The term “DNA” stands for Deoxyribonucleic Acid. DNA is a long thread-like molecule that serves as the blueprint for all life on Earth. It is a double-stranded molecule that is in the shape of a DOUBLE HELIX. The Double Helix structure of DNA was unveiled in the early 1950’s by two scientists named James D. Watson and Francis H. Crick. Along with the help of Maurice Wilkins and Rosalind Franklin who both contributed to the discovery with their work in X-Ray crystallography. To give you an idea of what a double helix structure looks like, imagine a long wooden step ladder, the kind that you have to lean up against a wall to climb. Now imagine that the ladder is flexible like rubber and you can twist it from top to bottom. When you twist both ends in opposite directions, the resulting shape of our imaginary step ladder would look very much like the double helix of the DNA molecule. The rungs of the ladder are representative of chemical subunits that make up the DNA molecule. These chemical subunits are called Nitrogenous Bases. There are 4 different kinds of Nitrogenous Bases (or just Bases as geneticists prefer to call them) that make up the rungs of the twisted DNA step ladder. They are: Guanine, Cytosine, Adenine and Thymine.
DNA and Cell Theory
In order to understand DNA we must first look at the cell theory. It was in 1665 a scientist named Robert Hooke observed the honeycomb-shaped structures of a wedge of cork under his primitive microscope. He described these structures, which were actually cell walls of dead plants, as cells. Hooke's first observations of cells eventually lead to the establishment of the cell theory.
As time passed, microscopes were improved and the biological knowledge base grew. In the 1830's, almost two centuries after Hooke's observations, Robert Brown observed a small and dark-staining sphere inside plant cells. He called this structure a nucleus. Brown's discovery was a key step in the development of the basic cell theory.
In the 1830's, Theodor Schwann and Mathias Schleiden (zoologist and botanist respectively) concluded that the nucleus plays a chief role in the growth and development of living cells. They established the basis of the cell theory. The modern cell theory states that all living organisms consist of one or more nucleated cells which are the fundamental unit of function of living organisms and that cells come from existing cells.
The structure of DNA
Once accepted and confirmed that DNA was the source of hereditary information it was a race to discover its structure in hopes of better understanding this fascinating molecule. There were many people who contributed to the discovery of the structure of DNA but it was James Watson and Francis Crick who ultimately developed the first three-dimensional model of a DNA strand. Rosalind Franklin made a significant contribution to the discovery of the structure of DNA with the information she gathered using during X-ray diffraction. At this time, scientists knew that DNA consisted of sugars (deoxyribose sugar), phosphates, and four different nitrogen bases: adenine, guanine cytosine, and thymine. Scientists also discovered, by comparing the DNA in various animals, that the proportion of each nitrogen base in the DNA of different species of animals varies but number of adenine molecules equals to the number of thymine molecules, and the number of guanine molecules equals to the number of cytosine molecules. Using these vital pieces of information, Watson and Crick worked together to evolve the first three-dimensional model of the structure of DNA in 1953. Watson and Crick's model is still in use today.
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