Transcription in Eukaryotes

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Transcription in Eukaryotes

• 3 RNA polymerases responsible for copying DNA
  template
• Pol I Transcribes large ribosomal RNAs
  (nucleolus)
• Pol II Transcribes mRNA
• Pol III Transcribes most small RNAs, tRNAs and
  RNAs involved in processing primary RNA
  transcripts

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Transcription in Eukaryotes mRNA modificaton and
processing

• primary transcript is modified in several ways
  before release into cytoplasm for translation
• 5 capping
• polyadenylation
• removal of intervening sequences (introns)

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Addition of 5methyl cap to transcript
Cap used for ribosome assembly during translation
iniitiation...
AAUAAA signal serves as recognition site for
cleavage factors and poly A polymerase
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AAAAAAA..
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Elements of the Promoter...
TATA or Goldberg-Hogness box
caat box
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General Transcription Factors interact with the
promoter to facilitate binding of RNA polymerase
IIe.g.
Specific Transcription factors can influence the
formation of a productive transcription complex...
STFs may bind at some distance from the
promoterenhancer sequences
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Intron RNA that is part of the primary
transcript which is removed from mature mRNA
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Exon part of the primary transcript that remains
in the mature mRNA
Primary transcript contains both introns and
exonsintrons must be removed...
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SNRPS small nuclear ribonucleoprotein particles
Macromolecular complex for RNA processing
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Intron removed
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Why Introns?

• Exon shuffling hypothesis exon/intron
  organization may facilitate evolution of new
  genes...
• In some cases, exons of a gene code for a
  functional domain- a peptide region that
  assumes a useful conformatione.g. binding a
  molecule
• ?-globin heme containing region
• introns allow ability to mix/assemble different
  functional cassettes into new combinations?

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Evolution of multigene families and gene
divergence

• Duplications in DNA could allow for genes to
  diverge and be incorporated into new
  physiological contexts
• e.g. alpha and beta globin gene families (pp.
  785-87)...

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Heterozygote advantage...
Gene duplication...
recombination
Favored by selection...
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• Genes expressed at different times during
  development, in different tissues
• Proteins are similar but have slightly different
  physiological properties
• Order of gene expression in development
  correlated with linear array in chromosomes

Pseudogenes duplications that are not functional
in the genometend to accumulate random mutations
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Summary of Eukaryotic transcription/translation .
translation
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Chromatin Structure

• Histones are highly conserved proteins that are
  intimately associated with the DNA in chromatin
• small in size
• carry a large number of basic residues
• complexed into a particle termed a nucleosome

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• nucleosome core particle
• Octamer of
• H2A
• H2B
• H3
• H4
• 200 bp DNA wrapped around twice (7 fold
  compaction)

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Further compaction can be achieved by formation
of a solenoid of nucleosomes...
H1 required for formation of 30 nm fiber...
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nucleosomes beads on a string in different
stages of condensation...
Low salt
Physiological ionic strength
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Higher order folding...
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Loops of DNA
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Mapping of scaffold attachment regions (matrix
attachment sites) in Drosophila...
interaction sites for topoisomerase II are also
observed in these regions, as are sites that may
be involved in transcriptional regulation...
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How does the configuration of chromatin affect
gene expression?

• It would seem to be difficult to transcribe DNA
  that was complexed in the nucleosome core RNA
  polymerase appears to displace histone octamers
  during transcription
• It is possible to distinguish active from
  inactive chromatin by susceptibility to enzyme
  digestion

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