DNA is a nucleic acid, which is the part of the cell that contains molecules that store information. DNA is made of a pattern of four different nucleotides, which are bound on one end to a phosphate group and on the other to a nitrogenous base. The nitrogenous base can be one of four types: adenine, cytosine, guanine or thymine. The nucleotides are joined in long strings to make up DNA and these two strands twist together to create DNA's characteristic double helix. Each strand has the sugar or phosphate end of the nucleotides. The sides are joined together by pairs of complementary nucleotides.
DNA stores the information that determines how you look, and that information is put into action by proteins. That means that DNA has to be able to build a protein, which is a chain of chemicals called amino acids. DNA has genes that know how to make necessary proteins. Twenty different amino acids can be used to create a protein, and DNA's four bases are responsible for arranging them in the right order for the desired protein. The bases are arranged in groups of three, called codons, and the groups work together.
One DNA molecule - - like those in E. coli bacteria, for instance - - can contain 3,000 genes. The space it takes to code all of that information would require a DNA molecule to measure about 1 millimeter long. But an E. coli bacterium is only about 3 microns long. DNA manages to fit inside and stay so small because it's tightly wound up into a circular chromosome. In complex organisms, there can be anywhere from 50,000 to 100,000 genes on the various chromosomes. In their cells, the DNA is actually wound around a protein called a histone, and the histones are coiled into chromosomes.
Cells must replicate - - or copy - - their DNA before they can reproduce. To do so, the DNA's double helix basically unzips and each side serves as a template. An enzyme called helicase unwinds the two strands of DNA and little proteins called single strand binding proteins attach to each side to keep them separated. Then, a different enzyme called DNA polymerase travels down each strand and attaches new nucleotides to match the existing ones. After the new nucleotides are added, a different part of the polymerase checks them to make sure they're right. Then the DNA ligase enzyme closes the whole thing off to create a long, continuous strand of DNA that winds itself up.
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