Question

3. What is meant by the term "central dogma” in the field of genetics? Include in your answer the following concepts: A) the three steps involved in the transfer of genetic information (DNA) is transfer to physical traits (proteins) in an individual organism; B) the final product of each step: C) errors generated during this process and how they could affect the individual organism.

Answer 1

Flow of biological information from DNA to protein via RNA is called central dogma of life (biology). It means every function of cell is regulated by DNA through protein synthesis.

• Replication is a process in which DNA copies itself to produce identical daughter molecule of DNA. DNA replication begins at specific site and usually proceeds bidirectionally. DNA synthesis always occur in 5'-3' direction because phosphodiester bond formed when a free 3'-OH group of growing chain react with $\alpha$ phosphate of incoming nucleotide.
• Transcription is the DNA dependent synthesis of any type of RNA molecule with the help of RNA Polymerase. Transcription always starts at a specific site called promoter. Promoter is the specific site on DNA where RNA Polymerase binds and initiates transcription. Synthesis of RNA occurs in 5'-3' direction and is continuous. In case of prokaryotes, single RNA Polymerase does all the functions whereas in case of eukaryotes three different RNA Polymerases are required. RNA Polymerase transcribe all types of rRNA, RNA Polymerase II transcribes all types of mRNA and RNA Polymerase III transcribes all types of tRNA. RNA Polymerase II is a large proteinaceous complex consisting of more than 12 subunits. RNA Polymerase binds to the core promoter and initiate transcription. During the elongation phase of transcription there is a continous movement of RNA Polymerase II on template and mRNA is syntheized in 5'-3' direction. For ribosomes rRNA is required which is transcribed by RNA Polymerase I. tRNA provides the site for attachment of amino acid and recognise base pairing with mRNA. Codon specifies an amino acid during translation (codon is simply a set of three nucleotides present in mRNA)
• Translation is the process of protein synthesis and completed in four sequential steps: activation of amino acid (tRNA) charging, initiation, elongation and termination. Small and large subunits of ribosomes are associated to each other during translation initiation whereas essentially dissociate during translation termination and tRNA transfer activated amino acid from cytoplasm to the active site of ribosomes.
• During the above processes many errors can occur for example incorrect nucleotides are incorporated during replication and escaped from proof reading but this can be repaired by mismatch repair system, it removes incorrect nucleotides only from daughter strand. Nucleotide exchange repair (NER) system operates for large scale DNA damage which destroys structure of DNA helix such as thymine dimers and any defect in proteins of NER cause Xeroderma pigmentosum. Missense mutation results in the change of single nucleotide as a result glutamic acid is replaced by valine, this further results in Sickle cell anaemia disease in humans.