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Home >>Molecular Maps of Animal Genome >>Sequence Tagged Sites (STS) and Expressed Sequence Tags (EST)
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Sequence Tagged Sites (STS) and Expressed Sequence Tags (EST) - Due to the availability of polymerase chain reaction (PCR) technology, in recent years strategies for mapping human genome have regularly been reviewed and scrutinized. Consequently sequence tagged sites (STSs) suggested by Olson et al. (1988), are becoming standard markers for the physical mapping of human genome.

In this technique a short tract of DNA from the clone that has been physically mapped is sequenced (300-500bp) and the site is now defined by this sequence and called an STS.

A PCR assay for an STS could be implemented simply by synthesizing two short (- 20 nucleotides) obligodeoxynucleotides, chosen to be complementary to opposite strands at the opposite ends of the STS sequence.

A DNA sample can then be tested for the presence of the STS sequence through its capacity to amplify using the above primers in PCR. While STSs represent unique sequences, which mayor may not be transcribed, recently analogous sequences called expressed sequence tags (ESTs) have been utilized, which would have an additional feature of representing only expressed genes.

In humans, cDNA libraries derived from brain mRNA have been utilized for this purpose. An EST is simply a segment of sequence from a cDNA clone that corresponds to a mRNA.It is shown that ESTs longer than 150bp (150-400bp) are useful even for search of similarity and for mapping by workers indifferent laboratories.

Using nucleotide sequence databank (GenBank) and protein sequence database (Protein Information Resource = PIR), the ESTs can also be matched with existing sequences and assigned to specific genes with the help of computer programmes.

Using EST sequences for preparing specific primers, and using hybrid cell lines of known chromosome constitution as template in PCR, an EST can also be assigned to a specific chromosome, by the mere presence of amplified DNA.

A large number of ESTs could be assigned to specific chromosomes using this technique. Thus ESTs will extend the physical mapping of expressed genes in the human genome at a much faster rate than hitherto conceived.

 
 

 

 
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