Translation

• what happens in translation?

  in translation, the sequence of codons on the mRNA is used to generate a specific sequence of amino acids, forming a polypeptide - it takes place on a ribosome and involves tRNA
• what are ribosomes made up of?

  2 subunits
• what does the larger subunit of a ribosome do?

  it has 2 sites for the attachment of tRNA molecules, so the 2 tRNA molecules are associated with a ribosome at any one time
• what does the smaller subunit of a ribosome do?

  it binds to the mRNA
• what does the ribosome do in translation?

  • acts as a framework moving along the mRNA and holding the codon-anticodon complex together, until the 2 amino acids attached to adjacent tRNA molecules bind
  • the ribosome moves along the mRNA, adding one amino acid at a time, until the polypeptide chain is assembled
  • the order of bases in the DNA has determined the order of amino acids in the polypeptide
• what are the 3 steps of translation?

  • initiation
  • elongation
  • termination
• what happens at the initiation stage of translation?

  • a ribosome attaches to a ‘start’ codon at one end of the mRNA molecule
  • the first tRNA, with an anticodon complimentary to the first codon on the mRNA, attaches to the ribosome-the 3 bases of the codon on the mRNA bond to the 3 complimentary bases of the anticodon on the tRNA, with hydrogen bonds
  • a 2nd tRNA, with an anticodon complimentary to the 2nd codon on the mRNA, attaches to the other attachment site and the codon and anticodon bond with the hydrogen bonds
• what happens at the elongation stage of translation?

  • the 2 amino acids are sufficiently close for a ribosomal enzyme to catalyse the formation of a peptide bond between them
  • the 1st tRNA leaves the ribosome, leaving its attachment site vacant - it returns to the cytoplasm to bind to another copy of its specific amino acid
  • the ribosome moves one codon along the mRNA strand
  • the next tRNA binds
• what happens at the termination stage of translation?

  • the sequence repeats until a ‘stop’ codon is reached
  • the ribosome-mRNA-polypeptide complex separates
• how many ribosomes bind to a single mRNA strand (translation)?

  several, each reading the coded information at the same time - this is called a polysome
• what is a polysome?

  a group of ribosomes bound to an mRNA molecule like ’beads’ on a ‘thread’
• what do the tRNA binding sites hold (translation)?

  • one holds the growing polypeptide
  • the other holds the tRNA carrying the next amino acids in the sequence
• what is this a diagram of?

  translation