Why adenine doesn't pair with cytosine in DNA

[Spectre5]

In DNA, the two purines are Adenine and Guanine and the two pyrimidines are Thymine and Cytosine.

NOw...in DNA...Adenine pairs with Thymine and Guanine pairs with Cytosine. A purine pairs with a pyrimidine. That explains why Adenine cannot pair with Guanine and why Thymine cannot pair with Cytosine.

However, I do not know why Adenine cannot pair with Cytosine or Guanine with Thymine...

Does anyone know why this is? If so I would greatly appreciate some insight on this. Thanks!

 

[Moonbear]

It's due to the structure of the nucleotides. Guanine and Cytosine can form three hydrogen bonds due to their conformation, and Adenine and Thymine can only form two hydrogen bonds due to their conformation.

This site shows an illustration of the locations of the hydrogen bonds that form between the two chains, which might help you understand.
http://www.morris.umn.edu/~goochv/BiolRhyt/Lectures/dna/dna.html
Look at the illustration between points 3 and 4.

 

[Spectre5]

Thanks for explaining it and giving me the link, I get it now