Mapping 'frozen accidents'

Article Abstract:

A new technique that allows tracing of maternal and paternal lineages during evolution has been introduced by the same laboratory that gave the world 'DNA fingerprinting'. The approach uses the fact that DNA sequences (small units of genetic material) from blood cells and sperm reflect past mutations ('frozen accidents'). The DNA sites of interest are called minisatellite loci, a series of repetitions of the same small DNA sequence where differences among alleles (the same position on the same chromosome, in different cells) are due to the number of repeats. These 'fingerprints within fingerprints' are unique to an individual. Using polymerase chain reaction techniques, the two parental alleles can be isolated, and shorter alleles with certain repeated segments can be deleted. Results from mapping one particular site (HaeIII site) show that mutation rates within an individual can be determined, that five assemblage groups can be identified, and that no specific assemblage is associated with a given geographical origin of an allele. The basis of different distributions of the site is discussed, as are the possible reasons why crossing over (the mechanism of genomic turnover) should result in creation of five assemblages. Results from experiments indicate that the repeated sequences of minisatellite regions are probably not 'hot-spots' for recombination during meiosis (the stage of cell division that leads to the formation of mature egg or sperm) and, since only two copies of the repeat unit are present in certain higher apes, it is likely that this mutation arose after the time when great apes diverged from the hominid line. Other interpretations are possible. Chromosomes continue to evolve, and each present version contains the past, as well. (Consumer Summary produced by Reliance Medical Information, Inc.)

Innovations, DNA testing, DNA identification, Chromosomes, Genetics, editorial

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Molecular genetic basis of the histo-blood group ABO system

Article Abstract:

The incorrect matching of the major blood group antigens, A, B, and O, when giving blood transfusions can lead to fatal reactions. The A, B, and O antigens contain three carbohydrate antigens, A, B, and H. Individuals who have type A, B or AB blood have an enzyme activity, known as glycosyltransferase activity, which transfers carbohydrate molecules, changing the H antigen into A or B antigens. Individuals who are type O lack this enzyme activity. The genes coding for the glycosyltransferase in type A, type B, and type O individuals have been isolated and the DNA sequences of these genes have been identified. A few differences in the sequence of the DNA molecules have been identified. In type A and type B individuals, there are differences which result in four different amino acids in the transferase molecule between the two types. These differences appear to be the reason why the transferase is different in these two blood types, leading to the different antigens. A single deletion in the DNA molecule was identified in the O gene, causing this gene to encode a very different protein which is inactive and cannot add sugars to the H antigen. Besides being present on blood cells, the ABO antigens are also on other cells, mainly epithelial cells. Changes in these molecules occur during development and also during the development of cancer. Therefore, the elucidation of the genetic mechanism of the expression of the A, B, and O antigens is important in the understanding of cellular development and cancer formation. (Consumer Summary produced by Reliance Medical Information, Inc.)

Antigens, Blood group antigens

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Disruption of a C/EBP binding site in the factor IX promoter is associated with haemophilia B

Article Abstract:

Hemophilia B, also known as the Christmas disease, is a disorder where individuals cannot stop bleeding. The disease is inherited and is passed from mothers to sons by the X chromosome. The cause of disease has been shown to be a mutation in one of the blood clotting factors, factor IX, or the plasma thromboplastin component. Mutations in the gene for factor IX have been examined and it was shown that in seven patients the mutations were located near the site were the gene is replicated, known transcription start site. The mutations interfere with the binding of a factor, known as C/EBP, which is necessary for transcription. This is the first time a mutation has been shown to disrupt a C/EBP binding site in a disease state. A rare subset of patients with hemophilia B have the disease as children, but improve once they reach puberty. The concentration of factor IX is less than 10 percent of the normal value in children with this disorder, and increases to 40 to 80 percent of normal after puberty. The reason for the increase in the levels of factor IX after puberty is not known, but it is thought that the synthesis of factor IX may be regulated by a sex hormone, such as testosterone. There may be a site on the gene encoding factor IX that is responsive to the hormone. These studies show the mechanisms behind the defect in blood clotting in individuals with hemophilia B and the importance of the binding of the factor C/EBP in controlling gene expression. (Consumer Summary produced by Reliance Medical Information, Inc.)

Causes of, Blood clotting disorders, Blood coagulation disorders, Hemophilia, Blood coagulation factors, Genetic disorders, Hemophilia B

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