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Molecular Basis of Heredity: Part 1. Nucleic Acids

Author(s): Raye L. Alford, PhD

The Molecular Components of DNA and RNA (II)

The bases of DNA and RNA are attached, as shown in the drawings, to the 1′ (one prime) carbon of a pentose (five carbon) sugar. This pentose sugar is part of the sugar-phosphate backbone of the DNA and RNA macromolecule.

Also attached to the pentose sugar is a triphosphate group. The triphosphate group is attached to the 5′ (five prime) carbon of the pentose sugar. Together, the triphosphate group, the pentose sugar and the base make a molecule called a nucleotide.

Nucleotides frequently are used as units of measure to describe the length of a DNA or RNA molecule. For example, a segment of DNA or RNA often is said to a specified number of nucleotides long. In addition, nucleotides can be a measure of distance. In this case, the nucleotide unit describes the number of intervening nucleotides between one position in a DNA or RNA molecule and another. For example, one might say the mutation in gene A that causes disease B is 25 nucleotides away from the first nucleotide of the gene.

A deoxynucleotide bearing an adenine base is called a 2'-deoxyadenosine-5'-triphosphate, or dATP. A deoxynucleotide bearing a cytosine base is called a 2'-deoxycytidine-5'-triphosphate, or dCTP. A deoxynucleotide bearing a guanine base is called a 2'-deoxyguanosine-5'-triphosphate, or dGTP. A deoxynucleotide bearing a thymine base is called a 2'-deoxythymidine-5'-triphospate, or dTTP.

A ribonucleotide bearing an adenine base is called an adenosine 5'-triphosphate, or ATP. A ribonucleotide bearing a cytosine base is called a cytidine 5'-triphosphate, or CTP. A ribonucleotide bearing a guanine base is called a guanosine 5'-triphosphate, or GTP. A ribonucleotide bearing a uracil base is called a uridine 5'-triphosphate, or UTP.

Note in the drawing on the left that there is only one hydroxyl group (-OH) attached to the pentose sugar, at the 3′ carbon position. This makes this pentose sugar a deoxyribose sugar. The deoxyribose sugar is the sugar that makes up DNA.

Note in the drawing on the right that there are two hydroxyl (-OH) groups attached to the pentose sugar, at the 2′ and 3′ carbon positions. This makes the pentose sugar a ribose sugar. The ribose sugar is the sugar that makes up RNA.