Protein Synthesis Process

Protein Synthesis

• Occurs in a large macromolecular assembly called the ribosome, composed of many proteins and several RNA molecules
• Involves tRNAs that interact with mRNA and are charged with one of the 20 common amino acids by 20 different aminoacyl tRNA synthetases
• Requires a dozen or more additional protein factors for initiation, elongation, and termination of translation
• Involves perhaps 100 additional enzymes that carry out a variety of modifications with many proteins

Crick's Adaptor Hypothesis

• Proposed by Francis Crick in the early 1950s, recognizing that protein sequence is encoded by the nucleotide sequence
• Suggests that the template is RNA (maybe rRNA), and the amino acid is carried to the template by an adaptor
• Adaptors may contain nucleotides

Properties of the Genetic Code

• Composed of nucleotide triplets (three nucleotides)
• Non-overlapping, meaning each nucleotide is part of only one codon
• Comma-free, meaning there are no punctuation marks
• Ordered, meaning similar amino acids are specified by related codons
• Contains start and stop signals
• Nearly universal, meaning the same code is used in all organisms
• Degenerate, meaning there is redundancy but no ambiguity

Codons

• Written in the 3' → 5' direction
• The 3nt base is less important
• Only Met and Trp have a single codon
• Some codons are better than others because of the abundance of such tRNAs

mRNA Codons and tRNA Anticodons

• The codon sequence is complementary with the anticodon sequence
• The codon in mRNA base pairs with the anticodon in tRNA via hydrogen bonding
• The alignment of two RNA segments is antiparallel

The Genetic Code

• A non-overlapping code, with each amino acid, plus polypeptide initiation and termination, specified by RNA codons, composed of three nucleotides
• Each of the 20 amino acids in proteins is specified by one or more nucleotide triplets in mRNA
• Of the 64 possible triplets, 61 specify amino acids and 3 signal chain termination
• The code is non-overlapping, with each nucleotide part of a single codon