The essentials
The genetic code is the relationship between the sequence of nucleotides in mRNA and the sequence of amino acids in a protein. George Gamow argued that a code of three bases would be needed for 20 amino acids, since 4³ = 64 combinations are possible. Har Gobind Khorana's chemical method of synthesising RNA with defined base combinations, and Marshall Nirenberg's cell-free protein synthesis system, helped decode it.
Features of the code:
- It is a triplet: 61 codons code for amino acids and 3 (UAA, UAG, UGA) are stop codons.
- It is unambiguous and specific: one codon codes for only one amino acid.
- It is degenerate: some amino acids are coded by more than one codon.
- It is read in a continuous fashion without punctuation (commaless).
- It is nearly universal: for example, UUU codes for phenylalanine in bacteria and humans, with some exceptions in mitochondria and some protozoans.
- AUG has a dual role: it codes for methionine and acts as the initiator codon.
tRNA is the adapter molecule. It has an anticodon loop with bases complementary to a codon, and an amino acid acceptor end. Its secondary structure looks like a clover leaf; the 3-D structure is an inverted L.
Translation: amino acids are first activated using ATP and linked to their tRNA (charging or aminoacylation). The ribosome, made of rRNA and proteins, is the cellular factory; its larger subunit has two sites for tRNAs and the rRNA in it (23S rRNA in bacteria) acts as a ribozyme to form peptide bonds. Translation begins when the small subunit binds mRNA at AUG. The ribosome moves along mRNA codon by codon, and a release factor binds the stop codon to end synthesis. mRNA also has untranslated regions (UTRs) at the 5′ and 3′ ends.
Exam tips
Count amino acids carefully: number of codons from AUG up to but not including the stop codon.