Deoxyribonucleic Acid (DNA) is the genetic material of many biological organisms, not only for humans, but also all animals, bacteria and many viruses. This means that it is DNA which stores the information required to construct proteins, cells and even entire organisms.

DNA is structured as a polymer, with a linear sequence of nucleotides. Similar to protein sequences, the sequences of DNA can vary, by including different nucleotides, and different orders of those same nucleotides. The different combinations of nucleotides give rise to the variability seen in DNA, different organisms, and different people will have slightly different DNA. This variability gives rise to people having differences in hair or eye color, or other physical attributes, and is also used in technologies like DNA fingerprinting or paternity testing.

DNA is similar to another nucleic acid, ribonucleic acid (RNA), which is also a linear polymer of nucleotides. There are though two things that distinguish DNA from RNA (Figure 1). Firstly, the DNA contains deoxyribose as its pentose sugar, so named, because it has one fewer oxygen atom than the ribose sugar found in RNA. Secondly, the DNA contains thymine as one of its bases, whereas RNA contains uracil. These two bases have the same role in these different molecules, both having complementarity with the adenine base.

Figure 1: Chemically, DNA and RNA differ in two ways. Firstly, DNA molecules contain deoxyribose as their pentose sugar, while RNA molecules contain ribose sugars. Secondly DNA molecules contain thymine molecules that form complementary base pairing with adenine molecules.

The organization and function of DNA is a highly complex subject to go into full detail. One important point about DNA is that it is typically found as a double helix (Figure 2). This means that two DNA polymers bind to each other and wrap around each other to form a helical structure. Individual nucleotides facilitate this interaction, with the guanine and cytosine bases forming strong interactions from their three bonds between them, and the thymine and adenine bases forming weak interactions through their two bonds.

Figure 2: The DNA double helix. DNA is typically found as a dimer, with two DNA sequences, which bind to each other with the nucleobases on the inside of the helix, which interact on the basis of charge complementarity, and in turn form a corkscrew like structure. The double helix has two grooves, the larger major groove, and the smaller minor groove, these separate the sugar phosphate backbones in the DNA, and the major groove allows the DNA to be exposed to interactions with proteins, and other biological molecules. The DNA double helix is the basis behind larger DNA structures such as chromosomes, which store and regulate the DNA in living organisms.

The DNA chains in the double helix are antiparallel. DNA chains have directionality, which is usually expressed in the terms of the 5° (5 prime) to the 3° (3 prime) ends of the DNA. This means the nucleotides are read in order from the end atom which has the phosphate groups attached to it (which is the 5 carbon of the pentose ring), finishing at the 3 carbon of the pentose ring. As the double helix is antiparallel, the DNA chains have opposite directionality along the length of the double helix (Figure 3). In genetics, only one of these chains encodes for any given protein (though the complementary strand may encode for another protein), which is known as the sense strand, while the chain complementary to it is called the antisense strand.ine bases forming weak interactions through their two bonds.

Figure 3: DNA dimerizes to a sequence which is a reverse complement of itself. The DNA sequence which is read to form a gene product (the sense strand) is read in a 5' to 3' manner, while the strand opposite it (the antisense strand) has complementary nucleotides that bind on the basis of base complementarity, and is in the opposite orientation, being in the 3' to 5' direction. In the case of this sequence (ACGT, chosen arbitrarily as a sequence of all the nucleotides in alphabetical order), the sequence is also palindromic, with the antisense strand being identical to the sense strand, but in the opposite orientation. This is not the case for all DNA sequences, but it is quite common in the sequences recognized by restriction enzymes used in molecular biology.

DNA is what is called the "genetic material", this means that it is the molecule that stores the instructions to create proteins, cells and even whole organisms. The study of how this information is processed is called genetics, and clinical genetics is the medical speciality that investigates how the DNA can affect human diseases. Another similar field is called molecular biology, which studies the chemistry involved in DNA, and the biological processes which allow DNA to function. As DNA usually is the core set of instructions for creating an organism, DNA is central to the study of wider biological systems.