Genes and chromosomes

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What is the human genome?

A human genome is all the DNA contained in the nucleus, or core, of a human cell. DNA molecules are very long, so DNA is packaged up tightly into chromosomes. Human cells have 23 pairs of chromosomes – one chromosome of each pair is inherited from each parent. This means that siblings (brothers and sisters) have about half of their DNA in common, while more distant relatives have less DNA in common (first cousins share about one quarter of their DNA).

Humans have two kinds of chromosome: sex chromosomes (the X and Y chromosomes), and autosomes. There are 22 paired autosomes and two sex chromosomes in each cell. Females have two X chromosomes and males have one X chromosome and one Y chromosome.

Each chromosome is made up of a single DNA molecule. DNA contains the instructions for building different parts of a cell and is in the form of a chemical code (the genetic code). DNA is made up of chains of four basic building blocks known as nucleotide bases. These bases are Adenine (A), which is always paired with Guanine (G), and Thymine (T) which is always paired with Cytosine (C).

The human genome is made up of over 3 billion of these genetic bases. Groups of these bases form genes which function to instruct the cell to build other molecules such as proteins. There are between 20,000 and 25,000 genes in the human genome.

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