Question

Describe the steps (Initiation, Elongation and Termination) involved in translation of mRNA to generate a protein, including the all the important molecules involved and how they interact. Diagrams MUST be included in your answer. (Draw on some paper, then photograph and insert the drawing below.) You may add to your answer using bullet points if you find it easier, but make sure they are in the correct order!

Answer 1

Translation is a process by which the genetic code contained within an mRNA molecule is decoded to produce the specific sequence of amino acids in a polypeptide chain.

The key components required for translation are mRNA, ribosomes, tRNA and aminoacyl-tRNA synthetases.Each tRNA molecule possesses an anticodon on the opposite end that is complementary to the mRNA codon. tRNA molecules are therefore responsible for bringing amino acids to the ribosome in the correct order ready for polypeptide assembly.Aminoacyl-tRNA synthetases are enzymes that link amino acids to their corresponding tRNA molecules. The resulting complex is charged and is referred to as an aminoacyl-tRNA.

It occurs in the cytoplasm following transcription and, like transcription, has three stages: initiation, elongation and termination.Let us now briefly understand each stages:

Initiation

During initiation, the mRNA, the tRNA, and the first amino acid all come together within the ribosome. The mRNA strand remains continuous, but the true initiation point is the start codon, AUG.The start codon is the set of three nucleotides that begins the coded sequence of a gene , also the start codon specifies the amino acid methionine. So, methionine is the name of the amino acid that is brought into the ribosome first.Methionine get itself to the ribosome by attaching to the tRNA that contains the right anticodon. The anticodon for AUG is UAC. We know that because of the rules of complementary base pairing. The tRNA with the anticodon UAC will automatically match to the codon AUG, bringing the methionine along for the ride. So, the mRNA is attached to tRNA, and tRNA is attached to methionine. At the 5’ cap of mRNA the small 40s subunit of the ribosome (with methionyl-tRNA) binds. For translation to start the start codon 5’AUG must be recognised. This is a codon specific to the amino acid methionine (anticodon on tRNA=5’CAU). The large 60s subunit of the ribosome then binds for elongation to occur.The ribosome has two tRNA binding sites; the P site which holds the peptide chain and the A site which accepts the tRNA.

Diagram :

Complex assembles on the 5' cap structure. Met-tRNA AUG LURRALDAJAURLAUALDAD-DAUDAU-LAURU-URO-THUAHUAHUAHULUPUHURU ATP 40s Ribosome Subunit ADP + P: Complex scans mRNA to find the start codon. ԱԿՍԱՀՄԱՆԱՊՄՀ-Սել-ՄԷՆ ՄԷ:Այն P Site A Site 60s Ribosome Subunit "Մ ՍԿՍ-ՍԱՄՎUA, HUAN ՍԿՍ-ՍԿ Initiation complex, with tRNAmet in the P site

Elongation

It is the addition of amino acids by the formation of peptide bonds. Elongation is just what it sounds like: a chain of amino acids grows longer and longer as more amino acids are added on. This will eventually create the polypeptide. While Met-tRNA occupies the P site, another aminoacyl-tRNA with an anticodon complementary to the next codon comes to occupy the A site. This process requires GTP. The enzyme peptidyl-transferase forms a peptide bond between methionine and the next aminoacyl-tRNA.The tRNA molecule in the P site becomes uncharged and leaves the ribosome. The ribosome then translocates along the mRNA molecule to the next codon. This opens up the A site for the next aminoacyl-tRNA. The polypeptide chain is built up in the direction from the N terminal to the C terminal.

Diagram:

Growing peptide chain Phe Lys Asp TRNA Incoming tRNA ONA bound to Amino Acid Outgoing empty tRNA ACC TRNA TRNA uuu laua UGGA AAGAUTU UC MessengerRNA Ribosome Peptide Synthesis

Termination

In this step one of the three stop codons enters the A site. No tRNA molecules bind to these codons so the peptide and tRNA in the P site become hydrolysed releasing the polypeptide into the cytoplasm.The small and large subunits of the ribosome dissociate ready for the next round of translation.The polypeptide produced undergoes post-translational modification before becoming the fully active protein.

Diagram:

new protein peptidyl transferase amino acid charged tRNA - mmmm new protein Release factor