Mitosis, Meiosis and Gametogenesis
Mitosis is the mechanism by which somatic (body) cells in higher
organisms replicate & divide.
each cell has 2 copies of each of each chromosome, one that is of paternal
origin, the other of maternal. These cells are said to be diploid or 2N, where
N is the number of chromosomes per set (one full set is a genome).
after mitosis both daughter cells have the same genetic composition.
Mitosis is dynamic, but is divided into stages for descriptive purposes
Interphase; chromosomes invisible, between divisions
Prophase: chromosomes condense and become visible
Each can be seen to have already doubled; the duplicates are referred to as chromatids
Chromatids are held together by a common centromere (kinetochore)
The number of chromosomes = the number of centromeres
Centriole splits, migrates apart to form poles and spindle fibers form
Metaphase: all chromosomes are aligned along the equator, attached at the
centrommere to spindle fibers from both poles
Anaphase: spindle fibers from each pole that are attached to the centromeres
begin to contract and separate the daughter chromatids
Telophase; two new nuclei form following cytokinesis (division of the cytoplasm)
animal cells "pinch" into two daughters
plant cells divide by laying down a dividing plate
Useful reviews of the stages of mitosis can be seen at several sites.
Several include "animations" to show the changes that occur.
http://www.biology.arizona.edu/cell_bio/tutorials/cell_cycle/main.html
http://gened.emc.maricopa.edu/bio/bio181/BIOBK/BioBookmito.html
http://www.csuchico.edu/~jbell/Biol207/animations/mitosis.html
http://galileo.physiology.uiowa.edu/animations/mitosis.htm
Meiosis occurs in specialized cells in "germline" tissues
ovaries produce eggs
testes produce sperm (in plants, anthers produce pollen)
As in mitosis, chromosomes have already replicated when the visible
events begin
The products of meiosis have one of each of the chromosomes; that is
they are haploid (1N)
Two meiotic divisions follow one replication
Meiosis I (MI): the first division reduces the number of chromosomes to 1N,
although each is still doubled
Meiosis II (MII): the centromeres split and daughter chromosomes separate
Prophase I: homologous chromosomes pair side by side. This pairing is
called synapsis.
Metaphase I: homologous chromosomes remain paired when they line up at
the equator. The paired chromosomes are often called a tetrad, since the
chromosome arms have already doubled, and each chromosome can be
seen to exist as a pair of chromatids, which are joined at the centromere.
Anaphase I: homologues centromeres are pulled to opposite poles, effectively
halving the number of chromosomes, but as indicated previously, each
chromosome is still doubled.
Telophase I may be very short, cytokinesis occurs but nuclei may not be fully
re-formed.
Meisosis II occurs in the daughter cells of meiosis I without
another round of replication. The steps are analogous to those in
mitosis.
Some good reviews of meiosis, some with animations or videos
can be found at:
http://gened.emc.maricopa.edu/bio/BIO181/BIOBK/BioBookmeiosis.html
http://mentor.lscf.ucsb.edu/mcdb4a/meiosis.htm
The movement of one member of each homologous pair of
chromosomes in MI to opposite poles explains Mendels law of
segregation; that is each gamete gets one allele from each gene.
Since only chance determines which member of any pair of
chromosomes goes to a specific pole, genes on different
chromosomes assort independently, accounting for Mendel's
second law (independent assortment)
Gametogenesis
Meiosis is just the beginning of gamete formation; using humans as an
example:
In males, all four products of meiosis can develop into sperm
meiosis is continuous in many cells after sexual maturity
takes about 74 days to convert to functional sperm
tail comes from centriole
head contains nucleus and acrosome from golgi
midpart is packed with mitochondria
In females, each meiotic division is followed by unequal division of
cytoplasm; one nucleus is shunted into a non-functional polar body
Meiosis is initiated in ovaries before birth of female, but arrests at prophase I
after pberty, one ovarian follicle per month resumes division
Secondary oocyte bursts into Fallopian tube
The second division of meiosis is not completed unless the oocyte is penetrated by
a sperm