RNA Polymerase Functions
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Questions and Answers

What is the direction of RNA synthesis during transcription?

  • It depends on the type of RNA being synthesized
  • Both directions are possible
  • 5’ to 3’ direction (correct)
  • 3’ to 5’ direction

    • What is the role of the NusA protein during transcription?

  • It is responsible for the termination of transcription
  • It aids in the elongation of RNA transcript (correct)
  • It helps in the initiation of transcription
  • It acts as a primer for RNA synthesis

    • What is the number of base pairs in the hybrid helix formed between the nascent RNA and the template DNA strand?

  • 20 base pairs
  • 10 base pairs
  • 17 base pairs (correct)
  • 12 base pairs

    • What happens to the nucleotide at the 5’ end of the RNA chain after several nucleotides are added?

    <p>It becomes unpaired from the base on the template strand</p> Signup and view all the answers

    What is the role of RNA polymerase during transcription?

    <p>It reads the template DNA strand and synthesizes the RNA chain</p> Signup and view all the answers

    What is the significance of the hybrid helix formed between the nascent RNA and the template DNA strand?

    <p>It helps in the transcription of RNA</p> Signup and view all the answers

    What is the result of the RNA polymerase losing its σ subunit during transcription?

    <p>The RNA polymerase becomes more processive</p> Signup and view all the answers

    What is the role of the σ subunit of RNA polymerase during transcription?

    <p>It is involved in the initiation of transcription</p> Signup and view all the answers

    What is the significance of the 5’ to 3’ direction of RNA synthesis?

    <p>It helps in the transcription of RNA</p> Signup and view all the answers

    Which of the following is a characteristic of tRNA and rRNA molecules?

    <p>They are much smaller than the primary transcripts.</p> Signup and view all the answers

    What type of modification occurs to bases and ribose in RNA?

    <p>Some bases are methylated in prokaryotes while ribose units are methylated in eukaryotes.</p> Signup and view all the answers

    What is the purpose of the cap structure added to the 5' end of mRNA in eukaryotes?

    <p>To stabilize the mRNA from phosphatases.</p> Signup and view all the answers

    What is the function of the poly A tail added to the 3' end of mRNA in eukaryotes?

    <p>To stabilize the mRNA from degradation.</p> Signup and view all the answers

    What is the function of the enhancer sequence in eukaryotes?

    <p>To increase the utilization of eukaryotic promoters.</p> Signup and view all the answers

    What is the purpose of splicing in mRNA processing?

    <p>To remove introns from the mRNA precursor.</p> Signup and view all the answers

    Where does transcription occur in eukaryotes?

    <p>In the nucleus.</p> Signup and view all the answers

    What is the difference between transcription in eukaryotes and prokaryotes?

    <p>Transcription is not coupled with translation in eukaryotes but is in prokaryotes.</p> Signup and view all the answers

    What is the purpose of the leader sequence in tRNA processing?

    <p>To remove the 5' leader sequence.</p> Signup and view all the answers

    What is the purpose of the 3' terminal CCA sequence in tRNA processing?

    <p>To replace the 3' terminal UU sequence.</p> Signup and view all the answers

    Study Notes

    RNA Polymerase I Functions

    • Searches for DNA initiation sites (promoters)
    • Unwinds a short stretch of double helical DNA to give single-strand DNA templates
    • Selects correct ribonucleoside triphosphates and catalyzes phosphodiester bond formation
    • Detects termination signals
    • Interacts with activator and repressor proteins to modulate transcription

    Differences between RNA Polymerase and DNA Polymerase

    • RNA polymerase does not require a primer, as RNA chains can be formed de novo
    • RNA polymerase does not have nuclease activity, resulting in low fidelity that can be tolerated
    • DNA template is fully conserved in RNA synthesis, but semi-conserved in DNA synthesis

    Structure of Prokaryotic RNA Polymerase

    • RNA polymerase of E. coli has a specific structure

    Bases in DNA and RNA

    • Adenine (A) is present in both DNA and RNA
    • Cytosine (C) is present in both DNA and RNA
    • Guanine (G) is present in both DNA and RNA
    • Thymine (T) is present in DNA, replaced by Uracil (U) in RNA

    Classes of Prokaryotic RNA

    • Ribosomal RNA (rRNA): 16S, 23S, and 5S
    • Transfer RNA (tRNA)
    • Messenger RNA (mRNA)

    Structure of Prokaryotic Messenger RNA

    • Shine-Dalgarno sequence facilitates initiation of protein synthesis
    • Sequence base-pairs with a pyrimidine-rich sequence in 16S rRNA

    RNA Transcription

    • Synthesis of RNA from DNA
    • RNA synthesis grows from 5' end to 3' end
    • Initiation of protein synthesis is facilitated by the Shine-Dalgarno sequence

    Types of RNA

    • Eukaryotic RNA: mRNA, tRNA, rRNA, and hnRNA
    • mRNA bears the genetic code (5%)
    • tRNA transfers amino acids to ribosomes (15%)
    • rRNA is responsible for protein synthesis (80%)
    • hnRNA is nuclear RNA before processing

    Transcription Process

    • Closed promoter complex forms
    • Open promoter complex forms
    • Initiation of transcription occurs
    • Elongation of RNA transcript occurs
    • Termination of transcription occurs
    • RNA product is formed

    Elongation of RNA Transcript

    • Nascent RNA forms a hybrid helix with the template DNA strand
    • After several nucleotides are added to the growing RNA chain, the nucleotide at the 5' end of the RNA chain becomes unpaired
    • RNA polymerase loses the σ subunit
    • NusA protein aids the elongation and binds to the RNA polymerase

    coli RNA Polymerase

    • The coli RNA polymerase is a multisubunit protein composed of subunits a, b, b', and s.
    • Subunit a: 2 units, role uncertain
    • Subunit b: 1 unit, forms phosphodiester bonds
    • Subunit b': 1 unit, binds DNA template
    • Subunit s: 1 unit, recognizes promoter and facilitates initiation

    Eukaryotic RNA Polymerases

    • There are three types of RNA polymerases in eukaryotes: I, II, and III
    • RNA polymerase I: localized in nucleolus, transcribes 18S, 5.8S, and 28S rRNA
    • RNA polymerase II: localized in nucleoplasm, transcribes mRNA precursors and snRNA
    • RNA polymerase III: localized in nucleoplasm, transcribes tRNA and 5S rRNA

    Template and Coding Strands

    • The strand that is transcribed into an RNA molecule is referred to as the template strand (or antisense strand)
    • The other DNA strand is referred to as the coding strand (or sense strand)
    • A given strand of double helical DNA can serve as a template strand for some genes and a coding strand for other genes

    Transcription of Genes from Both Strands

    • Genes can be transcribed from both strands of DNA
    • The template strand is always read in the 3' to 5' direction
    • The coding strand is identical except that U replaces T in the mRNA transcript

    Transcription Initiation

    • The sigma subunit binds to the -10 region, forming a closed promoter complex
    • The holoenzyme has a high affinity for promoter regions because of the sigma factor
    • Once initiation takes place, RNA polymerase does not need a high affinity for the promoter, and the sigma factor dissociates from the core polymerase

    Transcription Bubble

    • The transcription bubble is the region containing RNA polymerase, DNA, and nascent RNA
    • It is a locally melted (or "bubbled") region of DNA

    Termination of Transcription

    • Formation of phosphodiester bonds ceases
    • The RNA-DNA hybrid dissociates
    • The melted regions of DNA rewind to form a double helix
    • The core enzyme (devoid of s) has a low affinity for double helical DNA, so RNA polymerase leaves the DNA
    • The sigma factor rejoins the core enzyme, reforming the holoenzyme, which can repeat the transcription process

    RNA Processing

    • tRNA and rRNA molecules are much smaller than the primary transcripts
    • Modification of bases and ribose in RNA occurs, such as methylation of bases in prokaryotes and methylation of ribose units at the 2'-OH end in eukaryotes

    Processing of tRNA in Eukaryotes

    • The tRNA precursor undergoes a series of alterations to form mature tRNA:
      • Cleavage of the 5'-leader sequence
      • Splicing to remove an intron
      • Replacement of the 3'-terminal UU by CCA

    Transcription in Eukaryotes

    • Transcription of mRNA in eukaryotes is similar to that in bacteria, but with notable differences:
      • Eukaryotic cells contain 3 classes of nuclear RNA polymerase
      • Transcription in eukaryotes occurs in the nucleus, while translation occurs in the cytosol
      • In mRNA, both the 5' and 3' termini are modified
      • Splicing: introns are spliced out of mRNA precursors
      • Enhancers: nucleotide sequences that increase the utilization of eukaryotic promoters

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    Description

    This quiz covers the functions of RNA polymerase, including searching for initiation sites, unwinding DNA, and selecting ribonucleoside triphosphates. It also touches on the role of RNA polymerase in transcription and its interactions with activator and repressor proteins.

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