Chapter 6: Gene Expression - Translation Translation = conversion of a messenger RNA sequence into the amino acid sequence of a polypeptide (i.e., protein synthesis) Topics to be covered today: Peptide bond Amino acid biochemical properties Protein structure Genetic code Translation.
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Translation = conversion of a messenger RNA sequence into the amino acid sequence of a polypeptide (i.e., protein synthesis)
Topics to be covered today:
Amino acid biochemical properties
The sequence of AA gives the polypeptide its 3D shape and its properties in the cell.
Contains the following bonded to a central carbon atom.
Typically charged in the cell
(-NH3+ and -COO-)
Proteins show four hierarchical levels of structural organization:
Determined by the genetic code of the mRNA.
Result of weak H-bonds and electrostatic interactions
e.g., -helix (coiled) and -pleated sheet (zig-zag).
Results from the different R groups.
Occurs only with two or more polypeptides.
Discovered that frameshift mutations (insertion or deletion) resulted in a different sequence of amino acids.
Combination of three r+ mutants routinely yielded revertants, unlike other multiple combinations.
Fig. 6.6 - Three nearby insertions (+) restore the reading frame, giving normal or near-normal function.
Synthetic mRNA containing only one type of base:
UUU = Phe, CCC = Pro, AAA = Lys, GGG = ? (unstable)
Pro, Lys (already defined) + Asp, Glu, His, & Thr
Proportion (%AC) varied to determine exactly which codon specified which amino acid.
UCU CUC UCU CUC Ser Leu Ser Leu
1968: Robert Holley (Cornell), H. G. Khorana (Wisconsin-Madison),
and Marshall Nirenberg (NIH).
mRNA UUU codon
tRNA AAA (with Phe) anti-codon
mRNA UCU codon
tRNA AGU (with Ser) anti-codon
mRNA CUC codon
tRNA GAG (with Leu) anti-codon
Code is triplet. Each 3 codon in mRNA specifies 1 amino acid.
Code is comma free. mRNA is read continuously, 3 bases at a time without skipping bases (not always true, translational frameshifting is known to occur).
Code is non-overlapping. Each nucleotide is part of only one codon and is read only once.
Code is almost universal. Most codons have the same meaning in different organisms (e.g., not true for mitochondria of mammals).
Code is degenerate. 18 of 20 amino acids are coded by more than one codon. Met and Trp are the only exceptions. Many amino acids are four-fold degenerate at the third position.
Code has start and stop signals. ATG codes for Met and is the usual start signal. TAA, TAG, and TGA are stop codons and specify the the end of translation of a polypeptide.
Wobble occurs in the tRNA anti-codon. 3rd base is less constrained and pairs less specifically.
(Haig and Hurst 1991, J. Mol. Evol. 33:412-417).
fMet = formylmethionine (Met modified by transformylase; AUG at all other codon positions simply codes for Met)
mRNA 5’-AUG-3’ start codon
tRNA 3’-UAC-5’ anti-codon
Initiator Met is not modified in eukaryotes (but eukaryotes possess initiator tRNAs).
No Shine-Dalgarno sequence; but rather initiation factor (IF-4F) binds to the 5’-cap on the mature mRNA.
Eukaryote AUG codon is embedded in a short initiation sequence called the Kozak sequence.
Eukaryote poly-A tail stimulates translation by interacting with the 5’-cap/IF-4F, forming an mRNA circle; this is facilitated by poly-A binding protein (PABP).
Binding of the aminoacyl tRNA (charged tRNA) to the ribosome.
Formation of the peptide bond.
Translocation of the ribosome to the next codon.