The nucleotides, or building blocks of DNA, have three components: deoxyribose sugar, a phosphate group, and one of four nitrogenous bases---ringed or double-ringed structures built from carbon atoms and nitrogen atoms. Two of these nitrogenous bases, adenine (A) and guanine (G), contain two chemical rings and are called purines. The other two, each containing one chemical ring, are cytosine (C) and thymine (T), and they are called pyrimidines.
The chemical backbone of DNA, made from phosphate and deoxyribose, is held together by phosphodiester bonds between carbon atoms of neighboring deoxyribose units. Each strand of DNA has a direction based on these bonds. Two strands linked together by their bases, which are arranged like rungs of a ladder, will go in opposite directions--an antiparallel arrangement When many nucleotides are linked together like this, the DNA coils into a double helix.
Because of the shapes and arrangement of the four nitrogenous bases in DNA, two different base pairings are possible: A with T (A-T) and G with C (G-C). The former pair is stabilized with two hydrogen bonds, which are weak bonds that form between hydrogen and more electronegative atoms such as oxygen and nitrogen. The G-C pair is stabilized with three hydrogen bonds, so it is slightly tighter than an A-T pairing.
When DNA is in a relaxed state, its double helix makes one loop every 10.4 base pairs. When DNA twists like rope, it is called supercoiling. If the twist goes in the same direction as the underlying double helix, it is called positive supercoiling. If the supercoiling goes in the opposite direction, it's called negative supercoiling. Usually, DNA supercoiling is slightly negative.
Inside cells, supercoiled DNA and associated proteins form chromosomes, which are copied before cell division. Known as DNA replication, the process occurs in prokaryotes (organisms without nuclei in their cells, such as bacteria) and eukaryotes (animals, protozoans, plants and fungi). In eukaryotes, most of a cell's DNA exists in the nucleus. Mitochondria and chloroplasts in the cell have their own DNA, as they evolved from bacteria that once lived independently. Although eukaryotes can have many chromosomes, the DNA in prokaryotes is organized in a single, circular chromosome in the cell.
Genes are sequences of base pairs along a strand of DNA that form instruction manuals for making specific proteins. These proteins are responsible for the chemical reactions that are essential for life.