Structure of DNA

The millions of atoms in a DNA molecule are organized in a simple, elegant chain of building blocks called nucleotides. Each nucleotide consists of three parts: (1) a nitrogen-containing compound, called a base; (2) a 5-carbon sugar, named deoxyribose; and (3) a phosphate group. Both the nitrogenous base and the phosphate group are bonded to the sugar (Fig. 13.2). Four types of nucleotides occur in DNA. Each has a unique nitrogenous base, but all have the same phosphate group and deoxyribose sugar. Two of the bases—adenine (A) and guanine (G)—are called purines. Each has a molecular structure that resembles two linked rings. The other two bases— cytosine (C) and thymine (T)—are called pyrimidines. They each have a molecular structure consisting of a single ring.

The nucleotides in a DNA molecule are bonded to each other in such a way that they form a chain that looks like a ladder twisted into a spiral, or a helix. Each of the two sides of the

Structure Flowering Plants

Figure 13.2 Structure of a DNA molecule. In this enlargement of a small portion of a DNA molecule, the rungs of the twisted ladder formed by the two entwined spiraling strands consist of nitrogen-containing bases supported by alternating units of sugar (S) and phosphate (P) molecules. The purines adenine or guanine (A or G) occur opposite the pyrimidines thymine or cyto-sine (T or C). The purines and pyrimidines opposite each other are held together by hydrogen bonds linking the nitrogenous bases of the paired molecules. The double helix is 2 nanometers wide.

Figure 13.2 Structure of a DNA molecule. In this enlargement of a small portion of a DNA molecule, the rungs of the twisted ladder formed by the two entwined spiraling strands consist of nitrogen-containing bases supported by alternating units of sugar (S) and phosphate (P) molecules. The purines adenine or guanine (A or G) occur opposite the pyrimidines thymine or cyto-sine (T or C). The purines and pyrimidines opposite each other are held together by hydrogen bonds linking the nitrogenous bases of the paired molecules. The double helix is 2 nanometers wide.

"ladder" is composed of alternating sugar and phosphate groups (Fig. 13.3). Each sugar is also bound to a base. Hydrogen bonds hold each base on one side of the helix to another base on the other side, making the "rungs" of the ladder. Although the structure of a DNA molecule is very simple—alternating sugar and phosphate groups on the outside and pairs of bases in the middle—it provides all the genetic diversity on the planet. The variation comes in the form of base pairs. The sequence of base pairs on a DNA molecule determines whether, for example, that DNA will direct the synthesis of a chlorophyll molecule or a hemoglobin molecule.

As mentioned earlier, in the early 1950s, James Watson and Francis Crick began to unravel the mystery of DNA structure. Much of their work involved piecing together existing information to develop a model. They knew Linus Pauling had postulated that the structure of DNA might be similar to the structure of protein. Pauling had shown part of the structure of some proteins to be helical and maintained by hydrogen bonds between the amino acids. Watson and Crick also knew from X-ray work by Rosalind Franklin and Maurice Wilkins that DNA molecules are composed of regularly repeating units in a helical arrangement. Finally, studies

Genetics 233

Figure Watson And Creek Model Dna
Figure 13.3 The pairing of nucleotides in a tiny portion of a molecule of DNA. The variations in sequences of pairs are virtually unlimited.

by Erwin Chargaff indicated that the amount of pyrimidine nucleotides (cytosine + thymine) in DNA equals the amount of purine nucleotides (adenine + guanine). This suggested to Watson and Crick that purines pair with pyrimidines.

When they had put together all the facts, Watson and Crick concluded that a DNA molecule consists of a double helix whose two strands appear to be wrapped around an invisible pole—one strand spiraling in one direction and the other strand spiraling at the same angle in the opposite direction. They also concluded that nucleotides are linked in ladder-like fashion between the two strands. Furthermore, in order for the nucleotides to fit precisely in the DNA molecule, two linked purines or two linked pyrimidines would either be too wide or too narrow. However, a purine and a pyrmidine linked together would fit perfectly. Accordingly, they concluded that the ladder rungs had to consist exclusively of purine-pyrimidine pairs. Guanine always pairs with cytosine, and thymine always pairs with adenine. Watson and Crick's DNA molecule is now universally accepted as an authentic representation.

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