TRANSLATION – THE PROCESS OF MAKING PROTEIN FROM ITS CODON

Translation is the process of synthesizing protein using mRNA as template. It takes place in cytoplasm of cell. It is the mechanism which enables gene expression. Translation is the second step in genetic expression. The mRNA produced by transcription from DNA in the nucleus is transported into the cytoplasm where mRNA serves as template for protein synthesis.

CODON

Within the gene, the nucleotides are organized into three letter code words called codons and the collection of these codons makes up the genetic code.

Each codon is a triplet code made of a sequence of three nucleotides. Each of these codon corresponds to an individual amino acid which are the building blocks of protein. Twenty amino acids are required for the synthesis of proteins. There are 64 codons. (4³ = 64, as the genetic code is composed of triplets of four possible nucleotide bases- A, C, G, U)

61 codons encode 20 different amino acids while remaining three act as termination signal during translation, known as stop codons or nonsense codons (UAA, UAG, UGA).

KEY PROPERTIES OF CODON

• DEGENERACY – More than one codon can encode the same amino acid. For example, six different codons (UCU, UCC, UCA, UCG, AGU and AGC) code for the amino acid serine. (However, methionine and tryptophan have single codon).
• UNAMBIGUOUS – A specific codon codes for only one amino acid.
• NON OVERLAPPING – The reading of genetic code during the process of protein synthesis does not involve any overlap of codons. Codons are read in sequence as independent sets of three; a base in one codon is not used in the next.
• WITHOUT PUNCTUATION – There are no pauses between codons. The sequence is read continuously.
• UNIVERSAL – The genetic code is mostly identical across all organisms, from bacteria to humans. (Exception – four codons are read differently in mitochondria).

STEPS OF TRANSLATION

The translation of mRNA commences near its 5′ terminal with the formation of the corresponding amino terminal of the protein molecule. The message is read from 5′ to 3′, concluding with the formation of the carboxyl terminal of the protein.

The process of translation could be understood only after deciphering the genetic code. tRNA acts as adapter molecule for translation. tRNA works by recognizing specific codons by their anticodon arm and transferring codon specific amino acid to the ribosome-mRNA complex.

Anticodon of tRNA is complementary to codon on mRNA. For a give codon in the mRNA, only a single species of tRNA molecule possesses the proper anticodon. Cytoplasmic translation system has 31 tRNA species. The recognition and attachment of codon specific amino acid to tRNA is catalyzed by the enzyme, aminoacyl-tRNA synthetase.

In a tRNA, the acceptor arm is the site where specific amino acid attaches. The D arm helps in recognition of a given tRNA species by its proper aminoacyl-tRNA synthetase. The ribothymidine pseudouridine cytidine arm is involved in binding of aminoacyl-tRNA to the ribosomal surface at the site protein synthesis.

The anticodon region consists of seven nucleotides, and it recognizes the three letter codon in mRNA. Anticodon is read in 3′ to 5′ direction and genetic code is read in 5′ to 3′ direction ( they are antiparallel).

Translation occurs in three steps – initiation, elongation and termination.

INITIATION

Initiation of protein synthesis requires that an mRNA molecule be selected by a ribosome. Once mRNA binds to the ribosome, the correct reading frame on the mRNA is selected and translation begins.

• Formation of 43 S Pre-initiation complex – eukaryotic initiation factor 2 (eIF-2) binds to GTP. This binary complex then binds to met tRNA (tRNA responsible for delivering methionine). This ternary complex binds to the 40 S ribosomal subunit to form the 43 S pre-initiation complex.
• Formation of 48 S initiation complex – The methyl guanosine triphosphate cap of mRNA facilitates binding of mRNA to the 43 S pre-initiation complex to form 48 S initiation complex. The poly (A) tail stimulates recruitment of the 40 S ribosomal subunit to mRNA. The mRNA is then scanned in 5′ to 3′ direction for a suitable initiation codon. Generally, the initiator codon is AUG which codes for methionine.
• Formation of 80 S initiation complex – The 60 S subunit of ribosome associates with 40 S subunit to form 80 S initiation complex. During initiation, the met tRNA is on the P site of the ribosome.

The ribosome has three sites –

A site (Aminoacyl-tRNA binding site)- Site where new tRNA charged with amino acid enters. The tRNA binds here and its anticodon matches the mRNA codon.

P site (Peptidyl-tRNA site) – Occupied by peptidyl tRNA, the tRNA that carries the growing polypeptide chain.

E site (Exit site) – Exit site for tRNA after it has transferred the amino acid.

ELONGATION

• A new tRNA carrying an amino acid enters the A site of the ribosome. The anticodon of the incoming tRNA is matched with the codon of mRNA present on the A site.
• A peptide bond is formed between the adjacent amino acids on A and P sites.
• As the peptide bond is formed, the tRNA in the P site releases the amino acid onto the tRNA on the A site. The tRNA on P site is now empty.
• At the same time, the ribosome moves one triplet forward on the mRNA. As a result, the empty tRNA is now in the E site and the peptidyl tRNA is in the P site.
• A site is now unoccupied and is ready to accept a new tRNA.
• The cycle is repeated for each codon on the mRNA until a termination codon is reached.

TERMINATION

After multiple cycles of elongation, the stop codon of mRNA (UAA, UGA or UAG) appears in the A site. There is no tRNA with an anticodon capable of recognizing such a termination signal. Stop codon is identified by a releasing factor (RF 1), which causes hydrolysis of the bond between the polypeptide and tRNA occupying the P site. This hydrolysis releases the protein and the tRNA from the P site.

The 80 S ribosome dissociates into 40 S and 60 S subunits, which are then recycled. The mRNA is released from the ribosome.