DNA by means of its correct method of replication serves to carry genetic information from cell to cell and from generation to generation. The information is converted into proteins which find out the phenotype. Protein synthesis comprises how the information presents in the sequences of bases (that is, triplet codons) of the mRNA is converted into a sequence of amino acids in the proteins.
The Central Dogma:
Genes are the instructions for forming specific proteins. However a gene doesn't build a protein directly. The bridge between genetic information and protein synthesis is the RNA. The method of synthesis of protein comprises one of the central dogma of molecular biology; that postulates that genetic information flows from nucleic acids to protein. The primary step of the central dogma is termed as transcription and doesn't comprise a change of code since DNA and mRNA are complementary. The subsequent step comprises a change of code from nucleotide sequences to amino acid sequences and is termed as translation described as follows:
Duplication -> DNA -> (Transcription) -> RNA -> (Translation) -> Protein
Method of Protein Synthesis:
Protein synthesis is a much complex biochemical transformation functioned by cells resultant in the formation of a polypeptide chain. The method of protein synthesis can be splitted into the following three main steps: chain initiation, chain elongation and chain termination. All the three steps need protein factors (around 200 different proteins) generally enzymes which aid in mRNA, tRNA and ribosomes in the translation process. Chain initiation and elongation need energy generally given by GTP (that is, guanosine triphosphate), a molecule which is closely associated to ATP.
1) Chain Initiation:
The initiation phase brings altogether mRNA a tRNA bearing the first amino acid of the polypeptide chain and the two subunits of the ribosome. At first a small ribosomal subunit combines or binds to both mRNA and a special initiator tRNA. The small ribosomal subunit joins to the end of the mRNA (loading site). Downstream from the loading site is the initiation codon, AUG, where translation actually starts. The initiator tRNA that carries the amino acid methionine, joins to the initiation codon. The union of mRNA, imitator tRNA and small ribosomal subunit is followed by the attachment of a big ribosomal subunit to form a functional ribosome. Proteins called imitation factors are needed to bring these components altogether. The cell as well spends energy in the form of one GTP to form the initiator complex. At the completion of the initiation procedure, the initiator tRNA sits in the P site of the ribosome, and the vacant A site is ready for the subsequent tRNA.
2) Chain Elongation:
The elongation phase amino acids are added one by one to the initial amino acid attached by the peptide bond. Each addition comprises the participation of some proteins termed as elongation factors. The entire process takes place in a three-step cycle.
a) Codon Recognition:
The mRNA codon in A site of the ribosome forms hydrogen bonds having the anticodon of an incoming molecule of tRNA carrying its proper amino acid. An elongation factor employs the tRNA into the A site. This step needs the hydrolysis of a phosphate bond from GTP.
b) Peptide Bond Formation:
A component of the big ribosomal subunit catalyzes the formation of a peptide bond among the polypeptide extending from the P site and the newly arrived amino acid in the A site. In this step, the polypeptide separates from the tRNA to which it was bound and is transferred to the amino acid taken out by the tRNA in the A site.
The tRNA in the P site dissociates from the ribosome. The tRNA in A site, now joined to the growing polypeptide, is translocated to the P site. As the tRNA modifies sites, its anticodon remains hydrogen-bonded to the mRNA codon, let the mRNA and tRNA molecules to move as a unit. This movement brings the subsequent codon to be translated into the A site. The translocation step needs energy that is provided by hydrolysis of a GTP molecule.
3) Chain Termination:
The final phase of translation is the termination. Elongation carries on till a termination codon reaches the A site of the ribosome. Nonsense base triplets - UAA, UAG and UGA - don't code for amino acids however rather act as signals to stop the translation.
A protein termed as a release factor joins directly to the termination codon in the A site. The discharge factor causes the ribosome to add a water molecule rather than an amino acid to the polypeptide chain. This reaction hydrolyses the completed polypeptide from the tRNA that is in the P site, thus freeing the polypeptide from the ribosome. The ribosome then separates to its small and big subunits.
From Polypeptide to Functional Protein:
Throughout and after its synthesis, a polypeptide chain starts to coil and fold spontaneously, making a functional protein of specific conformation: a three-dimensional molecule having secondary and tertiary structures. A gene finds out primary structure and primary structure in turn finds out the conformation.
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