The Components of DNA

DNA is composed of purine (adenine and guanine) and pyrimidine (cyto-sine and thymine) bases, each connected through a ribose sugar to a phosphate backbone. Many variations are possible in the chemical structure of the bases and the sugar, and in the structural relationship of the base to the sugar that result in differences in helical shape and form. The most common DNA helix, B-DNA, is a double helix of two DNA strands with about 10.5 base pairs per helical turn.

Bases and Base Pairs. The four bases found in DNA are shown in Figures 1 and 2. The purines and pyrimidines are the informational molecules of the genetic blueprint for the cell. The two sides of the helix are held together by hydrogen bonds between base pairs. Hydrogen bonds are weak attractions between a hydrogen atom on one side and an oxygen or nitrogen atom on the other. Hydrogen atoms of amino groups serve as the hydrogen bond donor while the carbonyl oxygens and ring nitrogens serve as hydrogen bond acceptors. The specific location of hydrogen bond donor and acceptor groups gives the bases their specificity for hydrogen bonding in unique pairs. Thymine (T) pairs with adenine (A) through two hydrogen bonds, and cytosine (C) pairs with guanine (G) through three hydrogen bonds (Figure 2). T does not normally pair with G, nor does C normally pair with A.

base pairs two nucleotides (either DNA or RNA) linked by weak bonds hydrogen bonds weak bonds between the H of one molecule or group and a nitrogen or oxygen of another

nucleotide (base + sugar + PO4)

Figure 1. DNA nucleotides include a base, a deoxyribose sugar, and a phosphate. The carbons on the sugar are numbered 1' ("one prime") to 5'. The phosphate group links the 3' end of one nucleotide to the 5' end of the next. The phosphates are negatively charged, promoting their attraction to positively charged histone proteins (not shown).

i the Greek letter beta nucleotide the building block of RNA or DNA

Deoxyribose Sugar. In DNA the bases are connected to a ยก-D-2-deoxyri-bose sugar with a hydrogen atom at the 2' ("two prime") position. The sugar is a very dynamic part of the DNA molecule. Unlike the nucleotide bases, which are planar and rigid, the sugar ring is easily bent and twisted into various conformations (which exist in different structural forms of DNA). In canonical B-DNA, the accepted and most common form of DNA, the sugar configuration is known as C2' endo.

Nucleosides and Nucleotides. The term "nucleoside" refers to a base and sugar. "Nucleotide," on the other hand, refers to the base, sugar, and phosphate group (Figure 1). A bond, called the glycosidic bond, holds the base to the sugar and the 3'-5' ("three prime-five prime") phosphodiester bond holds the individual nucleotides together. Nucleotides are joined from the 3' carbon of the sugar in one nucleotide to the 5' carbon of the sugar of the adjacent nucleotide. The 3' and the 5' ends are chemically very distinct and have different reactive properties. During DNA replication, new nucleotides are added only to the 3' OH end of a DNA strand. This fact has important implications for replication.

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