In the coding DNA base sequence is well conserved in evolution, as changes (mutations) can be detrimental to the function and survival of the species. However, we also find in the coding DNA, but more often in non-coding DNA, that in the population are two alternative base pairs (two alleles) for approximately each 300-500. Base, in line with the variation in blood types. This variation was first diagnosed with so-called restriction enzyme cutting of total DNA, and the variety was called RFLPer,restriction fragment length polymorphism.
Later, it turned out that the majority of RFLPene depended on the change in a single base pair, and they now speak at all about SNPs (single nucleotide polymorphism): you multiply a few hundred base pairs around SNPen by PCR (a polymerase chain reaction) to approx. 1 million copies, and then cut those pieces with a restriction enzyme that cuts only if determined by two alternative bases in SNPen present. During the period in introns, often between functional genes, there is a significant proportion of DNA that different numbers of repetitions of the base sequences.
The so-called micro-satellite systems usually consist of two consecutive bases, often repeated numerous times between the unique base sequences on either side of the repetition. The repeat length of the unit may also consist of three (see Triplett diseases below), four or five bases. The significance of these micro-satellites, which are distributed over-all chromosomes, is still unknown, but they have become the main tool in the genetic mapping of chromosomes and has become a primary tool in genetic identification in criminal cases and paternity cases. As well as SNPene micro satellites (also called short tandem repeats, are approached) types today by multiply the relevant DNA region using PCR (polymerase chain reaction). It demonstrates found along all the chromosome DNA strands, the average of 5,000 base pars intervals. They have the greatest impact on the search for disease genes or gene's disease predisposing the so-called genome screen methods.
The so-called mini-satellites or VNTRer (variable number of tandem repeats) consists of a high number of repetitions of the base sequences with considerably longer due to the sequence, and these are the most by the end of chromosomes. They have such great variability from chromosome to chromosome that they have much higher identification potential than micro satellites, but require a more technically demanding analytical method.
By combining micro-satellite (STR) and VNTR systems to the Department of Forensic Medicine in Oslo identify all body parts and find family belonging to 139 of the 141 individuals who died in plane crash on Svalbard in August 1996.
The non-coding DNA also contains groups of even longer DNA sequence repeats: ALU-family, which is specific for human or hominoid DNA, contains a core sequence of 300 base pairs (which is a duplication of 150 base pairs). They are scattered over-all chromosomes and constitute about 5% of human DNA (about 300 000 copies). They seem to be inserted into the DNA preferably in the pre-existing repeats (see above) and has an end sequence that shows that they can be played by RNA and corresponds to transposable elements were first found in plants. They believed, therefore, to have the ability to continue to spread the chromosomes. Their significance is still unknown. They allow rapid identification of blood stains originated from humans.
The utilization of these is that the PCR oppformer the unique DNA sequence between two neighboring ALU areas, and obtain DNA probes that can specifically fluorescent farge each human chromosome. While ALU sequences are called "short paid-DNA = SINE,'" consisting "far paid-DNA = LINE" of a base sequence of about 2000 base pairs repeated 50 000 times spread over-all chromosomes. So far, they have not found the purpose of the LINE. Between these two length groups of repeated DNA, we find intermediate repetitive DNA, which is 400-450 functional genes, in so far as that coding for ribosomal RNA. They are localized to the short arm of chromosome stalk 13, 14, 15, 21 and 22. There are additional groups of DNA sequences that are repeated several times along the chromosome. A diverse group is the one found on the color (chromatin)-dense area in which the chromosomes divide and the last few similar compressed areas (heterocyclic chromatic) excluding cent's Romans on chromosomes 1, 9 and 16.
At the end of all chromosome arms (telomeres) are repeated unique base sequences called telomerisk DNA. These may contain the key to the "biological clock" because their combined length steadily decreases during cell divisions except in cancer cells. There are enzymes (telomerase) that affect these sequences. Further studies of this will have major implications for our understanding of life.
