Download Figure 10-9

The Genetic Material Must
Exhibit Four Characteristics
• For a molecule to serve as the genetic
material, it must be able to replicate, store
information, express information, and allow
variation by mutation.
• The central dogma of molecular genetics is
that DNA makes RNA, which makes proteins
(Figure 10.1).
Figure 10-1
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• The genetic material is physically
transmitted from parent to offspring. Proteins
and nucleic acids were the major candidates
for the genetic material.
• For a long time, protein was favored to be
the genetic material. It is abundant in cells, it
was the subject of the most active areas of
genetic research, and DNA was thought to be
too simple to be the genetic material, with
only four types of nucleotides as compared to
the 20 different amino acids of proteins.
• Griffith showed that avirulent strains of
Diplococcus pneumoniae could be
transformed to virulence (Figure 10.3). He
speculated that the transforming principle
could be part of the polysaccharide capsule or
some compound required for capsule
synthesis.
Figure 10-3
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• Avery, MacLeod, and McCarty demonstrated
that the transforming principle was DNA and
not protein (Figure 10.4).
Figure 10-4
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• Nucleotides are the building blocks of DNA.
They consist of a nitrogenous base, a pentose
sugar, and a phosphate group.
• The nitrogenous bases can be purines or
pyrimidines. The purines are adenine (A) and
guanine (G). The pyrimidines are cytosine
(C), thymine (T), and uracil (U) (Figure
10.9).
Figure 10-9
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Figure 10-9a
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Figure 10-9b
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• DNA and RNA both contain A, C, and G,
but only DNA contains T and only RNA
contains U.
• RNA contains ribose as its sugar; DNA
contains deoxyribose (Figure 10.9).
Figure 10-9
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• A nucleoside contains the nitrogenous base
and the pentose sugar. A nucleotide is a
nucleoside with a phosphate group added
(Figure 10.10).
Figure 10-10
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Figure 10-11
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• Nucleotides are linked by a phosphodiester
bond between the phosphate group at the C-5'
position and the OH group on the C-3'
position (Figure 10.12).
Figure 10-12a
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Figure 10-12b
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• Chargaff showed that the amount of A is
proportional to T and the amount of C is
proportional to G, but the percentage of C + G
does not necessarily equal the percentage of
A + T (Table 10.3).
Table 10-3
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• X-ray diffraction of DNA showed a 3.4
angstrom periodicity, characteristic of a
helical structure (Figure 10.13).
Figure 10-13
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• Watson and Crick proposed DNA is a righthanded double helix in which the two strands
are antiparallel and the bases are stacked on
one another. The two strands are connected by
A-T and G-C base pairing and there are 10
base pairs per helix turn (Figure 10.14).
Figure 10-14a
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Figure 10-14b
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• The A-T and G-C base pairing provides
complementarity of the two strands and
chemical stability to the helix.
• A-T base pairs form two hydrogen bonds
and G-C base pairs form three hydrogen
bonds (Figure 10.16).
Figure 10-16
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12.1 Viral and Bacterial
Chromosomes Are Relatively
Simple DNA Molecules
• Bacterial and viral chromosomes are usually
a single nucleic acid molecule, are largely
devoid of associated proteins, and are much
smaller than eukaryotic chromosomes.
• Bacterial chromosomes are double-stranded
DNA and are compacted into a nucleoid.
• DNA in bacteria may be associated with HU
and H DNA-binding proteins.
12.2 Supercoiling Is Common in
the DNA of Viral and Bacterial
Chromosomes
• Supercoiling compacts DNA (Figure 12.4).
Most closed circular DNA molecules in
bacteria are slightly underwound and
supercoiled.
Figure 12-4
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Figure 12-4a
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Figure 12-4b
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Figure 12-4c
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Figure 12-4d
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• Topoisomerases cut one or both DNA
strands and wind or unwind the helix before
resealing the ends.
12.4 DNA Is Organized into
Chromatin in Eukaryotes
• Eukaryotic chromosomes are complexed
into a nucleoprotein structure called
chromatin.
• Chromatin is bound up in nucleosomes with
histones H2A, H2B, H3, and H4.
• Nucleosomes are condensed several times to
form the intact chromatids (Figure 12.9).
Figure 12-9
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• Chromatin remodeling must occur to allow
the DNA to be accessed by DNA binding
proteins.
• Histone tails are important for histone
modifications such as acetylation,
methylation, and phosphorylation.
• Euchromatin is uncoiled and active, whereas
heterochromatin remains condensed and is
inactive.
12.5 Chromosome Banding
Differentiates Regions along the
Mitotic Chromosome
• Mitotic chromosomes have a characteristic
banding pattern. In C-banding, only the
centromeres are stained (Figure 12.11). Gbanding is due to differential staining along
the length of each chromosome (Figure
12.12).
Figure 12-11
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Figure 12-12
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• The differential staining reactions reflect the
heterogeneity and complexity of the
chromosome.
12.6 Eukaryotic Chromosomes
Demonstrate Complex
Organization Characterized by
Repetitive DNA
• Repetitive DNA sequences are repeated
many times within eukaryotic chromosomes,
and there are a number of categories of
repetitive DNA (Figure 12.14).
Figure 12-14
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• Satellite DNA is highly repetitive and
consists of short repeated sequences.
• Centromeres are the primary constrictions
along eukaryotic chromosomes and mediate
chromosomal migration during mitosis and
meiosis.
• There are two types of telomere sequences:
telomeric DNA sequences and telomereassociated sequences. Both consist of
repetitive sequences.
• Moderately repetitive DNA includes variable
number tandem repeats (VNTRs),
minisatellites, and microsatellites.
• Short interspersed elements (SINES), long
interspersed elements (LINES), and
transposable sequences are repetitive DNAs
that are dispersed throughout the genome
rather than tandemly repeated.
12.7 The Vast Majority of a
Eukaryotic Genome Does Not
Encode Functional Genes
• Highly repetitive and moderately repetitive
DNA constitute up to 40% of the human
genome. There are also a large number of
single-copy noncoding regions, some of
which are pseudogenes.