The main provisions of the program "human genome"
When the "Human Genome" program was created, three main goals of this program were identified: the creation of an accurate genetic map, the creation of a physical map of the human genome and the sequencing( definition) of the entire human genome.
Creation of the genetic map of the genome
An exact genetic map could be created if two conditions were met: the detection of a large number of polymorphic( multiple) genetic markers in the genome and the presence of a sufficient number of families to analyze the adhesion between these markers and establish their mutual arrangement. The problem of genetic markers was resolved after detection of different types of DNA polymorphisms. In order of their introduction into the genetic analysis, polymorphism of the length of restriction fragments( abbreviated RFLP) was first found, followed by polymorphism caused by a variable number of tandem repeats( VNTR polymorphism).Each of these species has its advantages and disadvantages, but all together they allow to label the genome of a person with very high density.
Even the first 4 types of polymorphisms allowed to create a genetic picture of the human genome. Set the relative position of the markers allowed a collection, or bank, of cell lines obtained from all members of several hundred families that included no less than three generations. This bank of cell lines was created in France to study polymorphism in the HLA system and was very useful for genetic mapping of the human genome. After each chromosome was mapped and the relative location of 10-15 polymorphic markers was established, work with subsequent markers was carried out on the material obtained from the members of these families.
Creation of a physical map of the genome
In order to create a physical map of the genome, cloned fragments of the human genome also needed to be labeled. This was done with the help of short sequences( DNA segments with a certain sequence), the so-called STS, the chromosome localization of which is precisely known. STS are readily identified by polymerase chain reaction( PCR).Received more than 50 000 STS.scattered over the genome, and during physical mapping, when the mutual arrangement of DNA fragments from genomic libraries is established, these STS are used as markers of overlapping DNA segments.
Definition of the human genome
Below, we will briefly outline the main results of the "draft version" of the human genome, which, as noted above, were published in the February 2001 issue of the journal Nature.
The concept of "draft version" refers primarily to the factthat the definition of the genome continues. In connection with this, at the present time, studies can not be arranged in order and orient many small sequenced sequences. The incompleteness of the sequence, naturally, creates problems for identification of genes of hereditary diseases, unique genes and other genetic structures.
It should be noted that over the last quarter of the 20th century. The genomes of 599 viruses and other microorganisms as well as macroorganisms( animals) were sequenced. The experience gained as a result of this work was fully used in sequencing the human genome.
The strategy of studying the human genome in a public project included obtaining a genetic and physical map of the human genome, the subsequent insertion into these maps of the sequencing results of individual clones of genomic DNA( the "clone by clone" sequencing strategy).The physical map of the human genome has a clonal basis, which was developed by the Olson scientist in 1981. The approach is as follows. The genome was cut into segments by partial digestion with enzymes-specific endonucleases. These large segments of DNA were placed in artificial bacterial chromosomes( BAC) and introduced into bacteria, where they were copied with each division of the bacterium. As a result, clones of identical DNA molecules were formed. Such clones covering the human genome should be approximately 20,000.
In addition, earlier developed STS cards were used to establish the order of clones. The result is a physical map of the genome. Individual BAC clones were cut into fragments and cloned. Subclones obtained as a result of cloning fragments were studied. Each time, a large number of identical subclones were determined to ensure that each fragment of the original BAC clone was analyzed several times and no errors were made. The sequences of the individual fragments were combined in order to obtain a sequence of nucleotides in each initial BAC clone. Finally, the sequence of the entire genome was collected by combining the sequences of the BAC set that overlapped the entire genome. The thus created map of the sequence of the human genome has more than 1000 discontinuities. This may be due to a number of reasons, in particular the fact that the original BAC clones did not overlap the whole genome, and overlap between the clones was missed due to the presence of large repetitions in the genome. The sequential map created as a result of the project implementation includes sequences containing on average several million nucleotide pairs in length. Segments of this length are sufficient to overlay a map based on clones on other maps with a lower resolution. The position on the genetic map of the sequenced sequences was also determined relative to the mapping of the STS.
In general, about 90% of euchromatic regions of the genome are sequenced and collected by computer programs in extended areas.
Despite the incompleteness of the sequence, a whole series of results obtained with its help already now is of undoubted interest. This applies primarily to deepening the notion of the organization of the human genome.