The human genome structure and organization

Genetic information of human is encoded in two genomes: nuclear and mitochondrial. 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 sapiens species about 100,000 years ago. From this reason human genome contains some features that are common for different groups of organisms and some features that are unique for Homo sapiens. 3.2 x 10(9) base pairs of human nuclear genome are packed into 23 chromosomes of different size. The smallest chromosome - 21st contains 5 x 10(7) base pairs while the biggest one -1st contains 2.63 x 10(8) base pairs. Despite the fact that the nucleotide 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 thousand genes. Coding sequences represent a few percent of human nuclear genome. The majority of the genome is represented by repetitiVe sequences (about 50%) and noncoding unique sequences. This part of the genome is frequently wrongly called "junk DNA". The distribution of genes on chromosomes is irregular, DNA fragments containing low percentage of GC pairs code lower number of genes than the fragments of high percentage of GC pairs.