Molecular Biology of the Gene

Introduction to DNA

DNA provides the blueprint for all traits an organism possesses. The structure of DNA serves to determine which traits are expressed.
Deoxyribonucleic acid (DNA) is an organic molecule containing coded instructions for the life processes of an organism, which consists of nucleotides bonded together to form two complementary strands. Whether the organism has brown fur, has 14 legs, has green eyes, or reaches sexual maturity after 10 years of growth, it relies on its DNA to provide instructions that generate these traits. In eukaryotic organisms, DNA is found in structures called chromosomes. Different species have different total numbers of chromosomes. For example, humans have 23 pairs, or 46 individual chromosomes. Chimpanzees, the closest living relatives of humans, have 48 individual chromosomes. Fruit flies, on the other hand, only have 8 individual chromosomes. There is even a species of fern that possesses more than 1,000 individual chromosomes. Prokaryotic cells also have DNA, but instead of having chromosomes, they have a circular piece of DNA within their cell membrane. This is called a plasmid. The structure of prokaryotic DNA is helpful when the organism reproduces. Because this DNA is not constrained by a nucleus, it can just be copied and then moved into the new cells.
Prokaryotes have a single circular strand of DNA, the molecule that determines every trait an organism expresses. Eukaryotes have multiple strands in the form of linear chromosomes, structures found in the nucleus.