Q. 17

‘‘Use of heavy is

Answer :

• To get to the conclusion that DNA followed a semi-conservative mode of replication, Meselson and Stahl grew E.coli in 15NH4Cl for many generations to get 15N incorporated into DNA.


• Then the cells were transferred into 14NH4Cl.


• The extracted DNA were centrifuged in CsCl and measured to get their densities.


• DNA extracted from the culture after one generation, 20 minutes showed intermediate hybrid density.


• DNA extracted after two generations, 40 minutes showed light DNA and hybrid DNA.



OR


Mechanism of translation in prokaryotes:


• First, charging of tRNA occurs by aminoacylation.


• Small subunit of ribosome binds to the 5’ end of the mRNA


• For initiation, the ribosome binds to the mRNA at the start codon (AUG) that is recognised only by initiator tRNA,


• In the elongation phase amino acid with tRNA sequentially bind to the appropriate codon on mRNA (forming complimentary base pairs with tRNA anticodon.)


• Ribosome moves from codon to codon along the mRNA and amino acids are added one by one in the two sites of the large subunit joined by peptide bond.


• Termination occurs when a release factor binds to the stop codon and releases the complete polypeptide.



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