Home › Learning Hub › IGCSE Biology › 17a Chromosomes, genes, cell division
Topic 17 · 17.1–17.3

Chromosomes, genes, mitosis and meiosis

Core and Extended · Papers 1–6

🎯What you need to be able to do

  • State that chromosomes are made of DNA containing genes; define gene and allele; describe the inheritance of sex with X and Y chromosomes.
  • Explain how the base sequence of a gene determines a protein, and outline protein synthesis EXTENDED.
  • Describe haploid and diploid nuclei, and state that human diploid cells have 23 pairs EXTENDED.
  • Describe mitosis and its roles, stem cells, and meiosis EXTENDED.

📚The biology

Chromosomes, genes and alleles

A cell with a nucleus containing chromosomes; one chromosome enlarged, with a highlighted band; that band enlarged as a section of DNA double helix. A gene is a length of DNA that codes for a protein; DNA contains the genetic information.
Nucleus → chromosome → DNA → gene.

Chromosomes are made of DNA, which contains genetic information in the form of genes. A gene is a length of DNA that codes for a protein. An allele is an alternative form of a gene (for example, the gene for eye colour has alleles for brown and for blue).

A Punnett square for the inheritance of sex. The mother is XX and makes only X eggs; the father is XY and makes X or Y sperm. The offspring are 2 XX, female, and 2 XY, male: a 1 to 1 ratio.
Females are XX, males XY. The sperm decides the sex of the child.

How genes control cells EXTENDED

EXTENDED The sequence of bases in a gene determines the sequence of amino acids in a specific protein. Different sequences of amino acids give proteins different shapes, and so different functions. By controlling which proteins are made — enzymes, membrane carriers, receptors for neurotransmitters — DNA controls how the cell works.

Protein synthesis. In the nucleus, a gene is copied to make mRNA while the gene itself stays in the nucleus. The mRNA moves out into the cytoplasm and passes through a ribosome, which joins free amino acids into a protein chain in the order set by the mRNA bases.
EXTENDED Gene → mRNA (in the nucleus) → ribosome (in the cytoplasm) → protein.
  • EXTENDED the gene coding for the protein remains in the nucleus;
  • messenger RNA (mRNA) is a copy of a gene; it is made in the nucleus and moves to the cytoplasm;
  • the mRNA passes through ribosomes, which assemble amino acids into protein molecules;
  • the specific sequence of amino acids is determined by the sequence of bases in the mRNA.

EXTENDED Most body cells contain the same genes, but many genes in a particular cell are not expressed: each cell makes only the proteins it needs. A red blood cell precursor expresses the haemoglobin gene; a stomach cell expresses the pepsin gene.

EXTENDED A haploid nucleus contains a single set of chromosomes; a diploid nucleus contains two sets. In a diploid cell there is a pair of each type of chromosome; a human diploid cell has 23 pairs (46 chromosomes).

Mitosis and meiosis EXTENDED

Left: a diploid cell with two pairs of chromosomes divides by mitosis into two genetically identical diploid cells. Right: a diploid cell divides by meiosis into four haploid cells, each with one chromosome from each pair, in different combinations.
EXTENDED Mitosis keeps the chromosome number; meiosis halves it and produces variation.
  • EXTENDED Mitosis is nuclear division giving rise to genetically identical cells. It is used for growth, repair of damaged tissues, replacement of cells and asexual reproduction. The chromosomes are exactly replicated before mitosis; during it the copies separate, so each daughter cell keeps the same chromosome number.
  • Stem cells are unspecialised cells that divide by mitosis to produce daughter cells that can become specialised for specific functions.
  • Meiosis is involved in producing gametes. It is a reduction division: the chromosome number is halved from diploid to haploid, and the cells produced are genetically different.

✏️Worked example

(a) Define the terms gene and allele. [2] (b) A couple have three sons. Calculate the probability that their next child is a daughter, and explain. [2] (c) EXTENDED A skin cell and a nerve cell from the same person contain the same genes but make different proteins. Explain why. [2] (d) EXTENDED A dog’s body cells contain 78 chromosomes. State the number in its sperm cells and in the cells made when its skin repairs a cut. [2]

(a) Gene: a length of DNA that codes for a protein. Allele: an alternative form of a gene.

(b) 0.5 (50%, 1 in 2). Each fertilisation is independent: half the father’s sperm carry X and half Y, whatever happened before.

(c) Only some genes are expressed in each cell; each cell makes only the proteins it needs, so different genes are switched on in skin and nerve cells.

(d) Sperm: 39 (made by meiosis, haploid). Repair cells: 78 (made by mitosis).

Check it. Haploid is always half the diploid number: 78 ÷ 2 = 39, and for humans 46 ÷ 2 = 23.
“They are due a girl.” Previous children do not change the probability; each birth has the same 1 : 1 chance.

📝Practise

In the style of the multiple-choice and theory papers. EXTENDED marks Supplement content.

1. (Multiple choice.) What are chromosomes made of? A: protein only. B: DNA. C: starch. D: amino acids only.
B. (DNA, packed with proteins.)
2. (Theory.) State the sex chromosomes of a human male and of a human female. [2]
Male: XY. Female: XX.
3. (Theory.) Draw a genetic diagram to show that the ratio of male to female offspring is expected to be 1 : 1. [3]
Parents XX × XY; gametes X and X, X and Y; offspring XX, XX, XY, XY; 2 female : 2 male = 1 : 1.
4. (Theory.) EXTENDED Explain how a change in the base sequence of a gene could stop an enzyme working. [3]
The base sequence determines the amino acid sequence; a change may alter the amino acid sequence, changing the enzyme’s shape, including its active site, so the substrate no longer fits.
5. (Theory.) EXTENDED State where in the cell (a) mRNA is made, (b) proteins are assembled. [2]
(a) The nucleus. (b) On ribosomes in the cytoplasm.
6. (Theory.) EXTENDED Give three roles of mitosis. [3]
Any three: growth; repair of damaged tissues; replacement of cells; asexual reproduction.
7. (Theory.) EXTENDED State two differences between mitosis and meiosis. [2]
Any two: mitosis gives genetically identical cells, meiosis genetically different ones; mitosis keeps the chromosome number (diploid), meiosis halves it (haploid); mitosis gives 2 cells, meiosis 4; meiosis makes gametes.
8. (Theory.) EXTENDED Describe what a stem cell is. [2]
An unspecialised cell that divides by mitosis to produce daughter cells that can become specialised for specific functions.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • yourgenome — animations of DNA, genes and protein synthesis
  • Learn Genetics (University of Utah) — interactive cell division