Page 147 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

5.2 Recombination: A Result of Crossing-Over During Meiosis 139

Figure 5.6 Evidence that recombination results from and species could never retain the same number of chromo-
reciprocal exchanges between homologous somes in successive generations.
chromosomes. Genetic recombination between the car and Bar The issue is that proper chromosome segregation re-
genes on the Drosophila X chromosome is accompanied by the quires homologous chromosomes to be pulled to opposite
exchange of physical markers observable in the microscope. Note spindle poles, which in turn requires the homologous chro-
that this depiction of crossing-over is a simplification, as genetic
recombination actually occurs after each chromosome has mosomes not only to pair with each other during prophase,
replicated into sister chromatids. but also to be linked to each other physically through meta-
phase until they separate at anaphase. The meiosis I spindle
Additional material from
part of the Y chromosome Discontinuity can exert tension on the chromosomes only if the homologs
car Bar are pulled in opposite directions but remain joined by a
Parental ( ) car +
chromosomes Bar + physical link. If the tension did not exist, homologous chro-
mosomes would not “know” that they were connected to
Meiosis
No crossing-over Crossing-over opposite spindle poles. Without tension, both chromo-
car Bar car Bar
somes therefore could often connect to fibers from the
car + Bar + car + Bar + same spindle pole, and nondisjunction would occur. What
then provides the physical link between homologous chro-
car Bar car Bar + mosomes until anaphase of meiosis I?
Chromosomes Parental Recombinant You might think from Fig. 4.16 that the synaptonemal
transmitted to
progeny ( ) car + Bar + car + Bar complex or recombination nodules form the necessary link
Parental Recombinant between homologous chromosomes. Figure 5.7a shows an
actual fluorescence micrograph of these structures during
the middle of prophase I (the pachytene substage). The syn-
Bar genes, and its physical marker consisted of part of aptonemal complexes that help homologous chromosomes
the Y chromosome that had become connected to the to pair with each other are shown in red. Although the DNA
X-chromosome centromere. of the chromosomes is not illustrated in this figure, each red
Figure 5.6 illustrates how the chromosomes in these line represents a bivalent (tetrad) made of two homologous
+
+
car Bar / car Bar females were transmitted to male prog- chromosomes, each of which has previously been dupli-
eny. According to the experimental results, all sons show- cated into sister chromatids. Studded at intervals along the
ing a phenotype determined by one or the other parental synaptonemal complex are recombination nodules that con-
+
+
combination of genes (either car Bar or car Bar ) had an tain the enzymes responsible for the actual crossing-over;
X chromosome that was structurally indistinguishable from one of these proteins is stained in green. However, contrary
one of the original X chromosomes in the mother. In re- to expectations, neither synaptonemal complexes nor re-
combinant sons, however, such as those that manifested combination nodules can link homologous chromosomes
+
carnation eye color and normal eye shape (car Bar / Y), an until anaphase I begins, for the simple reason that these
identifiable exchange of the abnormal features marking the structures both disappear by the end of prophase I.
ends of the homologous X chromosomes accompanied the Figure 5.7b and c illustrate that the homologous chro-
recombination of genes. The evidence thus tied an instance mosomes are still connected to each other even after the
of genetic recombination to the crossing-over of specifi- synaptonemal complexes and recombination nodules have
cally marked parts of particular chromosomes. This experi- dissolved. As discussed in Chapter 4, chiasmata mark
ment demonstrated elegantly that genetic recombination is the sites where recombination actually occurred earlier
associated with the actual reciprocal exchange of segments in prophase I (that is, where nonsister chromatids from
between homologous chromosomes during meiosis. homologous chromosomes exchanged places). However,
crossing-over by itself is insufficient to keep the homolo-
Why Recombination? gous chromosomes together. As seen in the artist’s diagram
in Fig. 5.7c, the physical linkage between homologous
In Chapter 4, we discussed one advantage that recombina- chromosomes involves molecular complexes called cohe-
tion provides for organisms on the earth measured over sin that make connections between sister chromatids soon
evolutionary time: Recombination contributes to genetic after the chromosomes have replicated. Once a crossover
diversity by reshuffling the alleles of genes between ho- takes place, it is cohesin complexes distal to the crossover
mologous chromosomes (review Fig. 4.17). However, point (that is, farther away from the centromere than the
crossing-over also plays another, even more crucial role to chiasmata) that keep the homologous chromosomes
ensure that chromosomes segregate properly when they are together at the metaphase plate and thus ensure proper
transmitted between parents and their progeny. As you will chromosome segregation.
see, if recombination did not occur, nondisjunction during Cohesin plays many roles in the biology of chromo-
meiosis I would be a common, rather than a rare, occurrence, somes; for example, not only is it found along chromosomal