Cell Division, revision notes

Overview

Cells divide to grow, repair damage and reproduce, and this chapter is built around two very different types of division. Mitosis produces identical body cells for growth and repair, while meiosis produces the gametes needed for sexual reproduction.

Both processes start from the same cell cycle, but they lead to cells with completely different chromosome numbers and completely different genetic content, which is exactly why examiners test the two processes side by side rather than in isolation. Understanding this distinction clearly from the start makes the whole chapter far easier to master.

The chapter opens with chromosomes themselves, thread-like structures made of DNA that carry the genes controlling an organism's characteristics. Human body cells are diploid, containing 23 pairs of chromosomes, one member of each pair inherited from each parent.

Understanding diploid and haploid numbers before studying mitosis and meiosis makes it far easier to explain why one division keeps the chromosome number constant while the other halves it, and why exam questions often ask you to state a chromosome number before and after a named division.

A separate content standard covers the cell cycle in detail: a long interphase in which the cell grows and its DNA replicates, followed by the much shorter mitotic phase, when the nucleus divides through prophase, metaphase, anaphase and telophase before the cytoplasm splits in cytokinesis. Meiosis follows a similar overall pattern but involves two successive divisions, which is what produces four cells instead of two.

Sketching the cell cycle as a labelled circular diagram helps you remember the order and relative length of each stage.

The final content standard links cell division to human health, focusing on what happens when the controls on the cell cycle fail. Cells that divide continuously without stopping form a tumour, and a malignant tumour can invade nearby tissue and spread to other parts of the body.

This section is examined by asking you to explain, in terms of cell division, why uncontrolled growth is dangerous, not by asking for medical treatment details. This reinforces the wider idea that cell biology explains the reasons behind a health condition, not just memorised medical facts.

Chromosomes and the diploid and haploid number (CS 6.1)

Content Standard 6.1 begins with the chromosome: a thread-like structure made of DNA, found in the nucleus, that carries the genes controlling an organism's characteristics. Chromosomes occur in matching pairs called homologous pairs, one member of each pair inherited from each parent.

A human body cell is diploid, written 2n, and contains 23 pairs, which is 46 chromosomes in all.

A gamete, by contrast, is haploid, written n, and carries only one chromosome from each homologous pair, so a human gamete contains 23 single chromosomes. Keeping the two terms apart is the first skill this chapter rewards, because later questions often begin by asking you to state whether a described cell is diploid or haploid, or to give its chromosome number before and after a named division.

Fixing the human numbers early makes the rest of the chapter easier. Mitosis keeps the diploid number constant, so each new body cell is also 2n with 46 chromosomes; meiosis halves it, so each gamete is n with 23.

When two gametes fuse at fertilisation, the diploid number of 46 is restored in the zygote, and the chromosome number stays constant across generations.

The cell cycle: interphase and the mitotic phase (CS 6.2)

Content Standard 6.2 sets out the cell cycle as two main stages. The first and longest is interphase, during which the cell grows, carries out its normal activities, and replicates its DNA so that every chromosome is copied into two identical sister chromatids joined at a centromere.

No visible division takes place during interphase; the cell is preparing for it.

The second stage is the mitotic phase, which is much shorter. Here the nucleus divides in a process called mitosis, passing through prophase, metaphase, anaphase and telophase, and then the cytoplasm divides in cytokinesis to give two separate daughter cells.

The fact that interphase lasts far longer than the mitotic phase reflects the time a cell needs to grow and to copy its DNA accurately.

A common exam point is the order of events: DNA is replicated during interphase, before the mitotic phase begins, so each chromosome already consists of two sister chromatids by the time division starts. Drawing the cell cycle as a labelled circle, with a long interphase and a short mitotic phase, helps fix both the sequence and the relative lengths.

Mitosis stage by stage (CS 6.2)

Mitosis is nuclear division that produces two daughter cells genetically identical to the parent cell and to each other, both diploid. It is used for growth, for replacing worn-out cells and for repairing damaged tissue.

The four stages run in a fixed order, remembered as PMAT.

In prophase the chromosomes condense and become visible, each seen as two sister chromatids, and the nuclear membrane breaks down. In metaphase the chromosomes line up along the equator of the cell, attached to spindle fibres.

In anaphase the sister chromatids are pulled apart to opposite poles of the cell, so that each pole receives a complete set. In telophase a new nuclear membrane forms around each set, giving two nuclei.

Cytokinesis then splits the cytoplasm, and here plant and animal cells differ: an animal cell pinches inwards by forming a cleavage furrow, while a plant cell builds a new cell plate across the middle because its rigid cell wall cannot pinch. Recognising a stage from a diagram, and describing what the chromosomes are doing, is one of the most frequently tested skills in this chapter.

Meiosis and why it halves the chromosome number (CS 6.3)

Content Standard 6.3 covers meiosis, a special nuclear division that occurs only in the reproductive organs, the testes and ovaries in animals and the anthers and ovaries in flowering plants. Unlike mitosis, meiosis involves two successive divisions, meiosis I and meiosis II, and produces four daughter cells rather than two.

In meiosis I the homologous chromosome pairs are separated, one chromosome of each pair going to each new cell; this is the step that halves the chromosome number from diploid to haploid. In meiosis II, which resembles mitosis, the sister chromatids of each chromosome are separated.

The outcome is four haploid cells, each with a single, unpaired set of chromosomes and each genetically different from the others.

Meiosis matters for two reasons the syllabus stresses. First, by halving the chromosome number it keeps that number constant from one generation to the next: if gametes were diploid, fertilisation would double the number every generation.

Second, the way homologous pairs separate independently shuffles the parents' chromosomes into new combinations, producing genetic variation among offspring, which is why siblings are not genetically identical and why a population has the raw material for natural selection.

Mitosis compared with meiosis

A side-by-side comparison is one of the most common ways this chapter is examined. Setting the two divisions out together keeps the differences clear, because a single table answers questions on the number of divisions, the number and ploidy of the daughter cells, genetic identity, location and purpose at once.

When cell cycle control fails: tumours and cancer (CS 6.4)

Content Standard 6.4 links cell division to human health. Normally the cell cycle is controlled so that a cell divides only when new cells are needed.

When this control fails, cells divide continuously and without stopping, and the extra cells build up into an abnormal mass called a tumour.

A tumour may be benign or malignant. A benign tumour stays in one place and does not spread, whereas a malignant tumour can invade the surrounding tissue and spread through the blood or lymph to other parts of the body, where it forms secondary growths.

It is a malignant tumour that is described as cancer, so a tumour and cancer are not simply the same thing.

This content standard is examined by asking you to explain, in terms of cell division, why uncontrolled growth is dangerous, not by asking for details of medical treatment. The danger comes from cells dividing when they should not, so that a growing tumour damages the organ it forms in and, if malignant, disrupts other organs it spreads to.

Factors that raise the risk, such as tobacco smoke, certain chemicals and prolonged ultraviolet exposure, are noted as influences on this loss of control rather than as a medical topic in their own right.

Key concepts to master

• Chromosomes, Chromosomes are thread-like structures made of DNA found in the nucleus, and each carries many genes that control an organism's characteristics. Human body cells are diploid (2n), containing 23 pairs of chromosomes, 46 in total, with one chromosome of each pair inherited from the mother and one from the father. Knowing this number is often needed to answer questions about gamete or daughter-cell chromosome counts.
• The cell cycle, The cell cycle has two main stages: interphase, during which the cell grows, carries out its normal functions, and replicates its DNA so each chromosome becomes two identical sister chromatids; and the mitotic phase, when the nucleus divides followed by cytokinesis, which splits the cytoplasm into two separate daughter cells. Interphase lasts considerably longer than the mitotic phase, reflecting the time needed for cell growth and careful DNA replication.
• Mitosis, Mitosis is nuclear division that produces two daughter cells genetically identical to the parent cell and to each other, both diploid. It proceeds through four stages in order, prophase, metaphase, anaphase and telophase, and is used for growth, for replacing worn-out cells, and for repairing damaged tissue. Recognising these four stages from a photograph or diagram is one of the most frequently tested skills in this chapter.
• Meiosis, Meiosis is a special type of nuclear division that occurs only in reproductive organs, involving two successive divisions that produce four daughter cells, each haploid (n) and genetically different from one another. It halves the chromosome number so that fusion of two gametes at fertilisation restores the full diploid number.
• Importance of meiosis, Meiosis keeps the chromosome number constant from one generation to the next; without it, fertilisation would double the chromosome number every generation. It also introduces genetic variation among offspring through the independent separation of chromosome pairs, which is one reason siblings are not genetically identical. Without this variation, populations would have far less raw material for natural selection to act on over time.
• Uncontrolled cell division and cancer, The cell cycle is normally controlled so that cells divide only when needed. When this control fails, cells divide continuously and without stopping, forming a mass of cells called a tumour. A malignant tumour can invade surrounding tissue and spread to other parts of the body, which is what makes it cancerous. Understanding the stages of the cell cycle helps pinpoint exactly where this normal growth control can fail.
• Diploid and haploid numbers, A diploid cell (2n) contains chromosomes in homologous pairs, one from each parent, and is the normal condition of body cells produced by mitosis. A haploid cell (n) contains only one chromosome of each pair and is found only in gametes produced by meiosis. Recognising which number a described cell has is often the first step in answering a genetics or cell-division question correctly.
• Stages of mitosis, In prophase the chromosomes condense and become visible, each already duplicated into two sister chromatids; in metaphase they line up at the equator of the cell; in anaphase the sister chromatids are pulled to opposite poles; and in telophase two new nuclei form before the cytoplasm divides.
• Chromosome behaviour in meiosis, In the first meiotic division, homologous chromosome pairs separate, halving the chromosome number; in the second division, which resembles mitosis, the sister chromatids of each chromosome separate. The result is four haploid cells, each with a single, unpaired set of chromosomes. This two-step separation is why meiosis, unlike mitosis, cannot be described by a single simple set of four stages.
• Growth, repair and reproduction, Mitosis and meiosis serve different purposes in the same organism: mitosis increases cell number for growth and replaces damaged cells during repair, while meiosis produces sex cells for reproduction. Confusing the two processes is one of the most common sources of lost marks in this chapter.

Quick recall checklist

1. Can you define and explain Chromosomes?
2. Can you define and explain The cell cycle?
3. Can you define and explain Mitosis?
4. Can you define and explain Meiosis?
5. Can you define and explain Importance of meiosis?
6. Can you define and explain Uncontrolled cell division and cancer?
7. Can you define and explain Diploid and haploid numbers?
8. Can you define and explain Stages of mitosis?
9. Can you define and explain Chromosome behaviour in meiosis?
10. Can you define and explain Growth, repair and reproduction?

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Source:SRC-DSKP-EN