesis.
a
B D c
In all sexually reproducing animals, the embryonic germ
cells (collectively known as the germ line) undergo a se-
Mitosis and Meiosis: A Comparison ries of mitotic divisions that yield a collection of special-
ized diploid cells, which subsequently divide by meiosis to
Mitosis occurs in all types of eukaryotic cells (that is, cells produce haploid cells. As with other biological processes,
with a membrane-bounded nucleus) and is a conservative many variations on this general pattern have been ob-
mechanism that preserves the genetic status quo. Mitosis served. In some species, the haploid cells resulting from
followed by cytokinesis produces growth by increasing the meiosis are the gametes themselves, while in other spe-
number of cells. It also promotes the continual replacement cies, those cells must undergo a specific plan of differen-
of roots, stems, and leaves in plants and the regeneration of tiation to fulfill that function. Moreover, in certain
blood cells, intestinal tissues, and skin in animals. organisms, the four haploid products of a single meiosis
Meiosis, on the other hand, occurs only in sexually do not all become gametes. Gamete formation, or gameto-
reproducing organisms, in just a few specialized germ genesis, thus gives rise to haploid gametes marked not
cells within the reproductive organs that produce haploid only by the events of meiosis per se but also by cellular
gametes. It is not a conservative mechanism; rather, the events that precede and follow meiosis. Here we illustrate
extensive combinatorial changes arising from meiosis are gametogenesis with a description of egg and sperm forma-
one source of the genetic variation that fuels evolution. tion in humans. The details of gamete formation in several
Table 4.3 illustrates the significant contrasts between the other organisms appear throughout the book in discus-
two mechanisms of cell division. sions of specific experimental studies.