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ABEEHA SARWAR

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Transcription:

  • Transcription, or RNA synthesis, is the process of creating an equivalent RNA copy of a sequence of DNA
  • Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA in the presence of the correct enzymes
  • During transcription, a DNA sequence is read by RNA polymerase, which produces a complementary, antiparallel RNA strand

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  • In eukaryotes several different enzymes are used to synthesize different types of RNA
  • They are called as:
    • RNA Polymerase I

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    • RNA Polymerase II

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    • RNA Polymerase III

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Stages of transcription:

  • Transcription is divided into 4 stages

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  • Formation of transcription complex

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  • Initiation

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  • Elongation

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  • Termination

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Formation of transcription complex:

  • RNA polymerase binds to specific sequence on DNA which are called promoters

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  • The promoter size is variable

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  • In prokaryotes ranges from 20-200 bases

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  • Sigma factor recognize the promoter region

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  • Following four steps occur
    • Sigma factor recognize the promoter sequence

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    • RNA polymerase attaches to promoter region

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    • RNA polymerase melts the helical structure and separates two strands of DNA locally

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    • RNA polymerase initiates RNA synthesis

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    • The site at which the first nucleotide is incorporated is called the start site or start point

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  • RNA polymerase is a core enzyme consisting of five subunits: 2 α subunits, 1 β subunit, 1 β subunit, and 1 omega subunit

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  • The core enzyme is associated with a sigma factor that aids in finding the appropriate -35 and -10 base pairs downstream of promoter sequences

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Promoter region:

  • DNA molecules contain regions of nucleotide called promoters that directs RNA polymerase to begin transcription
  • In bacterial cells, there are two sequences that commonly act as promoter

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  • One of these found is 35 nucleotide from the start of transcription and other one called Pribnow box is found 10 nucleotide from the start

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  • In eukaryotic cells, there are several promoter regions:
    • TATA Box (Hogness Box) found 25 nucleotide upstream from the start of transcription

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    • CAAT Box found about 75 nucleotide upstream

    • Enhancer sequence found for the right to left of start site

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Initiation:

  • Core enzyme starts initiation of transcription of the complex
  • As the enzyme moves along, the unwound region moves with it
  • The first base copied is always within six to nine bases of the conserved T of the Pribnow box on the unwound portion of 3’-5’ strand of DNA
  • RNA polymerase have two sites for the binding of nucleotide triphosphate

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  • The first incoming NTP binds to RNA polymerase at the start point of initiation site

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  • The second incoming NTP binds to the elongation site on the polymerase

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  • After this phosphodiester bond formation the σ factor is released

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Muhammad Wajid

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Elongation:

  • One strand of DNA, the template strand (or noncoding strand), is used as a template for RNA synthesis
  • As transcription proceeds, RNA polymerase traverses the template strand and uses base pairing complementarity with the DNA template to create an RNA copy
  • Although RNA polymerase traverses the template strand from 3 → 5, the coding (non-template) strand and newly-formed RNA can also be used as reference points, so transcription can be described as occurring 5 → 3

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    • This produces an RNA molecule from 5 → 3, an exact copy of the coding strand (except that thymines are replaced with uracils, and the nucleotides are composed of a ribose (5-carbon) sugar where DNA has deoxyribose (one less oxygen atom) in its sugar- phosphate backbone)
    • Unlike DNA replication, mRNA transcription can involve multiple RNA polymerases on a single DNA template and multiple rounds of transcription (amplification of particular mRNA), so many mRNA molecules can be rapidly produced from a single copy of a gene

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    • Elongation also involves a proofreading mechanism that can replace incorrectly incorporated bases

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Termination:

  • Bacteria use two different strategies for transcription termination:
    • Rho-independent termination
    • Rho-dependent termination
  • ρ-Independent termination:
  • In Rho-independent transcription termination, RNA transcription stops when the newly synthesized RNA molecule forms a G-C rich hairpin loop followed by a run of Us, which makes it detach from the DNA

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ρ-Independent termination:

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ρ-dependent termination:

  • In the "Rho-dependent“ type of termination, a protein factor called "Rho" destabilizes the interaction between the template and the RNA, thus releasing the newly synthesized mRNA from the elongation complex
  • rho (ρ) protein , which is a hexameric adenosine tri phosphatase (ATPase) with helicase activity
  • The ATP-dependent helicase activity of ρ separates the RNA-DNA hybrid helix, causing the release of the RNA

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Muhammad Wajid

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