The translation is the second step in gene expression. Gene expression occurs in 2 steps:
- Transcription: It refers to a copy of information from DNA to mRNA and it occurs in the nucleus.
- Translation: It refers to the polymerization of amino acids to form a polypeptide.
Structures Important for Translation:
- mRNA: It is responsible for carrying the information from DNA for the protein synthesis.
- Ribosomes: Site for protein synthesis.
- tRNA: Ribosomes read the codons on mRNA and tRNA transfers the specific amino acids on to ribosomes as per the sequence of codons on mRNA.
Site of Translation:
Eukaryotes—–>>Cytoplasm and across the endoplasmic reticulum
They act as the site of protein synthesis. It essentially consists of one large subunit and one small subunit.
The unit used to describe ribosomal subunit is Svedberg Unit, which is the measure of the rate of sedimentation during centrifugation.
In eukaryotes, the smaller subunit is the 40S and larger subunit is 60S and when they join together they are denoted as 80S.
These subunits lie separately in the cytoplasm until they need to come together for translation.
These subunits have different function as well, small subunit reads the codons on mRNA and large subunit joins the amino acids with the peptide bond to form a polypeptide.
Coding is basically a sequence of 3 DNA or RNA bases. When mRNA goes into ribosome it doesn’t read single cotton, but a triplet of codons that correspond to a specific amino acid.
Example—>> If the codon is UUU amino acid synthesized will be phenylalanine.
Signals the ribosome to start synthesizing proteins. Example- AUG codes for methionine
Signals the ribosome to stop synthesizing proteins. Example- UAA, UAG, UGA
Steps of Translation Mechanism:
(a) Activation of amino acids:
In the presence of enzyme aminoacyl-tRNA synthetase (E), specific amino acid (AA) binds with ATP.
(b) Charging of tRNA:
The AA1-AMP-E1 complex reacts with specific tRNA. Thus, amino acid is transferred to tRNA. As a result, the enzyme and AMP are liberated. It is also called aminoacylation of tRNA.
AA1+AMP-E1 complex+tRNA—->>AA1-tRNA+ AMP-E1
(c) Formation of polypeptide chain:
It requires 3 initiation factors in prokaryotes and 9 initiation factors in eukaryotes.
- Binding of mRNA with smaller subunit of ribosome (30S/40S).
In eukaryotes, there is formation of 40S-mRNA complex.
- Binding of 30S-mRNA with tRNA meth. non-formylated methionine is attached with tRNA in eukaryotes and formylated in prokaryotes.
30S-mRNA+tRNA fMeth+GTP—–>30S-mRNA-tRNA fMeth
- Attachment of larger subunit of ribosome. It is 50S in prokaryotes and 60S in eukaryotes.
2. Chain Elongation:
After the formation of complex riboome-mRNA-tRNA complex, an aminoacyl acceptor site (A-site) is established next to P-site. It exposes mRNA codon next to initation codon.
A new aminoacyl tRNA complex reaches A-site and forms codon-anticodon bonding. This requires elongation factors and energy i.e. GTP. A peptide bond is formed between COOH group of first amino acid and NH2 group of second amino acid.
If two charged tRNAs are close enough the formation of peptide bond between them will be favoured energetically.
The presence of a catalyst would enhance the rate of peptide bond formation.
It is catalyzed by an enzyme peptidyl transferase. The elongation factors are required in this process.
Translocation is the movement of tRNA on mRNA. The ribosome move from codon to codon along the mRNA. Amino acids are added one by one, translated into polypeptide sequence dictated by DNA and represented by mRNA.
A translated unit in mRNA is the sequence of RNA that is flanked by the start codon (AUG) and the stop codon and codes for a polypeptide.
The mRNA also has some additional sequences that are not translated and are referred to as untranslated regions (UTRs).
The UTRs are present at both 5’end before start codon and 3′ end after stop codon. They are required for the efficient translation process.
3. Chain Termination:
The termination of polypeptide is signalled by one of the three termination codons (UAA, UAG, UGA). A GTP-dependent factor known as release factor is associated with termination codon.
It is eRF1 in eukaryotes and RF1 and RF2 in prokaryotes, that helps in termination translation and releasing the complete polypeptide from ribosome.