The Unique Structure of DNA
DNA is the universal inherited material that controls the expression of genes through protein synthesis. In other words, DNA is a unique genetic blueprint. This specific information in this genetic blueprint is contained in our genes.
Genes are individual segments of a DNA molecule. Genes are further broken down into smaller, linear building blocks called nucleotides. Nucleotides are aptly named for the nitrogenous base they contain. Much of biology hinges on the chemistry of DNA.
Nucleotides are made of three separate chemical units. The first unit is a pentose sugar made of five carbon atoms. The second unit is a phosphate group made of four oxygen atoms bonded to a central phosphate atom. The third unit is one of four nitrogenous bases, adenine, thymine, cytosine, or guanine.
The four nitrogen-containing bases are sometimes represented as T, C, A, or G, and are always from of one of two chemical categories: Purines or pyrimidines. Purines form a double-ring, while pyrimidines are chemically a single ring. Guanine and adenine are classified as purines, whereas the pyrimidines are thymine and cytosine.
The chemistry of DNA hinges on precise bonding between pyrimidines and purines. This unique bonding pattern is what unites two DNA strands together. The structure formed from these precise chemical interactions is termed a double helix.
Double helix is the name given to a DNA molecule because of its twisted shape. A DNA molecule is chemically two strands of DNA held together by hydrogen bonding. One strand of DNA might look like a straight ladder cut down the middle, splitting it into a left and right side.
One strand complements the other in a way that is also related to chemical structure. The component nucleotides make the rungs of the ladder. The nucleotides in the first strand of DNA, the left side of the ladder, pair with nucleotides in the second DNA strand or the right side of the ladder. Together, the two DNA strands unite to form what looks like one single ladder.
The proper functioning of genes relies on the precise pairing of component nucleotides. The unique bonding pattern of nucleotides is a result of strict base-pair rules discovered by genetics pioneers, James D Watson and Francis Crick. These pairing rules require pyrimidines pair with purines. This means A always pairs with T and C always pairs with G.
The bonds formed as a result of nucleotide pairing are weak hydrogen bonds that also play an important part in the chemistry of DNA. Each molecule looks like a twisted ladder due to these and other complex chemical bonds between different molecular units. This unique shape is why a DNA molecule is termed a double helix.
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