Biology Reference
In-Depth Information
Meiosis has two unusual aspects: one of the most extraordinary aspects of
meiosis I is that the two homologous chromosomes that are destined to pair
and undergo recombination (crossing over) are able to find each other in a vast
set of nonhomologous sequences. How this is achieved is a matter of consider-
able interest ( Roeder 1997, Haber 1998 ). In Drosophila , pairing of homologous
chromosomes may be facilitated through specialized pairing sites on the chro-
mosomes. Heterochromatin, especially in the centromeres and telomeres, has
been implicated as a mechanism that facilitates chromosomal pairing ( Walker
and Hawley 2000 ). During the pairing of homologous chromosomes, an elabo-
rate ladder of protein called the synaptonemal complex is formed that helps to
hold them together ( Haber 1998 ).
A second extraordinary aspect of meiosis is the pairing of sister chromatids
until their disjunction. Protein complexes facilitate this cohesion ( van Heemst
and Heyting 2000 ). During pairing, recombination by crossing over occurs at a
100- to 1000-fold higher frequency in meiosis than in mitosis. Recombination
tends to occur at certain chromosomal loci called “hotspots” and occurs more
often between homologous chromosomes rather than between the sister
chromatids.
Crossing over occurs about twice per paired set of chromosomes and serves
two roles: the resulting recombination yields new combinations of alleles and
plays a mechanical role in separation (disjunction) of the homologous chromo-
somes at meiosis I ( van Heemst and Heytig 2000 ). Appropriate separation of
homologous chromosomes in meiotic anaphase I requires that paired homolo-
gous chromosomes, rather than individual chromosomes, line up on the meta-
phase I spindle. At anaphase I, the homologous chromosomes move to opposite
poles, resulting in meiosis I being the reductional division.
Each metaphase chromosome has a distinct morphology that is identifiable by
staining with lactic-acetic orcein or other stains. The location of the centromere
allows cytogeneticists to distinguish particular chromosomes. The arms of the
chromosome take up stains in a banding pattern that is characteristic, especially
in salivary gland chromosomes of dipterans; these chromosomes are polytene,
i.e., they contain multiple copies of the DNA ( Figure 3.5 ).
3.13 Chromosome Damage
Chromosome damage probably occurs continuously in all cells. Types of damage
range from single-base changes that result from mistakes made by DNA poly-
merases during replication, to chromosome breakage. DNA and chromosomes
are damaged by many factors, including the production of metabolic mutagens
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