Cell Cycle and Cell Division

Cells reproduce by dividing into two. The orderly sequence of events by which a cell duplicates its contents and divides into two daughter cells is called the cell cycle. In a typical human cell the cycle takes about 24 hours and has two broad stages: a long preparation stage called interphase and a short dividing stage called the M phase (mitotic phase).

Interphase is often called the resting phase, but the cell is actually very active — it grows and copies its DNA in readiness for division. It is divided into three parts:

• G1 phase (Gap 1) — the cell grows in size and is metabolically active; the DNA is not yet copied.
• S phase (Synthesis) — the DNA replicates, so the amount of DNA doubles. (The chromosome number stays the same; each chromosome now has two sister chromatids.)
• G2 phase (Gap 2) — the cell continues to grow and makes proteins, getting ready to divide.

After G2, the cell enters the M phase, in which the nucleus divides (karyokinesis) followed by the cytoplasm (cytokinesis). Some cells that no longer divide (like mature nerve cells) leave the cycle and enter an inactive stage called G0 (quiescent stage).

Mitosis is the division of the nucleus that produces two daughter cells genetically identical to the parent cell, each with the same number of chromosomes. It occurs in the body (somatic) cells and is therefore also called equational division. The nuclear division (karyokinesis) happens in four continuous stages:

• Prophase — chromosomes condense and become visible (each as two sister chromatids joined at a centromere); the nuclear membrane and nucleolus begin to disappear; the spindle starts to form.
• Metaphase — the chromosomes line up at the centre (equator) of the cell on the metaphase plate; spindle fibres attach to their centromeres.
• Anaphase — the centromeres split and the sister chromatids are pulled to opposite poles of the cell.
• Telophase — chromosomes reach the poles and decondense; nuclear membranes re-form around each set, giving two nuclei.

After karyokinesis comes cytokinesis — the division of the cytoplasm. In animal cells a furrow pinches the cell in two; in plant cells a cell plate forms in the middle and grows outward into a new wall.

Significance of mitosis: it allows growth of the body, repair and replacement of worn-out cells (e.g. skin, blood), and keeps the chromosome number constant in all body cells. It is also the basis of asexual reproduction in many organisms.

Meiosis is a special division that takes place in the reproductive cells and produces four daughter cells, each with half the chromosome number of the parent (haploid). It is therefore called reductional division. It involves two successive divisions — meiosis I and meiosis II — but the DNA is copied only once (before meiosis I).

Meiosis I is the reduction division. In its prophase I the homologous chromosomes (matching pairs, one from each parent) come together and pair up — a process called synapsis. While paired, they may exchange segments at points called chiasmata; this exchange is crossing over. Crossing over mixes the genes of the two parents, producing genetic recombination — new combinations of characters. The homologous chromosomes then separate to opposite poles, so each new cell gets only one of each pair (the chromosome number is halved).

Meiosis II is like mitosis: the sister chromatids of each chromosome separate, giving a total of four haploid cells.

Significance of meiosis: (1) it keeps the chromosome number constant from generation to generation — gametes are haploid, so when two fuse at fertilisation the normal (diploid) number is restored; and (2) crossing over and the random assortment of chromosomes create variation, which is the raw material for evolution.