Chromosomes

Chromosomes replicate themselves during the cell cycle
which consists of a short M phase during which mitosis occurs,
and a longer interphase. During interphase there is a G1 gap
phase, an S phase when DNA synthesis occurs and a G2 gap
phase. The stages of mitosis – prophase, prometaphase,
metaphase, anaphase and telophase – are followed by
cytokinesis when the cytoplasm divides to give two daughter
cells. The process of mitosis produces two identical diploid
daughter cells. Meiosis is also preceded by a single round of
DNA synthesis, but this is followed by two cell divisions to
produce the haploid gametes. The first division involves the
pairing and separation of maternal and paternal chromosome
homologs during which exchange of chromosomal material
takes place. This process of recombination separates groups of
genes that were originally located on the same chromosome
and gives rise to individual genetic variation. The second cell
division is the same as in mitosis, but there are only 23
chromosomes at the start of division. During spermatogenesis,
each spermatocyte produces four spermatozoa, but during
oogenesis there is unequal division of the cytoplasm, giving rise
to the first and second polar bodies with the production of only
one large mature egg cell.