With the exception of a few viruses, DNA forms the genetic material in all living organisms. In the prokaryotic cells DNA occurs in the cytoplasm. In the eukaryotic cells it is confined largely to the nucleus and forms the main component of chromosomes. A small amount of DNA is found in the cytoplasm in the organelles like mitochondria and plastids. It is called extra- nuclear DNA.
DNA is a polynucleotide of adenine, guanine, cytosine and thymine. Uracil nucleotides are absent. The DNA molecule is composed of two polynucleotide chains.
The structure of the DNA molecule and the arrangement of nitrogenous bases in the two polynucleotide chains, is very well explained by the double helix model proposed by Watson and Crick in 1953.
Watson and Crick
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DNA Double Helix: A Recent Discovery of Enormous Complexity
The DNA Double Helix is one of the greatest scientific discoveries of all time. First described by James Watson and Francis Crick in 1953, DNA is the famous molecule of genetics that establishes each organism’s physical characteristics. It wasn’t until mid-2001, that the Human Genome Project and Celera Genomics jointly presented the true nature and complexity of the digital code inherent in DNA. We now understand that each human DNA molecule is comprised of chemical bases arranged in approximately 3 billion precise sequences. Even the DNA molecule for the single-celled bacterium, E. coli, contains enough information to fill all the books in any of the world’s largest libraries.
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