The Cell Cycle
- 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.