Introduction:
Transcription is the first step in gene expression. Synthesis of an RNA molecule involves copying the DNA sequence of a gene.
RNA polymerases, which link nucleotides to create an RNA strand using a DNA strand as a template, carry out transcription.
Transcription has three stages:
initiation,
elongation,
termination.
Transcription is controlled separately for each gene in our genome.
RNA polymerase:
RNA polymerase, the primary enzyme responsible for transcription, builds a complementary strand of RNA using a template made of single-stranded DNA.
Specifically, RNA polymerase builds an RNA strand in the 5' to 3' direction, adding each new nucleotide to the 3' end of the strand.
Initiation:
RNA polymerase binds to a sequence of DNA called the promoter, found near the beginning of a gene.
Each gene has its own promoter.
Once bound, RNA polymerase opens the DNA strands, providing the single-stranded template needed for transcription.
Elongation:
One strand of DNA, the template strand, acts as a template for RNA polymerase. As it "reads" this template one base at a time, the polymerase builds an RNA molecule from complementary nucleotides, making a chain that grows in reverse, i.e. from 5' to 3'.
The RNA transcript uses the base uracil (U) rather than thymine (T), but it contains the same information as the non-template (coding) strand of DNA.
Termination:
Gene sequences called terminators signal that the RNA transcript is complete.
Once they are transcribed, they cause the transcript to be released from the RNA polymerase.
The transcript is called “pre mRNA,” which undergoes modifications.
Modifications of pre mRNA:
pre-mRNAs must have their ends modified, by addition of a 5' cap (at the beginning) and a 3' poly-A tail (at the end).
Many undergo splicing.
In this process, non essential parts of the pre-mRNA (called introns) are chopped out, and the remaining essential parts (called exons) are joined back together.
Modifications increase the stability of the mRNA, while splicing gives the mRNA its correct sequence.
Commonly Asked Questions.
Write a short note on RNA synthesis.
Write a short note on Transcription.