The human genome structure and organization

Genetic information of human is encoded in two genomes: nuclear and mitocho ndrial. Both of them reflect molecular evolution of human starting from the beginning of life (about 4.5 billion years ago) until the origin of Homo s apiens species about 100 000 years ago. From this reason human genome conta ins some features that are common for different groups of organisms and som e features that are unique for Homo sapiens. 3.2 X 10(9) base pairs of huma n nuclear genome are packed into 23 chromosomes of different size. The smal lest chromosome - 21st contains 5 X 10(7) base pairs while the biggest one - 1(st) contains 2.63 X 10(8) base pairs. Despite the fact that the nucleot ide sequence of all chromosomes is established, the organisation of nuclear genome put still questions: for example: the exact number of genes encoded by the human genome is still unknown giving estimations from 30 to 150 tho usand genes. Coding sequences represent a few percent of human nuclear geno me. The majority of the genome is represented by repetitive sequences (abou t 50%) and noncoding unique sequences. This part of the genome is frequentl y wrongly called "junk DNA". The distribution of genes on chromosomes is ir regular, DNA fragments containing low percentage of GC pairs code lower num ber of genes than the fragments of high percentage of GC pairs.