7.3.1 Translation

Initiation of translation involves assembly of the components that carry out the process.

• During translation, the specific sequence of messenger RNA (mRNA) is translated to produce a polypeptide chain consisting of amino acids
  • mRNA is a single stranded, linear, RNA molecule that transfers the information in DNA from the nucleus into the cytoplasm
• Translation is categorised into three stages: initiation, elongation and termination
• Translation occurs in the cytoplasm at complex molecules made of protein and RNA called ribosomes
  • Ribosomes have a two-subunit (large and small) structure that helps bind mRNA
  • Ribosomes have three tRNA binding sites termed “E” (exit), “P” (peptidyl) and “A” (aminoacyl)
    • At the A site the mRNA codon joins with the tRNA anticodon
    • At the P site the amino acids attached to the tRNA are joined by peptide bonds
    • At the E site the tRNA exits the ribosome
• Another key molecule in translation is transfer RNA (tRNA) that decodes mRNA
  
  • tRNA molecules are single stranded RNA molecules that fold to form a clover-shaped structure
    • The folded structure is held together by hydrogen bonds between bases at different points on the strand
    • tRNA molecules are the shortest of the RNA molecules, being only around 80 nucleotides in length
    • There are 20 different types of tRNA molecule, one for each of the amino acids involved in protein synthesis
  • tRNA molecules have a region that binds to a specific amino acid as well as a three-nucleotide region called an anticodon that is complementary to the codon on mRNA
  • The role of tRNA molecule is to carry a specific amino acid to the ribosome

• In eukaryotic cells, the mRNA molecule leaves the nucleus through the nuclear pores
• Translation is initiated by the following process
  • A small ribosomal subunit attaches to the 5’ end of mRNA
  • An initiator tRNA molecule carrying the amino acid methionine binds to the small ribosomal subunit
    • The initiator tRNA occupies the “P” site on the ribosome
  • The ribosome moves along the mRNA until it locates a start codon (AUG)
  • The large ribosomal subunit binds to the small subunit
    • Elongation of the polypeptide can begin

• The initiator tRNA currently occupies the “P” site, the next codon on the mRNA signals for the corresponding tRNA to bind at the “A” site
  • The two amino acids (attached to the tRNAs) are linked with a peptide bond, forming a dipeptide
• Synthesis of the peptide chain now involves a repeated cycle of events
  • In the cytoplasm, free tRNA molecules bind to their corresponding amino acids and transport them to the ribosome
  • The ribosome shifts along the mRNA one codon (three bases) at a time
    • The initiator tRNA in the “P” site moves to the “E” site which releases it
    • The tRNA carrying the peptide chain moves from the “A” site to the “P” site
    • The next mRNA codon is exposed and a tRNA with the complementary anticodon binds to the unoccupied “A” site whilst its amino acid is linked to the polypeptide chain
• The cyclical process is repeated as new amino acids are added to the growing chain

• The process of elongation continues until one of three ‘stop’ codons (UAA, UAG and UGA) on the mRNA molecule is reached
  • Stop codons do not code for a tRNA molecule but act as a signal for translation to stop
• The polypeptide chain and mRNA are released from the ribosome
• The ribosome disassembles back into two separate subunits
  • And can await the arrival of the next mRNA molecule

Following the initiation of protein synthesis, translation involves a repeated cycle of events to build the polypeptide chain, tRNA molecules move into the A, P and E sites as the ribosome reads the mRNA

• Amino acids are paired to specific tRNA molecules through the action of tRNA-activating enzymes
  • Each tRNA activating enzyme recognises a specific tRNA molecule
• tRNA-activating enzymes, in common with most enzymes, are substrate-specific and recognise the correct tRNA molecules by their shape
  • Nucleotide sequence variability between tRNA molecules results in variation in their three-dimensional structure
  • Active sites of tRNA-activating enzymes are optimised to bind a specific tRNA
• Initially, a tRNA-activating enzyme binds to ATP and a specific amino acid
• The active site of the enzyme attracts a conformationally-specific tRNA molecule
• The tRNA molecule is bound to the amino acid using ATP (phosphorylation) to create a high energy bond
  • The stored energy in this bond will be used later in peptide bond formation to link the amino acid to the growing polypeptide chain
  • This is an example of how an anabolic reaction like protein synthesis utilises the energy stored in ATP
• A tRNA molecule with an amino acid attached is called a charged tRNA

Specific tRNA-activating enzymes are involved in charging an amino acid to a specific tRNA molecule