Cell cycle: Class 9 Biology

Key points

The cell cycle is the series of events through which a cell grows, copies its DNA and divides into two daughter cells. It is the way by which living things grow, repair damage and replace old cells. It has two main parts: interphase, and the division phase (M phase).

Interphase is the long period between two divisions, when the cell is not dividing but is very active. It has three stages. In G1 the cell grows and makes proteins and organelles. In S phase the DNA is replicated, so each chromosome is copied. In G2 the cell prepares for division and makes the proteins needed. The M phase includes nuclear division (mitosis) and division of the cytoplasm (cytokinesis).

A chromosome is a thread like structure in the nucleus made of DNA and protein. It carries genes. Chromatin is the uncoiled form seen between divisions. Before division, each replicated chromosome has two identical sister chromatids joined at a centromere. Each species has a fixed number of chromosomes. Humans have 46 (23 pairs). Body cells are diploid (2n) with two sets, and gametes are haploid (n) with one set, so a human gamete has 23 chromosomes.

Mitosis is the division of a nucleus that produces two daughter nuclei, each identical to the parent and with the same chromosome number. It occurs in body (somatic) cells and is used for growth, repair and, in some organisms, asexual reproduction.

Phases of mitosis:

• Prophase: chromosomes coil and become visible as two chromatids. The nucleolus disappears and the nuclear envelope breaks down. In animal cells the centrioles move to opposite poles and spindle fibres form.
• Metaphase: chromosomes line up on the equator (the middle) of the cell, attached to spindle fibres at their centromeres.
• Anaphase: the centromeres split and the sister chromatids are pulled to opposite poles by the shortening spindle fibres.
• Telophase: chromatids (now called chromosomes) reach the poles and uncoil. The nuclear envelopes and nucleoli re-form and the spindle breaks down.

Cytokinesis is the division of the cytoplasm that follows telophase. In animal cells the membrane pinches inwards, forming a furrow that divides the cell in two. In plant cells a cell plate forms in the middle and becomes the new cell wall, because the rigid wall prevents furrowing.

Meiosis is a special division that makes four daughter cells, each with half of the chromosome number (haploid). It occurs in the reproductive organs to make gametes (sperm and egg in animals). It involves two successive divisions, meiosis I and meiosis II, with only one round of DNA replication.

Main events of meiosis:

• Meiosis I (reduction division): homologous chromosomes pair up in prophase I. Crossing over takes place, in which parts of non-sister chromatids are exchanged. The pairs line up at the equator in metaphase I, and in anaphase I the homologous chromosomes separate. Two haploid cells form.
• Meiosis II (like mitosis): the chromatids of each chromosome separate in anaphase II. The result is four haploid cells.

Meiosis keeps the chromosome number the same from generation to generation. When a haploid sperm and haploid egg join in fertilisation, the diploid number is restored in the zygote. Meiosis also produces variation, because crossing over and the random arrangement of chromosome pairs give new combinations of genes.

Mitosis compared with meiosis:

• Occurs in body cells for mitosis, in reproductive organs for meiosis.
• One division for mitosis, two divisions for meiosis.
• Two daughter cells for mitosis, four for meiosis.
• Daughter cells are identical and diploid in mitosis, and different and haploid in meiosis.
• No pairing of homologous chromosomes and no crossing over in mitosis, while both occur in meiosis.

Uncontrolled cell division leads to cancer. A tumour is a mass of cells that divide without control. Cancer cells may spread to other parts of the body. Causes include radiation, certain chemicals such as those in tobacco smoke, and some viruses.

Study tip: Draw each phase of mitosis in a small, neat diagram for a cell with four chromosomes and label the spindle, centromere and chromatids. A table of differences between mitosis and meiosis is almost certain to appear in tests, so learn at least five points. Remember that the chromosome number halves only in meiosis.

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Common mistakes

• Thinking interphase is a resting stage. The cell grows and copies its DNA in this stage.
• Mixing up the order of the phases. Remember: prophase, metaphase, anaphase, telophase.
• Saying that chromosomes split in anaphase I of meiosis. In anaphase I whole homologous chromosomes separate, and the chromatids separate in anaphase II.
• Calling the meiosis products identical to the parent. They are haploid and different.
• Writing that the cell plate forms in animal cells.

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