Q.Define transcription. In which phase of cell cycle does transcription occur in Eukaryotes? Describe briefly the process of transcription. OR
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Start your 14-day free trial to unlock the full solution →Transcription copies the genetic information of one strand of DNA into a complementary RNA molecule; in eukaryotes this happens in the nucleus, in the relatively relaxed chromatin state of interphase (mainly G1 and G2), and is switched off once the chromosomes condense for cell division.
Definition: Transcription is the process by which the enzyme RNA polymerase synthesises a single-stranded RNA molecule that is complementary in base sequence to one of the two strands of a DNA duplex (the template strand), copying the genetic information encoded in the DNA into an RNA transcript.
When it occurs in the eukaryotic cell cycle: For transcription to occur, RNA polymerase and the transcriptional machinery need physical access to the DNA double helix, which requires the chromatin to be in a relatively relaxed/decondensed state. This condition is met during interphase, and transcription is largely restricted to the G1 and G2 phases, when the DNA is available and not actively being copied. It is generally not favoured during the S phase, since the DNA replication machinery is occupying and unwinding the DNA at that time, creating a potential for conflict with transcription. Transcription effectively stops during the M phase (mitosis), because the DNA becomes tightly packed/condensed into visible chromosomes at that stage, making it physically inaccessible to RNA polymerase.
Process of transcription (in three main steps):
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Initiation: RNA polymerase binds to a specific region of DNA called the promoter, located upstream of the gene to be transcribed. This directs the enzyme to begin synthesising RNA at the correct starting point and identifies which of the two DNA strands is to act as the template.
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Elongation: The RNA polymerase moves along the template strand in the 3'→5' direction, unwinding the DNA double helix just ahead of it, and catalyses the addition of ribonucleotides (using the DNA as a guide, following complementary base-pairing rules, with uracil replacing thymine) to the growing RNA chain in the 5'→3' direction. As the enzyme progresses, the DNA re-forms its double-helix structure behind it.
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Termination: When RNA polymerase reaches a specific sequence on the DNA called the terminator, it stops adding further nucleotides, and both the enzyme and the newly synthesised RNA transcript are released from the DNA template.
In eukaryotes, three different RNA polymerases (I, II, and III) transcribe different classes of RNA (rRNA; hnRNA/mRNA; tRNA and other small RNAs, respectively), and the primary transcript (hnRNA in the case of mRNA) typically undergoes further post-transcriptional processing — capping, splicing (removal of introns), and polyadenylation — before it becomes a mature, functional mRNA.
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