23 - Molecular Biology of the Gene Hershey and Chase showed...

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Molecular Biology of the Gene Hershey and Chase showed that DNA, not protein, is the molecule that carries hereditary information. Deoxyribonucleic acid [DNA] is a long chain of nucleotides. A nucleotide contains a phosphate group, a 5-carbon sugar and nitrogen containing ring structure. The nitrogen ring structure is referred to a base because of the chemical composition and the entire nucleotide is often called a base There are four unique nucleotides, Adenine, Guanine, Thymine and Cytosine. The four nucleotides of differ by the structure of the nitrogenous base. Adenine and guanine have two rings whereas thymine and cytosine have a single ring. The sugar and phosphate groups are chemically identical. If we look at a single strand of DNA, the individual nucleotides are linked by covalent bonds between the phosphate group and the sugar. This is called the sugar-phosphate backbone. 1
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DNA is a double helix Watson and Crick determined that the structure of DNA is a double helix. This means that two single strands of DNA are bound to each other. The sugar-phosphate backbone is on the outside and the nitrogen bases are in the inside. Hydrogen bonds form between the nitrogen bases and hold the two strands of DNA together. Adenine binds to thymine by two hydrogen bonds. Cytosine binds to guanine by three hydrogen bonds. Why hydrogen bonds? Oxygen and nitrogen pull electrons into their orbit resulting in hydrogen bonding between nitrogen and oxygen molecules on one strand and the hydrogen molecules on the other strand.
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