6.4 Translation

Translation takes place on ribosomes. In both prokaryotes and eukaryotes, free ribosomes sit in the cytoplasm. Eukaryotes also have ribosomes attached to the cytoplasmic side of the rough endoplasmic reticulum (ER); those make proteins that will be sent to the membrane, to other organelles or out of the cell. In prokaryotes, which have no nucleus, ribosomes can start translating an mRNA while it's still being transcribed.

The mRNA is read in triplets called codons. With four bases there are 4³ = 64 possible codons. Sixty-one of them code for amino acids, and three (UAA, UAG and UGA) are stop codons. AUG is the start codon and also codes for methionine, so most new proteins begin with methionine.

There are only 20 amino acids, so many amino acids are coded by more than one codon. For example, both GAA and GAG code for glutamic acid. This redundancy is why some mutations don't change the protein (6.7).

You don't need to memorize the codon table. On the exam you'll be given one; you only need to know that AUG is the start codon.

The names of the proteins and factors that run each step are beyond the scope of the exam. Focus on the order of events and what each RNA does.

• Initiation: the small ribosomal subunit binds the mRNA, and the ribosome's rRNA helps line the mRNA up so that reading begins at the start codon, AUG. A tRNA carrying methionine pairs with AUG, and the large subunit joins.
• Elongation: the ribosome reads the next codon. A tRNA whose anticodon is complementary to that codon brings its amino acid. For example, the codon 5′-GGA-3′ pairs with the anticodon 3′-CCU-5′. The ribosome links the new amino acid to the growing chain with a peptide bond, then shifts one codon along the mRNA.
• Termination: when a stop codon enters the ribosome, no tRNA matches it. The finished polypeptide is released and the ribosome comes apart.

Bacteria, plants, fungi and animals nearly all use the same codon assignments. That's why a human insulin gene placed into bacteria can be translated into working human insulin. Such a shared, arbitrary code is hard to explain unless all organisms inherited it from a common ancestor, so it's strong evidence for common ancestry.

Usually information flows DNA → RNA → protein. Retroviruses, such as HIV, are the special case. Their genome is RNA, and they carry an enzyme called reverse transcriptase that copies the RNA genome into DNA. That viral DNA is inserted into the host cell's genome. The host then transcribes and translates it like its own genes, producing viral RNA genomes and proteins that assemble into new viruses. Because the viral DNA becomes part of the host genome, the infection can't simply be cleared, and drugs that block reverse transcriptase are one way to treat HIV.