The phases of the cell cycle and DNA content

A cell that divides again and again

A fertilised egg becomes a body of tens of trillions of cells. Every one of them came from another cell by division. A cell cannot simply split in two, though. It must first grow, then copy its DNA, and only then divide. The ordered series of events from one division to the next is the cell cycle.

The two parts of the cycle

The cycle has two main parts.

• Interphase is the long part between two divisions. The cell grows and copies its DNA.
• M phase (the mitotic phase) is the short part in which the cell divides. It has two jobs. Karyokinesis divides the nucleus, and cytokinesis divides the cytoplasm.

How long does each part last? NCERT takes a human cell in culture, which divides about once in 24 hours. The division itself lasts only about 1 hour. So interphase is more than 95 per cent of the cycle: $23 \div 24 = 95.8$ per cent. Other cells are quicker or slower. Yeast can go round the cycle in about 90 minutes.

Older books call interphase the "resting phase". That name is wrong. Interphase is the busiest time in the life of a cell.

The three phases of interphase

A textbook may split the 24 hours into hours for each phase. This course uses the split of Cooper and Hausman for a typical human cell in culture: G1 about 11 h, S about 8 h, G2 about 4 h and M about 1 h. OpenStax Biology gives G1 about 9 h, S about 10 h, G2 about 4.5 h and M about 0.5 h. Both add up to about 24 hours, and both show that M phase is short. NCERT prints only the 24 hours and the 1 hour. If a question gives its own times, use those.

In animal cells, the centriole (centrosome) is copied in S phase. That is why two centrosomes are ready to move to the poles in prophase. Cells of higher plants have no centrioles, but they still make a spindle.

C and n: two ways of counting

Two symbols are used all through this chapter.

• n is the number of chromosomes in one set. A diploid cell has 2n chromosomes. A human body cell has 2n = 46.
• C is the amount of DNA in one haploid set. A diploid cell in G1 has 2C of DNA.

S phase doubles the DNA: 2C becomes 4C. It does not change the number of chromosomes. The cell still has 2n chromosomes after S phase. Each chromosome now has two sister chromatids, joined at the centromere.

• G1: 2n chromosomes, one chromatid in each, and 2C of DNA.
• After S, in G2: 2n chromosomes, two chromatids in each, and 4C of DNA.
• Each daughter cell: 2n chromosomes, one chromatid in each, and 2C of DNA.

For a human cell, G2 has 46 chromosomes and 92 chromatids. The word chromatid is used only for the two halves of a copied chromosome. Once they separate, each half is a chromosome again.

G0: cells that leave the cycle

Some cells do not divide again. They leave the cycle at the end of G1 and enter an inactive stage called the quiescent stage (G0). A cell in G0 stays metabolically active, but it does not divide unless it is called on to do so. Heart muscle cells and most nerve cells stay in G0. Many other cells, such as those of the liver, divide only when cells are lost by injury or death.

You should see that the DNA stays at 2C through G1, rises only in S phase, and falls back to 2C when the cell divides. Make G1 longer. The cycle gets longer, and interphase is still more than 90 per cent of it.

Try it in Python

The program adds up the phases of one cell cycle and finds the share of each.

Practise with new numbers

Common mistakes

• Saying that S phase doubles the number of chromosomes. It doubles the DNA. The number of chromosomes stays 2n.
• Calling G1 a resting phase. The cell grows and works hard in G1.
• Mixing up G0 and G1. G0 is a stage outside the cycle. A cell in G1 is still in the cycle.
• Saying that M phase is the longest phase. In a human cell it is the shortest, about 1 hour of 24.
• Counting chromatids in G1. Before S phase each chromosome is one chromatid. Sister chromatids exist only after copying.

Remember these

• Cell cycle: interphase (G1, S, G2) and M phase. A human cell in culture takes about 24 hours, and M phase about 1 hour.
• Interphase is more than 95 per cent of the cycle. Yeast takes about 90 minutes.
• DNA is copied in S phase: 2C becomes 4C. The number of chromosomes stays 2n.
• G0 is the quiescent stage. Cells there are active but do not divide.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. A human cell in culture takes 24 hours to go round the cycle, and M phase takes 1 hour. What share of the cycle is M phase? What is left for interphase?

2. Name the three phases of interphase. Give the main event of each.

3. A cell has 2n = 8 chromosomes and 2C of DNA in G1. How many chromosomes, how many chromatids and how much DNA does it have at the end of S phase?

4. What is G0? Give two examples of cells that are in it.

5. Why is it wrong to call interphase a resting phase?

References

• NCERT (2023). Biology, Textbook for Class XI, chapter 10.
• Clark M. A., Douglas M. and Choi J. (2018). Biology 2e, OpenStax, chapters 10.2 and 10.3.
• Cooper G. M. and Hausman R. E. (2007). The Cell: A Molecular Approach, 4th edition, ASM Press and Sinauer, chapter 16, The Cell Cycle.