Pd

Deoxyguanylic acid

Deoxycytidylic acid

Figure 3-5

Figure 3-5

Figure 3-4

Deoxyadenylic acid, one of the nucleotides that make up DNA.

Figure 3-6

Arrangement of deoxyribose nucleotides in a double strand of DNA.

bound together to form two strands of DNA. The two strands are, in turn, loosely bonded with each other by weak cross-linkages, illustrated in Figure 3-6 by the central dashed lines. Note that the backbone of each DNA strand is comprised of alternating phosphoric acid and deoxyribose molecules. In turn, purine and pyrimidine bases are attached to the sides of the deoxyribose molecules. Then, by means of loose hydrogen bonds (dashed lines) between the purine and pyrimidine bases, the two respective DNA strands are held together. But note the following:

1. Each purine base adenine of one strand always bonds with a pyrimidine base thymine of the other strand, and

2. Each purine base guanine always bonds with a pyrimidine base cytosine.

Thus, in Figure 3-6, the sequence of complementary pairs of bases is CG, CG, GC,TA, CG, TA, GC, AT, and AT. Because of the looseness of the hydrogen bonds, the two strands can pull apart with ease, and they do

Symbols for the four nucleotides that combine to form DNA. Each nucleotide contains phosphoric acid (P), deoxyribose (D), and one of the four nucleotide bases: A, adenine; T, thymine; G, guanine; or C, cytosine.

so many times during the course of their function in the cell.

To put the DNA of Figure 3-6 into its proper physical perspective, one could merely pick up the two ends and twist them into a helix. Ten pairs of nucleotides are present in each full turn of the helix in the DNA molecule, as shown in Figure 3-2.

Genetic Code

The importance of DNA lies in its ability to control the formation of proteins in the cell. It does this by means of the so-called genetic code. That is, when the two strands of a DNA molecule are split apart, this exposes the purine and pyrimidine bases projecting to the side of each DNA strand, as shown by the top strand in Figure 3-7. It is these projecting bases that form the genetic code.

Symbols for the four nucleotides that combine to form DNA. Each nucleotide contains phosphoric acid (P), deoxyribose (D), and one of the four nucleotide bases: A, adenine; T, thymine; G, guanine; or C, cytosine.

Figure 3-6

Arrangement of deoxyribose nucleotides in a double strand of DNA.

P

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