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Chapter 16 Transcription and Translation

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in 2-step process. transcription of ... Wobble hypothesis (Crick) ... GUU (tRNA) can pair w/ CAA or CAG (glutamine) 3rd codon position is wobble position. tRNA ... – PowerPoint PPT presentation

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Title: Chapter 16 Transcription and Translation


1
Chapter 16Transcription and Translation
2
Outline
  • Proteins and overview
  • Transcription
  • RNA polymerase
  • initiation, elongation, termination
  • introns and exons
  • Translation
  • mRNA and the triplet code
  • tRNA
  • initiation, elongation, termination
  • modifications

3
Protein Functions
DNA
  • Proteins control
  • chemical reactions
  • physiological responses
  • developmental processes
  • structural support
  • DNA ? mRNA ? protein
  • (storage) (carrier) (machinery)
  • Proteins syn. in 2-step process
  • transcription of genes into mRNAs (in nucleus)
  • translation of mRNAs into proteins (in cytoplasm)

Information storage
TRANSCRIPTION
mRNA
Information carrier
mRNA
TRANSLATION
Active cell machinery
protein
4
Transcription in Bacteria
  • 1st step generation of mRNA by RNA polymerase
  • RNA polymerase reads DNA template strand and syn.
    mRNA
  • syn. occurs in 5 to 3 direction

5
RNA Polymerase
  • Prokaryotic RNA polymerase
  • large, globular enzyme w/ several channels and Mg
    in active site
  • holoenzyme core enzyme (syn. RNA) regulatory
    sigma protein (required for transcription
    initiation)

6
Transcription Initiation
  • Initiation phase of transcription
  • sigma factor binds to core enzyme
  • required for RNA polymerase to recognize promoter
  • responsible for identifying start site of gene
    (10 and 35 promoter sites)
  • RNA polymerase binds to promoters on DNA template
  • promoter short sequence of DNA that binds RNA
    polymerase so transcription can begin
  • 2 key regions -10 box (TATAAT box) and 35 box
  • sigma factor released from core enzyme and
    promoter after transcription begins

7
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8
Transcription Elongation and Termination
  • Elongation phase of transcription
  • RNA polymerase moves along DNA template in 3' to
    5' direction and syn. RNA in 5' ? 3' direction
  • driven by potential S in triphosphate bonds of
    NTP
  • complementary base pairing (no T, instead U)
  • Termination phase of transcription
  • specific sequences in template DNA act as
    termination sites
  • RNA polymerase encounters transcription
    termination signal causing RNA to form hairpin
    structure
  • RNA polymerase and mRNA strand released from DNA
    template

9
Transcription in Eukaryotes
  • Eukaryotic transcription similar to prokaryotic
    transcription
  • 3 different RNA polymerases in eukaryotes
  • RNA pol I transcribes genes that code for large
    rRNA subunit
  • RNA pol II transcribes genes that code for mRNAs,
    small nuclear ribonucleoproteins (snRNPs)
  • RNA pol III transcribes genes that code for tRNAs
    and small RNA subunit, snRNPs)
  • promoters more complex and varied
  • most have TATA box 30 bp upstream of
    transcription start site
  • basal transcription factors similar to sigma
    proteins (prokaryotes)
  • recognize promoters

10
Eukaryotic Genes Have Introns and Exons
  • Eukaryotic genes 1st transcribed into "primary
    transcript"
  • primary transcript RNA complement of exons and
    introns
  • exons protein-coding/translated region of genes
    (final mRNA)
  • introns stretches of non-coding/non-translated
    DNA
  • processing/editing generates a mature mRNA
    transcript
  • introns removed from mRNA by splicing
  • exons linked together
  • snRNPs form

    spliceosome and

    catalyze splicing

11
Post-Transcription Modification
  • Following transcription, 5' cap and poly-A tail
    added to mRNA in nucleus (eukaryotes)
  • cap modified G and P-P-P attached to 5' end
  • recognition signal for translation machinery
  • tail 100 to 250 adenines added to 3 end
  • protect mRNA from premature degradation by RNases

12
Translation
  • Transcription converts base sequence of DNA into
    mRNA sequence (in nucleus)
  • Translation converts base sequence of mRNA into
    AA sequence in protein (in cytoplasm)
  • mRNAs is released from DNA
  • mRNAs processed in nucleus
  • mRNAs associate w/ ribosomes in cytoplasm
  • ribosomes catalyze translation of mRNA sequence
    into protein

13
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14
tRNA
  • Adapter molecule transfer RNA (tRNA) proposed by
    Crick
  • AAs bind to tRNAs and transfer AA to proteins on
    ribosomes
  • tRNA attached to AA aminoacyl tRNA
  • aminoacyl tRNA synthetase catalyze addition of
    AA to tRNAs
  • each tRNA carries specific AA
  • AA coded by 1 codon have 1 tRNA
  • AA coded by 1 codons have 1 tRNAs

15
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16
tRNA
  • tRNA structure
  • 75 - 85 nucleotides long
  • cloverleaf secondary structure
  • double-stranded stems, single-stranded loops
  • CCA at 3' end binding site for AA
  • triplet of bases on one loop varies
    in each tRNA
    (anticodon)
  • pairs with mRNA codon

17
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18
tRNA
  • 61 different mRNA codons but only 40 tRNAs
  • Thus how can mRNA codon, for which there is no
    tRNA, be translated?
  • Wobble hypothesis (Crick)
  • 3rd position of tRNA anticodon can form
    nonstandard base pair w/ codon
  • one tRNA can pair w/ 1 codon
  • codons for same AA tend to have same 1st and 2nd
    nucleotides
  • GUU (tRNA) can pair w/ CAA or CAG (glutamine)
  • 3rd codon position is wobble position
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