Detailed Notes on Translation

Translation

  • Translation is the process where the messenger RNA (mRNA) is read by a ribosome.
  • The ribosome builds a polypeptide.
  • Two ribosome subunits clamp onto the mRNA.
  • Transfer RNA (tRNA) brings amino acids to the ribosome.
  • Each codon on the mRNA matches a specific anticodon on a tRNA.
  • Amino acids join in the correct sequence via peptide bonds.

Process

  1. mRNA Strand: An mRNA strand (e.g., AUGACU) is fed into the ribosome.
  2. Ribosome Structure: The ribosome has two subunits and two active sites.
  3. tRNA Binding:
    • tRNA with an anticodon complementary to the mRNA codon binds weakly via hydrogen bonds.
    • Example: Anticodon for AUG.
    • Complementary base pairing: A to U, C to G, U to A.
  4. Peptide Bond Formation: When the tRNA binds, it causes a peptide bond to form between the amino acids.
  5. Continuing the Process: The ribosome continues to move along the mRNA, adding amino acids.

Example: DNA to Protein

  • Given DNA sequence: A long sequence with triplets.
  • Transcription yields an mRNA sequence.
  • Example: If DNA has triplets, transcription involves:
    • T bonding with A
    • A bonding with U (in RNA)
    • C bonding with G
  • This creates a series of codons, including the start codon, and multiple codons in total.

Using a Codon Table

Codon Table Basics
  • The codon table is used to translate mRNA codons into amino acids.
  • It uses codons (three-nucleotide sequences) to determine the corresponding amino acid.
  • Base 1, Base 2, and Base 3 positions are used to find the specific codon.
Example: AUG
  1. Find the first base (A) in the table.
  2. Find the second base (U) in the appropriate section.
  3. Find the third base (G) to identify the codon AUG.
  • AUG corresponds to methionine (start codon).
Examples
  • GGC: Glycine
  • UCC: Serine (Ser)
  • AUC: Isoleucine

Translation Example

Consider the mRNA sequence translated:

  • AUG - Methionine (start)
  • ACU - Threonine (Thr)

Complete Translation Example

  • Given mRNA sequence: AUG GGC UCC AUC GGC GCA UAA
    • AUG: Methionine (start)
    • GGC: Glycine
    • UCC: Serine
    • AUC: Isoleucine
    • GGC: Glycine
    • GCA: Alanine
    • UAA: Stop codon (releasing factor)
  • This sequence represents the primary structure of a protein.

Ribosome Sites: P Site and A Site

The ribosome has a P site and an A site where tRNA molecules bind and peptide bonds form.

Elongation and Termination

  • Elongation: Peptide bonds form, and the amino acid chain grows.
  • Termination: Occurs when a stop codon (UAA, UAG, UGA) is reached, causing the release of the polypeptide.

End Products of Protein Synthesis

  • The end product is a primary structure of a protein: a sequence of amino acids bonded together by peptide bonds.

From Anticodon to DNA

  • Given an anticodon (e.g., UAC), convert to mRNA, then to DNA.
  • Example:
    • Anticodon: UAC
    • mRNA: AUG
    • DNA: TAC
  • Be familiar with going in both directions: anticodon → codon → DNA triplet.

Polyribosomes

  • Polyribosomes: groups of ribosomes reading the same mRNA simultaneously.
  • This produces many polypeptides at the same time.
  • Multiple ribosomes can read the same mRNA strand until the mRNA is damaged, creating the same protein repeatedly.