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A Winding Staircase
A Winding Staircase
A DNA molecule is a nucleic acid. It is shaped like a spiral staircase and is composed of two parallel strands of linked subunits. This spiral shape is known as a double helix. Each strand is made up of linked subunits called nucleotides.
A DNA molecule is a nucleic acid. It is shaped like a spiral staircase and is composed of two parallel strands of linked subunits. This spiral shape is known as a double helix. Each strand is made up of linked subunits called nucleotides.
Parts of the Nucleotide
Parts of the Nucleotide
Each nucleotide is made up of three parts: a phosphate group, a five-carbon sugar molecule, and a nitrogen base.
The phosphate groups and the sugar molecules of nucleotides link together to form a “backbone” for a DNA strand. The five-carbon sugar in DNA is called deoxyribose, from which DNA gets its full name, deoxyribonucleic acid. The bases of nucleotides pair together to connect the two strands.
Nitrogenous Bases
Nitrogenous Bases
In DNA, each nucleotide has the same sugar molecule and phosphate group, but the nucleotide can have one of four nitrogenous bases. The four kinds of bases are adenine (A), guanine (G), thymine (T), and cytosine (C).
Base-Pairing Rules
Base-Pairing Rules
A nitrogen base on one strand of a DNA molecule is always paired with a nitrogen base on the other strand. More specifically, adenine always pairs with thymine, and guanine always pairs with cytosine. These base-pairing rules are dictated by the chemicalstructure of the bases. The structure and size of the nitrogenous bases allow for only these two pair combinations. The base pairs are held together by weak hydrogen bonds. The hydrogen bonds between bases keep the two strands of DNA together.
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