Page 33 - Genetics_From_Genes_to_Genomes_6th_FULL_Part2

192 Chapter 6 DNA Structure, Replication, and Recombination

Figure 6.12 Z DNA is one variant of the double helix. molecule than a highly evolved protein and for this reason
(a) Typical Watson-Crick B-form DNA forms a right-handed helix with reveals its secrets more easily.”
a smooth backbone. (b) Z-form DNA is left-handed and has an Four basic DNA secrets are embodied in the following
irregular backbone. four questions:
(a) (b)
3' 5' 5' 3' 1. How does the molecule carry information?
2. How is that information copied for transmission to
future generations?
3. What mechanisms allow the information to change?
4. How does DNA-encoded information govern the
expression of phenotype?
The double-helical structure of DNA provides a poten-
tial answer to each of these questions, endowing the mole-
cule with the capacity to carry out all the crucial functions
5' 3' required of the genetic material.
3' 5'
B DNA Right-handed Z DNA Left-handed In the remainder of this chapter, we describe how
DNA DNA DNA’s structure enables it to carry genetic information,
replicate that information with great fidelity, and reorgan-
helixes before replication and expression—these are minor ize the information through recombination. How the infor-
variations on the double-helical theme. Despite such exper- mation changes through mutation and how the information
imentally determined departures of detail, the Watson-Crick determines phenotype are the subjects of Chapters 7 and 8.
double helix remains the model for thinking about DNA struc-
ture. This model describes those features of the molecule that essential concepts
have been preserved through billions of years of evolution.
• The DNA molecule is a double helix composed of two
antiparallel strands, in each of which nucleotides are
DNA Structure Is the Foundation joined by phosphodiester bonds. Hydrogen bonding
of Genetic Function between the complementary bases—A with T, and G with
C—holds the two strands together.
Without sophisticated computational tools for analyzing • Antiparallel means that one strand is oriented in the 5′-to-3′
base sequence, one cannot distinguish bacterial DNA from direction, while the other, complementary strand is oriented
human DNA. The reason is that all DNA molecules have in the 3′-to-5′ direction.
the same general chemical properties and physical struc- • Most eukaryotes have double-stranded, linear DNA, but
ture. Proteins, by comparison, are a much more diverse prokaryotes, chloroplasts and mitochondria, and some
group of molecules with a much greater complexity of viruses have double-stranded circular DNA. Certain other
structure and function. In his account of the discovery of viruses contain a single-stranded DNA that can be linear
the double helix, Crick referred to this difference when he or circular.
said that “DNA is, at bottom, a much less sophisticated

Figure 6.13 DNA molecules may be linear or circular, double-stranded or single-stranded. These electron micrographs of
naturally occurring DNA molecules show: (a) a fragment of a long, linear, double-stranded human chromosome, (b) a circular double-stranded
papovavirus chromosome, (c) a linear single-stranded parvovirus chromosome, and (d) circular single-stranded bacteriophage M13 chromosomes.
a: © Biophoto Associates/Science Source; b: © Yoav Levy/Phototake; c: © Ross Inman & Maria Schnös, University of Wisconsin, Madison, WI; d: © Jack D. Griffith/
University of North Carolina Lineberger Comprehensive Cancer Center

(a) (b) (c) (d)