Learn about the primary molecular structure of DNA (Deoxyribonucleic acid).
The Basic DNA Structure
Although DNA is known as "the molecule of heredity", DNA macromolecules are usually not found in nature as completely single molecules but as pairs of complementary molecules linked together to form a double helix.
DNA is a double straned helix with anti-parralel strands. Two complementary DNA molecules are held together by a 4-bond arrangement. Interestingly with DNA, either strand contains all the information essential for DNA replication. The double stranded state is actually a "resting state". During DNA relication and transcription, regions of DNA can exist as single stranded forms.
Diagram 1. The structure of DNA. Note the double helix of DNA composed of two antiparallel and complimentary strands of deoxyribonucleic acid. Each nitrogenous base binds through hydrogen bonding to a complementary nitrogenous base (A binds to T, and C binds to B and vice versa).
DNA has significant secondary structure. Each double helix of DNA consists of chemically linked chains of nucleotides. Each nucleotide in DNA consists of a sugar, a phosphate group, and a nucleobase (or base). As DNA is composed of multiple nucleotide subunits, DNA is also a polymer of nucleotides. The sugar units form part of the hydrophillic polar external sugar-phosphate backbone. The nucleotides form a hydrophobic core of bases: Adenine or A, Thymine or T, Guanine or G, Cytosine or C.
The Sugar-Phosphate Backbone of DNA
DNA can be divided structurally into two portions: the external sugar-phosphate backbone, and the internal nucleotide bases. The sugar-phosphate backbone is the major structural component of the DNA molecule. The backbone is composed of alternating deoxy-ribose sugar molecules and phosphate molecules which are both highly polar. The backbone is hydrophillic due to its polarity, meaning it likes water.
The Hydrophobic Complimentary Core of Nucleotides
The interior portion "ladder" of DNA is composed of 4 nitrogenous bases: Adenine (A), Guanine (G), Thymine (T), and Cytosine (C). These bases are non-polar and are thus hyrdophobic (they don't like water). Inside a DNA molecule these bases pair up, A to T and C to G, forming hydrogen bonds that stabilize the DNA molecule. Because the interior bases pair up in this manner, we say the DNA double helix is complimentary. It is this sequence of bases inside the DNA double helix that we refer to as the genetic code.
Interestingly, humans are approximately 70% composed of water (similar to other life forms). For every DNA molecule in the cell, there are billions of water H2O molecules. A key question is how can you have two base pairs hydrogen bonding with each other and not to water molecules?
The first hydrogen bond "pays" the most entropic energy for the cost of the interaction.
See the DNA Molecule in 3-Dimensions