Molecular Biology of the Gene

Structure of DNA

DNA has two complementary strands that form a double helix.
The structure of DNA is called a double helix. It looks like a ladder that has been twisted at the top and the bottom. It is twisted to the right and has both major (large) and minor (small) grooves. The major groove is where the DNA backbones lie far apart; the minor groove is where they lie close together. DNA is composed of three main parts:
  • a 5-carbon sugar called deoxyribose
  • a phosphate group
  • a nitrogenous base
The building blocks of DNA are called nucleotides. Each consists of a central 5-carbon sugar attached on one side to a phosphate group, and on the other side to a nitrogenous base. Neighboring nucleotides are attached to each other at the phosphates and sugars, forming the sugar phosphate backbone of a DNA strand. The nitrogenous (nitrogen-containing) bases line up on each side of the molecule, and each is a complementary strand, a strand of DNA that has complementary nitrogenous bases to another strand. The four nitrogenous bases found in DNA are adenine, guanine, thymine, and cytosine. A purine is one of the two types of nitrogenous bases of nucleic acids, which consists of two joined rings containing carbon and nitrogen and includes adenine and guanine. A pyrimidine is one of the two types of nitrogenous bases of nucleic acids, which consists of a single ring containing carbon and nitrogen. These include thymine, uracil, and cytosine.
The double-ringed purines bind with the single-ringed pyrimidine. Adenine can only bind to thymine, forming two hydrogen bonds. Guanine can only bind with cytosine, forming three hydrogen bonds.
A DNA strand has what is known as 3' and 5' ends. This naming convention references which carbon atoms of the terminal sugar are free or attached to a terminal phosphate. One side of the sugar molecule runs from 3' to 5' and the other runs from 5' to 3'. The strands are called antiparallel, meaning they run opposite to each other. Chargaff's rule states that for double-stranded DNA, a relationship can be observed where the amount of adenine equals the amount of thymine, and the amount of guanine equals the amount of cytosine. Watson and Crick built on this understanding to determine that, if the sequence of nitrogenous bases on one side of the DNA molecule is ATTGCGATC, according to Chargaff's rule, the corresponding sequence of bases would be TAACGCTAG. This designation of DNA's ends will become important when discussing how DNA replicates itself. The complementary strands of DNA are held together by hydrogen bonds that form between the nitrogen bases. Although these bonds are very weak, they work well to hold the strands together.

The Double Helix

The sequence of nitrogen bases in the DNA molecule is responsible for the traits an organism expresses. Two different views of the molecule are present here. The upper shows the helical nature of the molecule. The lower shows the sequence of bases.