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http://www.youtube.com/watch?v=3QJ4CjFsflA

This is a link to Hershey and Chase experiment. According to this experiment, we conclude that DNA is the genetic material. But how do we conclude that DNA is the genetic material? They only proved that DNA of virus enters into the bacteria, not the protein coat of virus.

The DNA of the virus later destroys the bacteria because the virus reproduces inside them. Does this demonstrate that DNA is the genetic material?

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It is a matter of deduction. We know that:

  1. Viruses entering a cell are capable of making more copies of themselves.
  2. The only part of a virus that actually enters the cell is its DNA (or RNA, depending on the virus in question).

Based on these two facts, we can conclude that the DNA ̣--since it is the only part of the virus to enter the cell-- somehow contains the information necessary to make new copies of the virus and is, therefore, the genetic material of the virus.

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  • $\begingroup$ @MuhammadRafique, I did not mean to push you to accept my answer (it is often a good idea to wait a while in case a better one comes along), I was just pointing out the etiquette on these sites :). Thank you comments are discouraged. $\endgroup$
    – terdon
    Commented Dec 13, 2012 at 18:34
  • $\begingroup$ i was not pushed, don't worry....:)...and i didn't know commnets are discouraged, so bye bye...:p $\endgroup$
    – Rafique
    Commented Dec 13, 2012 at 18:38
  • $\begingroup$ @alanboyd, ouch,thanks for the edit, durned iphone... $\endgroup$
    – terdon
    Commented Feb 18, 2013 at 5:43
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This was an important experiment, but did not convince everyone.

Not even Hershey and Chase claimed this conclusion, although it convinced many. In most mol bio classes or when reading "The Double Helix" you can see how there were lots of facts pointing to DNA, but doubts lingered. Some viruses contained RNA only and not DNA, but still replicated. There was also a school of thought which felt that proteins could be the genetic material.

It was Watson and Crick that finally convinced everyone that DNA was the medium of genetic inheritance. The reasons for this are varied, one of them is probably that atomic structures are so clearly understandable and seem so uncomplicated compared to a wet lab experiment which is built upon so many other experiments. Questions keep coming up: "how do you know you shook the flask long enough?", "What if a small amount of residual viral protein got in?" For every revelation like Hershey and Chase, there are many cases where someone stopped an incubation too quickly or wrote down an illegible number and was found out after publishing.

And you can see how much the structure gave us in one fell swoop: the structure of DNA inspired a mechanism by which replication of the DNA could be produced with some error correction, the central dogma (DNA -> RNA -> protein). There were several years more effort to demonstrate that the genetic code existed and then parsing out the exact codons and the fact that they were 3 bases long. But all of this was predicted when the structure was found. So much clearer as to what is going on.

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    $\begingroup$ Perhaps worth adding this quote from Watson: "the Hershey-Chase experiment had a much broader impact than most confirmatory announcements and made me ever more certain that finding the three-dimensional structure of DNA was biology's next important objective. The finding of the double helix by Francis Crick and me came only 11 months after my receipt of a long Hershey letter describing his blender experiment results." $\endgroup$
    – Alan Boyd
    Commented Feb 13, 2013 at 21:32

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