Question

Discuss the process of translation in detail.

Answer 1

Translation is the process of protein synthesis i.e. the process of polymerisation of amino acids to for the polypeptide known as protein.

The process is divided into three steps:

1. Initiation, 2. Elongation, 3. Termination

In this process there are two important rules:

1. mRNA is translated 5’ to 3’
2. proteins are synthesize from the N-terminus to the C-terminus

1. Process of Initiation:

• In this phase of protein synthesis, a functional ribosome assembled to mRNA, which has been positioned correctly so that its start codon is positioned in the P site and is paired with the initiator tRNA.
• The following factors are essential for the process:

1. m RNA
2. two ribosomal subunits: 30S and 50S
3. three initiation factors: IF-1, IF-2 (GTP) and IF-3
4. initiator Met-tRNAiMet

• The process always starts from Met (methionine).
• IF-3 promotes the dissociation of ribosomes into its two subunits. The presence of IF-3 permits the assembly of the initiation complex.
• IF-1 assists IF-3, which increases the dissociation rate of 30S and 50S subunits of ribosomes.
• At P site start codon is correctly positioned by IF-3, which assists mRNA to bind with 30S subunit of ribosome. So that the base-pairing between the 3’ end of 16S rRNA with the Shine-Dalgarno sequence, which bring ribosome to the initiation codon.
• IF-3 can dissociate after the 30S initiation complex is complete.
• As IF-3 is released, the 50S subunit of the ribosome binds to the complete the initiation complex.
• During this process, GTP hydrolysis occurs on IF-2, which is required for the dissociation of IF-2.
• After IF-2 and IF-1 are released, the process of translation can proceed further.

2. Process of Elongation:

• A ribosome contains two major tRNA binding site A and P.
• There are three elongation factors: EF-Tu (GTP), EF-Ts and EF-G (GTP).
• This phase is a cyclic phase, where new aminoacyl-tRNA is positioned in ribosomes, the amino acid is transferred to the C-terminus of the growinf peptide chain and whole assembly moves further with ribosome.
• The elongation factor, EF-Tu (GTP) binds to aminoacyl-tRNA and brings it to ribosome.
• As the correct aminoacyl-tRNa is positioned, GTP is hydrolyzed and EF-Tu (GDP) dissociates from the ribosome.
• EF-Tu (GDP) is an inactive but has a higher affinity for GDP than GTP.
• In order to recycle EF-Tu, the elongation factor EF-Ts binds to the EF-Tu (GDP) complex to displace GDP.
• At P site polypeptide chain grows by the attachment to the tRNA which is paired at A site. The new amino acid is added to the C-terminal end of the polypeptide chain.

3. Process of termination:

• This phase is also known as translocation phase, where ribosomes are translocated along the mRNA.
• This step requires the elongation factor EF-G(GTP).
• Here GTP is hydrolyzed during this process which is required for the dissociation of EF-G.
• EF-G blocks the binding of aminoacyl tRNA to A site as well as the binding of releasing factors.