The cell cycle and mitosis (SACE Stage 2 Biology)

What this dot point is asking

You need to describe the stages of the cell cycle, outline the phases of mitosis, and explain why mitosis produces genetically identical cells. You should also link loss of control to cancer.

The cell cycle

The cell cycle has two main parts: a long growth period called interphase, then the division phase (mitosis plus cytokinesis).

Interphase has three sub-stages:

• G1 - the cell grows and makes proteins and organelles.
• S - DNA replication, so each chromosome becomes two identical sister chromatids joined at a centromere.
• G2 - further growth and preparation for division.

Because DNA is replicated during interphase, each chromosome already has a full copy ready to be separated during mitosis.

Mitosis

Mitosis divides the nucleus so that each daughter nucleus receives a complete, identical set of chromosomes. It is conventionally described in four phases:

1. Prophase. Chromosomes condense and become visible as two sister chromatids; the nuclear membrane breaks down and spindle fibres form.
2. Metaphase. Chromosomes line up along the middle (equator) of the cell, attached to spindle fibres at their centromeres.
3. Anaphase. The centromeres split and the sister chromatids are pulled apart to opposite poles of the cell.
4. Telophase. A nuclear membrane reforms around each set of chromosomes, which begin to uncoil.

Cytokinesis then divides the cytoplasm, producing two separate daughter cells.

What mitosis is used for

Mitosis is used for growth, repair of damaged tissue, and asexual reproduction in some organisms. It always produces genetically identical cells, which is why this dot point contrasts with meiosis (which produces genetically varied gametes).

Control of the cell cycle and cancer

The cell cycle is tightly regulated at checkpoints that confirm conditions are right before the cell proceeds, for example checking that DNA has replicated correctly and is undamaged. Genes control these checkpoints.

If mutations damage the genes that control the cycle, a cell may divide uncontrollably. A mass of such cells forms a tumour, and a malignant tumour is cancer. This connects the cell cycle to the earlier work on mutations and mutagens (many mutagens are carcinogens).

Two classes of gene normally police the cycle. Proto-oncogenes encode proteins that promote division when growth is needed; a gain-of-function mutation can turn one into an oncogene that drives division continuously, like an accelerator stuck on. Tumour-suppressor genes (such as p53) encode proteins that halt the cycle at a checkpoint if DNA is damaged or conditions are wrong; a loss-of-function mutation removes this brake. Cancer typically requires several such mutations to accumulate in one cell line, which is why incidence rises with age and with exposure to carcinogens that increase mutation rate.

Tracking DNA content through the cycle

Quantitative SACE items often plot DNA mass against time. The pattern to remember: DNA content is constant through G1, doubles smoothly during S phase as replication proceeds, stays at the doubled level through G2 and mitosis (because the copies are still in one cell), then halves abruptly at cytokinesis as each daughter cell receives one full set. If a question gives the daughter-cell DNA mass as $m$, then a G2 cell holds $2m$ and each chromosome is present as two sister chromatids. Reading these graphs correctly is mostly about matching the doubling to S phase and the halving to division.

Mitosis versus meiosis

A frequent comparison: mitosis produces two diploid daughter cells genetically identical to the parent and is used for growth, repair and asexual reproduction; meiosis produces four genetically varied haploid gametes through two divisions, halving the chromosome number and generating variation by crossing over and independent assortment. The shared starting point is a single round of DNA replication in S phase; the difference is the number of divisions that follow and whether homologous chromosomes are separated.