In eukaryotic organisms, the majority of DNA is found within the nucleus of the cell in the form of a chain of chromosomes. Eukaryotes also have extra chromosomal DNA that is found within various cell organelles located outside of the nucleus, primarily in the mitochondria and chloroplasts.
The mitochondria contains only a very limited amount of DNA, which is arranged in small, circular molecules. These molecules of mitochondrial DNA contain only a few mitochondrial genes, which are used to synthesize proteins. All organisms that are able to perform photosynthesis also have a small amount of DNA within their chloroplasts. This DNA can come in the form of either a circular molecule or a linear chain. Both types are also used in certain synthesis functions, just as the mitochondrial DNA.
The DNA in the chromosome comes in the form of two strands that are joined together in a spiral shaped double helix. Each strand is made up of various nucleotides, each of which is also made up of several separate parts. The nucleotides each consist of a phosphate group, a sugar and one of four different nitrogen bases: adenine, guanine, cytosine or thymine. When these nucleotides join together to form a strand of DNA, the order of the specific nitrogen bases determines what genes are expressed.