What is a genome?

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A genome is an organism’s complete set of genetic instructions. Each genome contains all of the information needed to build that organism and allow it to grow and develop.

  • Organisms are made up of millions of cells (100,000,000,000,000), each with their own complete set of instructions for building and sustaining that living thing.
  • This set of instructions is known as our genome and is made up of DNA. Each cell in an organism, for example, a skin cell or a liver cell, contains this same set of instructions.

From genome to genes


  • The instructions in our genome are made up of DNA.
  • Within DNA is a unique chemical code that guides an organism’s growth, development and health.
  • This code is determined by the order of the four nucleotide bases that make up DNA, adenine, thymine, cytosine and guanine.
  • DNA has a twisted structure in the shape of a double helix.
  • Single strands of DNA are coiled up into structures called chromosomes.
  • In animals, chromosomes are located in the nucleus within each cell.
  • Within chromosomes, sections of DNA are organised into genes.
  • Genes control different characteristics such as eye colour and height.
  • All living things have a unique genome.


The human genome


  • The human genome is made of 3.2 billion bases of DNA but other organisms have different genome sizes.
  • If printed out the 3.2 billion letters in your genome would:
    • Fill a stack of paperback books 200 feet (61 m) high
    • Fill 200 telephone directories, each with 500 pages.
    • Take a century to recite, if we recited at one letter per second for 24 hours a day
    • Extend 3,000 km (1,864 miles) – approximately the distance from London to the Canary Islands, Washington to Guatemala, or from New Delhi to Hanoi.
If printed out 1 mm apart, the DNA letters in your genome would extend 3,000 km. Credit: Laura Olivares Boldú / Wellcome Connecting Science

How does a cell use the information held in its genome? Find out on our next Introduction to Genomics page.