DNA Base Pairing

In every base pairing, a Purine always pairs with a Pyrimidine. In DNA replication, the DNA strands pair with its complementary base. A purine pairs with a Pyrimidine.

The Purines are: Adenine (A) and Guanine (G).  they have a larger molecular structure than pyrimidines.

The pyrimidines are Cytosine (C) and Thymine (T). their molecular structure is smaller when compared with the Purines.

In essence, a large molecule pairs with a small molecule (Purine-Pyrimidine or Pyrimidine-Purine).  The Purine-Purine or Pyrimidine-Pyrimidine bond does not form because it would anot fit into the double-helix structure of a DNA. This is called Watson-Crick pairing.

 A always pairs with T and C with G

Each bas pair is tightly bound to one another by Hydrogen bonds. This is a very strong type of bond.

A – Adenine

C – Cytosine

T – Thymine

G – Guanine

REFERENCES.

http://wps.prenhall.com/esm_krogh_biology_3/17/4438/1136322.cw/index.html

http://wps.prenhall.com/esm_krogh_biology_3/17/4439/1136522.cw/index.html

http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/B/BasePairing.html