The human genome
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Overview
The nucleus of a human cell contains between 30 000 and 40 000 genes. This complete set is called the human genome. Each gene is a code made up of pairs of bases carried on the DNA molecule.

In June 2000, researchers announced that they had completed a draft of the entire human genome sequence. This is the map for a human being. In this e–source, you can see what a huge task this has been and why it is such a valuable piece of research.

Zoom in flash
Click here to take a journey into the cell using flash.
Picture1. Different magnifications of the inside of a cell. Drag the magnifier along the scale to see all 8 pictures in order.
Question 1
Give a definition of the term 'genome'.
Cells
Cells are the basic building blocks of all living things. The bulk of a human cell is made from cytoplasm and in the centre is the nucleus.
The nucleus
Inside the nucleus of nearly all types of cell are chromosomes. The nucleus of a human cell contains 23 pairs of chromosomes.
Chromosomes
The chromosomes contain the genes that carry the information that determines a person's characteristics. Each chromosome consists of one piece of DNA about 4 cm long.
Coils of coils
A chromosome is about 0.004 mm long. To fit inside, the 4cm piece of DNA is squashed by a factor of 10 000.

It twists into a string (shown in yellow), which forms into clumps (shown in blue). These coil up like a snake to fit inside the chromosome.
Coils of DNA
The single piece of DNA molecule winds itself around special proteins to form into very tight coils like a string (shown in yellow).
DNA molecule
The genes are made from a sequence of bases on DNA (deoxyribonucleic acid).
DNA molecule
This shows two pieces of DNA with about 50 bases on each one. A single gene is made from a sequence of thousands of bases.
DNA molecule
The DNA molecule is made from two strands, twisted together in a double helix. The links between each strand are called bases.
Using the magnifier
The picture shows some human cheek cells. Drag the magnifier to the right to zoom into its DNA.