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The Genetic Code
- is composed of nucleotide triplets
- is NON-OVERLAPPING
- is COMMA-FREE
- is DEGENERATE
- contains START and STOP codons
- is almost universal
- '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
How many different tRNAs are there?
30-50 different tRNAs
- 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)
A particular tRNA accepts ONLY ONE TYPE of amino acid.
BUT, more than one tRNA may accept the SAME amino acid!
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
rRNA is processed in both prokaryotes and eukaryotes
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!
Translation in Prokaryotes
- 1. Initation
- 2. Elongation
- 3. Termination
*Numerous accessory FACTORS are required for each!
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
Shine - Dalgarno Sequence (proks only!)
- Recognized by 16S rRNA, a major component of the 30S subunit.
- By complementary base pairing!
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.
- IF3 binds the 30S ribosomal subunit and keeps it dissociated from the 50S subunit
- IF-1 also helps to keep this subunit open
- 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