NEET Botany Molecular Basis of Inheritance Questions Solved

BOARD

(a) What is central dogma? Who proposed it?

(b) Describe Meselson and Stahl's experiment to prove that the DNA replication is

semi-conservative.

(a) Francis Crick proposed the Central Dogma in molecular biology. It states that 

the genetic information flows from DNA RNAProteins.

(b) 

Messelson and Stahl's experiment: 

(i) They grew E.coli in a medium containing NH415Cl as the only nitorgen source for many generations. The result was that N15 was incorporated into newly synthesised DNA.

(ii) The heavy DNA molecule could be distinguished  from the normal DNA by centrifugation in a cesium chloride density gradient.

                                  (Separation of DNA by centrifugation)

                                   Messelson's and Stahl's experiment 

(iii) They transferred the cells into a medium with normal NH414Cl and took samples at various definite time intervals as the cells multiplied, and extracted the DNA that remained remained as doubled-stranded helicals.  

(iv) Thus, the DNA that was extracted from the culture one generation after the transfer from N15 to N14  medium (i.e., E.coli divides in 20 minutes) had hybrid or intermediate density.

(v) DNA extracted from the culture after another generation (that is after 40 minutes, II generation) was composed of equal amounts of the hybrid DNA and light DNA.

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