Genetic Code and Translation: Structure, Function, and Mutation

Genetic Code and Translation

Overview of Translation

Translation is the process by which the genetic information encoded in messenger RNA (mRNA) is used to synthesize proteins. It occurs in three main stages: initiation, elongation, and termination.

• Initiation: The small ribosomal subunit binds to the mRNA at the start site, followed by the joining of the large subunit and aminoacyl-tRNA.

• Elongation: The ribosome moves along the mRNA, extending the protein by transferring the growing polypeptide chain to aminoacyl-tRNA (translocation).

• Termination: The polypeptide chain is released when a stop codon is encountered, and the ribosome dissociates from the mRNA.

Termination Codons and Protein Factors

Termination codons signal the end of translation and are recognized by protein release factors, not by aminoacyl-tRNAs.

• Release Factors (RFs):

  • RF1: Recognizes UAA and UAG stop codons in bacteria.

  • RF2: Recognizes UAA and UGA stop codons in bacteria.

  • RF3: Facilitates the function of RF1 and RF2.

• The structure of release factors mimics aminoacyl-tRNA, allowing them to interact with the ribosome.

tRNA Charging and Aminoacyl-tRNA Synthetases

Function and Specificity

Aminoacyl-tRNA synthetases are enzymes that attach amino acids to their corresponding tRNAs in a two-step reaction requiring ATP.

• Each synthetase aminoacylates all tRNAs in an isoaccepting group (cognate group) for a specific amino acid.

• Recognition of tRNA by synthetases depends on a unique set of nucleotides called the tRNA identity set, often found in the acceptor stem and anticodon loop.

The Genetic Code

Universality and Exceptions

The genetic code is nearly universal, having been established early in evolution. However, exceptions exist in some prokaryotes, fungi, and protists.

Organism	Codon Exceptions
Prokaryotes	Some codons reassigned
Fungi	Rare codon changes
Protists	Rare codon changes
Most Eukaryotes	Standard code

Additional info: Table inferred from slide showing codon usage exceptions.

Structure of the Genetic Code

• There are 64 codons: 61 encode amino acids, 3 are stop codons.

• Each codon consists of three nucleotides (triplet code).

• One amino acid can be encoded by multiple codons (redundancy).

• Stop codons do not have corresponding tRNAs.

Codon	Amino Acid
AUG	Met (Start)
UAA, UAG, UGA	Stop
UUU, UUC	Phe
Other codons	Various amino acids

Additional info: Table simplified from standard codon chart.

Redundancy and Degeneracy

The genetic code is described as redundant or degenerate because multiple codons can encode the same amino acid.

• The third position of the codon is often less important (wobble position).

• Exceptions: Methionine (AUG), Tryptophan (UGG), and stop codons have unique third positions.

• Codons for the same amino acid are called synonymous codons.

Wobble Rule

The wobble rule describes the flexibility in base pairing between the third base of the codon and the first base of the anticodon, allowing alternative tRNAs to recognize the same codon.

• Wobble occurs only at the third codon position.

• Allows for efficient translation and use of fewer tRNAs.

• No wobble for Methionine and Tryptophan codons.

Example: GCU, GCC, GCA, and GCG all encode Alanine; tRNAs with different anticodons can recognize these via wobble pairing.

Codon-Anticodon Recognition

Pairing between the first base of the anticodon and the third base of the codon can deviate from standard Watson-Crick base pairing, following specific wobble rules.

• Allows for non-standard base pairs such as G-U.

• Increases the efficiency of translation.

Mutations and Codon Usage

Synonymous and Silent Mutations

A synonymous mutation changes a codon to another codon that encodes the same amino acid. Most synonymous mutations are silent, meaning they do not alter the protein sequence.

• Some synonymous mutations can affect translation efficiency or mRNA stability.

• Example: A mutation from GAA to GAG (both encode Glutamate) is synonymous.

Codon Usage Bias

Different organisms prefer certain codons over others for the same amino acid, a phenomenon known as codon usage bias.

• The abundance of each tRNA correlates with the frequency of its corresponding codon in mRNA.

• Differential tRNA concentrations can affect translation speed and protein folding.

Codon	Human	Drosophila	E. coli
AGA	22%	1%	0%
AGG	21%	1%	0%
CGA	11%	6%	5%
CGC	18%	18%	36%
CGG	18%	1%	11%
CGU	10%	73%	48%

Additional info: Table inferred from slide showing codon usage for Arginine in different organisms.

Polymorphisms and Functional Effects

Polymorphisms are genetic variations that do not affect viability or fertility. Synonymous mutations can sometimes affect protein folding, translation speed, or membrane insertion, as shown in the MDR1 gene example.

• Rare codons can slow translation, affecting protein conformation.

• Changes in timing of folding and membrane insertion can alter protein function.

Conservative Mutations

Conservative mutations are missense mutations that change a codon to encode an amino acid with similar biochemical properties, often minimizing the impact on protein function.

• Protein structure and function depend on the chemical properties of amino acids.

• Conservative mutations are less likely to disrupt protein function than non-conservative missense mutations.

• Example: Substitution of lysine (positively charged) for arginine (also positively charged).

General Rules of Codon Recognition

• Each amino acid can have multiple codons and tRNAs (redundancy/degeneracy).

• Each codon has a corresponding tRNA with a complementary anticodon.

• One codon can be recognized by multiple tRNAs (wobble).

• Each codon is only recognized by tRNAs encoding the same amino acid (no wobble for wrong amino acid).

Suppressor tRNA

Genetic Suppressors

A genetic suppressor is a mutation in one gene that compensates for a mutation in another gene. Suppressor tRNAs can recognize mutated codons and restore translation.

• Nonsense suppressor tRNA: Suppresses premature stop codons by inserting an amino acid at the stop codon, allowing translation to continue.

• Missense suppressor tRNA: Suppresses missense mutations by inserting the original or a similar amino acid at the mutated codon.

Example: A tRNA with a mutated anticodon can recognize a stop codon and insert an amino acid, preventing premature termination.

Recap of Module 3

• RNA transcription

• RNA processing

• RNA degradation

• RNA localization

• RNA translation in polypeptide

• The genetic code