Chromosomes: The nucleus of a cell contains chromosomes made of DNA molecules. Each chromosome carries a large number of genes.

The idea

The nucleus is the control centre of a eukaryotic cell and holds the genetic material. That material is organised into chromosomes, and each chromosome is one very long DNA molecule folded and packaged with proteins. Along the DNA molecule are genes: short sections that code for particular proteins, and therefore influence characteristics. Because a single chromosome is enormous compared with one gene, each chromosome carries a large number of genes, not just one. For example, a human body cell has 46 chromosomes but around 20 000 genes, so genes are densely packed along each DNA molecule. Understanding this hierarchy — nucleus, chromosome, DNA, gene — lets you explain inheritance, protein synthesis and mutation.

What earns the marks

• The nucleus contains the cell's genetic material in the form of chromosomes.
• Chromosomes are made of DNA molecules, so DNA is the molecule of inheritance.
• A gene is a section of a DNA molecule that codes for a protein.
• Each chromosome carries a large number of genes, not a single gene.
• The relationship is hierarchical: nucleus contains chromosomes, chromosomes contain DNA, and genes are sections of that DNA.
• Chromosomes are long DNA molecules, so many genes can be present along one chromosome.

Common mistakes and how to put them right

• Saying a chromosome is made of genes.

  Put it right by stating a chromosome is made of DNA, and genes are sections along that DNA.

• Thinking one chromosome carries one gene.

  Put it right by noting each chromosome carries a large number of genes.

• Confusing the nucleus with the chromosome.

  Put it right by describing the nucleus as the structure that contains the chromosomes.

Exam tips

• Use the hierarchy nucleus → chromosome → DNA → gene when explaining where genetic material is found.
• Define a gene as a section of DNA that codes for a protein, then link it to a characteristic.
• If asked to compare numbers, note that there are far more genes than chromosomes in a body cell.