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3 main types of RNA

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mRNA: encode AA sequence of one of more polypeptides from a gene or set of genes ... genes from oocytes of the salamander Pleurodeles waltl undergoing transcription. ... – PowerPoint PPT presentation

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Title: 3 main types of RNA


1
3 main types of RNA
Transcription
  • All are made from a DNA Template
  • mRNA encode AA sequence of one of more
    polypeptides from a gene or set of genes
  • tRNA reads info. In mRNA and transfers
    appropriate AA to growing peptide chain
  • rRNA part of ribosome, cellular machines
    synthesize proteins

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Synthesis
  • Resembles DNA replication in its chemical
    mechanism, polarity, use of template
  • Has initiation, elongation, termination phase
  • Does not need a primer, only involves limited
    segment of DNA molecule

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RNA Polymerase
  • DNA directed RNA polymerase
  • DNA template
  • Nucleotides (ATP, CTP, UTP, GTP)
  • Builds in the 5 to 3 direction

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RNA Polymerase
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Binds to initiation site through sequences called
promoter elements that are recognized by ? factor
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Once initiation occurs ? factor dissociates
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Recognized by ?70
Recognized by ?32
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RNA Polymerase has helicase activity
Gyrases/topoisomerases are needed to prevent
positive supercoiling ahead of the transcription
bubble
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An electron micrograph of three contiguous
ribosomal genes from oocytes of the salamander
Pleurodeles waltl undergoing transcription.
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Transcription is tightly regulated
Not all genes are transcribed equally
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Transcription Factor
  • Upstream of RNAP II initiation site, different
    combinations of specific DNA sequences each is
    recognized by corresponding site specific DNA
    binding protein
  • Each combination of DNA sequence and cognate DNA
    binding protein represent a control module

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The lac operon
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Eukaryotic transcription
Heterochromatin transcriptionally
inactive Euchromatin may be transcriptionally
active
Eukaryotic RNAP have almost no affinity for
promoters on their own
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Eukaryotic RNA Polymerases
  • RNAP I synthesis of transcripts called
    preribosomal RNA precursor 18s, 5.8s, 28s RNA
  • RNAP II synthesis of mRNA
  • RNAP III synthesize tRNA

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RNA Polymerase II
  • Has 12 subunits
  • Needs transcription factors that stimulate the
    enzyme to bind to a nearby promoter
  • General TF required at every RNAP II promoter

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Functionally equivalent to ? factor
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Gaining access to the DNA template
Histones must be modified to access DNA
Histone acetyltransferases decrease
charge Histone deacetylases increase charge
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Lysine
Acetyllysine
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Gene Silencing
5 of cytidine residues are methylated at the 5
position
These are most often found at CG sequences
Hypermethylation is associated with
heterochromatin
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Recruiting RNA polymerase
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Regulated Transcription
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Regulated Transcription
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RNA Processing
  • Most RNA is processed after synthesis
  • Ribozymes catalyze post-transcriptional
    processing
  • Primary Transcript newly synthesized RNA
    molecule occurs in mRNA and tRNA contains
    sequence encoding one gene but encoded
    polypeptide not continuous
  • Noncoding Regions introns
  • Coding Region exons

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Post-transcriptional control
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RNA Editing Changes the Meaning
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RNA Editing Changes the Meaning
C to U
A to I
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There are many ways to regulate the meaning of
genes after transcription
Prevent translation
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There are many ways to regulate the meaning of
genes after transcription
Degrade the message
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Target the mRNA
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What about mistakes? Nonsense Mediated Decay
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