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translation protein synthesis in prokaryotes


Translation: Protein Synthesis | NEET Biology Notes

Translation is the process of protein synthesis. It involves the formation of a polypeptide chain by polymerisation of amino acids according to the information present in mRNA.

In simple terms, translation involves the conversion of the nucleotide sequence or "language" of mRNA into the amino-acid sequence of a protein.

1. Translation Unit

A part of mRNA having coded information for a polypeptide is called a translation unit.

A translation unit consists of:

  • Initiation codon
  • Coding region
  • Termination codon

The initiation codon is generally AUG, which codes for methionine.

2. Components Required for Translation

The major components involved in protein synthesis are:

  1. mRNA
  2. tRNA
  3. Ribosomes
  4. Amino acids
  5. Aminoacyl-tRNA synthetase
  6. Energy in the form of ATP and GTP

The ribosome acts as the site of protein synthesis.

3. Charging of tRNA

The process by which an amino acid becomes attached to its specific tRNA is called charging of tRNA or aminoacylation.

The reaction can be represented as:

Amino acid + tRNA + ATP → Aminoacyl-tRNA

This reaction is catalysed by the enzyme aminoacyl-tRNA synthetase.

The charged tRNA carries its specific amino acid to the ribosome during translation.

4. Ribosome

The ribosome is the site of translation. It consists of a small subunit and a large subunit.

Prokaryotic Ribosome

70S = 30S + 50S

Eukaryotic Ribosome

80S = 40S + 60S

The ribosome contains three important functional sites:

  • A site — Aminoacyl site
  • P site — Peptidyl site
  • E site — Exit site

5. Role of Ribosome in Protein Synthesis

  • The mRNA binds to the ribosome.
  • The anticodon of tRNA pairs with the complementary codon of mRNA.
  • Amino acids are brought to the ribosome by tRNA.
  • Peptide bonds are formed between successive amino acids.
  • The growing polypeptide chain is released after termination.

6. Initiation of Translation

Translation begins with the formation of an initiation complex.

  1. The small ribosomal subunit binds to mRNA.
  2. The initiator tRNA recognizes the initiation codon, usually AUG.
  3. The initiator tRNA carries methionine.
  4. The large ribosomal subunit then joins the complex.

7. Elongation

During elongation:

  1. Charged tRNA enters the A site.
  2. Its anticodon pairs with the complementary codon on mRNA.
  3. A peptide bond is formed between amino acids.
  4. The growing polypeptide chain is transferred to the tRNA at the A site.
  5. The ribosome moves along the mRNA in the 5′ → 3′ direction.
  6. The empty tRNA leaves through the E site.

This process continues until a termination codon is reached.

8. Termination of Translation

Translation terminates when a termination codon enters the A site.

The three termination codons are:

  • UAA
  • UAG
  • UGA

These codons do not code for any amino acid. Instead, they are recognized by release factors, which help release the newly synthesized polypeptide chain.

9. Polyribosome / Polysome

Several ribosomes can simultaneously translate a single mRNA molecule. Such a structure is called a polyribosome or polysome.

Polysomes allow multiple copies of the same polypeptide to be synthesized from a single mRNA molecule.

10. Important Inhibitors of Translation

Important inhibitors associated with protein synthesis include:

  • Streptomycin
  • Tetracycline
  • Chloramphenicol
  • Puromycin

These antibiotics or inhibitors interfere with different stages of protein synthesis.

11. Important Directions in Translation

mRNA is read in the direction: 5′ → 3′

Polypeptide synthesis: N-terminus → C-terminus

Quick Revision: Translation

Feature Key Point
Process Protein synthesis
Template mRNA
Site Ribosome
Initiation codon AUG
Stop codons UAA, UAG, UGA
mRNA reading direction 5′ → 3′
Polypeptide direction N-terminus → C-terminus

Conclusion

Translation is a fundamental step in gene expression in which the nucleotide sequence of mRNA is decoded at the ribosome to produce a specific amino-acid sequence and ultimately a protein.

These notes are useful for NEET Biology, Class 12 Biology, CBSE Biology and revision of the chapter Molecular Basis of Inheritance.

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