The cell cycle
Actively dividing eukaryote cells pass through a series of stages known collectively as the cell cycle: two gap phases (G1 and G2); an S (for synthesis) phase, in which the genetic material is duplicated; and an M phase, in which mitosis partitions the genetic material and the cell divides.
Mitosis is a form of eukaryotic cell division that produces two daughter cells with the same genetic component as the parent cell. Chromosomes replicated during the S phase are divided in such a way as to ensure that each daughter cell receives a copy of every chromosome. In actively dividing animal cells, the whole process takes about one hour.
A chromatid is one-half of two identical copies of a replicated chromosome. During cell division, the identical copies are joined together at the region of the chromosome called the centromere. Joined chromatids are known as sister chromatids. Once the joined sister chromatids separate from one another in anaphase of mitosis, each is known as a daughter chromosome.
- G1 phase. Metabolic changes prepare the cell for division. At a certain point - the restriction point - the cell is committed to division and moves into the S phase.
- S phase. DNA synthesis replicates the genetic material. Each chromosome now consists of two sister chromatids.
- G2 phase. Metabolic changes assemble the cytoplasmic materials necessary for mitosis and cytokinesis.
- M phase. A nuclear division (mitosis) followed by a cell division (cytokinesis).
The period between mitotic divisions - that is, G1, S and G2 - is known as interphase.
The replicated chromosomes are attached to a 'mitotic apparatus' that aligns them and then separates the sister chromatids to produce an even partitioning of the genetic material. This separation of the genetic material in a mitotic nuclear division (or karyokinesis) is followed by a separation of the cell cytoplasm in a cellular division (or cytokinesis) to produce two daughter cells.
In some single-celled organisms mitosis forms the basis of asexual reproduction. In diploid multicellular organisms sexual reproduction involves the fusion of two haploid gametes to produce a diploid zygote. Mitotic divisions of the zygote and daughter cells are then responsible for the subsequent growth and development of the organism. In the adult organism, mitosis plays a role in cell replacement, wound healing and tumour formation.
Mitosis, although a continuous process, is conventionally divided into five stages: prophase, prometaphase, metaphase, anaphase and telophase.