Translation

• Overview • RNA in Translation• mRNA • Genetic code• Process of translation• V. Eukaryotic modifications

Translation

• Overview • DNA---> RNA --->

Polypeptide • Orientation of

nucleotide with polypeptide

RNA in Translation• mRNA • rRNA

– ribosomes and proteins

– prokaryotic and eukaryotic differences

• tRNA – structure – loops/domains – charging of tRNA with

amino acid

rRNA

tRNA

CCA-3’ end

Charging of tRNA with amino acid

• 2 step process• ATP & Amino acid• Releases PPi

• Correct tRNA comes into active site

Genetic code

• deciphering the code • define a codon• triplets, start and stop

codons• universality • degeneracy • wobble hypothesis

Exceptions

Deciphering the code

Anti-codon/Codon rulesWobble hypothesis

Proposed by Crick in 1966, first two ribonucleotides critical.Third position would be less constrained

Translation

• Initiation • Elongation • Translocation • Termination

Process of translation: Initiation

• Initiation Complex • binding of ribosome • Shine-Dalgarno

sequence (on mRNA)• fmet

• subunits • initiation factors

Initiation

• 30s subunit complexed with IF and GTP binds to mRNA and fMet tRNA

• Then 50s subunit binds• Requires GTP hydrolysis– IF2

Elongation

• Peptidyl transferase(23s Ribosome).

• 15 amino acids/ second

• Requires GTP hydrolysis for each round of elongation

Peptide bond formation

Translocation

• Requires GTP hydrolysis for each round of translocation.

• EF-Tu facilitates amino acid entry into A site

• EF-G stimulates translocation

Termination

• Release factors• Requires GTP

hydrolysis• Suppressor mutations

Polysomes

Eukaryotic modifications • Ribosomal binding: 5’

cap, CBP (cap binding protein) and Kozaksequence surrounding AUG codon, initatortRNA scans mRNA

• no fmet• Poly A tail loops toward

5’ end to help initiation• signal peptide/signal

hypothesis • cellular compartment

and ER

LDL receptor exons

Modifications of polypeptides• The polypeptide chain that results from mRNA

translation is often subject to chemical modifications. Examples of post-translational modifications include:

• • glycosylation• • addition of lipid groups (e.g. fatty acyl or prenyl

groups)• • phosphorylation• • hydroxylation• • addition of co-factors (e.g. a heme molecule)• • or proteolytic cleavage• The type of modification a protein undergoes depends

on its function and sub-cellular location.