DNA Structure

DNA is a double stranded molecule twisted into a helix. Each spiraling strand is comprised of a sugar phosphate backbone and attached bases connected to a complimentary strand. The complimentary strand is connected by non-covalent hydrogen bonding between paired bases. The bases are adenine, thymine, ctyosine, and gaunine. The fundamental building block of DNA is the nucleotide.

The nucleotide has three parts. A nitrogen-containing pyrimidine or purine base, a deoxyribose sugar, and a phosphate group that acts as a bridge between the deoxyribose sugars that are adjacent to it. Each deoxyribose sugar contains five carbon atoms joined to an oxygen atom. The carbon atoms are sequentially numbered from 1-5. Each is considered a prime and is labeled such as this: 1'. The first carbon atom is the carbon atom covalently attached to one of the four bases. Guanine and adenenine are purines and cytosine and thymine are pyrymidines.

The phosphate groups are attached to the (3') third and (5') fifth carbon atoms. When speaking of DNA the term nucleotide refers to the complete assembly of a nitrogenous base, a five carbon deoxyribose sugar, and a phosphate group.

A DNA molecule is composed of two unbranched polynucleotide chains that wind about each other into a structure called a double helix. Each revolution is 34 A, there are ten base pairs in each revolution. The structure of DNA is an advantage because of it's simplicity. This structure lessens the chance of mutation during replication.

Robert D. Stewart, Ph.D.

A Few Words About DNA and Chromatin

Dosimetry research and Technology (DRT) Group K3-55

Pacific Northwest National Laboratory (PNNL)