Q.18 Expressed Sequence Tag is defined as

• (A) A partial sequence of a cDNA randomly selected from cDNA library
• (B) The characteristic gene expressed in the cell
• (C) The protein coding DNA sequence of a gene
• (D) Uncharacterized fragment of DNA present in the cell

  Expressed Sequence Tags (ESTs) revolutionized gene discovery by providing snapshots of actively transcribed genes. These short, single-pass sequences from cDNA libraries identify expressed genes without full genome sequencing. This article solves the MCQ for biotechnology students.

  Correct Answer: (A) A partial sequence of a cDNA randomly selected from cDNA library
  ESTs are short (200-800 bp) partial sequences obtained by single-pass sequencing of randomly selected cDNA clones from mRNA-derived libraries. They represent expressed genes at specific tissues/conditions.

  EST Generation Process

  1. Isolate mRNA from target tissue (poly-A selection)

  2. Reverse transcribe to cDNA library

  3. Randomly pick clones and perform single-pass sequencing (5′ or 3′ end)

  4. Result: ESTs—short, error-prone sequences identifying active transcripts

  ESTs bypass introns, directly showing exon structure and expression patterns.

  Explanation of All Options

  Each option’s relation to EST definition:

  • (A) A partial sequence of a cDNA randomly selected from cDNA library
    Correct. Classic EST workflow: random cDNA clone → single-pass sequencing → short tag representing expressed gene.

  • (B) The characteristic gene expressed in the cell
    Incorrect. Too vague; ESTs represent many genes, not one “characteristic” gene.

  • (C) The protein coding DNA sequence of a gene
    Incorrect. ESTs are partial cDNAs (UTRs + partial CDS), not complete coding sequences.

  • (D) Uncharacterized fragment of DNA present in the cell
    Incorrect. ESTs are characterized expressed sequences from genomic DNA, not random fragments.

  Option	EST Definition Match?	Key Characteristics	Accuracy
  (A) Partial cDNA sequence	Yes	Random clone, single-pass	Correct
  (B) Characteristic gene	No	Too specific	Wrong
  (C) Complete CDS	No	ESTs are partial	Wrong
  (D) Uncharacterized DNA	No	ESTs = expressed	Wrong

  Bioinformatics Applications

  EST utilities:

  • Gene discovery (74M+ ESTs in dbEST)

  • Expression profiling (tissue-specific)

  • Genome annotation (exon identification)

  • SNP discovery (polymorphisms in transcripts)

  Example workflow: EST → BLAST → UniGene clustering → full-length cDNA cloning

  Exam essential: ESTs = random partial cDNA sequences from libraries, not complete genes or genomic fragments.