The total genetic material in human cells is termed the human genome, composed of the nuclear genome and the mitochondrial genome. The mitochondrial genome codes for a handful of genes involved in oxidative phosphorylation, located on 16.5 kb circular DNA molecules found in the mitochondria. The much larger nuclear genome provides the vast majority of genetic information, stored on the chromosomes. The human genome consists of between 50 and 100 000 genes, but only approximately five per cent of the genome actually codes for messenger RNA. The remaining noncoding DNA consists of introns, pseudogenes, simple sequence repetitive DNA, mobile DNA elements and unclassified spacer DNA between transcription units. Pseudogenes are non-functional genes derived from normal genes by duplication then loss of critical nucleotide sequenees. Introns in most cases have no known function and their origin is uncertain. Mobile DNA elements move via RNA intermediates to new genetic locations and are sometimes termed "selfish DNA".
Throughout the human genome neutral molecular polymorphisms (different forms of the same gene) are found resulting in the production of the same protein, due to the degenerate nature of the genetic code. Many more polymorphisms with no functional consequence occur in the non-coding parts of the genome. These DNA polymorphisms can be used as markers to map human genes and mutations. A particularly useful type of variation is that seen in tandemly repeated simple sequence DNA. These arrays of non functional repeats, often of only one to five bases, vary enormously from individual to individual.
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