DNA Structure

Each DNA molecule has the following features:

• The building blocks of DNA are nucleotides

• Each nucleotide consists of a sugar (deoxyribose), a nitrogen-containing base and a phosphate group

• There are two types of N-containing bases, purines which are double ring structures, and pyrimidines that are single ring. Two purines are found in DNA, adenine and guanine and there are two pyrimidines, thymine and cytosine

• Each nucleotide has a OH group attached to the 3' carbon and a phosphate attached to the 5' carbon of the deoxyribose

• Nucleotides are linked into a long strand by phosphodiester bonds that joins the 3' carbon of one sugar to the 5' carbon of the next. The alternating sugar-phosphates are termed the backbone and each backbone will have a 3' OH group at one end and a 5' phosphate at the other

• The complete DNA molecule is composed of two strands which wind around each other to form a right-handed helix

• The two strands run in opposite directions (antiparallel) so that the 3' end of one strand is facing the 5' end of the other strand

• The bases on the two strands are found in specific pairings. An adenine on one strand is always opposite a thymine on the other and a guanine is always paired with a cytosine. These base pairs are held together by hydrogen bonds, three for a GC pair and two for an AT pair

• The DNA molecule can be any length, is 20 Angstroms in diameter and makes a complete turn every 34 Angstroms