The double helix in genetics describes the shape of the genetic material known as DNA. The double helix resembles a ladder that is twisted into a spiral. The upright portion of the DNA is composed of sugar and phosphate, and the "rungs" are nucleotides called adenine, thymine, guanine and cytosine.
The DNA molecule contained in the double helix consists of only four nucleotides, which are the "letters" of the genetic alphabet. The nucleotides, known as A, T, G and C in short, are the code that program a cell. Nucleotides' specific sequencing determines the message that a gene sends to a cell. The order of the letters affects the physical characteristics of an organism.
The structure of the double helix was discovered in 1953 by James Watson and Francis Crick. It's now understood that cells' structure and function are determined by the order of the nucleotides that form a gene. Scientists are able to manipulate organisms' structure, function and appearance by manipulating the sequence of the nucleotides in the DNA molecule. Additionally, the totality of human genes were mapped in a project known as the Human Genome Project, completed in 2003. The project was aimed at better understanding the master codes of life and helping to find cures for disease. It is now known that genes are not static, and that our genes are not simply deterministic of human life. Rather, they can be turned off and on depending on environmental circumstances.