3.1 Essential ideas

3.1.2 Chromosomes

  • All chromosomes are made of a single DNA molecule, but the number and structure of prokaryotic and eukaryotic chromosomes differ.

Figure 3.1.2a – Chromosomes in prokaryotes and eukaryotesFigure 3.1.2a – Chromosomes in prokaryotes and eukaryotes




Number of chromosomes


  • Circular
  • Linear

Association with proteins

  • Histone proteins (for organisation and packaging of DNA)

Presence of plasmids

  • No plasmids
  • Plasmids are a unique feature of prokaryotes. A plasmid is a small piece of circular DNA which codes for additional genes that are not needed for normal growth or development.

Figure 3.1.2b – Bacterial plasmids are not chromosomesFigure 3.1.2b – Bacterial plasmids are not chromosomes

  • For example, genes for antibiotic resistance are usually located on plasmids.

Figure 3.1.2c – Conjugation involves replication and transfer of plasmid DNAFigure 3.1.2c – Conjugation involves replication and transfer of plasmid DNA

  • DNA replication in plasmids occurs independently of the bacterial chromosome. Plasmids can be transferred between bacterial cells, or even between different species.

Features of eukaryotic chromosomes

Figure 3.1.2d – Karyograms for human female (left) and male (right)Figure 3.1.2d – Karyograms for human female (left) and male (right)

  • Homologous chromosomes are identical in shape and structure and carry the same sequence of genes.
  • One chromosome is inherited from the mother and the other is inherited from the father, so that although they carry the same sequence of genes, the alleles of genes on homologous chromosomes may differ.
  • In humans, there are 23 pairs of chromosomes.
  • Chromosomes that determine an individual’s sex are called sex chromosomes.
  • In humans, females have a pair of X chromosomes, and males have one X and one Y chromosome. The Y chromosome is much smaller than the X chromosome, as you can see from Figure 3.2.1d.
  • Chromosomes that do not determine sex are called autosomes.
  • In humans, there are 22 pairs of autosomes, numbered from 1 to 22. The sex chromosomes are the 23rd pair.

Haploid and diploid nuclei

Figure 3.1.2e – Haploid and diploid nucleiFigure 3.1.2e – Haploid (N) and diploid nuclei (2N)

Application: Haploid and diploid numbers

Copy the table and fill in the missing numbers.

Latin binomial

Common name

Number of chromosomes in somatic cells

Number of chromosomes in gametes

Homo sapiens




Pan troglodytes




Canis familiaris




Oryza sativa




Parascaris equorum

Horse threadworm



Key idea

Chromosomes carry genes in a linear sequence that is shared by members of a species.

Figure 3.2.1f – GametesFigure 3.2.1f – Gametes
Gametes have haploid nuclei. The nuclei of haploid cells contain only one chromosome from each homologous pair.

Figure 3.2.1g – X and YFigure 3.2.1g – X and Y
Electron micrograph of human X chromosome (left) and Y chromosome (right).

Food for thought

Are the sex chromosomes in a human male, X and Y, a homologous pair?

International mindedness

Sequencing of the rice genome involved cooperation between biologists in 10 countries.

Figure 3.2.1h – MicrographFigure 3.2.1h – Micrograph
Micrograph showing that transfer of plasmids can be transferred between cells.

Course links

  • Populations of bacteria quickly become resistant to antibiotics because plasmids are easily transferred between cells – see 5.1.2.
  • Plasmids are used to insert genes for the purpose of genetic modification of organisms – see 3.1.5.
  • HL students will learn more about the function of histones in 7.1.1 and 7.1.2.