The genome

The genome is the entire genetic material of an organism. It is found in the nucleus of a cell, and is composed of a chemical called DNA. The study of the structure and function of the genome is called genomics.

DNA is a polymer - a large and complex molecule. The monomers that make it are called nucleotides. DNA is made up of two strands forming a twisted ladder structure called a double helix. The nucleotides are arranged along the DNA to form a code. This genetic code, determines the characteristics of a living organism.

Genomes of different organisms

Genetic material for eukaryotes is in their nucleus. Prokaryotes such as bacteria do not have nuclei. Their genetic material is in the cytoplasm and includes small loops of DNA called plasmids.

Genes and chromosomes

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

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

Diagram showing a chromosome and its DNA

Scientists now know that genes only make up a small part of an organism's DNA. Most DNA - 98.5 per cent in humans - does not code for proteins. Instead, it seems to be important in controlling gene expression. This means it controls when genes are 'read' to make proteins.

The genes and non-coding DNA are mixed together in the chromosomes. These are long threads of DNA, which carry the genetic code. An organism's genetic code is made up of many genes.