Chater 15

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  1. The Genetic Code
    • is composed of nucleotide triplets
    • is COMMA-FREE
    • contains START and STOP codons
    • is almost universal
  2. Transfer RNAs
    • 'link' the mRNA to the polypeptide (Crick's hypthesis)
    • Each is specific to ONE amino acid:
    • - covalently attached to 3' end
    • H-bonds to a CODON by complementary base pairing
    • - through its anticodon
  3. How many different tRNAs are there?
    30-50 different tRNAs
  4. Remember!
    • The genetic code is DEGENERATE.
    • Numerous codons vary only in the THIRD POSITION, and encode the same amino acid.
    • Such codons MAY be recognized by a SINGLE tRNA
    • Wobble Hypothesis (Crick)
  5. Key Concept
    A particular tRNA accepts ONLY ONE TYPE of amino acid.

    BUT, more than one tRNA may accept the SAME amino acid!

    Isoaccepting tRNAs
  6. Translation Occurs Within Ribosomes
    • ~1/3 protein, ~2/3 RNA (rRNA)
    • The components are named according to how they SEPARATE
    • based on weight and shape (Svedberg units)
    • Ex: 60S and 40S subunits
    • Ex: 28S and 18S rRNA
  7. Remember!
    rRNA is processed in both prokaryotes and eukaryotes
  8. Assembling Ribosomal Subunits
    • Most cells are FULL of ribosomes!
    • There are 100's-1000's of copies of the rRNA genes for rapid transcription
    • In euks, rRNA is transcribed by RNA pol I in a special area of the nucleus
    • Ribosome assembly occurs in the nucleous!
  9. Translation in Prokaryotes
    • 1. Initation
    • 2. Elongation
    • 3. Termination

    *Numerous accessory FACTORS are required for each!
  10. Translation Initiation Components
    • 1. mRNA
    • 2. small (30s) and large (50s) ribosomal subunits
    • 3. Initiation factors 1,2,3: IF-1, IF-2, IF-3
    • 4. Initiator tRNA
    • 5. GTP
  11. Shine - Dalgarno Sequence (proks only!)
    • Recognized by 16S rRNA, a major component of the 30S subunit.
    • By complementary base pairing!
  12. Translation: Initiation
    Involves a special "initiator tRNA" that carries the first amino acid of the polypeptide.

    It carries a modified methionine bearing a formyl group.

    fMet-tRNAMet or fMet-tRNA

    fmet-tRNA binds IF-2 and GTP

    THIS IF-2/f-met tRNA/GTP complex will bind to the robosome.
  13. Translation: Initiation
    • IF3 binds the 30S ribosomal subunit and keeps it dissociated from the 50S subunit
    • IF-1 also helps to keep this subunit open
  14. Translation: Initiation
    • 1) the 30S ribosomal subunit, bound by IF-3, allows an mRNA to bind:
    • -via the Shine-Dalgarno sequence to 16S rRNA
    • 2. When AUG s correctly positioned on 30S, fmet-tRNA/IF-2/GTP can base pair.

    the 30s initiation complex
Card Set:
Chater 15
2011-11-30 16:46:44
Genetics Chapter 15

The Components of Translation
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