The cell cycle & its regulation

The cell cycle at a glance

The cell cycle is the sequence of events from when a cell is formed by division until it divides itself. It has two main parts: interphase, when the cell grows and copies its DNA, and the mitotic (M) phase, when it divides. A typical human cell growing in culture takes about 24 hours to go round once, and most of that time is interphase.

• G₁ phase (first gap): the cell grows, makes proteins and organelles, and builds up the materials it needs to copy its DNA.
• S phase (synthesis): the cell replicates its DNA. Each chromosome now consists of two identical sister chromatids joined at the centromere. The centrosome, which will organise the spindle, is also duplicated.
• G₂ phase (second gap): the cell keeps growing and makes the proteins it needs for division.
• M phase: mitosis (division of the nucleus) and then cytokinesis (division of the cytoplasm).

Not every cell keeps cycling. Cells can leave G₁ and enter a non-dividing state called G₀. Some stay there only until they're signalled to divide again; others, such as most mature nerve cells, stay there for good.

Mitosis and cytokinesis

Mitosis separates the sister chromatids so that each new cell gets one complete, identical set of chromosomes. It runs continuously, but biologists divide it into stages:

1. 1Prophase: chromosomes condense into visible, compact structures, and the mitotic spindle, made of microtubules, starts forming between the two centrosomes.
2. 2Prometaphase: the nuclear envelope breaks down. Spindle microtubules attach to the kinetochores, protein structures at each centromere; the two sister chromatids attach to opposite poles.
3. 3Metaphase: the chromosomes line up in a single plane across the middle of the cell, the metaphase plate.
4. 4Anaphase: the proteins holding sister chromatids together (cohesins) are cut. The chromatids separate and are pulled to opposite poles. Each is now counted as a chromosome in its own right.
5. 5Telophase: a nuclear envelope forms around each set of chromosomes, and the chromosomes loosen again.

Cytokinesis usually starts during anaphase or telophase. In animal cells, a ring of actin filaments just inside the membrane contracts and pinches the cell in two, forming a cleavage furrow. Plant cells can't pinch, because of their walls; instead, vesicles from the Golgi gather in the middle and fuse into a cell plate, which becomes a new wall between the two cells.

The engine: cyclins and CDKs

The cycle is driven forward by protein kinases called cyclin-dependent kinases (CDKs). As the name says, a CDK only works when it's bound to a cyclin. When it is (and when it's been phosphorylated in the right places), the complex phosphorylates target proteins that trigger the next stage, such as starting DNA replication or breaking down the nuclear envelope.

Here's the key point: the amount of CDK in the cell stays roughly constant. It's the cyclins that come and go. Each type of cyclin is made at a particular stage, builds up, activates its CDK partner, and then is rapidly destroyed. Without its cyclin, the CDK falls silent. So CDK activity rises and falls through the cycle, but only because cyclin levels do. That's exactly what Hunt saw in his sea urchin eggs.

Different cyclin–CDK pairs run different transitions. One important one is MPF (maturation- or mitosis-promoting factor), which is cyclin B bound to CDK1. As cyclin B builds up during G₂, MPF activity rises and pushes the cell into mitosis. From the start of anaphase, cyclin B is destroyed, MPF activity crashes, and the cell can finish mitosis and divide.

The checkpoints

A checkpoint is a control point where the cycle can be stopped until certain conditions are met. Hartwell introduced the idea when he found yeast genes whose job is to delay division when something has gone wrong. There are three major ones:

• G₁/S checkpoint: is the cell big enough, are there enough resources, is there a signal to divide, and is the DNA undamaged? This is the main decision point. Once past it (in animal cells it's called the restriction point), the cell is usually committed to dividing. If the answer is no, it may pause or enter G₀.
• G₂/M checkpoint: has all the DNA been replicated, and is it undamaged? If not, the cell waits and repairs before starting mitosis.
• Spindle assembly (M) checkpoint: is every chromosome's kinetochore attached to the spindle, with sisters pulled to opposite poles? Anaphase can't be undone, so the cell won't start it until every chromosome is correctly attached.

If the spindle checkpoint fails, sister chromatids can go to the wrong cell. The daughter cells then have too many or too few chromosomes, a condition called aneuploidy, which is common in cancer cells.

Signals that press the accelerator and the brake

Most cells in your body don't divide unless they're told to. The instruction usually comes from outside as a growth factor, a signal that binds a receptor tyrosine kinase and starts the Ras–MAP kinase pathway from the signal transduction topic. The end result is more G₁ cyclins, which lets the cell pass the G₁/S checkpoint.

Two proteins act as brakes, and they're among the most important in the whole topic.

• Rb (retinoblastoma protein) guards the G₁/S checkpoint. Unphosphorylated Rb binds a transcription factor called E2F and keeps it from switching on the genes needed for S phase. When G₁ cyclin–CDK complexes build up, they phosphorylate Rb, which lets go of E2F, and the cell moves into S phase.
• p53 responds to DNA damage. When DNA is damaged, p53 builds up and switches on genes, including one for a protein called p21. p21 binds and inhibits cyclin–CDK complexes, halting the cycle so that repairs can be made. If the damage is too great to repair, p53 can trigger apoptosis, the cell's own orderly self-destruction.

When control fails: cancer

Cancer is uncontrolled cell division, and it arises when mutations disable the cycle's controls. The genes involved fall into two groups, like the two pedals of a car.

Proto-oncogenes are normal genes whose products push the cycle forward: growth factor receptors, Ras, cyclins. A gain-of-function mutation can turn one into an oncogene, a version that pushes too hard or all the time, like an accelerator stuck to the floor. One mutated copy is enough, because the overactive protein works whatever the other copy is doing.

Tumour-suppressor genes make proteins that hold the cycle back or trigger repair and apoptosis: Rb, p53, p21. Loss-of-function mutations disable these brakes. Usually both copies must be lost, because one working copy still makes enough of the brake protein. Mutated p53 is found in more than half of human tumours.

A single mutation is rarely enough. Cancer usually develops as a cell lineage builds up several mutations in turn, each passed to the daughter cells: perhaps an oncogene first, then the loss of a tumour suppressor, then loss of p53, which makes further mutations more likely to survive.

Key terms

Interphase (G₁, S, G₂)

The part of the cycle when the cell grows (G₁, G₂) and replicates its DNA (S).

Mitosis / cytokinesis

Division of the nucleus / division of the cytoplasm.

Sister chromatids

The two identical copies of a replicated chromosome, joined at the centromere.

Cyclin

A protein whose level rises and falls through the cycle; it activates a CDK.

Cyclin-dependent kinase (CDK)

A kinase present at steady levels that is active only when bound to its cyclin.

MPF

Cyclin B–CDK1, the complex that drives the cell into mitosis.

Checkpoint

A control point where the cycle stops unless certain conditions are met.

Proto-oncogene / oncogene

A normal gene that promotes division / its mutated, overactive form.

Tumour-suppressor gene

A gene whose product restrains division, e.g. Rb or p53; both copies usually must be lost.

Check yourself

Try answering in your head before you open each answer.

• 1.A scientist measures the amount of CDK1 protein and the activity of CDK1 in cells over three cycles. Sketch what each graph would look like and explain the difference.Show answerHide

  The amount of CDK1 would be a roughly flat line, because CDK is made steadily. CDK1 activity would rise to a peak at the start of each mitosis and fall sharply in anaphase, three times. The difference is cyclin B: CDK1 is active only when bound to cyclin B, and cyclin B is made and then destroyed each cycle.

• 2.Cells with a drug that stops the spindle forming are found stuck in mitosis with condensed chromosomes. Which checkpoint is responsible, and why is the arrest useful?Show answerHide

  The spindle assembly (M) checkpoint. Kinetochores that aren't attached to the spindle keep anaphase from starting. The arrest is useful because if anaphase went ahead, the chromatids would be shared out wrongly, and the daughter cells would be aneuploid.

• 3.A mutation in the Rb gene means Rb protein can no longer bind E2F. Is Rb a proto-oncogene or a tumour suppressor, and how many copies must be mutated before G₁/S control is lost?Show answerHide

  Rb is a tumour suppressor: it's a brake that keeps E2F from switching on S-phase genes. One normal copy still makes Rb that can hold E2F, so usually both copies must be mutated before E2F is always free and the cell enters S phase without the proper signals.