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Location & Composition of Chromosomes

DNA, also known as deoxyribonucleic acid, is a nucleic acid that stores and transmits genetic information from one generation to the next by coding for the production of a cell's proteins. It is contained within the nucleus of an animal cell and carries the genetic code for the organism. DNA is a large complex molecule composed of carbon, oxygen, hydrogen, nitrogen, and phosphorus atoms. Contained within DNA are four nitrogen bases, two of which belong to a group of compounds known as purines: adenine and guanine. The other two, cytosine and thymine, are known as pyrimidines.

Watson and Crick developed the double helix model of DNA structure, which we can see in this animation. After observing an x-ray diffraction photograph taken by Franklin, which showed the DNA pattern and demonstrated that there were probably two strands of DNA, Watson and Crick imagined the two strands twisted around one another, forming the double helix. In their model, the nitrogen bases were located on the inside of the helix, like the rungs of a twisted ladder. Furthermore, they proposed that the two strands of the double helix were held together because each purine (shown in light blue) paired with a pyrimidine (pink) using hydrogen bonds. Bases that pair with each other are referred to as complimentary bases. In DNA, adenine (often abbreviated as A) is complimentary to thymine (T) while cytosine (C) is complimentary to guanine (G). We now know that each animal chromosome consists of a single, long DNA double helix.