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Q.
All the given statements are true, except
NTA AbhyasNTA Abhyas 2020Molecular Basis of Inheritance
Solution:
DNA is the nucleic acid that forms the genetic material in all organisms. The structure of deoxyribose nucleic acid was elucidated by Watson and Crick. The DNA is a double helical structure having two anti-parallel strands of polynucleotide chains joined to each other by hydrogen bonding formed between the N-bases. It is more stable than ribonucleic acid (RNA) as it has thymine (5-methyl uracil) which has greater resistance to photochemical mutation and deoxyribose (less reactive than ribose).
DNA replication is the process of making new DNA strand from the old one and this process is semi-conservative. Synthesis of new DNA occurs simultaneously and independently on both strands but in one direction as the DNA polymerase used in the process of elongation of the strand of DNA during replication process proceeds in the 5' $ \rightarrow $ 3' direction only. Of the two strand synthesis, the one that is continuous and occurs without stopping from one end to the other is called leading strand, while the one that is discontinuous in that it occurs in several small chunks one after another is called lagging strand and each short sequence is called Okazaki fragment. The replication process is bidirectional in that it goes in both ways from the point of origin of replication in the DNA. Meselson and Stahl proved that DNA replication is semi conservative in bacteria by experimenting with E. coli. Herbert Taylor Colchicine demonstrated semi-conservative DNA replication in eukaryotes by using bean root tip.
On been cleaved by the protein subtilisin, DNA polymerase I produces a large fragment called Klenow fragment which contains polymerase activity and 3' $ \rightarrow $ 5' exonuclease activity but not the 5' $ \rightarrow $ 3' exonuclease activity.