DNA contains the instructions for building and maintaining living organisms. It is found in the cells of all living things and takes the form of a double helix composed of nucleotide base pairs (A, T, C, G) linked by hydrogen bonds. DNA can store vast amounts of information - a single gram has the capacity to hold 215 million gigabytes of data, while the entire human genome requires just 3 gigabytes of storage. DNA ensures that all humans are 99.9% similar genetically while still allowing for the unique differences between individuals.