Translation
Translation
is a fundamental procedure that is essential to protein synthesis in living
cells. It is the second step in the
central dogma of molecular biology, following transcription, where a DNA
sequence is transcribed into an mRNA molecule.
The mRNA
molecule, which contains the genetic information contained in its nucleotide
sequence, interacts with ribosomes, the component of the cell responsible for
protein synthesis, during translation.
Codons (sets
of three nucleotides) present on mRNA sequence that each code for a distinct
amino acid, are read by ribosomes. There are 20 different amino acids found in
proteins, and the genetic code in mRNA determines the order and type of amino
acids incorporated into the growing protein chain. Transfer RNA (tRNA)
molecules act as adapters in this process, bringing the appropriate amino acids
to the ribosome based on the mRNA codons. The ribosome catalyses the formation
of peptide bonds between neighbouring amino acids as it moves along the mRNA
strand, forming a polypeptide chain. Eventually, this polypeptide chain forms a
functional protein.
Process of translational
The
information contained in messenger RNA (mRNA) is used to create proteins during
the translational process, which is a crucial stage in gene expression. The
process of translation is divided into the following three steps:-
1) Initiation
Protein synthesis is what
triggers the translational process. This takes place in the cytoplasm of
eukaryotic cells, while in prokaryotes it takes place in the
cytoplasm as well but is coupled with transcription. Several important
participants are involved in the initiation process:-
a) Small ribosomal subunit: At the start codon, the small ribosomal
subunit attaches to the mRNA molecule. This usually corresponds to the AUG
codon in eukaryotes, which codes for the amino acid methionine.
b) Initiation Factors: These are
the proteins that help put together the ribosome and other translation-related
parts. Eukaryotic initiation factor 2 (eIF2) is one of the most significant
initiation factors in eukaryotes.
c) Initiator tRNA: This
transfer RNA molecule recognises the start codon on the mRNA and transports the
amino acid methionine (or formylmethionine in prokaryotes).
d) Big Ribosomal Subunit: After
joining the complex, the big ribosomal subunit completes the ribosome. The
peptidyl transferase centre, which is in charge of catalysing the creation of
peptide bonds between amino acids, is located in this big subunit.
2) Elongation
After
initiation, the ribosome proceeds in a 5' to 3' orientation along the mRNA
molecule, reading each codon and adding amino acids to the expanding
polypeptide chain. The following crucial processes are part of the elongation
phase:
a) Codon regulation:
Aminoacyl-tRNA molecules, each carrying a unique amino acid, enter the ribosome
and base-pair with the mRNA codon in the A-site (aminoacyl site) of the
ribosome. This process is known as codon recognition. The expanding polypeptide
chain is joined to the tRNA molecule at the P-site (peptidyl site), which is
close to the A-site.
b) Peptide bond
formation:
A peptide bond is created between the amino acid in the A-site and the
expanding polypeptide chain at the P-site by the ribosome. The polypeptide
chain is moved during this step from the amino acid on the tRNA in the A-site
to the tRNA in the P-site.
c) Translocation: Following
the synthesis of peptide bonds, the ribosome translocates one codon along the
mRNA. The tRNAs are moved from their A- and P-sites to their P- and E-sites
(exit sites), respectively, by this movement.
d) Release Factor: When a stop
codon (such as UAA, UAG, or UGA) is found on the mRNA, it signifies the
completion of translation rather than coding for an amino acid. In order to
facilitate the release of the finished polypeptide chain from the tRNA in the
P-site and the separation of the ribosome from the mRNA, a release factor
protein binds to the A-site.
3) Termination
The translational process comes to a halt at
this point. A release factor enters the A-site and aids in the release of the
freshly synthesised protein when a stop codon is met. The mRNA is then released
as the ribosome splits into its big and tiny subunits.
4) Post-Translational changes
To become a
functional protein, the freshly synthesised polypeptide chain may go through a
number of post-translational changes after translation. These alterations may
consist of:
a) Folding: Many
proteins need to be folded correctly in order to form their useful
three-dimensional structures. In this process, chaperone proteins may help to
ensure that the protein adopts the proper shape.
b) Cleavage: Some
proteins must be split into their active forms after being synthesised as
bigger precursor molecules. These precursor proteins are broken down at
particular locations by enzymes referred to as proteases.
c) Adding Functional
Groups:
Proteins may have particular functional groups or chemical alterations, such as
phosphorylation or glycosylation, added to them. These modifications might
affect the activity and location of the protein inside the cell.
5) Protein Transport and
Localization
Proteins are
frequently transported to their particular cellular sites following
post-translational modifications. This process may involve dispersal to various
cellular compartments or transport into organelles such as the nucleus,
mitochondria, or endoplasmic reticulum.
6) The role of proteins
The protein
can perform its precise duties inside the cell or organism once it has been
correctly folded, changed, and localised. A variety of biological functions,
such as enzymatic reactions, structural support, signaling, and molecular
transportation, depend on proteins.
7) Regulation
The
translational process is tightly controlled on many different levels. Cells can
regulate the rate of protein synthesis by controlling the availability of
ribosomes, tRNAs, and initiation factors. In addition, it is possible to
regulate the accessibility and stability of the ribosome-binding site of mRNA.
Protein levels and activity are also regulated by post-translational changes
and protein breakdown.
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