The genome is the list of instructions, encoded in DNA (deoxyribonucleic acid), needed to make a human.

DNA and protein

• The four letters in the DNA alphabet - A, C, G and T - are used to carry the instructions for making an organism. The order (or sequence) of these letters holds the code - just like the order of letters that makes words mean something. Each set of three letters corresponds to a single amino acid.
• There are 20 different building blocks - amino acids - used in a bewildering array of combinations to produce our proteins. The different combinations make proteins as different as keratin in hair and haemoglobin in blood.

The genome

• The human genome is made up of 3 billion (3 000 000 000) bases of DNA, split into 24 chromosomes.

This information...

• would fill a stack of paperback books 200 ft (61 m) high
• would fill two hundred 500-page telephone directories
• would take a century to recite, if we recited at one letter per second for 24 hours a day
• if spread out 1 mm apart, would extend 3000 km (1864 miles) or about 7000 times the height of the Empire State Building.

Human cells

• The human body is made up of 100 trillion cells. Each cell has at least one nucleus, which houses the chromosomes.
• There is 1.8 m of DNA in each of our cells packed into a structure only 0.0001 cm across (it would easily fit on the head of a pin).
• If all the DNA in the 100 trillion cells of the human body was put end to end it would reach to the sun and back over 600 times [100 trillion × 6 ft (1.8 m) divided by 92 million miles (148 800 000 km) = 1200].
• Most human cells contain 46 chromosomes: pairs of chromosomes 1-22, and a pair of sex chromosomes (females have two Xs; males an X and a Y). Sperm and eggs contain one of each chromosome.

Genes and variation

• Housed along each chromosome is a selection of genes. The human genome contains about 20 000-25 000 genes.
• Mice also have about 20 000 genes; in the nematode (C. elegans), the number is around 19 000; in yeast (S. cerevisiae) there are approximately 6000 genes; and the microbe responsible for tuberculosis has around 4000.
• Between humans, our DNA differs by only 0.2 per cent, or 1 in 500 base (letters). (This takes into account that human cells have two copies of the genome.)
• Human DNA is 98 per cent identical to chimpanzees.