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Cell cycle: Class 11 Biology
The stages of the cell cycle, chromosome structure, mitosis and cytokinesis in animal and plant cells, meiosis and its stages, crossing over, differences between mitosis and meiosis, and cancer as uncontrolled division.
Key points
The cell cycle is the series of events in a cell from one division to the next. It has two main parts: interphase, when the cell grows and copies its DNA, and the M phase, when the nucleus divides (mitosis) and the cytoplasm divides (cytokinesis). Cells spend most of their life in interphase.
Stages of interphase:
- G1 phase: the cell grows, makes proteins and organelles and carries out its normal work.
- S phase: DNA is replicated, so each chromosome now has two identical sister chromatids joined at a centromere.
- G2 phase: the cell makes the proteins needed for division and checks the copied DNA.
- Cells that stop dividing, such as nerve cells, rest in a state called G0.
Checkpoints control the cycle. They test if the cell is large enough, the DNA is complete and undamaged, and the chromosomes are lined up properly. Proteins called cyclins and enzymes called cyclin dependent kinases drive the cell through the stages. If damage cannot be repaired the cell may self destruct by apoptosis.
Chromatin is DNA wrapped around histone proteins. In division it coils into thick chromosomes. Human body cells are diploid (2n = 46), with 23 pairs of homologous chromosomes, one of each pair from each parent. Gametes are haploid (n = 23).
Mitosis, the stages in order:
- Prophase: chromosomes condense and become visible, the nucleolus disappears and the nuclear envelope breaks down. Spindle fibres form, and in animals centrioles move to opposite poles.
- Metaphase: chromosomes line up at the equator of the cell, attached to the spindle by their centromeres.
- Anaphase: the centromeres split and sister chromatids are pulled to opposite poles. Each is now a chromosome.
- Telophase: chromosomes uncoil, nuclear envelopes form around the two sets and the nucleoli reappear.
Cytokinesis follows mitosis. In animal cells the membrane pinches inward to make a cleavage furrow. In plant cells a cell plate forms at the middle from vesicles, grows outward and becomes a new cell wall. Mitosis gives two daughter cells that are genetically identical to the parent and have the same chromosome number.
Mitosis is needed for growth, replacement of dead cells, repair of wounds, and asexual reproduction in organisms such as Hydra and yeast. In plants, mitosis occurs mainly in meristems at root and shoot tips.
Meiosis is the division that makes gametes. It happens in the gonads. One diploid cell divides twice, meiosis I and meiosis II, and makes four haploid cells that are genetically different. It keeps the chromosome number constant from one generation to the next, because fertilisation restores the diploid number.
Meiosis I and II:
- Prophase I: homologous chromosomes pair to form bivalents. Crossing over takes place between non-sister chromatids at points called chiasmata, which exchanges segments of DNA.
- Metaphase I: bivalents line up at the equator, in random orientation. Anaphase I: homologous chromosomes, not chromatids, separate. Telophase I gives two haploid cells.
- Meiosis II is like mitosis: chromatids separate and four haploid cells result.
Variation arises in meiosis from crossing over and from independent assortment of chromosomes. In contrast to mitosis, meiosis gives four unlike cells with half the chromosome number. Cancer happens when cells divide without control after mutations in genes that regulate the cycle. A mass of such cells forms a tumour. Smoking, radiation and some chemicals raise the risk.
Study tip: Practise drawing every stage of mitosis for a cell with 2n = 4, and then meiosis for the same cell. Count the chromosomes and chromatids at each stage, as examiners test this. Learn the table of differences between mitosis and meiosis, with the site, number of divisions, number of cells and variation.
Definitions and flashcards
Say the meaning first, then open the card to check.
Interphase
The period between divisions when the cell grows and copies its DNA.
Chromatid
One of the two identical copies of a replicated chromosome.
Centromere
The region where sister chromatids are joined.
Homologous chromosomes
A pair of chromosomes of the same size and genes, one from each parent.
Diploid
Having two sets of chromosomes (2n).
Haploid
Having one set of chromosomes (n).
Crossing over
Exchange of segments between non-sister chromatids in prophase I.
Cytokinesis
Division of the cytoplasm after nuclear division.
Chiasma
The point where crossing over takes place between homologous chromosomes.
Short questions with model answers
These are the kind of questions asked for 2, 3 and 5 marks. Try each one before opening the answer.
In which phase of interphase is DNA replicated? (2 marks)
DNA is replicated in the S (synthesis) phase. After this each chromosome has two sister chromatids.
State the importance of mitosis. (2 marks)
Mitosis gives growth of the body, replaces worn out cells, repairs wounds, and is the basis of asexual reproduction. It keeps the same chromosome number and genes in all body cells.
How does cytokinesis differ in plant and animal cells? (3 marks)
In animal cells the plasma membrane is pulled inward by a ring of protein fibres and forms a cleavage furrow, which pinches the cell in two. In plant cells the rigid wall prevents this, so a cell plate forms in the middle from vesicles and grows outward to make a new wall between the daughter cells.
Give three differences between mitosis and meiosis. (3 marks)
Mitosis has one division and gives two cells, while meiosis has two divisions and gives four. Mitosis daughter cells are diploid and identical to the parent, while meiosis cells are haploid and different. Mitosis occurs in body cells, and meiosis in reproductive organs. Crossing over occurs only in meiosis.
Describe the stages of mitosis. (5 marks)
In prophase the chromosomes condense, the nuclear envelope and nucleolus disappear, and spindle fibres form. In metaphase the chromosomes line up on the equator with centromeres attached to the spindle. In anaphase the centromeres divide and the chromatids are pulled to opposite poles by the shortening spindle fibres. In telophase the chromosomes uncoil at the poles, new nuclear envelopes form and the nucleoli return. Cytokinesis then divides the cytoplasm to give two identical diploid daughter cells.
How does meiosis produce genetic variation? (3 marks)
In prophase I crossing over exchanges pieces of DNA between non-sister chromatids, which makes new combinations of alleles. In metaphase I the bivalents line up at random, so the maternal and paternal chromosomes are sorted independently into the gametes. Together these make every gamete different.
Common mistakes
- Saying that chromosomes duplicate in prophase. DNA is copied in the S phase of interphase.
- Confusing separation of chromatids (anaphase of mitosis and anaphase II) with separation of homologous chromosomes (anaphase I).
- Writing that meiosis gives two cells, or that mitosis gives four.
- Calling interphase a resting phase. The cell is very active.
- Saying crossing over happens between sister chromatids. It is between non-sister chromatids of homologues.