Which is the following is correct for bonding between the purines and pyrimidines? 1. Pyrimidines have a double-ringed structure.
2. In DNA, pyrimidines hydrogen bond to other pyrimidine.
3. Adenine and guanine are purines.
4. Uracil is a purine.

3,

Explanation:

Both purines and Pyrimidines are organic Nitrogen  bases in DNA and RNA molecules. Adenine and Guanine are purines because they have double ring structure.Pyrimidine  are three- cytosine,thymine and Uracil.The uracil is lacking in DNA,but present in RNA. furthermore thymine is only present ni DNA but not in RNA. However, cytocine is present  in DNA and RNA. They are made up of single bonds.and are smaller than the Purines.

For perfect structure of DNA molecule,there must be complementary paring of Purines with corresponding pyrimidine. This paring ensures that for each purine (adenine) it must pair with corresponding(Thymine).While for each Guanine it must fit with corresponding Cytosine.(pyrimidine).

Two hydrogen bonds Hold A-T,while three hydrogen bonds hold G-T.

There above complementary paring is based on Chargaff’s Rule of compliment paring. Which states that the ratio of A-T and C-G will always constant in DNA molecule.That is the rates of these purines to pyrimidines will always be a fixed ratio.Thus each strand of DNA molecule will always complement one another for each A-T and C-G.

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