From DNA to genomes

The genetic material in the nucleus of a cell is made up of a chemical called DNA. DNA is a polymer, made of many smaller units called nucleotides. A nucleotide is made of a sugar and a phosphate group, with one of four different bases, A, C, T or G, attached. The nucleotides join together, forming two strands. These, in turn, form a double helix structure. The double helix is held together by weak hydrogen bonding between complementary base pairs. Base A always pairs with T, and C always pairs with G forming a twisted ladder structure called a double helix. It carries the genetic code, which determines the characteristics of a living organism.


The cell's nucleus contains chromosomes. These are long threads of DNA, which are made up of many genes.


A gene is a small section of DNA in a chromosome. Each gene codes for a particular sequence of amino acids in order to make a specific protein. It is the unit of heredity, and may be copied and passed on to the next generation.

The diagram shows the relationship between the cell, its nucleus, chromosomes in the nucleus, and genes.

Diagram showing the relationship between the cell, its nucleus, chromosomes in the nucleus, and genes


The genome is one copy of all an organism's DNA. In humans this is all the DNA that makes up the 23 pairs of chromosomes found in all diploid body cells. That is all the cells except sex cells or gametes, which only have half of a person's genome.

The Human Genome Project

The Human Genome Project, or HGP for short, was started at the end of the last century. It was very ambitious and had several aims, including:

  • to work out the order or sequence of all the three billion base pairs in the human genome
  • to identify all the genes
  • to develop faster methods for sequencing DNA

The sequencing project was finished in 2001, and work continues to identify all the genes in the human genome. The HGP used the DNA of several people to get a sort of average sequence, but each person has a unique sequence (unless they have an identical twin).

Mapping of a person's genome can help in predicting how likely they are to develop certain conditions. Scientists and doctors are also now beginning to use the information they have discovered in the HGP to help make more effective medicines as the effectiveness of medicines can be affected by variations in alleles.