Nucleic Acids (AHL) - Translation

Translation is the process of protein synthesis where the genetic information encoded in mRNA is translated into a sequence of amino acids. It occurs in the $5' \rightarrow 3'$ direction.

Ribosomes consist of a small and a large subunit, made of RNA and proteins. They contain three binding sites for tRNA: the $A$ (aminoacyl) site, the $P$ (peptidyl) site, and the $E$ (exit) site.

tRNA activation involves the enzyme tRNA activating enzyme (aminoacyl-tRNA synthetase), which attaches a specific amino acid to the $3'$ end of the tRNA using energy from $ATP$.

Initiation of translation involves the assembly of the mRNA, the first tRNA (carrying methionine, codon $AUG$), and the ribosomal subunits.

Elongation is a repeated cycle where new aminoacyl-tRNAs bind to the $A$ site, a peptide bond forms between amino acids, and the ribosome translocates along the mRNA.

Termination occurs when a stop codon ($UAA$, $UAG$, or $UGA$) is reached. A release factor fills the $A$ site, leading to the disassembly of the translation complex.

In prokaryotes, translation can occur immediately after transcription because there is no nuclear envelope separating the processes, often forming polysomes (multiple ribosomes on one mRNA).

Protein structure levels: Primary (amino acid sequence), Secondary ($\alpha$-helices and $\beta$-pleated sheets via $H$-bonds), Tertiary ($3D$ folding via $R$-group interactions), and Quaternary (multiple polypeptide chains).