Mitosis

Interphase: Cells may appear inactive during this stage, but they are quite the opposite. This is the longest period of the complete cell cycle during which DNA replicates, the centrioles divide, and proteins are actively produced. For a complete description of the events during Interphase,

Prophase: During this stage, the nucleolus fades and chromatin (replicated DNA and associated proteins) condenses into chromosomes. Each replicated chromosome comprises two chromatids, both with the same genes. Microtubules of the cytoskeleton, cause for cell shape, motility and attachment to other cells during interphase, disassemble. And the building blocks of these microtubules are used to grow the mitotic spindle from the region of the centrosomes.

Prometaphase: In this stage the nuclear envelope breaks down so there is no longer a recognizable nucleus. Some mitotic spindle fibers elongate from the centrosomes and attach to kinetochores, protein bundles at the centromere region on the chromosomes where sister chromatids are joined. Other spindle fibers elongate but instead of attaching to chromosomes, overlap each other at the cell center.

Metaphase: Tension applied by the spindle fibers aligns all chromosomes in one plane at the center of the cell.

Anaphase: Spindle fibers shorten, the kinetochores separate, and the chromatids (daughter chromosomes) are pulled apart and begin moving to the cell poles.

Telophase: The daughter chromosomes arrive at the poles and the spindle fibers that have seperated them apart disappear.

Cytokinesis: The spindle fibers not attached to chromosomes begin breaking down until only that portion of overlap is left. It is in this region that a contractile ring cleaves the cell into two daughter cells. Microtubules then reorganize into a new cytoskeleton for the return to interphase.

Meiosis

Diploid Cell (2N): From a preceding mitotic division, the Oogonium (Spermatogonium) enters meiosis with DIPLOID (2N) chromosomes but TETRAPLOID (4N) DNA. Chromosomes then duplicate to produce SISTER CHROMATIDS (or HOMOLOGOUS DYADS).

Prophase I: Dyad pairs align to create "TETRADS", non-sister chromatids connect and trade sections at a "CHIASMA", a process called "CROSSING OVER".

Metaphase I: SPINDLE FIBERS attach to each dyad at the KINETOCHORE. Tension from spindle fibers aligns the tetrads at the cell equator.

Anaphase I: Chiasmata break apart and sister chromatids begin migrating toward opposite poles.

Telophase I: CLEAVAGE FURROW forms beginning the process of CYTOKINESIS (cell division). Resulting daughter cells are HAPLOID (1N).

Prophase II: Spindle formation begins and centrosomes begin moving toward poles.

Metaphase II: Tension from spindle fibers aligns chromosomes at the metaphase plate.

Anaphase II: CHROMATIDS separate and begin moving to the poles.

Telophase II: CLEAVAGE FURROW forms beginning CYTOKINESIS.

Gamete (1N): NUCLEAR ENVELOPES form and chromosomes disperse as CHROMATIN. Meiosis has produced 4 DAUGHTER CELLS, each with 1N chromosomes and 1N DNA. Later, in fertilization, male and female 1N gametes will fuse to form a 2N ZYGOT

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