The central dogma illustrates the flow of genetic information in cells, the DNA replication, coding for the RNA through transcription process and further RNA codes for the proteins by translation.
Since the new DNA strands formed, have one strand of the parent DNA and the other is newly synthesized, this process is called semiconservative DNA replication.
Transcription is the process by which the information is transferred from one strand of the DNA to another strand of the RNA. The DNA strand which undergoes this process consists of three parts namely promoter, structural gene, and a terminator.
- The strand which synthesizes the RNA is called the template strand and the other strand is called the coding strand. The DNA-dependent RNA polymerase gets bind to the promoter and catalyzes the polymerization in the direction of 3′ to 5′ on the template strand.
- As it approaches the terminator sequence, it terminates and releases the newly synthesized RNA strands.
- The newly released RNA strands undergo post-transcriptional modifications.
Genetic code contains the information of the protein manufactured from RNA. We basically have three nucleotides and four nitrogenous bases collectively form a triplet codon that codes for one amino acid. Therefore, the number possible amino acids range to 4 x 4 x 4 = 64 amino acids. But we have 20 naturally existing amino acids. This was explained by the features of the genetic code, as per which some amino acids are coded by more than one codon thus making them degenerate. Each codon codes for only one specific amino acid and the codes are universal irrespective of the type of organism. Out of the 64 codons, 3 are stop codons which stop the process of transcription and one of the codons is an initiator codon i.e. AUG coding for Methionine.
Translation is the process by which the mRNA codes for a specific protein. It is an active process which requires the expense of ATP. This energy is provided by the charged tRNA. The complete machinery for translation is contained in the ribosomes. The ribosomes consist of a larger sub-unit and a smaller subunit. The larger sub-unit, in turn, consists of two tRNA molecules placed close enough so that peptide bond can be formed at the expense of enough energy. The mRNA enters the smaller sub-unit which is then held by the tRNA molecules of the complementary codon present in the larger subunit. Thus, two codons are held by two tRNA molecules placed close to each other and a peptide bond is formed between them. As this process repeats, long polypeptide chains of amino acids are synthesized.
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