Cell cycle

The cell cycle is the cyclic sequence of events involved in cell growth, DNA replication and cell division. In eukaryotes, the cell cycle can broadly be divided into two phases - the interphase (described in this article) and the mitotic (M) phase.

The M phase is the stage of nuclear and cell division; mitosis has its own sub-phases which are described in detail in the wiki-article on mitosis. The interphase, then, is the sequence of events preceding (and, in a cyclic manner, immediately following) mitosis in which a cell grows and synthesises the materials needed for mitosis.

The interphase can be divided into three key phases:

  1. Gap phase 1 (G1): biosynthesis resumes its rapid rate, having slowed down during mitosis. In particular, there is a lot of enzyme synthesis for the enzymes needed in DNA replication, which is a key component of the proceeding S phase.
    1. Gap phase 0 (G0): sometimes the cell may exit the cell cycle and enter a state of rest called G0 where synthesis and division ceases. The cell is said to be in a quiescent state. Some cells, such as nerve cells, remain in the G0 phase as soon as they are fully differentiated, some enter the G0 phase for a while but return to the G1 phase when ready to continue dividing, as in the case of liver cells which divide only once or twice per year.
  2. Synthesis phase (S): DNA is replicated causing each chromosome to have two sister chromatids; however, the ploidy of the cell remains unchanged. Rates of transcription and translation are low, with the exception of histone synthesis as more histones are now needed for the packaging of the newly-replicated DNA.
  3. Gap phase 2 (G2): biosynthesis resumes, synthesising microtubules for the spindle fibres of mitosis (or meiosis) and duplicating organelles for the replicated cell.
  4. From here the cell may enter the mitotic (M) phase (mitosis) or, in some cases, meiosis (although meiosis is not strictly considered part of the cell cycle, as it is a one way process).

During interphase, chromosomes exist as uncondensed chromatin, meaning they are not visible as defined structures under the microscope. Each chromosome occupies a distinct nuclear compartment, called a territory. The regions between territories are called interchromosomal domains. The chromosomes are continually rearranged, cycling transcriptionally active regions to the territory borders; the pre-mRNA transcripts accumulate in the interchromosomal domains for post-transcriptional processing. Transcriptionally active genes are located differentially with respect to nearby heterochromatin according to cell type, since heterochromain can silence nearby genes.

The cell cycle is tightly regulated at events called cell cycle checkpoints. The first of these checkpoints is the restriction checkpoint at the end of the G1 phase. From this checkpoint, the cell may enter G0. At this checkpoint, various mechanisms ensure that the cell is prepared to enter the S phase and DNA replication. If DNA damage is detected, then the cell will not normally be permitted to pass to the S phase.

The next checkpoint is at the end of the G2 phase, before the cell enters mitosis. Proteins called cyclin-dependent kinases (CDKs) activate the proteins involved in mitosis, only when another protein called mitosis promoting factor (MPF) activates them. The MPF is usually inactivated by the presence of phosphate groups, and these phosphate groups are only removed, allowing MPF to become active, if conditions are favourable for mitosis.

A third checkpoint - the mitotic spindle checkpoint - occurs during mitosis, and is described in more detail in the wiki-article on mitosis.