tRNA (Transfer RNA) in Protein Synthesis

tRNA Structure and Function

• tRNA is a single-stranded molecule with bases U, A, C, and G.
• It has two attachment sites: one for amino acids and one for anti-codons.
• tRNA acts as an adapter molecule, linking amino acids to their specific codons in mRNA.
• Aminoacylation of tRNA is the first step in protein synthesis.
• tRNA is specific to each amino acid.

Ribosome Structure

• Prokaryotic (bacterial) ribosomes have a sedimentation constant of 70S (2.7×10^6 Daltons) with small subunit 30S and large subunit 50S.
• Eukaryotic ribosomes have 80S ribosomes (4×10^6 Daltons) with small subunit 40S and large subunit 60S.
• Ribosomes have two subunits: small and large.

Protein Synthesis (Translation)

• Translation is the process of converting mRNA into an amino acid chain.
• DNA stores the information for proteins in its nucleotide sequence.
• There are three steps to translation: initiation, elongation, and termination.

Initiation

• The small subunit of the ribosome attaches to the 5' end of the mRNA strand.
• The small subunit reads the mRNA nucleotides in groups of three (codons) until it runs into the start codon (AUG).
• The ribosome recruits the large ribosomal subunit and the transfer RNA (tRNA).
• Key ingredients for translation to start: ribosome, mRNA with codon, and tRNA carrying the first amino acid (methionine).

Elongation

• The complete ribosome travels along the mRNA strand and reads the nucleotides in groups of three (codons).
• The ribosome grabs tRNAs from the surrounding areas and matches their anticodons to the respective codons.
• Amino acids are joined together by peptide bonds, and the tRNAs leave.
• Elongation always goes from the 5' end of the mRNA molecule towards the 3' end.

Termination

• The third step of translation is when the ribosome runs into a stop codon (UAA, UAG, or UGA).
• The ribosome places the last amino acid and severs the protein.
• The ribosome breaks off from the mRNA strand.
• There are three places on the ribosome where tRNAs bind: A, P, and E site.

tRNA Binding

• tRNAs bind to mRNAs inside the ribosome.
• As tRNAs enter slots in the ribosome and bind to codons, their amino acids are linked to the growing polypeptide chain in a chemical reaction.
• The end result is a polypeptide whose amino acid sequence mirrors the sequence of codons in the mRNA.