SUMMARY

• 33.1 A Nucleic Acid Consists of Bases Linked to a Sugar–Phosphate Backbone

  DNA and RNA are linear polymers of a limited number of monomers. In DNA, the repeating units are nucleotides with the sugar being a deoxyribose and the bases being adenine, thymine, guanine, and cytosine. In RNA, the sugar is a ribose and the base uracil is used in place of thymine. DNA is the molecule of heredity in all prokaryotic and eukaryotic organisms.

• 33.2 Nucleic Acid Strands Can Form a Double-Helical Structure

  All cellular DNA consists of two very long, helical polynucleotide strands coiled around a common axis. The sugar–phosphate backbone of each strand is on the outside of the double helix, whereas the purine and pyrimidine bases are on the inside. The two strands of the double helix run in opposite directions. The two strands are held together by hydrogen bonds between pairs of bases: adenine is always paired with thymine, and guanine is always paired with cytosine. The sequence of one strand thus determines the sequence of the other. This sequence determination makes DNA an especially appropriate molecule for storing genetic information, which is the precise sequence of bases along a strand. The strands can be separated, and each separated strand can be used to make a double helix identical with the original molecule.

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• 33.3 DNA Double Helices Can Adopt Multiple Forms

  DNA is a structurally dynamic molecule that can exist in a variety of helical forms: A-DNA, B-DNA (the classic Watson–Crick helix), and Z-DNA. DNA can be bent, kinked, and unwound. In A-, B-, and Z-DNA, two antiparallel strands are held together by Watson–Crick base pairs and by stacking forces between bases in the same strand. The sugar–phosphate backbone is on the outside of the helix, and the bases are inside. A- and B-DNA are right-handed helices. In B-DNA, the base pairs are nearly perpendicular to the helix axis. An important structural feature of the B helix is the presence of major and minor grooves, which display different potential hydrogen-bond acceptors and donors according to the base sequence. Z-DNA is a left-handed helix. Most of the DNA in a cell is in the B form.

  Double-stranded DNA can also wrap around itself to form supercoiled structures. The supercoiling of DNA has two important consequences. Supercoiling compacts the DNA and, because supercoiling is left-handed, the DNA is partly unwound and more accessible for processes such as replication and transcription.

• 33.4 Eukaryotic DNA Is Associated with Specific Proteins

  Eukaryotic DNA is tightly bound to proteins, most notably to a group of small basic proteins called histones. The entire complex of a cell’s DNA and associated protein is called chromatin. Five major histones are present in chromatin: four histones—H2A, H2B, H3, and H4—associate with one another; the other histone is H1. Chromatin is made up of nucleosomes—repeating units, each containing 200 bp of DNA and two copies each of H2A, H2B, H3, and H4. The DNA forms a left-handed superhelix as it wraps around the outside of a nucleosome. The winding of DNA around the nucleosome core contributes to DNA’s packing. The nucleosomes themselves are arranged into two intertwined left-handed helical stacks.

• 33.5 RNA Can Adopt Elaborate Structures

  Although RNA is usually single stranded, the single strand folds back on itself to form elaborate structures. The simplest structure is the stem-loop stabilized by Watson–Crick base pairs. In other cases, hydrogen-bond donors and acceptors that are not normal participants in Watson–Crick base pairs form hydrogen bonds in nonstandard pairings.