Replication and division of nuclei and cells

Biology AS · The mitotic cell cycle. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

A chromosome is one very long DNA molecule wound around histone proteins. Before a cell divides, its DNA is copied. Each chromosome then consists of two identical sister chromatids joined at the centromere.

The mitotic cell cycle has three parts: interphase, mitosis (division of the nucleus) and cytokinesis (division of the cell). Most of the cycle is interphase. Mitosis gives two daughter cells that are genetically identical to the parent cell and to each other.

Cell division is normally controlled. If the genes that control it mutate, cells may divide without stopping and form a tumour.

Rules to remember

Interphase: G₁ (growth, making proteins and organelles), S (DNA replication), G₂ (more growth and preparation for mitosis)
The mass of DNA in a cell doubles during S phase and returns to the original value in each daughter cell
Mitosis is needed for growth of multicellular organisms, replacement of damaged or dead cells, repair of tissues and asexual reproduction
Telomeres are repeated non-coding DNA sequences at the ends of chromosomes; a little DNA is lost from the ends at each replication, and telomeres mean that no genes are lost
Stem cells can divide by mitosis many times; some daughter cells stay as stem cells and others specialise to replace cells and repair tissue
Tumour: mutations in genes that control the cell cycle → uncontrolled mitosis → a mass of abnormal cells

Common mistake

Saying that DNA is replicated during mitosis. Replication happens in S phase of interphase, before mitosis begins.

Worked example

Cells in a root tip have a cell cycle lasting 20 hours. In a stained sample, 5% of the cells are in mitosis. Estimate how long mitosis lasts.

If all the cells are dividing and are not in step with each other, the fraction of cells seen in a stage equals the fraction of the cycle spent in that stage.
5% of 20 hours = 0.05 × 20 = 1 hour.
The other 95% of cells are not in mitosis, and nearly all of these are in interphase: 0.95 × 20 = 19 hours.

Answer: Mitosis lasts about 1 hour; interphase takes about 19 hours of the cycle.

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