question_answer

Represent diagrammatically Hershey-Chase experiment. What did this Experiment prove?

OR

Write the salient features of double helix structure of DNA.

Answer:

Alfred Hershey and Martha Chase in 1952 performed an experiment on viruses that infect bacteria called bacteriophages. They grew some viruses on a medium that contained radioactive phosphorus and some others on medium that contained radioactive form

These radioactive phases were allowed to attach E.coli bacteria. This experiment was performed in three steps:

1. Infection

2. Blending

3. Centrifugation

Bacteria which was infected with viruses bacteriophages that had radioactive DNA were radioactive, indicating that DNA was the material that passed from the virus to the bacteria. Bacteria that were infected with viruses that had radioactive proteins were not radioactive. These findings indicated that protein did not enter the bacteria from the viruses but DNA from virus particle enters bacteria as genetic material because of which radioactivity is discovered in bacteria.

OR

Salient features of double helix structure of DNA:

(a) It is made up of two strands. The backbone is made up of sugar phosphate molecules and hydrogen bonds with base pairs projected inside.

(b) The two chains have antiparallel position. One chain has polarity \[5'\to 3'\] the other has polarity \[3'\to 5'\]

(c) The bases in two strands are paired through hydrogen bonds (H-bonds) forming base pairs (bp). The back always binds from double bond of hydrogen between adenine and thymine and triple bond between cytosine and guanine.

(d) The two chains are coiled in a right-handed fashion. The single turn on DNA have ten base pairs with total length of 3.4 nm or\[34\overset{o}{\mathop{A}}\,\].

(e) The diameter of DNA molecule is 2 nm or\[20\overset{o}{\mathop{A}}\,\].

(f) The base pair stacks one above the other in a double helix. This in addition to hydrogen bonds provides stability to helical structure.