Molecular Markers,Isozymes,Chromosome Mapping,Restriction Enzymes,Regions of Chromosomes,Human Genome,Genome Mapping

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	Molecular Markers Isozymes (electrophoretic variants of enzymes) were and DNA sequences are used as molecular markers in chromosome mapping. Therefore, for all practical purposes, a molecular marker may be defined as a DNA sequence used for chromosome mapping as it can be located at a specific site in a chromosome.
A molecular marker may be either (1) anonymous or (2) defined. An anonymous marker is a cloned random DNA fragment whose function or specific features are not known. But a defined marker may contain a gene, or some other specific feature, e.g., restriction sites for rare cutting restriction enzymes, etc.
Such markers are said to be polymorphic markers if they have different structures in different individuals of a species; the structural differences in a given marker are usually detectable as differential mobilities in gel electrophoresis. Some of the common molecular markers are as follows: (1) restriction fragment length polymorphism (RFLP), (2) random amplified polymorphic DNA (RAPD), (3) variable number of tandem repeat (VNTR) DNA. RFLPs and RAPDs are described in Appendices 4-II and 4-III, while VNTRs are considered here briefly.
VNTRs are dispersed throughout the genome and are made up of a variable number of end-to-end duplications of identical or almost identical sequences of 2-80 bp each. Polymorphism in VNTRs is usually associated with the number of repeats, i.e., different individuals show different numbers of repeat units at a given locus or position in a chromosome; these variations constitute the alleles of VNTR loci. The VNTRs are generally classified into two groups: (i) minisatellite and (ii) microsatellite DNAs; these together constitute the hypervariable region or DNA. Minisatellite DNAs are usually 0.2 to 2 kb long and consist of tandem repeats of 15 to 60 bp sequences.
	In case of humans, minisatellite DNAs are concentrated in the pro terminal regions of chromosomes; therefore, they do not constitute a good marker system for human genome. Microsatellite DNAs are short sequences of usually less than 100 bp and are made up of tandem repeats of 2 to 7 bp; therefore, they are also called short tandem repeats (STRs), simple sequence repeats and simple sequence- length polymorphism (SSLP). STRs are random and frequently distributed in all eukaryotic genomes (except yeast) examined so far. They show a large, stable polymorphism due to variation in the number of repeat units and are almost ideal as molecular markers for genome mapping. So far, over 52,000 STSs (sequence tagged sites; mostly SSRs/STRs) have been physically mapped in human genome.