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The genome is the total DNA present in the nucleus of every cell of an organism. It corresponds to all the organism's chromosomes. If you compare DNA to a string of four different types of pearls (representing the four DNA bases: A, T, C and G), our genome is a chain of 3.4 billion pearls. The genomes of two different people differ on average by one 'pearl' in one thousand.
Genes represent only about 5% of the human genome. The role of all the non-coding sequences that make up the remaining 95% is today unknown, but researchers have nonetheless been able to assign functions to some of those sequences : some regulate DNA replication and transcription; others contribute to chromosomal structuring. The Human Genome Project, launched in the early 1990s, has succeeded in completing the first rough draft of the DNA sequence of our entire genome. The final version should be ready by 2003. Sequencing a genome is the first step taken by researchers in the field of genomics.