Human Genomes

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Humans contain in their cells two distinct genomes, the nuclear genome that resides is a cell’s nucleus and the mitochondrial genome in the mitochondrion.

 
The nuclear genome contains almost all (99.5%) of the DNA of a cell in its linear DNA molecules, the chromosomes, while the remainder of a cell’s DNA is contained in the mitochondria in a cell’s cytoplasm. The DNA of the nuclear genome is linear and arranged into 23 pairs of chromosomes, while the DNA contained in mitochondria are circular molecules. The vast majority of forensic analysis uses nuclear DNA. However, in cases where nuclear DNA has degraded mitochondrial DNA is used. The mitochondrial genome will be discussed in detail in chapter 6.
 
Genome variations are differences in the sequence of DNA that differ from one person to the next. People are in fact unique because their genomes are unique. Most genome variations are relatively small and simple, involving only a few bases. The more closely related two people are, the more similar their genomes. It is estimated that the genomes of non-related people, thus any two people taken at random from a population, differ at about 1 in every 1 200 to 1 500 DNA bases.
The size of a single set of human genomes, a haploid set of 23 chromosomes (basic chromosome set) and one mitochondrial genome, a haploid set, comprises about 3.2 × 109 base pairs of DNA. The number of genes in the human genome has been the subject of much speculation; while the small mitochondrial genome is known to have precisely 37 genes, the precise number of nuclear genes remains unknown. Theoretical calculations estimate about 65 000 – 80 000 nuclear genes. However, more recently scholars have stated that 30 000 is probably a more realistic estimate. It is estimated that only 2 – 3 % of the nuclear genome consists of coding DNA. The vast majority is either non-coding DNA or extragenic DNA.
 
Different classes of DNA of the human genomes

 

(Kb = kilobases and Gb = gigabases)  (modified from Strachan and Read 1996).
 
The human nuclear genome comprises only a small fraction of gene-containing and gene related DNA (25%), the vast majority is non-coding DNA (75%) (extragenic). Within the regions containing genes, there also exist many non-coding sequences (90%); such as pseudogenes, introns, untranslated DNA and gene fragments. The repetitive DNA or extragenic DNA contains low copy repeated sequences and, unlike other complex eukaryotes, a large amount of highly repeated DNA sequence families. These repetitive DNA sequences are of great importance to forensic DNA typing.

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