Molecular Genetics

Polymorphisms in the lengths of tandemly repeated short sequences can be used as molecular markers on the genetic chromosome.

Facts

Short identical segments of DNA aligned head to tail in a repeating fashion are interspersed in the human genome. The numer of repeated segments at a locus varies between individuals. Such sequence regions are called VNTR's (variable number of tandem repeats). Some VNTR sequence segments are found at only a single locus in the human genome. Probes made of these sequences are single locus probes and yield patterns such as those at left when used to probe RFLP blots of DNAs of six individuals. Other VNTR sequence segments occur at many loci in the human genome. These loci are dispersed among the chromosomes. Multi-locus VNTR probes yield patterns such as those at right.

Interpretations

Images produced by single-locus VNTR probes are much easier to interpret since each individual should have no more than two clear bands.

Images produced by multi-locus VNTR probes yield more information since one is simultaneously probing ten to 30 loci.

• The term VNTR refers to the variable sequence rather than to the method used to detect it. In the description above, RFLP is used to detect the variability in the number of repeats. Several PCR-based methods can also be used to detect VNTR's.

Further information

Single-locus VNTR analysis was used in the USA for forensic investigation and paternity analysis. Single-locus VNTR is now the preferrred approachals elsewhere.

Multi-locus VNTR analysis was used in Europe for forensic investigation and paternity analysis.

• VNTR's are used in DNA fingerprinting and are also called STR's (simple tandem repeats)
• PCR has supplanted RFLP by Southern blotting and hybridization in analysis of VNTR's. In addition, the use of capillary electrophoresis and fluorescent dyes (attached to primers) has led to increased reliability and reporducibility. Note, in the result below (from http://bioinformatics.ucd.ie/shields/kjohnston/img6.JPG):
  • Three chromatograms are shown, one for each of three distinct fluorophores.
  • For each fluorophore, three or four loci are examined.
  • The PCR primers are designed to give products with non-overlapping sizes for loci using the same fluorophore.
  • Note two peaks for most loci (heterozygous) and one peak for one locus.
  • Simple sequence repeats can be used as genetic markers.
  • Details on many VNTR markers used by the US FBI are available.

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  This is page 11125 of Molecular Genetics by Ulrich Melcher, ©1997, 1998, 1999, 2000, 2007
  
  

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  E-mail inquiries to U. Melcher------------Last Updated: 4 December, 2007