The Chemistry of DNA
DNA is the inherited genetic material that controls gene expression through protein synthesis. This is a fancy way of saying that DNA is a chemical form of a genetic blueprint. The genetic instructions contained in DNA are housed in 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.
A nucleotide is a chemical unit that has three main components. The first is a sugar with five carbon atoms, aptly named a five-carbon sugar or pentose sugar. The second is a phosphate atom bonded to four oxygen atoms, called a phosphate group. The third is one of four possible nitrogen-containing bases called A for adenine, C for cytosine, T for thymine, or G for guanine.
The four bases, A, C, T, or G, are categorized into two types: Purines and pyrimidines. Purines are chemically a double-ringed structure, whereas pyrimidines form a single ring. Adenine and guanine are classified as purines, while cytosine and thymine are pyrimidines.
The chemical structure of DNA depends on the correct chemical interaction between purines and pyrimidines. This interaction allows certain bonding to occur that holds two strands of DNA together. The result is what is known as a double helix.
A DNA molecule is termed a double helix due to its twisted shape. The chemical structure is essentially two strands of DNA held together by weak hydrogen bonds. Each DNA strand looks like one half of a ladder sliced down the middle, but together the strands unite to form a single ladder.
Each strand is a chemical complement of the other. The nucleotides that pair together represent the rungs of the ladder. So, when the halves of the ladder bond, a single ladder is formed.
Properly-functioning genes require the correct pairing of nucleotides. This pairing is based on a strict set of base-pair rules pioneered by James Watson and Francis Crick. Base-pair rules dictate the union of a purine to a pyrimidine. This results in only possible pairings of A with T and C 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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