Biomedical Engineering Reference
In-Depth Information
to mitosis. However, at this point there are only 23 chromosomes in
each of the cells that undergoes meiosis II. Each chromosome has
two sister chromatids connected by a centromere. The chromosomes
line up at a metaphase plate (
metaphase-2
), the centromeres split,
the chromatids travel to opposite poles of the cell (
anaphase-2
),
the chromatids reach the opposite poles and the cytoplasm divides
(
telophase-2
) .
There are several important consequences of meiosis and sexual reproduc-
tion that arise and lead to a genetic reshuffl ing of the genomic gene pool each
time it occurs. Some of these are: (i) the chromosome number is halved; (ii)
meiosis facilitates segregation of alleles; (iii) there is independent assortment
of homologous chromosomes so each gamete has both maternal and pater-
nal chromosomes; and (iv) because of crossing over of material from homol-
ogous chromosomes, which occurs during each meiotic event, there is a good
chance that each chromosome has both maternal and paternal genes.
Gametogenesis
Just as with mitosis, genetic and chromosomal mutations can occur during
the process of meiosis, leading to germ line changes that can have a dra-
matic phenotypic effect on offspring.
There are also several important differences that occur during the mei-
otic process of gametogenesis in the different sexes that are important to
note ( Fig. 7.11 ).
In males,
spermatogenesis
occurs in the seminiferous tubules of the tes-
tes. It commences at puberty and continues throughout the lifetime of the
male. The precursor male germ cell is the spermatogonia. This produces
the primary spermatogonia, each of which goes through meiosis I, result-
ing in two secondary spermatocytes. These two cells then undergo meiosis
II to produce four spermatids. After about 75 days the spermatids mature
into sperm. Spermatogenesis occurs throughout the male's lifetime and the
primary spermatogonia are continuously undergoing the meiotic process.
Since mutational rates are linked to cell division, there is always the possi-
bility of genetic or chromosomal mutations arising with each meiotic divi-
sion and some genetic diseases, such as achondroplastic dwarfi sm, actually
show an increase in frequency with advanced paternal age because of this
phenomenon.
In females
oogenesis
is quite different from spermatogenesis in several
important aspects. Expansion of female germ cell precursors occurs only
in the fi rst few months after conception. The primary oocytes begin meio-
sis I at the third month of gestation and no primary oocytes develop after
birth. Primary oocytes are arrested in prophase-1 (dictyotene) before birth
