6.4 - Translation

Translation is the synthesis of a polypeptide (protein) using the information encoded in the mRNA. It converts a nucleotide sequence into an amino acid sequence.

Key Components

• Ribosomes: The site of translation, composed of two subunits (large and small). Translation can occur in the cytoplasm or on the Rough ER in eukaryotes.

• mRNA (Messenger RNA): Carries the genetic information from DNA to the ribosome.

• tRNA (Transfer RNA): Key player in translating the mRNA into an amino acid sequence. Each tRNA has an anticodon region that is complementary and antiparallel to a codon in the mRNA.

Process of Translation

1. Initiation:

   • Translation begins when rRNA in the ribosome interacts with the mRNA at the start codon (AUG).

   • The start codon codes for the amino acid methionine (Met), and the anticodon on the tRNA that pairs with AUG is UAC.

   • The small ribosomal subunit binds to the mRNA, followed by the large subunit, forming the complete ribosome.

2. Elongation:

   • The mRNA moves through the ribosome, and its codons are read one at a time.

   • Each codon specifies an amino acid, which is brought to the ribosome by a corresponding tRNA.

   • The amino acids are linked together by peptide bonds, forming a growing polypeptide chain.

   • Codon charts are used to determine the amino acid specified by each codon.

   • The universality of the genetic code supports the idea of common ancestry among all organisms.

3. Termination:

   • Translation ends when a stop codon (UAA, UAG, UGA) appears in the mRNA.

   • Stop codons do not code for any amino acids.

   • The stop codon signals for a release factor, which promotes the release of the polypeptide chain from the ribosome.

Key Points

• Codons and Anticodons:

  • Codon: A three-letter code in mRNA that specifies an amino acid.

  • Anticodon: A complementary and antiparallel three-letter sequence in tRNA that pairs with the mRNA codon.

• Stages of Translation:

  • Initiation: Assembly of the ribosome, mRNA, and the first tRNA at the start codon.

  • Elongation: Sequential addition of amino acids to the growing polypeptide chain as codons are read by the ribosome.

  • Termination: Release of the completed polypeptide chain when a stop codon is encountered.

Visualization of the Process

1. Initiation:

   • Start codon (AUG) on mRNA.

   • tRNA with anticodon UAC carrying methionine.

2. Elongation:

   • Ribosome moves along mRNA.

   • tRNA brings amino acids based on codon sequence.

   • Peptide bonds form between amino acids.

3. Termination:

   • Stop codon encountered.

   • Release factor binds.

   • Polypeptide chain released.

Universal Genetic Code

• The genetic code is universal among all organisms, emphasizing the common evolutionary origin of all life.