Molecular Basis of Inheritance
RNA world, Replication
- The first biocatalysts were RNA and it was the first genetic material.
- Now also, certain enzymes are made up of RNA eg. Ribozyme.
- It is a fact that, in the early unstable environment, RNA worked well.
- Later on, RNA was modified into enzymes and proteins which were more stable then RNA to increase the metabolic activity and enhanced evolution of complex organisms.
- Delbruck suggested three methods for DNA replication (i) dispersive (ii) conservative and (iii) semi conservative (widely accepted).
- Meselson and stahl performed experiments with E. coli and different isotopes of nitrogen and proved the semi conservation replication model through centrifugation procedures.
- DNA replication in E. coli starts from ORI (origin of replication) site in DNA. [Prokaryote – mono repliconin ; Eukaryote – Multirepliconic]
- A, T, G, C nucleotides have to be converted to triphosphates before attaching it to newly synthesizing DNA.
- According to semi conservative model, the 2 strands of DNA has to be divided. New strands are synthesized individually from respective single stranded DNA.
- Parent DNA strands are separated with the enzyme Helicase at ORI site.
- Topoisomerase acts on the opposite end to present the supercoiling of DNA.
- To prevent the binding of divided SS DNA. Single stranded Binding proteins are attached to the parent DNA (SS).
- DNA replicates from 5’ end to 3’ end, always.
- DNA replication shorts with RNA primer.
- The strand which starts from 5’ end is called leading strand and it is completed faster and that it requires only one primer. To extend the DNA polynucleotide chain, enzyme DNA polymerase III is required.
- Lagging strand starts from 3’ end to 5’ end which requires more than 1 primer to form okozaki fragments, which is then connected to each other with the help of DNA ligase.
- After both the strands extend, DNA polymerase I and II gets activated for proof reading.
- Co -factor required for DNA polymerase is Mg2+
- Leading strand formation is continuous process ; Lagging strand formation is discontinuous process. Hence DNA replication process in semi – discontinuous process.
Part-1: Watch this video for the topic from 0:56 to 44:45
Part-2: Watch this video for the topic from 0:40 to 54:25
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