Control, Cancer and Counting Through the Cycle: Quick Revision

A brief note on Control, Cancer and Counting Through the Cycle. Read it before a test. Try each question at the end before you open its answer.

Control of the cycle

• G1 checkpoint: size, food, growth factors and DNA damage. A cell may pause, repair or enter G0.
• G2 checkpoint: all the DNA copied and undamaged.
• Spindle checkpoint: at metaphase, every chromosome attached to the spindle.
• CDK: a kinase with a steady amount, inactive alone. Cyclin: its amount rises and falls, and it switches the CDK on.
• M cyclin and CDK (MPF) start mitosis. The M cyclin is broken down at the end.
• Brakes: p53 (halts, calls in repair, can start apoptosis, raises p21), p21 (blocks CDK and cyclin), Rb (holds E2F).

Cancer

• Cancer: uncontrolled division, no contact inhibition, escape from apoptosis, invasion and metastasis.
• Proto-oncogene to oncogene: a gain of function, an accelerator that is stuck on.
• Tumour suppressor gene (Rb, p53): a loss of function, a brake that is cut. Usually several mutations are needed.
• Carcinogens: physical, chemical and biological. Benign tumours stay put. Malignant tumours spread.
• Chemotherapy harms all fast dividing cells, so hair, gut lining and bone marrow are hurt.

Counting

• Count the centromeres for the chromosomes and the DNA molecules for the chromatids.
• Human cell: G2 has 46 chromosomes, 92 chromatids and 4C. Anaphase has 92 chromosomes and 4C. A cell in anaphase II has 46 chromosomes and 2C. A gamete has 23 chromosomes and 1C.
• Mitotic index = cells in mitosis divided by all the cells counted, times 100.
• Time in a phase = the share of the cells in it times the length of the cycle (for cells that divide at random).
• One cell gives $2^n$ cells in $n$ cycles, and $N - 1$ divisions make $N$ cells.

A worked example

A field of 500 cells has 60 in mitosis, and the cycle takes 20 h. What is the mitotic index and the time of M phase?

1. Mitotic index: $\dfrac{60}{500} \times 100 = 12$ per cent.
2. Time of M phase: $0.12 \times 20\ \text{h} = 2.4\ \text{h}$.

Practice with new numbers

A program to try

The program finds the mitotic index and the time of M phase from the counts.

Mistakes to avoid

• Saying that a cyclin is a kinase. The CDK is the kinase.
• Saying that cancer needs one mutation. Usually several are needed.
• Giving a cell in anaphase II n chromosomes. It has 2n and 2C.
• Using the mitotic index for cells that divide in step. The cells must be at random stages.

Check yourself

1. Which proteins form the pair that starts mitosis?

2. What is the difference between an oncogene and a tumour suppressor gene?

3. How many chromosomes and how much DNA does a human cell have in anaphase of mitosis?

4. How many divisions take place in making 64 cells from one cell?

Sources

• Clark M. A., Douglas M. and Choi J. (2018). Biology 2e, OpenStax, chapters 10.3 and 10.4.
• NCERT (2023). Biology, Textbook for Class XI, chapter 10.