The members of each base pair can fit together within the double helix only if the two strands of the helix are antiparallel—that is, only if the polarity of one strand is oriented opposite to that of the other strand (see Figures 4–3 and 4–4). A consequence of DNA’s structure and base-pairing requirements is that each strand of a DNA molecule contains a sequence of nucleotides that is exactly complementary to the nucleotide sequence of its partner strand.
Figure 4–3 DNA and its building blocks.
DNA is made of four types of nucleotides, which are linked covalently into a polynucleotide chain (a DNA strand) with a sugar-phosphate backbone from which the bases (A, C, g, and T) extend. A DNA molecule is composed of two antiparallel DNA strands held together by hydrogen bonds between the paired bases. The arrowheads at the ends of the DNA strands indicate the polarities of the two strands. In the diagram at the bottom left of the figure, the DNA molecule is shown straightened out; in reality, it is twisted into a double helix, as shown on the right. For details, see Figure 4–5 and Movie 4.1.
The Structure of DNA Provides a mechanism for Heredity
The discovery of the structure of DNA immediately suggested answers to the two most fundamental questions about heredity. First, how could the information to specify an organism be carried in a chemical form? And second, how could this information be duplicated and copied from generation to generation?
The answer to the first question came from the realization that DNA is a linear polymer of four different kinds of monomer, strung out in a defined sequence like the letters of a document written in an alphabetic script.
The answer to the second question came from the double-stranded nature of the structure: because each strand of DNA contains a sequence of nucleotides that is exactly complementary to the nucleotide sequence of its partner strand, each strand can act as a template, or mold, for the synthesis of a new complementary strand. In other words, if we designate the two DNA strands as S and Sʹ, strand 0.34 nm
Figure 4–4 Complementary base pairs in the DNA double helix. The shapes and chemical structures of the bases allow hydrogen bonds to form efficiently only between A and T and between g and C, because atoms that are able to form hydrogen bonds (see Panel 2–3, pp. 94–95) can then be brought close together without distorting the double helix. As indicated, two hydrogen bonds form between A and T, while three form between g and C. The bases can pair in this way only if the two polynucleotide chains that contain them are antiparallel to each other.
Figure 4–5 The DNA double helix.
A space-filling model of 1.5 turns of the DNA double helix. Each turn of DNA is made up of 10.4 nucleotide pairs, and the center-to-center distance between adjacent nucleotide pairs is 0.34 nm. The coiling of the two strands around each other creates two grooves in the double helix: the wider groove is called the major groove, and the smaller the minor groove, as indicated.
A short section of the double helix viewed from its side, showing four base pairs. The nucleotides are linked together covalently by phosphodiester bonds that join the 3ʹ-hydroxyl (–OH) group of one sugar to the 5ʹ-hydroxyl group of the next sugar. Thus, each polynucleotide strand has a chemical polarity; that is, its two ends are chemically different. The 5ʹ end of the DNA polymer is by convention often illustrated carrying a phosphate group, while the 3ʹ end is shown with a hydroxyl.
S can serve as a template for making a new strand Sʹ, while strand Sʹ can serve as a template for making a new strand S (Figure 4–6). Thus, the genetic information in DNA can be accurately copied by the beautifully simple process in which strand S separates from strand Sʹ, and each separated strand then serves as a template for the production of a new complementary partner strand that is identical to its former partner.