Cell Division, practice questions

B, A human body cell is diploid with 23 pairs, which is 46 chromosomes; a gamete has 23.

B, Meiosis occurs only in the reproductive organs, such as the testes and ovaries; body cells divide by mitosis.

D, Meiosis has two successive divisions, so it produces four haploid cells; mitosis produces two.

B, The fixed order is prophase, metaphase, anaphase, telophase, remembered as PMAT.

C, In anaphase the centromeres separate and the sister chromatids are pulled to opposite poles of the cell.

A, DNA is replicated in interphase, before the mitotic phase, so chromosomes enter mitosis already doubled.

B, Haploid gametes restore, rather than double, the diploid number when they fuse at fertilisation.

B, A malignant tumour can invade surrounding tissue and spread, which is why it is classified as cancer.

C, Mitosis produces identical diploid cells for growth, repair and replacement; meiosis produces gametes and variation.

B, Homologous pairs separate in meiosis I, which halves the chromosome number; meiosis II separates sister chromatids.

• (a) The four stages in order are prophase, metaphase, anaphase and telophase.
• (b) 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, so each pole receives a complete set.
• (c) In an animal cell the cytoplasm divides by forming a cleavage furrow that pinches inwards.
• In a plant cell a new cell plate forms across the middle because the rigid cell wall cannot pinch.
• Each daughter cell produced is diploid and genetically identical to the parent cell.

• (a) One meiosis produces four haploid daughter cells.
• (b) Gametes must be haploid so that the diploid number is restored, not doubled, when two gametes fuse at fertilisation.
• If gametes were diploid, the chromosome number would double every generation.
• (c) During meiosis I the homologous chromosome pairs separate independently of one another.
• This shuffles the parents' chromosomes into new combinations in the gametes.
• As a result the offspring are genetically different from one another and from their parents.

• (a) When control of the cell cycle fails, cells divide continuously and without stopping.
• The extra cells build up into an abnormal mass called a tumour.
• (b) A benign tumour stays in one place and does not spread, while a malignant tumour can invade tissue and spread to other parts of the body.
• (c) A malignant tumour is dangerous because its cells keep dividing when they should not.
• It invades and damages the tissue it forms in, and can spread through the blood or lymph to disrupt other organs.
• It is the malignant tumour, able to spread, that is classified as cancer.

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 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 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 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.

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.

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.

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.

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.

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.

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.