3) Outline what occurs during translation. [9]
Translation is the process of protein synthesis which occurs at the ribosomes. It translates the m-RNA code that was once found in DNA into the amino acid language of polypeptides.
The ribosome (r-RNA) is made of one small and a second larger ribosomal subunit. Both of these fit onto the promoter code or initiation site on m-RNA (always the m-RNA code AUG) to begin translation.
Transfer RNA (t-RNA) picks up and carries specific amino acids to the ribosome.
The codon is the 3 nucleotide sequence on m-RNA that codes for one amino acid. The anticodon is the 3 nucleotide sequence on t-RNA that must be fit against the codon by the ribosome.
The ribosome then reads the codon on m-RNA and matches it to the complementary 3 nucleotide anticodon of t-RNA.
This matching of codon to anticodon codes allows only the amino acid specified by the original DNA to be lined up in the correct sequence.
Once the first t-RNA with its amino acid is lined up by the ribosome, it reads the next codon and places the second t-RNA and its amino acid into the empty space on the ribosome.
The two adjacent amino acids form a peptide bond and the new polypeptide is “born”.
The first amino acid is detached from the first t-RNA, which then returns to the cytoplasm to pick up the same amino acid there.
The ribosome then translocates to the next empty codon along the m-RNA. It then places the t-RNA that has the complementary anticodon on that codon. The third amino acid bonds to the second, and the polypeptide elongates (grows longer).
The ribosome keeps reading the m-RNA in the 5’-3’ direction, adding the amino acids specified by the codons.
Translation continues until the ribosome reaches a terminator code or three nucleotide sequence of m-RNA known as a stop codon. The polypeptide is released (eventually to go to the golgi apparatus to be finished and packaged in a vesicle. The ribosomal subunits disengage from the m-RNA and seek new m-RNA to begin translating.
A polysome is a piece of m-RNA that is being translated by multiple ribosomes, creating many polypeptides at a much faster rate.