The Cell Cycle and Mitosis: Quick Revision

A brief note on The Cell Cycle and Mitosis. Read it before a test. Try each question at the end before you open its answer.

The cell cycle

• Cycle: interphase (G1, S, G2) and M phase. A human cell in culture takes about 24 hours, and M phase about 1 hour.
• Interphase: more than 95 per cent of the cycle. Yeast takes about 90 minutes.
• G1: growth, no DNA copying. S: DNA copied, 2C to 4C, centrosome copied. G2: proteins for mitosis.
• G0: the quiescent stage. The cell is active but does not divide.
• Counting: S phase doubles the DNA, not the chromosomes. A cell with 2n chromosomes in G1 has 2n chromosomes and 4n chromatids in G2.

Mitosis

• Prophase: chromosomes condense, two chromatids each, spindle starts, envelope and nucleolus disappear at the end.
• Prometaphase: the envelope breaks into vesicles, spindle fibres join the kinetochores. NCERT does not name it.
• Metaphase: chromosomes on the metaphase plate, best seen here.
• Anaphase: centromeres split, chromatids move to opposite poles. The cell has 4n chromosomes for a short time.
• Telophase: chromosomes decondense, envelopes, nucleolus, Golgi complex and ER reform.

Cytokinesis and the work of mitosis

• Animal cell: a furrow cuts in from the outside.
• Plant cell: the cell plate grows out from the centre and becomes the middle lamella.
• Syncytium: karyokinesis without cytokinesis, for example the liquid endosperm of a coconut.
• Work: growth, repair, the nucleo-cytoplasmic ratio, growth of meristems and reproduction of single celled organisms.

A worked example

A human cell has 46 chromosomes in G1. How many chromatids and how much DNA does it have in G2?

1. S phase gives every chromosome a second chromatid: $2 \times 46 = 92$ chromatids.
2. The DNA doubles from $2$C to $4$C.

Practice with new numbers

A program to try

The program lists the counts of a human cell at three points of the cycle.

Mistakes to avoid

• Saying that S phase doubles the chromosomes. It doubles the DNA.
• Saying that G0 and G1 are the same. G0 is outside the cycle.
• Putting the splitting of the centromere in metaphase. It starts anaphase.
• Mixing up the two kinds of cytokinesis. Furrow for animals, cell plate for plants.

Check yourself

1. How many chromatids does a cell with 8 chromosomes in G1 have at metaphase?

2. What does the cell plate become?

3. Which stage of mitosis shows the chromosomes best?

4. In which phase is the DNA copied?

Sources

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