Figure 6.2 DNA structure. (a) DNA is a double-helical structure composed of sugars, phosphates, and nitrogenous bases. (b) Each strand of the helix is composed of repeating units of sugars and phosphates, making the sugar-phosphate backbone, and nitrogenous bases. (c) A phosphate, a sugar, and a nitrogenous base comprise the structure of a nucleotide. Adenine and guanine are purines, which have a two-ring structure, and cytosine and thymine are pyrimidines, with a single-ring structure.
helix consists of a long series of chemical building blocks called nucleotides. A nucleotide is made up of a sugar, a phosphate, and a nitrogen-containing base. The sugar in DNA is the 5-carbon sugar deoxyribose. The nitrogen-containing bases, or nitrogenous bases, of DNA have one of four different chemical structures, each with a different name: adenine, guanine, thymine, and cytosine. Nucleotides are joined to each other along the length of the helix by a type of chemical bond formed by the hydroxyl (OH) and phosphate (P) groups—called a phosphodiester bond.
Nitrogenous bases form hydrogen bonds with each other across the width of the helix. Adenine (A) on one strand always pairs with a thymine (T) on the opposite strand. Likewise, guanine (G) always pairs with cytosine (C). The term complementary is sometimes used to describe these pairings. For example, A is complementary to T, and C is complementary to G. Therefore, the order of nu-cleotides on one strand of the DNA helix predicts the order of nucleotides on the other strand. Thus, if one strand of the DNA molecule is composed of nu-cleotides AACGATCCG, we know that the order of nucleotides on the other strand is TTGCTAGGC.
As a result of this base-pairing rule (A pairs with T; G pairs with C), the width of the DNA helix is uniform. There are no bulges or dimples in the structure of the DNA helix since A and G, called purines, are structures composed of two rings while C and T are single-ring structures called pyrimidine. A purine always pairs with a pyrimidine and vice versa, so there are always a total of three rings across the width of the helix. Nitrogenous bases are held together across the width of the helix by weak bonds called hydrogen bonds. A:T base pairs have two hydrogen bonds holding them together. G:C pairs have three hydrogen bonds holding them together.
Each strand of the helix thus consists of a series of sugars and phosphates alternating along the length of the helix, the sugar-phosphate backbone. The strands of the helix align so the nucleotides face "up" on one side of the helix and "down" on the other side of the helix. For this reason the two strands of the helix are said to be anti-parallel.
The overall structure of a DNA molecule can be likened to a rope ladder that is twisted, with the sides of the ladder (the hand rails) composed of sugars and phosphates (the sugar-phosphate backbone) and the rungs of the ladder composed of the nitrogenous-base sequences A,C, G, and T. When the DNA is copied, or replicated, the ladder is split up the middle of the rungs, and base pairs are added according to the base-pairing rule.
As we learned in Chapter 4, every individual who is not an identical twin has a unique set of genes, and therefore a unique sequence of nucleotides, which make up genes.
Scientists used the fact that different individuals have distinct nucleotide sequences to test their hypothesis that the bones buried in the Ekaterinburg grave belonged to the Romanov family. The scientists had to answer the following questions:
1. Which of the bones from the pile were actually different bones from the same individuals?
2. Which of the adult bones could have been from the Romanovs, and which bones could have belonged to their servants?
3. Which two Romanov children were missing from the grave?
4. Are these bones actually from the Romanovs, not some other related set of individuals?
All of these questions were answered using a very powerful technique that takes advantage of differences in DNA sequence, called DNA fingerprinting.
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