The cell cycle and mitosisEdexcel A-Level Biology B: Revision notes

The cell cycle is a regulated sequence of events in which a cell grows, copies its DNA and divides into two genetically identical daughter cells. It has three main stages:

• Interphase, the longest stage: G1 (growth, protein synthesis), S (DNA replication) and G2 (further growth, preparation for division)
• Mitosis: nuclear division in four stages
• Cytokinesis: division of the cytoplasm

The cycle is controlled at checkpoints, which check that DNA has been copied accurately and is undamaged before the cell continues. If control is lost, cells divide uncontrollably and a tumour can form.

In G1 each chromosome is a single, long DNA molecule. In S phase the DNA is replicated, so each chromosome consists of two identical sister chromatids joined at the centromere. A human cell has 46 chromosomes in G1 but 92 chromatids after S phase, and the DNA content doubles.

As mitosis begins the chromosomes condense (supercoil), which makes them short enough to be moved without tangling. After mitosis each daughter cell has 46 chromosomes and the original amount of DNA.

• Prophase: chromosomes condense and become visible as two chromatids; the nucleolus disappears; the nuclear envelope breaks down; spindle fibres form
• Metaphase: chromosomes line up on the equator, each attached to a spindle fibre by its centromere
• Anaphase: the centromeres divide and the spindle fibres shorten, pulling the sister chromatids to opposite poles
• Telophase: chromosomes reach the poles and uncoil; nuclear envelopes reform around each set; the spindle disappears

After telophase the cytoplasm divides. In animal cells the cell surface membrane pulls inwards to form a cleavage furrow. In plant cells vesicles fuse across the equator to form a cell plate, which becomes the new cell wall and membrane. This gives two cells, each with one nucleus.

Because mitosis produces genetically identical cells, it is used for:

• Growth: increasing cell number, from a zygote to a whole organism
• Repair and replacement: replacing damaged or worn-out cells, such as skin or gut lining
• Asexual reproduction: producing offspring (clones) identical to the parent, as in spider plant plantlets, strawberry runners or budding in yeast

Asexual reproduction is fast and needs no mate, but gives no genetic variation, so a whole population can be hit by one disease.

1. Cut about 1 cm from the tip of a growing root (the meristem, where cells divide rapidly)
2. Warm it in hydrochloric acid (about 1 mol dm⁻³, 60 °C, a few minutes) to break the middle lamella and separate the cells
3. Place the tip on a slide, add a stain (acetic orcein or toluidine blue) that binds to chromosomes, and leave
4. Add a coverslip and squash firmly, using a paper towel, to make a thin single layer
5. Examine under the light microscope, starting at low power, and identify the stage of each cell

Mitotic index = number of cells in mitosis ÷ total number of cells counted. Count several fields of view to make the result reliable.