Transcription in Prokaryotes: Transcription is the process of copying genetic information from one strand of the DNA into RNA. The principle of complementarity governs the process of transcription, except the adenosine now forms base pair with uracil instead of l thymine. In transcription, only a segment of DNA is duplicated and on Iv one of the strands is copied into RNA. Both the strands are not copied.

Transcription in Prokaryotes 

• The process of synthesis of RNA by copying the template strand of DNA is called transcription.
• During replication entire genome is copied but in transcription only the selected portion of genome is copied.
• The enzyme involved in transcription is RNA polymerase. Unlike DNA polymerase it can initiate transcription by itself, it does not require primase. More exactly it is a DNA dependent RNA polymerase.
• The steps of transcription completes in three major steps
  • Initiation: RNA polymerase attaches to the DNA molecule and moves along the DNA strand until it recognises a promoter sequence. These are known as the transcription start sites. The DNA double helix then unwinds and all the bases on each of the DNA strands are exposed. This acts as a template for a new mRNA strand.
  • Elongation: Ribonucleotides are added to the template strand that enables the growth of mRNA growth.
  • Termination: RNA polymerase encounters a terminator sequence and the transcription stops. RNA polymerase then releases the DNA template.

Prokaryotic DNA Polymerase Types and Function

There are five DNA polymerases identified in E.coli. All the DNA polymerases differ in structure, functions and rate of polymerization and processivity.

DNA Polymerase I is coded by polA gene. It is a single polypeptide and has a role in recombination and repair. It has both 5’→3’ and 3’→5’ exonuclease activity. DNA polymerase Ⅰ removes the RNA primer from lagging strand by 5’→3’ exonuclease activity and also fills the gap.

DNA Polymerase II is coded by polB gene. It is made up of 7 subunits. Its main role is in repair and also a backup of DNA polymerase III. It has 3’→5’ exonuclease activity.

DNA Polymerase III is the main enzyme for replication in E.coli. It is coded by polC gene. The polymerization and processivity rate is maximum in DNA polymerase III. It also has proofreading 3’→5’ exonuclease activity.

DNA polymerase III of E.coli is made up of a total of 13 subunits, which comprises 9 different types of subunits.

• It consists of two core domains made up of 𝜶, 𝟄, and 𝞱 subunits. It is attached to the 𝝲 complex or clamp-loading complex, which is made up of five subunits, 𝞽₂𝝲𝝳𝝳’. Additional subunits 𝟀 and 𝟁 are attached to the clamp-loading complex. 𝞫 subunits make two clamps with a dimer each. They increase the processivity of the DNA polymerase III.

DNA Polymerase IV is coded by dinB gene. Its main role is in DNA repair during SOS response, when DNA replication is stalled at the replication fork. DNA polymerase II, IV and V are translesion polymerases.

DNA Polymerase V is also involved in translesion synthesis during SOS response and DNA repair. It is made up of UmuC monomer and UmuD dimer.

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By Team Learning Mantras