Reality → Life → Genetics → Genome
The genome is the totality of genetic information encoded in the DNA of an organism's complete set of chromosomes [1] . The first human genome became available in 2003 [2] . It showed that the individual human chromosomes have from 50 to 250 million base pairs, and the total chromosomal set has about 3 billion base pairs [3] . The number of protein-coding genes is believed to be within a range of 20,000 to 30,000 [4] . The genes are interspersed with non-coding sequences, and between genes lie vast stretches of repetitive and other sequences of no or unknown genetic value. It is estimated that protein-coding genes represent only about 2 percent of the human genome. Role and origin of non-coding segments are still debated. It is becoming increasingly apparent that DNA and the genome can only partially explain the complexities of genetic and developmental processes. Genomic research has, however, triggered dramatic progress in the development of new technologies for DNA sequencing, resulting in fast test results at much reduced costs, accelerated development of new pharmaceuticals and medical procedures, and new ways of studying evolutionary processes.
The genome can be portrayed as a huge map (or list) showing the identity and sequence of all nucleobases (or base pairs) contained in all chromosomes of an individual or a species. The genome varies in detail with every individual (a species' average therefore varies with number and selection of samples).
The Human Genome Project was a US government sponsored 'Megaproject' implemented from 1988 to 2013. It generated a huge database, pushed advances in lab technologies, and started a 'omics revolution'. Comparison of the human and other genomes shows some astonishing, still largely unexplained facts: e.g., major chromosomes of chimpanzees and humans may be 98% identical, a mouse may have a genome 85% the size of a human, and some fishes and plants have genomes significantly larger than humans.
While the human genome relates to a set of 23 different chromosomes (or DNA molecules), the full count of human chromosomes is 46, consisting of 2 sets of 23 chromosomes, one set from the father and one set from the mother (of the 23 chromosome pairs, one pair determines the sex, XX for females and XY for males). Stretched out, the chain of 50-250 million base pairs (or nucleotides) would be up to 10 cm long. To accommodate the molecule into the tiny space of a cell nucleus (typically about 10 μm diameter, or 1/10,000th of the DNA strand's length), chromosomes become extremely compacted during the time of cell division (the only time they are visible under a light microscope). At that time, the length of a chromosome is reduced to about 7 μm, through extreme coiling and supercoiling of the DNA around small protein (histone) packages.
A protein-coding gene is a stretch of DNA that codes for the sequence of amino acids of a specific protein. The genome also contains RNA-coding genes which code for functional RNA. Genes are interspersed with long stretches of non-coding DNA. It is very difficult to identify a gene on a DNA sequence, and the number of genes in a genome is therefore relatively uncertain.