Translation

Follows Transcription

Genetic coding

Bases are read in groups of three, which produces enough combinations to code for all 20 amino acids. In general, the third base is not important for coding, referred to as the wobble position. When translating, codons aren't reused for the next amino acid (non-overlapping).

The sequence AUG is used at the translational start site, whereas UAA, UGA, and UAG are used as the stop site.

tRNA

Carries amino acids to the ribosome. tRNA is expanded using aminoacyl-tRNA synthetase. There is a unique synthetase for every amino acid. tRNA synthetase will also proofread by destroying incorrect amino acids.

Nomenclature

A prefix will denote the amino acid is actually attached to the tRNA, such as in the case of $\text{Met-tRNA}^{Met}$. The suffix simply specifies which amino acid the tRNA is specific to.

Methionine is added at the beginning of the peptide. Its rRNA will be denoted as $tRNA_i^{Met}$, with the i noting an initiator is being used to add the first amino acid (Met). The i is dropped for amino acids added other places ($tRNA^{Met}$).

Ribosomes

28S rRNA: Constructs peptide bonds between amino acids (via peptidyl transferase)
18S rRNA: Holds mRNA in place and aligns the start codon (AUG).

Multiple ribosomes working on the same mRNA is called a polysome.

Steps

Initiation

1. eIF2 (bound to GTP) adds $\text{Met-tRNA}_i^{Met}$, matching the AUG codon
2. eIF4F binds the 5' cap, slides the initiation complex to the correct start codon
3. Large (60S) ribosomal unit completes formation of the initiation complex

Elongation

1. eF1 alpha-GTP captures the following amino acid (hydrolyzing GTP → GDP)
2. Peptide bond formed between amino acids (via 28S rRNA)
3. eEF2 translocates the mRNA, freeing up a spot for the next amino acid

Termination

Release factors recognize different stop codons. eRF1 recognizes all stop codons, assisted by eRF3. After the peptide is release to go do the lord's work somewhere, the large and small ribosomal subunit are separated by EF-G, IF-3, and ribosomal recycling factor (RRF).