Cell Division
Meiosis Mitosis
Paternal & Maternal Homolog
DNA replication
& pairing of duplicated homologous chromosomes
but now
genetic recombination in meoisis
Bivalents line up next to each other on the
spindle
Cell Division I
Bivalents line up next to each other
Bivalents line up
on spindle
Cell Division II
Cell Division
Meiosis
A haploid gamete produced from a
diploid cell through meiosis contains half the original number of chromosomes.
In other words, it contains only one chromosome (either the maternal or paternal
copy) in place of each homologous pair of chromosomes. This is
accomplished in 2 separate cell divisions. In division I of meiosis each
chromosome in the diploid cell replicates to produce 2 sister chromatids
just as in a mitotic cell division. Each duplicated chromosome pairs with its
duplicated homolog forming a structure called a bivalent which contains 4
chromatids. This pairing allows genetic recombination to occur, where a fragment
of a maternal chromatid can be exchanged for a corresponding fragment of a
homologous paternal chromatid. The 2 duplicated homologus (each consisting of 2
sister chromatids) separate form each other and the cell divides.
In division II of meiosis, the sister chromatids separate to produce cells with a haploid DNA content.
MITOSIS
(1) Prophase: The replicated chromosomes each consisting of two closely associated sister chromatids, condense. Outside the nucleus, the mitotic spindle assembles between the two centrosomes (which is a microtubule organizing center that surrounds a pair of centrioles.
(2) Prometaphase: starts with the breakdown of the nuclear envelope through MPF phosphorylation of nuclear lamins. Chromosomes can then attach to spindle microtubles via their kinetochores (complex protein machine that assembles onto the highly condensed DNA at the centromere.
(3) Metaphase: alignment of the chromosomes at the equator of the spindle. This is a spindle attachment checkpoint which insures that cells do not prematurely enter anaphase until all chromosomes are attached and aligned properly.
(4) Anaphase: degradation of cohesions through enzyme called separase. This transition is triggered by the activation of anaphase promoting complex (APC) which cleaves and inactivates the M-phase cyclin (M-cyclin) thereby inactivating M-Cdk and it cleaves an inhibitory protein (securin) thereby activating separase. Sometimes anaphase is divided up into 2 parts;
-
Anaphase A which denotes kinetochore microtubule shortening and
-
Anaphase B to denote the spindle poles which move apart.
(5) Telephase: arrival of the daughter chromosomes at the poles of the spindle and decondensation. Dephosphorylation of MPF and reformation of the nuclear envelope.
(6) Cytokinesis: the cytoplasm is divided in two by a contractile ring of actin and myosin filaments which pinches the cell in two.