6.4 Translation

Explain how the phenotype of an organism is determined by its genotype.

• T ranslation of the mRNA to generate a polypeptide occurs on ribosomes that are present in the cytoplasm of both prokaryotic and eukaryotic cells, as well as the cytoplasmic surface of the rough ER of eukaryotic cells.
• In prokaryotic organisms, translation of the mRNA molecule occurs while it is being transcribed.
• T ranslation involves many sequential steps, including initiation, elongation, and termination. The salient features of translation include:
  • i. T ranslation is initiated when the rRNA in the ribosome int eracts with the mRNA at the start codon (AUG, coding for the amino acid methionine).
  • ii. The sequence of nucleotides on the mRNA is read in triplets, c alled codons.
  • iii. Each codon encodes a specific amino acid, which can be deduc ed by using a genetic code chart. Many amino acids are encoded by more than one codon.
  • iv. Nearly all living organisms use the same genetic code , which is evidence for the common ancestry of all living organisms.
  • Exclusion: The details and names of the enzymes and factors involved in each of these steps are beyond the scope of the AP Exam. Memorization of the genetic code, with the exception of the start codon AUG, is beyond the scope of the AP Exam.
• Genetic information in retroviruses is a special case and has an alternate flow of information: from RNA to DNA, made possible by reverse transcriptase, an enzyme that copies the viral RNA genome into DNA. This DNA integrates into the host genome and is transcribed and translated for the assembly of new viral progeny.