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  4. A DNA molecule having labelled thymidine in one of its strand is allowed to replicate in a radioactive medium. Calculate the number of strands with labelled thymine in the DNA after three successive divisions.

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Q. A DNA molecule having labelled thymidine in one of its strand is allowed to replicate in a radioactive medium. Calculate the number of strands with labelled thymine in the DNA after three successive divisions.

NTA AbhyasNTA Abhyas 2020Molecular Basis of Inheritance

Solution:

DNA replication is the process of making a new double stranded nucleic acid. This process is semi-conservative where one is a new strand synthesized and the other is the old strand from the parent DNA used as template for replication. The given dsDNA- one strand was labelled with labelled thymidine and other was non-labelled. This dsDNA was replicated in a medium of labelled nucleotide. In subsequent 3 generations of replication, only one strand (parent dsDNA) will remain unlabeled and rest all will be labelled as the new nucleotides used for making new strand are from the labelled medium. Therefore, after 3 divisions there will 16 strands from 8 dsDNA but 15 strands will be labelled.