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How does genetic information sit on chromosomes, and how is it passed on when cells divide?

Describe chromosomes, genes and DNA and explain the role of cell division in growth and reproduction

A focused answer to the O-Level Biology outcome on chromosomes and cell division. The link between DNA, genes and chromosomes, and the roles of mitosis (growth) and meiosis (making gametes).

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What this dot point is asking

SEAB wants you to describe the relationship between DNA, genes and chromosomes, and to explain the roles of cell division: mitosis for growth and repair (making identical cells) and meiosis for making gametes (making cells with half the chromosomes). You should explain why halving the chromosome number in gametes matters.

The answer

DNA, genes and chromosomes

Inside the nucleus of a cell is the genetic material, arranged as chromosomes.

  • A chromosome is made of a long, coiled molecule of DNA.
  • A gene is a short section of DNA on a chromosome that codes for a particular characteristic (by coding for a protein).
  • DNA carries the instructions, in a code, for making the proteins that build and run the body.

So: DNA makes up the chromosomes, and genes are sections of that DNA. Different genes control different features, such as eye colour.

Chromosome number

Human body cells contain 46 chromosomes, arranged in 23 pairs. One of each pair came from the mother and one from the father.

Mitosis: division for growth and repair

Mitosis is the type of cell division that makes new body cells for growth and the repair of tissues, and for asexual reproduction. In mitosis:

  • One cell divides into two.
  • The two new cells are genetically identical to the parent cell and to each other.
  • Each new cell has the full number of chromosomes (46 in humans).

Meiosis: division to make gametes

Meiosis is the type of cell division that makes gametes (sperm and egg cells) in the reproductive organs. In meiosis:

  • One cell divides to make four cells.
  • The cells have half the number of chromosomes (23 in humans).
  • The cells are not genetically identical, so meiosis produces variation.

Why halving the number matters

Because gametes have half the chromosomes, when a sperm (23) fuses with an egg (23) at fertilisation, the zygote has the full 46 again, the correct number for a body cell. If gametes had the full number, the chromosome number would double each generation, which would be harmful.

Examples in context

Example 1. Healing a cut. When skin is cut, the surrounding cells divide by mitosis to make new, identical skin cells that fill the gap and repair the wound. This shows mitosis at work in repair.

Example 2. Why siblings differ. Meiosis makes gametes that are all genetically different, so each egg and sperm carries a different combination of genes. This, combined with fertilisation, is why brothers and sisters are not identical.

Try this

Q1. State the relationship between a gene and a chromosome. [1 mark]

  • Cue. A gene is a short section of DNA on a chromosome (a chromosome carries many genes).

Q2. State how many chromosomes are in a human body cell and in a human gamete. [2 marks]

  • Cue. A body cell has 46; a gamete has 23 (half the number).

Q3. Explain why gametes must have half the number of chromosomes. [2 marks]

  • Cue. So that when two gametes fuse at fertilisation, the zygote has the full number again, keeping the chromosome number constant each generation.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original5 marks(a) State the relationship between DNA, a gene and a chromosome. (b) A human body cell has 46 chromosomes. State how many chromosomes are in a human sperm cell, and explain why this number is important.
Show worked answer →

(a) A chromosome is made of a long molecule of DNA. A gene is a short section of the DNA on a chromosome that codes for a particular characteristic (or protein). So genes are parts of the DNA, and the DNA makes up the chromosomes.

(b) A human sperm cell has 23 chromosomes (half of 46). This is important because the sperm and egg each carry half the number, so when they fuse at fertilisation the zygote has the full 46 again, the correct number for a human body cell.

Markers reward the chromosome being made of DNA with genes as sections of it, the sperm having 23 chromosomes, and the explanation that halving the number lets fertilisation restore the full set.

Original4 marksCompare mitosis and meiosis by stating where each occurs, the number of cells produced, and whether the cells are genetically identical to the parent cell.
Show worked answer →

Mitosis occurs in body cells for growth and repair; it produces two cells; the cells are genetically identical to the parent cell (and to each other), with the full number of chromosomes.

Meiosis occurs in the reproductive organs to make gametes (sperm and eggs); it produces four cells; the cells are not genetically identical and have half the number of chromosomes.

Markers reward the matched comparison: mitosis for growth or repair making two identical cells with the full chromosome number, versus meiosis making gametes, four non-identical cells with half the number.

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