The cell cycle and mitosisAQA A-Level Biology: Revision notes

Not all cells in a multicellular organism keep the ability to divide. Many specialised cells, such as mature neurones, no longer divide, whereas others, such as skin, gut lining and root tip cells, divide throughout life.

Eukaryotic cells that do divide show a cell cycle, a repeating sequence made of interphase (the longest phase), then mitosis (nuclear division) and cytokinesis (division of the cytoplasm).

DNA replication occurs during interphase, before mitosis begins. Each chromosome is copied, so it then consists of two identical sister chromatids joined at a centromere. The cell also makes organelles and proteins and increases its ATP supply.

Because replication happens first, mitosis can produce two daughter cells, each with identical copies of the DNA produced by the parent cell during replication.

Prophase: chromosomes condense, becoming shorter and thicker so they are visible; the nuclear envelope breaks down.

Metaphase: chromosomes line up across the equator of the cell, each attached by its centromere to spindle fibres.

Anaphase: the centromeres divide and the spindle fibres shorten, pulling the sister chromatids, centromere first, to opposite poles.

Telophase: chromatids (now chromosomes) reach the poles and uncoil; nuclear envelopes reform around each set.

Cytokinesis usually follows telophase. The cytoplasm divides, producing two new cells. In animal cells the cell-surface membrane pinches in; in plant cells a new cell wall forms across the middle.

Cytokinesis does not always occur, so mitosis can occasionally produce one cell with two nuclei.

You must recognise the stage of a cell and explain its appearance. Clues: visible chromosomes with no nuclear envelope (prophase); chromosomes at the equator (metaphase); V-shaped chromatids moving apart (anaphase, shape caused by centromeres leading); two groups at opposite poles with nuclear envelopes reforming (telophase).

Mitosis is a controlled process. Mutations in genes that control the cell cycle can cause uncontrolled division, forming a tumour; if cells spread it is called cancer. Many cancer treatments control the rate of cell division, for example by stopping DNA replication or preventing spindle formation. These also affect healthy dividing cells, which causes side effects.

Method: cut about 1 cm from the root tip, soften it by warming in hydrochloric acid, place on a slide, add a stain that binds to chromosomes (e.g. toluidine blue or acetic orcein), cover with a coverslip and squash it to make a single thin layer. Examine with an optical microscope, identify stages and count cells.

Mitotic index = number of cells in mitosis ÷ total number of cells observed.

Worked example: 27 cells in mitosis out of 180 gives 27 ÷ 180 = 0.15. A high index suggests rapid growth or a tumour. Count several fields of view to improve reliability.

Meiosis is not part of this subtopic.