Page 114 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

106 Chapter 4 The Chromosome Theory of Inheritance

of chromosomes, after which each one consists of two sister Prophase I: Homologs condense and pair,
chromatids. A key to understanding meiosis I is the observa- and crossing-over occurs
tion that the centromeres of these sister chromatids remain Among the crucial events of prophase I are the condensa-
connected throughout the entire division, rather than separat- tion of chromatin, the pairing of homologous chromo-
ing from each other as in mitosis. somes, and the reciprocal exchange of genetic information
As meiosis I proceeds, homologous chromosomes between these paired homologs. Figure 4.15 shows a gen-
align across the cellular equator to form a coupling that eralized view of prophase I; however, research suggests
ensures proper chromosome segregation later in the divi- that the exact sequence of events may vary in different spe-
sion. Moreover, during the time homologous chromo- cies. These complicated processes can take many days,
somes face each other across the equator, the maternal months, or even years to complete. For example, in the fe-
and paternal chromosomes of each homologous pair may male germ cells of several species, including humans, mei-
exchange parts, creating new combinations of alleles at osis is suspended at prophase I for many years until
different genes along the chromosomes. Afterward, the ovulation (as will be discussed further in Section 4.5).
two homologous chromosomes, each still consisting of Leptotene (from the Greek for thin and delicate) is the
two sister chromatids connected at their centromeres, are first definable substage of prophase I, the time when the
pulled to opposite poles of the spindle. As a result, it is long, thin chromosomes begin to thicken (see Fig. 4.16a for
homologous chromosomes (rather than sister chromatids a more detailed view). Each chromosome has already dupli-
as in mitosis) that segregate into different daughter cells cated prior to prophase I (as in mitosis) and thus consists of
at the conclusion of the first meiotic division. With this two sister chromatids affixed at their centromeres. At this
overview in mind, let us take a closer look at the specific point, however, these sister chromatids are so tightly bound
events of meiosis I, remembering that we analyze a dy- together that they are not yet visible as separate entities.
namic, flowing sequence of cellular events by breaking it Zygotene (from the Greek for conjugation) begins as
down somewhat arbitrarily into the easily pictured, tradi- each chromosome seeks out its homologous partner and
tional phases. the matching chromosomes become zipped together in a

Figure 4.16 Prophase I of meiosis at very high magnification.
Sister chromatid 1
+
Sister chromatid 2 Synaptonemal
complex
Synaptonemal
complex
Homologous
chromosomes Sister chromatid 3
+ Recombination
Sister chromatid 4 nodules

(a) Leptotene: Threadlike chromosomes begin (b) Zygotene: Chromosomes are clearly (c) Pachytene: Full synapsis of homologs.
to condense and thicken, becoming visible visible and begin pairing with homologous Recombination nodules appear along the
as discrete structures. Although the chromosomes along the synaptonemal synaptonemal complex.
chromosomes have duplicated, the sister complex to form a bivalent, or tetrad.
chromatids of each chromosome are not
yet visible in the microscope.

Chiasmata

(d) Diplotene: Bivalent pulls apart slightly, (e) Diakinesis: Further condensation
but homologous chromosomes remain of the bivalent.
connected due to recombination at
crossover sites (chiasmata).