The cell cycle, its control, and haploid and diploid cells

1Why some cells never stop dividing

The cells lining your small intestine are worn away and replaced every few days, so the stem cells at the base of the gut lining divide over and over again. Most nerve cells in your brain, by contrast, stopped dividing before you were born and will never divide again. Your liver cells sit somewhere in between: they rarely divide, but if part of the liver is removed they start again until the organ has regrown. Each cell decides whether and when to divide, and the decision is checked at several points. In Senior 3 you met the stages of the cell cycle; this lesson explains how the cycle is controlled, and how biologists describe the number of chromosome sets in a cell.

2The phases of the cell cycle

• G1 (first gap): the cell grows, makes proteins and organelles, and carries out its normal functions. It is the most variable phase: hours in a fast-dividing cell, years in a slow one.
• S (synthesis): the DNA is replicated, as you saw in Unit 3, and new histones are made. Each chromosome now consists of two sister chromatids.
• G2 (second gap): the cell grows further, checks the copied DNA, and makes the proteins needed for division, such as tubulin for the spindle.
• M phase: mitosis divides the nucleus, and cytokinesis divides the cytoplasm.

3How the cycle is controlled

Each checkpoint works like a guard at a gate:

• G1 checkpoint (the restriction point): is the cell big enough, are there enough nutrients, have growth factors signalled "divide", and is the DNA undamaged? If not, the cell pauses, enters G0, or repairs its DNA. The protein p53 stops the cycle here when DNA is damaged and, if the damage cannot be repaired, makes the cell destroy itself (apoptosis).
• G2 checkpoint: has all the DNA been replicated, and correctly?
• Spindle checkpoint (in metaphase): is every chromosome attached to spindle fibres from both poles? Anaphase cannot start until it is.

The signals that move a cell past each checkpoint are proteins called cyclins and cyclin-dependent kinases (CDKs). A CDK is an enzyme that adds phosphate groups to other proteins, switching them on or off, but it is active only when bound to a cyclin. Different cyclins build up and are destroyed at different stages of the cycle, so different CDKs are switched on in turn and drive the cell from one phase to the next. Leland Hartwell, Tim Hunt and Paul Nurse shared the 2001 Nobel Prize in Physiology or Medicine for discovering these controls. When the controls fail, cells can divide without stopping, which is the start of cancer (Lesson 2).

4Haploid and diploid cells

Two different counts change during the cycle, and A-level questions test whether you can keep them apart:

• The number of chromosomes (count the centromeres) stays the same through interphase: a diploid human cell has 46 in G1 and still 46 in G2.
• The amount of DNA doubles in S phase. Biologists call the DNA in one haploid set "1C": a diploid cell has 2C in G1 and 4C in G2, and a gamete has 1C.

Many crop plants are polyploid. Bread wheat is hexaploid, with six sets of 7 chromosomes (42 in all). The cooking and dessert bananas grown in Rwanda are triploid, with three sets of 11 chromosomes (33 in all); Lesson 4 explains why this makes them seedless.