Chapter 16 Transcription and Translation

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Chapter 16Transcription and Translation
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Outline

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

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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
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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

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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)

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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

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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

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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

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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

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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

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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

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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

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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

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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