Molecular Basis of Inheritance
- Copying the genetic information from DNA to RNA with the help of RNA polymerase is called Trasnscription.
- Coding strand/sense strand/ 5′ – 3′ strand of the DNA does not get converted into RNA. The strand of DNA that gets converted into RNA is called Template/ Antisense/ 3′ - 5′ strand.
- Three parts of the template strand are (i) Promoter (to which RNA polymerase bonds) (ii) Terminator (Rho protein) and (iii) Transcription unit (Structural gene that gets converted to RNA).
- Promoter is a A and T rich sequence, called as TATA box. In prokaryotes. It is called as pribnow box and in Eukaryotes, it is called as Hogness box
- Structural gene can produce one protein (monocistronic) in prokaryotes.
- Prokaryotic transcription requires a single RNA polymerase to produce 3 types of RNA (r, m, t RNA). But eukaryotes require RNA pol I for rRNA, RNA pol II for mRNA and and RNA pol III for t-RNA ; Sn RNA.
- Sigma factor from RNA pol gets attached to promoter and rest of the enzyme moves over the transcription unit. (Therefore RNA polymerase – sigma factor = core enzyme).
- Extersion is similar to that of DNA replication.
- Newly formed RNA is by itself active in prokaryotes. But in eukaryotes, post transcriptional modification occur to convert inactive RNA / itranscript / heterogenous RNA into a functional mRNA.
- PTM involues capping at 5′ end by 7 methyl Guanosine triphosphate to prevent the itranscript from Nuclease activity, Tailing by Adenosine with poly ‘A’ polymerase at 3′ end to prevent it from exonuclease activity and splicing the introns (unnecessary / non coding regions of RNA), Later connecting all the exone by ligase.
- Translation is a process of formation of protein from the m-RNA, that carries the signal for protein synthesis.
- Ribosomes are the protein machinery, which contains three sites namely (i) P site (Peptidy transfer / donor) (ii) A site (acceptor) and (iii) Esite (Exit).
- Initially amino acid has to be activated, or in other words, tRNA is to be charged by binding to the amino acid with the help of amino acid tRNA synthetase.
- m-RNA is then bound to 40-S of ribosome with the help of Initiation Factor (IF) 1 and a GTP and then the charged t RNA binds with 40 S – mRNA complex with IF2 (Prokaryotes) (or) IF3 (Eukaryotes) followed by the attachment with 60-S subunit of ribosome with the help of IF 1 and 4 (Eukaryotes) and now the whole setup is ready for extension.
- The start codon of mRNA (AUG – That codes for methionine in eukaryotes) goes and fits at the p site initially.
- The tRNA carries the respective amino acid (Met) to the p site and it is released over the p site and the enzyme translocase moves the ribosome over the RNA and the codons of A site reaches P site and P site reaches Esite and the process of f-RNA carrying the next amino acids continue till the Psite encounters the stop codon of MRNA (UAA, UAG or UGA) and reaches termination step of translation.
- In prokaryotes, AUG codes for Formyl – methionine, where the formyl group is defromylated by aminopeptidase.
- After the P site encounters stop codon, both subunits of ribosome gets dissociated.
- Formation of same polypeptide, for the number of times with the help of polysomes is called Translational amplification.
- For every single amino acid incorporated, 1 ATP and 2 GTP are used.
- All the amino acids are connected by peptide bond with the help of peptidyl transferase to form a functional protein.
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