DNA Contains Four Deoxynucleotides

The chemical nature of the monomeric deoxynucleo-tide units of DNA—deoxyadenylate, deoxyguanylate, deoxycytidylate, and thymidylate—is described in Chapter 33. These monomeric units of DNA are held in polymeric form by 3',5'-phosphodiester bridges constituting a single strand, as depicted in Figure 35-1.

The informational content of DNA (the genetic code) resides in the sequence in which these monomers— purine and pyrimidine deoxyribonucleotides—are ordered. The polymer as depicted possesses a polarity; one end has a 5'-hydroxyl or phosphate terminal while the other has a 3'-phosphate or hydroxyl terminal. The importance of this polarity will become evident. Since the genetic information resides in the order of the monomeric units within the polymers, there must exist a mechanism of reproducing or replicating this specific information with a high degree of fidelity. That requirement, together with x-ray diffraction data from the DNA molecule and the observation of Chargaff that in DNA molecules the concentration of de-oxyadenosine (A) nucleotides equals that of thymidine (T) nucleotides (A = T), while the concentration of de-oxyguanosine (G) nucleotides equals that of deoxycyti-dine (C) nucleotides (G = C), led Watson, Crick, and Wilkins to propose in the early 1950s a model of a double-stranded DNA molecule. The model they proposed is depicted in Figure 35-2. The two strands of this double-stranded helix are held in register by hydrogen bonds between the purine and pyrimidine bases of the respective linear molecules. The pairings between the purine and pyrimidine nucleotides on the opposite strands are very specific and are dependent upon hydrogen bonding of A with T and G with C (Figure 35-3).

This common form of DNA is said to be right-handed because as one looks down the double helix the base residues form a spiral in a clockwise direction. In the double-stranded molecule, restrictions imposed by the rotation about the phosphodiester bond, the favored anti configuration of the glycosidic bond (Figure 33-8), and the predominant tautomers (see Figure 33-3) of the four bases (A, G, T, and C) allow A to pair only with T and G only with C, as depicted in Figure 35-3. This base-pairing restriction explains the earlier observation that in a double-stranded DNA molecule the content of A equals that of T and the content of G equals that of C. The two strands of the double-helical molecule, each of which possesses a polarity, are antiparallel; ie, one strand runs in the 5' to 3' direction and the other in the 3' to 5' direction. This is analogous to two parallel streets, each running one way but carrying traffic in opposite directions. In the double-stranded DNA molecules, the genetic information re o

Figure 35-1. A segment of one strand of a DNA molecule in which the purine and pyrimidine bases guanine (G), cytosine (C), thymine (T), and adenine (A) are held together by a phosphodiester backbone between 2'-de-oxyribosyl moieties attached to the nucleobases by an W-glycosidic bond. Note that the backbone has a polarity (ie, a direction). Convention dictates that a single-stranded DNA sequence is written in the 5' to 3' direction (ie, pGpCpTpA, where G, C, T, and A represent the four bases and p represents the interconnecting phosphates).

Figure 35-1. A segment of one strand of a DNA molecule in which the purine and pyrimidine bases guanine (G), cytosine (C), thymine (T), and adenine (A) are held together by a phosphodiester backbone between 2'-de-oxyribosyl moieties attached to the nucleobases by an W-glycosidic bond. Note that the backbone has a polarity (ie, a direction). Convention dictates that a single-stranded DNA sequence is written in the 5' to 3' direction (ie, pGpCpTpA, where G, C, T, and A represent the four bases and p represents the interconnecting phosphates).

sides in the sequence of nucleotides on one strand, the template strand. This is the strand of DNA that is copied during nucleic acid synthesis. It is sometimes referred to as the noncoding strand. The opposite strand is considered the coding strand because it matches the RNA transcript that encodes the protein.

The two strands, in which opposing bases are held together by hydrogen bonds, wind around a central axis in the form of a double helix. Double-stranded DNA exists in at least six forms (A-E and Z). The B form is usually found under physiologic conditions (low salt, high degree of hydration). A single turn of B-DNA about the axis of the molecule contains ten base pairs. The distance spanned by one turn of B-DNA is 3.4 nm. The width (helical diameter) of the double helix in B-DNA is 2 nm.

As depicted in Figure 35-3, three hydrogen bonds hold the deoxyguanosine nucleotide to the deoxycyti-

dine nucleotide, whereas the other pair, the A-T pair, is held together by two hydrogen bonds. Thus, the G-C bonds are much more resistant to denaturation, or "melting," than A-T-rich regions.

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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