Year 1 · Organs and cells · Section 2.5

The cell nucleus

Chromosomes

The nuclei of human body cells usually contain chromosomes: long structures carrying genetic information that helps control cell activities. They are usually difficult to distinguish with a light microscope.

Before cell division, the chromosomes become shorter and thicker and can be seen more clearly. Most human body cells contain 46 chromosomes. Other species can have different numbers. Some specialised human cells, such as mature red blood cells, have no nucleus.

The structure of DNA

Chromosomes contain DNA packaged with proteins. DNA resembles a twisted ladder with millions of rungs. Each rung contains two bases.

There are four types of base: A, T, C and G. They form specific pairs: A pairs with T, and C pairs with G. Each pair is called a base pair.

The function of DNA

DNA carries information involved in your characteristics, such as eye colour. The information is stored in the sequence of bases. As with letters in words, a different sequence can carry different information.

DNA helps determine which substances cells make, such as substances involved in producing pigments in the iris.

Inherited characteristics

You inherit DNA from your parents: approximately half of your nuclear DNA comes from each parent. Characteristics influenced by this information are called inherited characteristics, such as aspects of face shape or the ability to digest certain substances.

The environment and lifestyle can also influence how characteristics develop.

Genes

A gene is a section of DNA containing information used by cells and contributing to one or more characteristics. Genes vary in length and may contain thousands of base pairs.

Genes are located on chromosomes; several genes on different chromosomes can contribute to one characteristic. Most body cells contain the same set of genes, but use different genes according to their function and needs.