The DNA structure

Even before the structure of DNA was elucidated, genetic studies indicated that the hereditary material had to have three key properties:

1. Because essentially every cell in the body of an organism has the same genetic makeup, it is crucial that the genetic material be faithfully replicated at every cell division. Thus the structural features of DNA must allow faithful replication. These structural features will be considered later in this chapter.

2. Because it must encode the constellation of proteins expressed by an organism, the genetic material must have informational content. How the information coded in DNA is deciphered to produce proteins will be the subject of Chapters 8 and 9.

3. Because hereditary changes, called mutations, provide the raw material for evolutionary selection, the genetic material must be able to change on rare occasion. Nevertheless, the structure of DNA must be relatively stable so that organisms can rely on its encoded information. We will consider the mechanisms of mutation in Chapter 14.

DNA structure before Watson and Crick

Consider the discovery of the double-helical structure of

DNA by Watson and Crick as the solution to a compli

6 Chapter 7 • DNA: Structure and Replication cated three-dimensional puzzle. Incredibly, Watson and Crick were able to put this puzzle together without doing a single experiment. Rather, they used a process called "model building" in which they assembled the results of earlier experiments (the puzzle pieces) to form the three-dimensional puzzle (the double-helix model). To understand how they did so, we first need to know what pieces of the puzzle were available to Watson and Crick in 1953.

THE BUILDING BLOCKS OF DNA The first piece of the puzzle was knowledge of the basic building blocks of DNA. As a chemical, DNA is quite simple. It contains three types of chemical components: (1) phosphate, (2) a sugar called deoxyribose, and (3) four nitrogenous bases —adenine, guanine, cytosine, and thymine. The carbon atoms in the bases are assigned numbers for ease of reference. The carbon atoms in the sugar group also are assigned numbers, in this case followed by a prime (1', 2', and so forth). The sugar in DNA is called "deoxyribose" because it has only a hydrogen atom (H) at the 2' carbon atom, unlike ribose (of RNA), which has a hydroxyl (OH) group at that position. Two of the bases, adenine and guanine, have a double-ring structure characteristic of a type of chemical called a purine. The other two bases, cytosine and thymine, have a single-ring structure of a type called a pyrimidine. The chemical components of DNA are arranged into groups called nucleotides, each composed of a phosphate group, a deoxyribose sugar molecule, and any one of the four bases (Figure 7-5). It is convenient to refer to each nucleotide by the first letter of the name of its base: A, G, C, or T. The nucleotide with the adenine base is called deoxyadenosine 5'-monophosphate, where the 5' refers to the position of the carbon atom in the sugar ring to which the single (mono) phosphate group is attached.

CHARGAFF'S RULES OF BASE COMPOSITION The second piece of the puzzle used by Watson and Crick came from work done several years earlier by Erwin Chargaff. Studying a large selection of DNAs from different organisms (Table 7-1), Chargaff established cer

Phosphate Group PuzzlesPurine Table

Figure 7-5 Chemical structures of the four nucleotides found in DNA. These nucleotides, two with purine bases and two with pyrimidine bases, are the fundamental building blocks of DNA. The sugar is called deoxyribose because it is a variation of a common sugar, ribose, that has one more oxygen atom (position indicated by the red arrow).

Figure 7-5 Chemical structures of the four nucleotides found in DNA. These nucleotides, two with purine bases and two with pyrimidine bases, are the fundamental building blocks of DNA. The sugar is called deoxyribose because it is a variation of a common sugar, ribose, that has one more oxygen atom (position indicated by the red arrow).

7.2 The DNA Structure

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