Translation and Protein Synthesis

TRANSLATION AND PROTEIN SYNTHESIS

The process of protein synthesis is called translation. There are 20 amino acids that together make all the proteins of life. Each is associated with a triplet of base pairs, called a codon. There are sixty-four possible combinations of base pairs so there are 64 codon types. A total of 61 of them code for amino acids, while the rest are either stop codons or start codons. The codon AUG is a start codon. Because there are 20 amino acids, there is more than one codon for the same amino acid. Essentially all forms of life have the same codons matching for the same amino acids.

Ribosomes are made from ribosomal RNA and protein material. The different organisms have structurally different ribosomes. Prokaryotes are said to have a 70S ribosome, while eukaryotes are said to have an 80S ribosome. There are two subunits. The small subunit binds messenger RNA, while the large subunit binds transfer RNA.

The messenger RNA is read from the five-prime end to the three-prime end, making a protein that starts with an amino group and ends with a carboxyl group. A polyribosome is a ribosome plus a messenger RNA segment. In prokaryotes, translation and transcription happen at the same time. This means that prokaryotes can respond quickly to environmental signals.

Transfer RNA is a special type of RNA that specifically helps to extend and elongate the polypeptide chain. It has a unique three-dimensional shape. Figure 46 shows what transfer RNA looks like:

Figure 46.

The shape is made by different base pairs connecting to one another in the same strand. It allows for positioning of the amino acid binding site. The anticodon is what binds to the messenger RNA codon in order to align these segments. Transfer RNA is either charged with an amino acid or it is not charged. There are enzymes that connect the amino acid to the transfer RNA called aminoacyl transfer RNA synthetases.

There are similarities between the translation process of prokaryotes and eukaryotes. Initiation starts with the making of an initiation complex. This requires GTP as an energy source. Initiation starts with the start codon, which is AUG. There is a specialized amino acid added to every protein at the time of initiation, called methionine.

Figure 47 shows what translation looks like:

Figure 47.

Elongation is the same in all organisms. Peptide bonds are formed to a growing peptide chain. The charged transfer RNAs enter the A site in the ribosome, get moved over to the P site, and leave at the E site or exit site. Peptidyl transferase is the enzyme that makes the peptide bond. Termination happens when a stop codon or nonsense codon is reached. This releases the peptide chain and the process starts all over again.