DNA: Replication, Transcription and Translation
- DNA replication
- The process of producing identical replicas of DNA from original DNA molecule in known as DNA replication.
- It is the most important part of inheritance and occurs in all living organisms.
- DNA consist of two strands which are complementary to each other during the process of DNA replication these tow strand get separated and serve as template for the synthesis of new complementary strand this is known as semiconservative replication.
- After cellular proofreading and error checking mechanism new DNA molecule is synthesized with one strand from parent DNA molecule and another is fresh.
- Each cell of a human being contains 3 X 109 base pairs of DNA distributed over 23 pairs of chromosomes.
- In human genome 95% region do not code for any protein known as introns.
- According to central dogma DNA makes RNA and RNA makes protein.
- The process of producing RNA from DNA is called transcription and synthesis of protein from RNA is called translation
- DNA replication
- When cell divides, the double stranded DNA splits into single strand and makes new copy of the genome which gets transferred into each cell.
- Base pairing is according to the Watson-Crick paring of the DNA strand.
- It is a complex process which includes array of enzymes.
- Synthesis of DNA proceeds in the 5’-3’ direction and for attachment of DNA polymerase first DNA strand has to be unwind which is regulate by the helicase enzyme.
- During the synthesis of the new strands by the DNA polymerase one strand is synthesized continuously and called leading strand on the other hand the opposite strand known as lagging strand synthesised in to the small fragments called okazaki fragments.
- In bacteria there are three distinct DNA polymerase: pol I, pol II and Pol III. Pol III largely involve in chain elongation. DNA polymerase itself cannot initiate DNA synthesis it require a short primer with free 3’ hydroxyl group.
- Once the primer attaches to the DNA strand Pol III then take over and synthesis of new strand begins.
- The process by which DNA is copied to mRNA is called transcription.
- The mRNA contains information for the protein synthesis.
- Transcription occurs in two steps. In first step pre mRNA is synthesized with the help of RNA polymerase, the resultant RNA strand is reverse complement of the original DNA sequence. Then after pre messenger RNA is edited and forms the mRNA molecule by the process of RNA splicing.
- The pre-messenger synthesized contains introns and exons in it, introns are the sequence which does not code for any protein hence it is not required in protein synthesis.
- Thus the pre- messenger RNA is chopped up and introns are removed from it which synthesise messenger RNA contain only exons.
- The mRNA synthesised in the transcription process is now transported outside the nucleus into cytoplasm to the ribosome.
- Messenger RNA does not directly involve in the protein synthesis transfer RNA is required the process of protein synthesis is called translation.
- The mRNA contains three base stretch called codon and each codon contains information for a specific amino-acid. While the ribosome passes through the mRNA each transfer RNA molecule anticodon interacts with codon of mRNA.
- The transfer RNA contains amino acid at 3’ end which is added to the growing protein chain and the t-RNA expelled from the ribosome.
- Transfer RNA is tertiary structure which is represented in two dimensions as cloverleaf shape.
- Each amino acid has its own unique t-RNA like t-RNA for phenylalanine is different from that of histidine.
- Attachment of amino acid is at 3’- OH group of t-RNA.