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Genetic Code & Protein Synthesis - Coggle Diagram
Genetic Code & Protein Synthesis
Genetic Code
Definition
Translation = Protein synthesis from mRNA
Genetic code = Relation between DNA/RNA bases and protein amino acids
Features
Triplet Code
3 nucleotides = 1 codon = 1 amino acid
Nonoverlapping
Each base read once
No Punctuation
Continuous reading
Directionality
Read 5′ → 3′
Degeneracy
18 amino acids encoded by ≥2 codons
Trp, Met: only 1 codon
Allows tolerance to mutations
Universality
Nearly universal across organisms
Exceptions: mitochondrial code
tRNA and Wobble
tRNA Structure
73–93 nucleotides
Cloverleaf 2D, L-shaped 3D
5′ phosphorylated, 3′ CCA –OH
Anticodon loop near center
Modified Bases
e.g., Inosine (I) from A
Wobble Hypothesis
3rd codon base: flexible pairing
1st anticodon base:
C or A → 1 codon
U or G → 2 codons
I → 3 codons
Aminoacyl-tRNA Synthetases
Function
Link specific amino acid to its correct tRNA
“Translators” of the genetic code
Activation Process
Amino acid + ATP → Aminoacyl-AMP
Aminoacyl group transferred to tRNA
2 ATP equivalents consumed
Proofreading
Editing site removes wrong amino acids
Double sieve mechanism
Recognition
Recognises anticodon loop and acceptor stem
Ribosome Structure
Composition
Prokaryotic: 70S = 50S + 30S
Eukaryotic: 80S = 60S + 40S
rRNA
2/3 of mass
Catalytic role (ribozymes)
Sites
A (aminoacyl-tRNA), P (peptidyl-tRNA), E (exit)
Steps of Protein Synthesis
Initiation
Start Codon: AUG (methionine)
Prokaryotes
Shine-Dalgarno sequence binds 16S rRNA
fMet-tRNAf + 30S + IF1, IF2 (GTP), IF3 → 30S initiation complex
50S joins → 70S initiation complex
Eukaryotes
5′ cap recognition
Scanning for 1st AUG
Initiator tRNAi (Met), many eIFs
mRNA circularised
Elongation
EF-Tu (prokaryotes)
Delivers aminoacyl-tRNA to A site
Peptide Bond Formation
Peptidyl transferase activity of 23S rRNA
Translocation
EF-G (translocase) + GTP hydrolysis
Shift mRNA by 1 codon
Cycle
Repeats: 40 amino acids/sec in E. coli
Termination
Stop Codons: UAA, UAG, UGA
Release Factors (RFs)
RF1, RF2, RF3
Hydrolysis of ester bond → release protein
Special Topics
Polysomes
Multiple ribosomes on 1 mRNA
Secretory Pathway
Signal sequence recognised by SRP → Rough ER docking → Translocation into ER
Regulation of Translation
Iron regulation via IRE-IREBP system
RNAi
siRNA, miRNA regulation of mRNA degradation or translation inhibition