Terms related to Meiosis … and some perspective.
Define the following terms:
1.
2.
3.
4.
5.
6.
7.
8.
9.
Homolgous chromosomes (homologs)
Haploid
Diploid
Synapsis
Tetrad
Crossing over
Chaiasma(ta)
Gamete
Somatic cell
In some organisms (fungi and the spore plants - mosses and ferns), meiosis does not
produce egg (oogenesis) and sperm (spermatogenesis), but for the present, we will focus
our attention on meiosis in the context of gametogenesis (formation of egg/sperm… the
gender-neutral term).
Meiosis is a two part division (Meiosis I and Meiosis II) of germ line cells…those that
are destined to become egg or sperm. Meiosis begins just as mitosis does; with the
replication of DNA and condensing of chromosomes. The difference is evident in
PROPHASE I. Homologous chromosomes (maternally and paternally derived
chromosomes carrying equivalent genetic information) somehow find each other and
become intimately entwined. This is synapsis, and since there are 4 (total) sister
chromatids in this formation, it is called a tetrad. It is during this time (prophase I) that
portions of a maternal chromatid and a paternal chromatid break off at a very specific
location - the chaiasma - and exchange these chromosome regions. It is important to note
that 1. Precise chromosome packaging is critical, and 2. The chaiasma can occur at
numerous locations along the chromatids. However, the chaiasma MUST be at the exact
same point in both. This exchange of portions of chromatids and the genes that go with
them is called crossing over. Meiosis proceeds through metaphase, anaphase, and
telophase.
During meiosis I, sister chromatids DO NOT SEPARATE, however, homologous
chromosomes go to either one or the other of the daughter cells. The parent cell was
diploid (both homologous chromosomes present), but the two cells that result from
meiosis I are haploid (only one or the other of each chromosome pair).
During meiosis II, the sister chromatids are separated at the centromere. The result of
meiosis II is 4 genetically unique haploid cells that (in animals) mature to egg or sperm
(gametes).