Expressed Sequence Tags (ESTs) versus Serial Analysis of Gene Expression

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Expressed Sequence Tags (ESTs) versus Serial Analysis of Gene Expression (SAGE)

INTRODUCTION Expressed sequence tag or also known as EST are 200 to 800 unedited nucleotide bases in length, and randomly selected single pass sequence reads which are derived from the cDNA libraries while serial analysis of gene expression or SAGE have more shorter sequence tags with only 10 to 20 base pairs. But even it is short it still have enough information to uniquely identify a transcript, especially if it is obtained from a unique position in each of the transcripts. Serial analysis of gene expression is a method or technique designed to gain a direct and quantitative measure of overall gene expression pattern. It is basically a sequence based
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ETS also can determine definitively the number of copies of each transcript per cell and highlight any differential gene expression. EST includes an inefficient sequencing step, in which one sequencing process yields only one cDNA sequence. Although the more recent methods of hybridization based analyses (DNA microarray) using immobilized cDNAs (Schena et al., 1995) or oligonucleotides (Lockhart et al., 1996) can potentially examine the expression patterns of a relatively large number of genes, the method can only examine expressed sequences that have already been identified. In contrast, the SAGE method allows for a quantitative and simultaneous analysis of a large number of transcripts in any particular cells or tissues, without prior knowledge of the genes (Velculescu et al., 1995).

METHODOLOGY The SAGE method is based on the isolation of unique sequence tags from individual mRNAs and concatenation of tags serially into long DNA molecules for lump sum sequencing. It can be applied to the studies exploring virtually any kinds of biological phenomena in which the changes in cellular transcription are responsible. SAGE is based mainly on three principles, firstly, a short sequence tags, 10 to 15 base pairs contains sufficient information to uniquely identify a transcript provided that the tag is obtained from a unique position within each transcript to allow the efficient sequencing analysis. Secondly

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