Translation

Introduction

Translation is the biological process by which the genetic information present in messenger RNA (mRNA) is decoded to synthesize proteins. It occurs on ribosomes in the cytoplasm. During translation, the nucleotide sequence of mRNA determines the sequence of amino acids in a polypeptide chain according to the genetic code.

Protein synthesis is essential for growth, repair, metabolism, and regulation of all cellular activities.

Genetic Code

The genetic code is the relationship between the sequence of nucleotides in mRNA and the sequence of amino acids in proteins.

• The genetic code consists of triplet codons.

• Each codon is made up of three nitrogenous bases.

• There are 64 codons in total:

  1. Sixty-one (61) codons specify amino acids.

  2. Three (3) codons act as stop codons.

Important Features of Genetic Code

1. Triplet Nature

   • Three nucleotides form one codon.

2. Universal

   • Almost all organisms use the same genetic code.

3. Non-overlapping

   • One nucleotide belongs to only one codon.

4. Comma-less

   • Codons are read continuously without gaps.

5. Specific

   • Each codon specifies only one amino acid.

6. Start Codon

   • AUG codes for methionine and initiates translation.

7. Stop Codons

   • UAA, UAG, and UGA terminate translation.

Degeneracy of the Genetic Code

The genetic code is said to be degenerate because more than one codon can code for the same amino acid.

Examples:

• Leucine is coded by six codons.

• Glycine is coded by four codons.

Significance

• Reduces the harmful effects of mutations.

• Provides stability to protein synthesis.

Wobble Hypothesis

The Wobble Hypothesis was proposed by Francis Crick in 1966.

According to this hypothesis:

• The first two bases of the codon pair strictly with the anticodon.

• The third base allows flexible pairing or “wobbling.”

Thus, one tRNA can recognize multiple codons.

Importance

• Explains degeneracy of the genetic code.

• Reduces the number of tRNA molecules required.

Aminoacyl tRNA Synthetases and Charging of tRNA

Aminoacyl tRNA synthetases are enzymes responsible for attaching the correct amino acid to its corresponding tRNA molecule.

This process is called charging or aminoacylation of tRNA.

Steps in Charging of tRNA

1. Activation of amino acid using ATP.

2. Binding of activated amino acid to specific tRNA.

3. Formation of aminoacyl-tRNA.

Reaction

Amino acid + tRNA + ATP → Aminoacyl-tRNA + AMP + PPi

Significance

• Ensures accuracy in protein synthesis.

• Prevents incorporation of incorrect amino acids.

Proteins Involved in Translation

Translation occurs in three major stages:

1. Initiation

2. Elongation

3. Termination

Initiation of Translation

Initiation is the assembly of ribosomal subunits, mRNA, and initiator tRNA.

Important Components

• Small ribosomal subunit

• mRNA

• Initiator tRNA carrying methionine

• Initiation factors

In Prokaryotes

Important initiation factors:

• IF-1

• IF-2

• IF-3

In Eukaryotes

Initiation involves several eukaryotic initiation factors (eIFs).

Elongation of Translation

During elongation, amino acids are added one by one to the growing polypeptide chain.

Major Steps

1. Entry of aminoacyl-tRNA into A-site.

2. Peptide bond formation by peptidyl transferase.

3. Translocation of ribosome along mRNA.

Elongation Factors

• EF-Tu and EF-G in prokaryotes.

• eEFs in eukaryotes.

Termination of Translation

Termination occurs when a stop codon reaches the A-site of ribosome.

Stop Codons

• UAA

• UAG

• UGA

These codons do not code for amino acids.

Release Factors

Release factors bind to stop codons and release the completed polypeptide chain.

Mechanism of Translation

1. Activation of Amino Acids

Amino acids become attached to specific tRNA molecules.

2. Initiation

Ribosomal subunits assemble on mRNA at the start codon.

3. Elongation

Repeated cycles of codon recognition, peptide bond formation, and translocation occur.

4. Termination

Translation ends at stop codons and the synthesized protein is released.

Inhibitors of Protein Synthesis

Certain antibiotics and toxins inhibit translation.

Inhibitor	Target	Action
Streptomycin	Prokaryotic ribosome	Causes misreading of mRNA
Tetracycline	30S subunit	Blocks attachment of tRNA
Chloramphenicol	50S subunit	Inhibits peptidyl transferase
Erythromycin	50S subunit	Prevents translocation
Puromycin	Both	Causes premature termination
Cycloheximide	Eukaryotic ribosome	Inhibits elongation

Importance

• Useful in medicine as antibiotics.

• Helpful in studying protein synthesis mechanisms.

Conclusion

Translation is a highly coordinated process that converts genetic information into functional proteins. The genetic code, tRNA charging, ribosomal activity, and translation factors together ensure accurate protein synthesis. Understanding translation and its inhibitors is important in molecular biology, medicine, and biotechnology.

Exam-Oriented Questions

Short Questions

1. Define translation.

2. What is genetic code?

3. What is degeneracy of genetic code?

4. State the Wobble Hypothesis.

5. What are stop codons?

6. Define aminoacyl tRNA synthetase.

7. What is charging of tRNA?

8. Name the stages of translation.

9. What are initiation factors?

10. Name two inhibitors of protein synthesis.

Long Questions

1. Describe the mechanism of translation in detail.

2. Explain the features of genetic code.

3. Discuss the Wobble Hypothesis and its significance.

4. Describe the role of aminoacyl tRNA synthetases.

5. Write an account of inhibitors of protein synthesis.

MCQs

1. The start codon is:

   • a) UAA

   • b) AUG

   • c) UAG

   • d) UGA
     Answer: b) AUG

2. The Wobble Hypothesis was proposed by:

   • a) Watson

   • b) Crick

   • c) Mendel

   • d) Darwin
     Answer: b) Crick

3. Which enzyme attaches amino acid to tRNA?

   • a) DNA polymerase

   • b) RNA polymerase

   • c) Aminoacyl tRNA synthetase

   • d) Ligase
     Answer: c) Aminoacyl tRNA synthetase

4. Which is a stop codon?

   • a) AUG

   • b) GGG

   • c) UAA

   • d) CCC
     Answer: c) UAA

5. Tetracycline inhibits:

   • a) DNA replication

   • b) Translation

   • c) Transcription

   • d) Mutation
     Answer: b) Translation

References

1. Alberts B. et al. Molecular Biology of the Cell.

2. Watson J.D. et al. Molecular Biology of the Gene.

3. Lodish H. et al. Molecular Cell Biology.

4. Lewin B. Genes.

5. Karp G. Cell and Molecular Biology.