Overview: What Is DNA?
Genes are made of DNA, short for deoxyribonucleic acid. It is a very long molecule made of the nitrogen-carrying bases adenine, guanine, thymine, and cytosine, phosphates, and the sugar deoxyribose. The long molecule is in an unique double helix shape, which allows a large amount of chemical information to be processed, stored, and replicated.
How Was It Discovered?
Scientists knew that the cell nucleus was made up of proteins and a substance called nucleic acid about thirty years before the model of the DNA molecule was put together, but they first thought that genes were contained in proteins within the cell nucleus. Researchers realized that genes were contained in nucleic acid after a series of experiments that involved strains of bacteria. One strain (S) caused mice to develop pneumonia and die but the other, closely-related strain (R) did not. When the S strain was killed and injected into mice, they did not die. However, if the inactive S strain was combined with the R strain, mice developed pneumonia and died. When DNA from the S strain was combined with the R strain, the R strain began developing lethal bacteria, leading scientists to conclude that the DNA, not protein carried genetic instructions.
What Was Learned about DNA?
In the early 1950s, several things were known about DNA. Its chemical composition was known, as well as its long, thin shape. Wilkins and Franklin discovered that the molecule had a distinctive cross-hatching pattern. They used a technique of first crystallizing DNA, then photographing it with X-rays. The molecule formed a long spiral.
What Was the Model of DNA?
Watson and Crick developed a model for the DNA molecule that was based on existing knowledge. It was shaped in a double helix, with long side rails of sugars and phosphates connected by rungs of nitrogen-carrying bases. Adenine was only paired with thymine, and guanine with cytosine to account for the known chemical ratios. Even the smallest virus contains thousands of nucleotide pairs that are arranged in order to carry coded genetic instructions.
What Is DNA Replication?
Every new cell has a complete and exact copy of the parent cell, copied through a replication process. DNA replicates before mitosis, as well as once during mitosis. The molecule splits down the middle, and enzymes cause nucleotides to join together with a new strand. Since adenine can only join with thymine, and guanine with cytosine, the sequence is preserved for the new cell.
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