How was a heavy isotope of nitrogen used to provide experimental evidence to semiconservative mode of DNA replication?
class 12th CBSE Biology sample paper
Answers
(1)They grew E. coli in a medium containing 15NH4Cl (15N is the heavy
isotope of nitrogen) as the only nitrogen source for many
generations. The result was that 15N was incorporated into newly
synthesised DNA (as well as other nitrogen containing compounds).
This heavy DNA molecule could be distinguished from the normal
DNA by centrifugation in a cesium chloride (CsCl) density gradient.
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(2)Then they transferred the cells into a medium with normal
14NH4Cl and took samples at various definite time intervals as
the cells multiplied, and extracted the DNA that remained as
double-stranded helices. The various samples were separated
independently on CsCl gradients to measure the densities of
DNA.
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(3)Thus, the DNA that was extracted from the culture one
generation after the transfer from 15N to 14N medium [that is
after 20 minutes; E. coli divides in 20 minutes] had a hybrid or
intermediate density. DNA extracted from the culture after
another generation [that is after 40 minutes, II generation] was
composed of equal amounts of this hybrid DNA and of ‘light’
DNA.
(Please note that 15N is not a radioactive isotope, and it can be
separated from 14N only based on densities).
DNA Replication
Explanation:
- Semiconservative replication describes the component of DNA replication in completely referred to cells
- This process is known as semi-preservationist replication since two duplicates of the original DNA particle is delivered. Each copy contains one unique strand and one new synthesized strand
- Four normally occurring nitrogenous bases, there are four unique sorts of DNA nucleotides such as adenine (A), thymine (T), guanine (G), and cytosine (C).As the DNA polymerase descends the single-stranded DNA, it utilizes the arrangement of nucleotides in that strand as a template for replication