Decoding Protein Synthesis: tRNA, Ribosomes, and Translation Steps

Translation: The Core of Protein Synthesis

The transfer RNA (tRNA) possesses a sequence complementary to a specific messenger RNA (mRNA) triplet. This recognition sequence is simply called the anticodon. No tRNA recognizes a triplet; rather, it must carry its corresponding amino acid. The binding of each amino acid to its tRNA is catalyzed by an enzyme called aminoacyl-tRNA synthetase. This enzyme binds to a specific section of the tRNA and, with the expenditure of energy (ATP), incorporates the amino acid, renaming the tRNA as aminoacyl-tRNA.

There are 20 essential amino acids, requiring at least 20 different tRNAs and an equal number of aminoacyl-tRNA synthetases. These adapters convert the mRNA sequence into a protein. The genetic code is translated sequentially through decoders acting on the mRNA. The union of mRNA and tRNA must relate them somehow. The ribosome has three small subunit binding sites for RNA: two for tRNA and one for mRNA.

Phases of Protein Synthesis (Translation)

Translation involves three main phases:

1. Initiation

The small ribosomal subunit binds to the 5' end of the mRNA, positioning the start codon. The P site is occupied by the complementary aminoacyl-tRNA (UAC) carrying Methionine (Met) in eukaryotes or formylMethionine (fMet) in prokaryotes. The entire complex (mRNA, initiator tRNA, and small subunit) forms. The large ribosomal subunit then joins this complex. This phase is catalyzed by proteins called initiation factors.

2. Elongation

This phase has three sub-phases:

1. The next mRNA codon is exposed at the A site (Aminoacyl site), where the corresponding aminoacyl-tRNA is placed.
2. Peptidyl transferase catalyzes the mechanism where the amino acid located in the P site is separated from its tRNA and forms a peptide bond with the amino acid incorporated at the A site. This enzyme is actually the large ribosomal subunit rRNA possessing catalytic activity.
3. While the aminoacyl-tRNA in the A site shifts to the P site, the tRNA in the E site (Exit site) is ejected. The ribosome moves exactly three nucleotides along the mRNA, leaving a new codon at the A site, allowing the cycle to begin anew. Different elongation factors act as catalysts in this phase.

3. Termination

A stop codon is encountered at the A site. If no complementary tRNA is present, a release factor binds to the A site, catalyzing the separation of the polypeptide chain. Subsequently, the mRNA is separated from the ribosome.

The translation of a given mRNA by numerous ribosomes acting simultaneously implies that once the first ribosome moves about 80 nucleotides from the start codon, a new ribosome initiates translation. This results in a structure called a polyribosome or polysome, consisting of an mRNA strand bound to numerous ribosomes.