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6.3 Genetic Information in Nucleotide Sequence 193

6.3 Genetic Information in synthesizing a stretch of RNA (a process called transcrip-
Nucleotide Sequence tion) or DNA (a process called replication) complemen-
tary to a specific sequence.

learning objectives Some Genetic Information Is Accessible
1. Explain how DNA stores complex information. Without Unwinding DNA
2. Compare the two ways in which the information in DNA Some proteins can recognize and bind to specific base pair
may be accessed by proteins. sequences in double-stranded DNA (Fig. 6.15). This infor-
3. Describe the structural differences between DNA and RNA. mation emerges mainly from differences between the four
bases that appear in the major and minor grooves. Within
the grooves, certain atoms at the periphery of the bases are
The information content of DNA resides in the sequence of exposed, and particularly in the major groove, these atoms
its bases. The four bases in each chain are like the letters of may assume spatial patterns that provide chemical infor-
an alphabet; they may follow each other in any order, and mation. Proteins can access this information to sense the
different sequences spell out different “words.” Each word base sequence in a stretch of DNA without disassembling
has its own meaning, that is, its own effect on phenotype. the double helix. Sequence-specific DNA-binding proteins
AGTCAT, for example, means one thing, while CTAGGT include transcription factors that turn genes on and off
means another. Although DNA has only four different let- (Chapters 16 and 17) as well as bacterial restriction en-
ters, or building blocks, the potential for different combina- zymes that cut DNA at particular sites (Chapter 9).
tions and thus different sets of information in a long chain
of nucleotides is staggering. Some human chromosomes,
for example, are composed of chains that are 250 million In Some Viruses, RNA Is the Repository of
nucleotides long; because the different bases may follow Genetic Information
each other in any order, such chains could contain any one
of 4 250,000,000 (which translates to 1 followed by 150,515,000 DNA carries the genetic information in all cellular forms of
zeros) potential nucleotide sequences. life and in many viruses. Prokaryotes such as Escherichia
coli bacteria carry their DNA in a double-stranded, cova-
lently closed circular chromosome. Eukaryotic cells pack-
Most Genetic Information Is Read age their DNA in double-stranded linear chromosomes.
from Unwound DNA Chains DNA viruses carry it in small molecules that are single- or
double-stranded, circular, or linear.
The unwinding of a DNA molecule exposes a single By contrast, retroviruses, which include those that
sequence of bases on each of two strands (Fig. 6.14). Pro- cause polio and AIDS, use ribonucleic acid, or RNA as
teins read the information in a single DNA strand by their genetic material.

Figure 6.14 DNA stores information in the sequence of Figure 6.15 Proteins bind to specific sequences in DNA.
its bases. A partially unwound DNA double helix. Note that Computer artwork of the E. coli catabolite gene activator protein
different structural information is available in the double-stranded (CAP) bound to DNA (green and orange). The structure of CAP is
and unwound regions of the molecule. shown as a series of cylinders and ribbons. CAP can recognize
Base Icons specific sites in the major groove of double-helical DNA.
Purines © Dr. Tim Evans/Science Source
=
Adenine (A)
=
Guanine (G)
Pyrimidines
=
A T
Thymine (T) G C
= T A
Cytosine (C) G C
A T
T A