Mrh

Uracil (U)

(present in RNA)

Phosphate

110.11 A nucleotide contains a phosphate group.

and G), which differ in the positions of their double bonds and in the groups attached to the six-sided ring. There are three pyrimidines found in nucleic acids: cytosine (C), thymine (T), and uracil (U). Cytosine is present in both DNA and RNA; however, thymine is restricted to DNA, and uracil is found only in RNA. The three pyrimidines differ in the groups or atoms attached to the carbon atoms of the ring and in the number of double bonds in the ring. In a nucleotide, the nitrogenous base always forms a cova-lent bond with the 1'-carbon atom of the sugar (see Figure 10.9). A deoxyribose (or ribose) sugar and a base together are referred to as a nucleoside.

The third component of a nucleotide is the phosphate group, which consists of a phosphorus atom bonded to four oxygen atoms ( FIGURE 10.11). Phosphate groups are found in every nucleotide and frequently carry a negative charge, which makes DNA acidic. The phosphate is always bonded to the 5'-carbon atom of the sugar in a nucleotide (see Figure 10.9).

The DNA nucleotides are properly known as deoxyri-bonucleotides or deoxyribonucleoside 5'-monophosphates. Because there are four types of bases, there are four different kinds of DNA nucleotides ( FIGURE 10.12). The equivalent RNA nucleotides are termed ribonu-cleotides or ribonucleoside 5'-monophosphates. RNA molecules sometimes contain additional rare bases, which are modified forms of the four common bases. These modified bases will be discussed in more detail when we examine the function of RNA molecules in Chapter 14.

Concepts)"

The primary structure of DNA consists of a string of nucleotides. Each nucleotide consists of a five-carbon sugar, a phosphate, and a base. There are two types of DNA bases: purines (adenine and guanine) and pyrimidines (thymine and cytosine).

Polynucleotide strands DNA is made up of many nucleotides connected by covalent bonds, which join the 5'-phosphate group of one nucleotide to the 3'-carbon atom of the next nucleotide ( FIGURE 10.13). These bonds, called phosphodiester linkages, are relatively strong cova-lent bonds; a series of nucleotides linked in this way constitutes a polynucleotide strand. The backbone of the polynucleotide strand is composed of alternating sugars and phosphates; the bases project away from the long axis of the strand. The negative charges of the phosphate groups are frequently neutralized by the association of positive charges on proteins, metals, or other molecules.

An important characteristic of the polynucleotide strand is its direction, or polarity. At one end of the strand a phosphate group is attached only to the 5'-carbon atom of the sugar in the nucleotide. This end of the strand is therefore referred to as the 5'end. The other end of the strand, referred to as the 3'end, has an OH group attached to the 3'-carbon atom of the sugar.

RNA nucleotides also are connected by phosphodiester linkages to form similar polynucleotide strands (see Figure 10.13).

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