The characteristics of all living organisms, including humans, are essentially determined by information contained within DNA that they inherit from their parents. The molecular structure of DNA can be imagined as a zipper with each tooth represented by one of four letters (A, C, G, or T), and with opposite teeth forming one of two pairs, either A-T or G-C. The letters A, C, G, and T stand for adenine, cytosine, guanine, and thymine, the basic building blocks of DNA.
The information contained in DNA is determined primarily by the sequence of letters along the zipper. For example, the sequence ACGCT represents different information than the sequence AGTCC in the same way that the word "POST" has a different meaning from "STOP" or "POTS," even though they use the same letters. The traits of a human being are the result of information contained in the DNA code.
Living organisms that look different or have different characteristics also have different DNA sequences. The more varied the organisms, the more varied the DNA sequences. DNA fingerprinting is a very quick way to compare the DNA sequences of any two living organisms.