Genes


Genes are specific sequences of nucleotides, encoding information required for constructing proteins, the main structural and functional components of cells. Genes are fundamental physical and functional subunits of heredity.


Genome is made of three different types of DNA :
Single Copy DNA consist of genes, found in only one or few places in the genome (75% of genome). It also includes multiple copy genes such as those coding for rRNA or Histones which exist as large clusters of multiple copies (50-10000 copies).
Repetitive dispersed DNA fractions are characteristic short sequences (6-10 np) repeated 100000 - 1,000,000 times in disparate places throughout the genome (15% of genome).
Satellite DNA (10% of genome) is made of highly repetitive sequences, basically confined to the chromosomes centromere and telomeres.


The human genome is estimated to comprise about 3 billions nucleotide pairs and at least 100000 genes. However it seems that there is no systematic correlation between complexity of an organism and the number of nucleotide pairs (np) per haploid genome. Some plants and amphibian organisms have total DNA amount of about 100 billions np (30 times more than human).

Actually the genome of higher eukaryotes seems to contain a large excess of DNA. In mammals, only about 10% of the genome is known to be expressed in protein encoding or in regulation processing. This excess of DNA is supposed to serve as hiding package, preventing genes from accidental mutations.

Size of genes vary widely in length, from some hundreds to several thousands nucleotides pairs. However, even in longer genes only a small part of the sequence is used to encode informations. The coding regions are named exons and the non-coding interrupting sequences are called introns.

The presence of introns in prokaryotic genes is extremely rare. Generally the more complex and recently evolved the organism, the more numerous and larger the introns.

Some specific DNA regions are dedicated to the control of genes expression. These regulatory sequences are often located at the beginning of the gene (5'side), at its end
(3'side) but also sometimes in introns or in exons.






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