Teaching the nature of genetic material to the first semester M.Sc students is a challenge since they have been learning about DNA right from school. Thus, when I talk about Hershey and Chase experiment or Meselson-Stahl experiment they already know about it and are bored. So I have to find ways to make it interesting for them.
One such thing is Chargaff's rule.
Erwin Chargaff's experiment were a key point in understanding the structure of DNA. He showed that DNA from any organism/cell had a 1:1 ratio of purines to pyrimidines. This was the first rule. Further, the amount of adenine was equal to that of thymine while the amount of guanine was equal to that of cytosine. This was the second rule.
Chargaff did not conclude that the DNA was a double-helix from his experiments because the nature of DNA was not known at that point of time. His rules were used, however, by Watson and Crick to solve the structure of DNA. In fact Linus Pauling lost the race because he failed to take Chargaff's rule into account. He proposed a triple helix structure of DNA which did not follow Chargaff's rule.
But the question really is whether Chargaff's rule applies uniformly. Further, from the data can we always conclude whether the given genome is double-stranded or single-stranded?
The question I posed before the class was what if the amount of adenine, thymine, guanine, and cytosine each is 25%. Can you then figure out whether the genome is double-stranded or single-stranded?
It turns out that Chargaff's rule has exceptions. The rule does not apply to single-stranded DNA genomes as well as to mitochondrial genome (one strand is C rich and the other strand is G rich- so the first law does not apply).
The answer to the question I posed is that you cannot predict whether the DNA is single-stranded or double-stranded. That is the fun and challenge of Chargaff's rule.
One such thing is Chargaff's rule.
Erwin Chargaff's experiment were a key point in understanding the structure of DNA. He showed that DNA from any organism/cell had a 1:1 ratio of purines to pyrimidines. This was the first rule. Further, the amount of adenine was equal to that of thymine while the amount of guanine was equal to that of cytosine. This was the second rule.
Chargaff did not conclude that the DNA was a double-helix from his experiments because the nature of DNA was not known at that point of time. His rules were used, however, by Watson and Crick to solve the structure of DNA. In fact Linus Pauling lost the race because he failed to take Chargaff's rule into account. He proposed a triple helix structure of DNA which did not follow Chargaff's rule.
But the question really is whether Chargaff's rule applies uniformly. Further, from the data can we always conclude whether the given genome is double-stranded or single-stranded?
The question I posed before the class was what if the amount of adenine, thymine, guanine, and cytosine each is 25%. Can you then figure out whether the genome is double-stranded or single-stranded?
It turns out that Chargaff's rule has exceptions. The rule does not apply to single-stranded DNA genomes as well as to mitochondrial genome (one strand is C rich and the other strand is G rich- so the first law does not apply).
The answer to the question I posed is that you cannot predict whether the DNA is single-stranded or double-stranded. That is the fun and challenge of Chargaff's rule.
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