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Renin Angiotensin Mechanism.

Transcription or RNA synthesis.

 

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.

  1. Write a short note on RNA synthesis.

  2. Write a short note on Transcription.
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