Cell Cycle

Progression through the cell cycle requires the coordination of  variety of macromolecular syntheses, assemblies and movements. The chromosomes must be replicated, condensed, segregated, and decondensed. The spindle poles must duplicate, separated, and migrate to opposite ends of the nucleus. Coordination of these complex processes is thought to be achieved by  series of changes in the cyclin-dependent kinases (CDKs). The active forms of the CDKs are a complex of at lest two proteins,  kinase and  cyclin. These complexes undergo changes in the kinase and cyclin components that are believed to drive the cell from one stage of the cell cycle to another. According to this paradigm, cell cycle stage is determined by the constellation of proteins activated or inactivated by phosphorylation as  result of the activity of the CDKs during that stage.

Progression through the cell cycle is tightly controlled by cell-cycle checkpoints in phases G₁ and G₂.

Interphase:

The period when the cell is not in mitosis is referred to as "interphase". Chromosomes are replicated beginning at the origins of replication and proceed bidirectionally from the origins across the chromosome.  Euchromatin makes up most of interphase chromosomes. Interphase contains the following phases:

• G₁ Phase: is the phase between M phase (below) and S phase. Unlike transit through the S, G2, and M phases, G1 progression normally relies on sitmulation by mitogens and can be blocked by antiproliferative cytokines.

• S Phase: In contrast to bacteria which replicate their DNA continually, DNA replication in most eucaryotic cells occurs only during a specific part of the cell division cycle, called the DNA synthesis phase or S phase. In a mammalian cell, the S phase typically lasts for around 8 hours.

• G₂ Phase: is the phase between S phase and mitosis.

Mitosis (M phase):

Chromosomes become highly condensed and then are separated and distributed to the two daughter nuclei. The condensation of interphase chromosomes into mitotic chromosomes requires a class of proteins called condensins which are large protein complexes that contain SMC proteins; these are long, dimeric protein molecules that use the energy of ATP hydrolysis to make large right handed loops in DNA.

When the 46 human chromosomes at mitosis are stained with dyes, one can see a reproducible banding patter.

The centromere attaches to the duplicated chromosomes to the mitotic spindle so that one copy is distributed to each daughter cell.

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