a1.2.12 - purine-to-pyrimidine bonding

Question 1

compare and contrast the structures of purines and pyrimidines.

Answer 1

both purines and pyrimidines are nitrogenous bases of DNA and RNA.

pyrimidine: single ring nitrogenous bases
- cytosine
- thymine
- uracil

purine: double ring nitrogenous bases
- guanine
- adenine

Question 2

state that in DNA, a purine forms hydrogen bonds with a pyrimidine.

Answer 2

• in DNA, a purine complementary base pairs to a pyrimidine using hydrogen bonds
• in DNA and RNA, guanine bonds with cytosine with three hydrogen bonds.
• in DNA, adenine bonds with thymine with two hydrogen bonds.
• in RNA, adenine bonds with uracil with two hydrogen bonds

Question 3

given a diagram of DNA, identify the four bases of DNA based on purine or pyrimidine and the number of hydrogen bonds.

Answer 3

• purines have two rings. if it can form 2 H-bonds it is adenine and if it can form 3 H-bonds it is guanine.
• pyrimidines have one ring. if it can form 2 H-bonds it is thymine and if it can form 3 H-bonds it is cytosine.