The following components of deoxyribonucleotide (DNA) have been described: 1. A Five Carbon Ring 2. Nitrogenous Base 3. A Phosphate Group.
1. A Five Carbon Ring:
Deoxyribose is a pentose sugar consisting of five carbon atoms. The first and fourth carbon atoms of this sugar combine with one oxygen atom and form a ring. The fifth atom (5′) forms –CH2 group which is present outside this ring.
Three -OH groups are attached at position 1′, 3′ and 5′ and the hydrogen atoms combine at position 1′, 2′, 3′ and 4′ of carbon atoms (Fig 5.1). In ribonucleotides, the pentose sugar is ribose which is similar to deoxyribose except that there is an -OH group instead of -H at 2′ carbon atom. The absence of -OH group in DNA makes it chemically more stable than the RNA.
2. Nitrogenous Base:
There are two nitrogenous bases, purines and pyrimidines. The purines are double ring compounds that consist of 5- membered imidazole ring with nitrogen at 1′, 3′, 7′ and 9′ positions. The pyrimidines are single ring compounds, the nitrogen being at position r and 3′ in 6- membered benzene ring (Fig 5.2).
A single base is attached to I’-carbon atom of pentose sugar by an AT-glycosidic bond. Purines are of two types, adenine (A) and guanine (G), and pyrimidines are also of two types, thymine (T) and cytosine (C). Uracil (U) is a third pyrimidine (Fig 5.3). A, G and C are common in both DNA and RNA. U is found only in RNA.
3. A Phosphate Group:
In DNA a phosphate group (PO4-3) is attached to the 3′-carbon of deoxyribose sugar and 5′-carbon of another sugar. Therefore, each strand contains 3′ end and 5′ end arranged in an alternate manner. Strong negative charges of nucleic acid are due to the presence of phosphate groups. A nucleotide is a nucleoside phosphate which contains its bond at 3′ and 5′ carbon atoms of pentose sugar that is called phosphodiester (Fig 5.4).
