Genome & DNA

  1: What is DNA?
  2: What is a genome?

The word genome designates the ensemble of hereditary information of an organism. This information is present in totality in every cell of the organism (1). When a cell divides, this information is transmitted to the daughter cells.

The genome contains all the instructions necessary for the development, function, reproduction and maintenance of the integrity of cells and organisms. These instructions are called genes.

The material support for the genetic information is DNA (deoxyribonucleic acid). The genome is composed of giant DNA molecules associated with other types of molecules named proteins to form the chromosomes. A human being possesses 23 pairs of chromosomes, i.e. two complete sets of instructions, each of which is inherited from one of her/his parents (2).

Two living organisms of different species present genomes which differ in their size and in their number, and in the order and the nature of the instructions which they contain. Two individuals of the same species, on the other hand, possess the same catalog of instructions, even though they may exist in versions which are slightly different (from one individual to the next, or in a single individual when the copies inherited from the father and the mother are different). It is in this sense that we speak of the human genome, shared by all human beings with its unique gene baggage. In the strict sense, the genome of each human being is unique (with the exception of those of identical twins), but it only differs by 0.1% from that of an unrelated individual.

The size of genomes, measured in number of bases (see “What is a DNA sequence?”), is extremely variable: several tens of thousands of bases on average for a virus genome, several million bases for a bacterium, three billion bases for the human genome—and 16 billion for the wheat genome! The number of genes contained in the genomes varies less: several thousand in a bacterium, 13 000 in Drosophila, 25 000 in humans. It is difficult to correlate the complexity of organisms with their number of genes, and even more difficult to correlate the size of their genomes. There are lots of exceptions.

  (1)With a few exceptions, such as human red blood cells
  (2)Individuals of the masculine sex possess one pair of dissimilar sex chromosomes, which have a different set of instructions. Furthermore abnormalities in the number of chromosomes exist, such as trisomy 21.