The nucleotide is the basic building block of nucleic acids.

DNA Structure

­DNA is o­ne of the nucleic acids, information-containing molecules in the cell (ribonucleic acid, or RNA, is the other nucleic acid). DNA is found in the nucleus of every human cell. (See the sidebar at the bottom of the page for more about RNA and different types of cells). The information in DNA:

  • guides the cell (along with RNA) in making new proteins that determine all of our biological traits
  • gets passed (copied) from one generation to the next

The key to all of these functions is found in the molecular structure of DNA, as described by Watson and Crick.

Although it may look complicated, the DNA in a cell is really just a pattern made up of four different parts called nucleotides. Imagine a set of blocks that has only four shapes, or an alphabet that has only four letters. DNA is a long string of these blocks or letters. Each nucleotide consists of a sugar (deoxyribose) bound on one side to a phosphate group and bound on the other side to a nitrogenous base.

There are two classes of nitrogen bases called purines (double-ringed structures) and pyrimidines (single-ringed structures). The four bases in DNA's alphabet are:

  • adenine (A) - a purine
  • cytosine(C) - a pyrimidine
  • guanine (G) - a purine
  • thymine (T) - a pyrimidine

Strands of DNA are made of the sugar and phosphate portions of the nucleotides, while the middle parts are made of the nitrogenous bases. The nitrogenous bases on the two strands of DNA pair up, purine with pyrimidine (A with T, G with C), and are held together by weak hydrogen bonds.

Watson and Crick discovered that DNA had two sides, or strands, and that these strands were twisted together like a twisted ladder -- the double helix. The sides of the ladder comprise the sugar-phosphate portions of adjacent nucleotides bonded together. The phosphate of one nucleotide is covalently bound (a bond in which one or more pairs of electrons are shared by two atoms) to the sugar of the next nucleotide. The hydrogen bonds between phosphates cause the DNA strand to twist. The nitrogenous bases point inward on the ladder and form pairs with bases on the other side, like rungs. Each base pair is formed from two complementary nucleotides (purine with pyrimidine) bound together by hydrogen bonds. The base pairs in DNA are adenine with thymine and cytosine with guanine.

DNA has a spiral staircase-like structure. The steps are formed by the nitrogen bases of the nucleotides where adenine pairs with thymine and cytosine with guanine.

Photo courtesy U.S. National Library of Medicine

In the next section we'll find out how long DNA strands fit inside a tiny cell.