Cambridge International AS and A Level Biology 5: The mitotic cell cycle

The Mitotic Cell Cycle is Cambridge International Biology 9700 Topic 5. It covers chromosome structure, interphase, mitosis, cytokinesis, telomeres, stem cells, tumours and chromosome behaviour in plant and animal cells. Theory owns stage mechanisms and image interpretation; microscope preparation and drawing technique remain in the practical hub.

1. Chromosome structure

A eukaryotic chromosome contains a long DNA molecule associated with histone proteins. Histones help package DNA into chromatin. Before DNA replication, a chromosome contains one DNA molecule. After S phase, it consists of two sister chromatids, normally with matching DNA sequences, held together by cohesion, particularly near the centromeric regions.

Each sister chromatid has a centromeric region where a kinetochore assembles for spindle-microtubule attachment. Telomeres are repetitive DNA at chromosome ends. They protect coding information from progressive end loss during DNA replication.

For the standard chromosome counts used here, a joined sister-chromatid pair counts as one replicated chromosome until the sisters separate. This convention does not mean that centromeric DNA first replicates at anaphase.

2. Interphase and the cell cycle

The cell cycle includes interphase, mitosis and cytokinesis. In G1, the cell grows, synthesises proteins and increases organelle content. In S phase, DNA replicates. In G2, further growth and preparation for nuclear division occur.

Interphase is metabolically active and is not a resting stage. Chromosomes are generally uncondensed and individually indistinct in light micrographs, but nuclear DNA replication in this normal mitotic cycle occurs in S phase.

Mitosis divides the nucleus so each daughter nucleus receives one copy of every chromosome. Cytokinesis divides the cytoplasm, producing two cells.

3. Why mitosis matters

Mitosis produces genetically identical daughter cells, subject to rare mutation. It supports growth of multicellular organisms, replacement of dead or damaged cells, tissue repair and asexual reproduction.

Genetic identity preserves chromosome number and cell-lineage information. Growth occurs by increasing cell number, not by making every existing cell indefinitely larger.

4. Prophase and metaphase

In this four-stage model of open mitosis, prophase includes chromosome condensation, nucleolar disappearance, nuclear-envelope breakdown and spindle development. A more detailed scheme distinguishes prometaphase, including kinetochore attachment after nuclear-envelope breakdown; not all eukaryotes undergo open mitosis. In animal cells, centrioles are associated with spindle organisation; plant cells form spindles without relying on centrioles in the same way.