The nucleus of a cell contains chromosomes made of DNA molecules.

The idea

Think of the nucleus as the cell's control room, and inside it the genetic material is packaged into chromosomes. Each chromosome is one very long DNA molecule wound tightly around proteins, so a molecule far too thin to see becomes a compact, visible structure during cell division. DNA itself is a polymer built from four bases whose order forms a code. That code is read in sections, and each section is a gene. In a human body cell there are 46 chromosomes, arranged as 23 pairs, and every one of them is DNA. A useful method is to zoom in mentally: cell, then nucleus, then chromosome, then DNA molecule, then gene. This hierarchy matters because it explains why the nucleus controls cell activities and why DNA can be copied and passed on.

What earns the marks

• States that the nucleus contains the cell's genetic material in the form of chromosomes.
• Describes a chromosome as a structure made of a DNA molecule, typically coiled and associated with protein.
• Links DNA to a code or sequence of bases that carries genetic information.
• Places the structures in order of size: nucleus larger than chromosome, chromosome larger than DNA molecule.
• Uses the human example of 46 chromosomes in a body cell to make the idea concrete.

Common mistakes and how to put them right

• Saying the nucleus contains genes but omitting chromosomes.

  Put it right: State that genes are carried on chromosomes, which are made of DNA.

• Treating DNA and a chromosome as identical.

  Put it right: A chromosome is a DNA molecule packaged with protein, so DNA is the molecule and the chromosome is the packaged structure.

• Writing that chromosomes float freely in the cytoplasm.

  Put it right: In a eukaryotic cell the chromosomes are found inside the nucleus.

Exam tips

• Use the sequence nucleus, chromosome, DNA, gene when explaining where genetic material is found, so each level is clearly placed.
• If asked to describe the relationship, give a size order as well as a location, because both are commonly needed for full credit.
• Check that any diagram is labelled with straight lines and that the label chromosome points to a structure inside the nucleus, not to the nuclear membrane.