Chapter 9
DNA and the Molecular Structure of
Chromosomes
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Chapter Outline
Functions of the Genetic Material
Proof That Genetic Information Is
Stored in DNA
The Structures of DNA and RNA
Chromosome Structure in Prokaryotes
and Viruses
Chromosome Structure in Eukaryotes
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Functions of the Genetic Material
The genetic material must
replicate, control the growth and
development of the organism,
and allow the organism to adapt
to changes in the environment.
Genotypic Function: Replication
Phenotypic Function: Gene Expression
Evolutionary Function: Mutation (Gene
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Chromosomes
Genes are located on chromosomes.
Chromosomes contain proteins and nucleic
acids.
The nucleic acids are deoxyribonucleic acid
(DNA) and ribonucleic acid (RNA).
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Proof that Genetic Information
is Stored in DNA
In most organisms, the genetic
information is encoded in DNA. In
some viruses, RNA Is the genetic
material. Proteins?
Chromosomes-nuclear fraction-DNA
Cytosol-proteins-RNA
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DNA Mediates Transformation
Griffith’s experiment
(in vivo)
Mice
Streptococcus pneumoniae
--type IIIS (virulent)
--type IIR (avirulent)
“Transforming principles”
Sia and Dawson’s experiment
(in vitro)
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DNA Mediates Transformation
Griffith’s experiment
(in vivo)
Mice
Streptococcus pneumoniae
--type IIIS (virulent)
--type IIR (avirulent)
“Transforming principles”
Sia and Dawson’s experiment
(in vitro)
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Avery, MacLeod and McCarty’s experiment (in vitro)
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The Genetic Material of Bacteriophage T2 is DNA
Hesley-Chase’s experiment (in vitro)-----------> Transfection ( Infection)
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The Genetic Material of Tobacco
Mosaic Virus (TMV) is RNA
Retroviruses ( RNA------> DNA)
(in vitro)
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• The genetic information of most living
organisms is stored in deoxyribonucleic
acid (DNA).
• In some viruses, the genetic information is
present in ribonucleic acid (RNA).
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The Structures of DNA and RNA
DNA is double-stranded,
with adenine paired with thymine
and guanine paired with cytosine.
RNA is usually single-stranded
and contains uracil in place of
thymine.
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Deoxyribonucleotides
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Structure of a Polynucleotide Chain
DNA composition: [A]=[T] ;
[C]=[G]
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Chargaff’s Rules
%A = %T and %G = %C.
%A ~ %T and %G ~ %C are valid for each of the two DNA strands
methyltransferases methylate and/or deaminate
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Structure of a Polynucleotide Chain
DNA composition: [A]=[T] ;
[C]=[G]
X-ray Diffraction Pattern of DNA
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The Double Helix
Phosphodiester
[C-O-P-O-C]
Polynucleotide chain
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Complementary and Antiparallel
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DNA Structure
Complementary Base Pairs (A with T,
G with C)
Anti-parallel Strands
Right-handed double helix (B-DNA)
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DNA replication
Elevated content
of C:G
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DNA-RNA duplexes
DNA Structure:
Supercoils
Unique structure of a DNA molecule
Is produced after one or both strand
of DNA are cleaved
DNA will rotate or twist
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• DNA usually exists as a double helix, with
the two strands held together by hydrogen
bonds between the complementary base
pairs: adenine paired with thymine and
guanine paired with cytosine.
• The complementarity of the two strands of a
double helix makes DNA uniquely suited to
store and transmit genetic information.
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• The two strands of a DNA double helix have
opposite chemical polarity.
• RNA usually exists as a single-stranded
molecule containing uracil instead of thymine.
• The functional DNA molecules in cells are
negatively supercoiled.
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Chromosome Structure in
Prokaryotes and Viruses
The DNA molecules of prokaryotes and viruses
(RNA?) are organized into negatively
supercoiled domains.
Prokaryotes are monoploid and are part of the
nucleoids.
Most viruses and prokaryotes have a single set of
genes stored in a single chromosome, which
contains a single ©molecule
of nucleic acid.
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The E. coli Chromosome
Folded genome: is the functional state of a isolated bacterial chromosome
Mild conditions
(no ionic detergents)
1M salt
Polyamines(-)
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Model of E. coli Chromosome
folded=coiled
protein
Nicked=single strand
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• The DNA molecules in prokaryotic and viral
chromosomes are organized into negatively
supercoiled domains (loops).
• Bacterial chromosomes contain circular
molecules of DNA segregated into 50 to 100
domains.
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Chromosome Structure in Eukaryotes
Eukaryotic chromosomes contain huge molecules
of DNA that are highly condensed during
mitosis and meiosis.
The centromeres and telomeres of eukaryotic
chromosomes have unique structures.
Each chromosome is unineme (Theory)
Each chromosome contains a single large double
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helix (strand) of DNA molecule
Chromatin Composition
Histones:
H1, H2a, H2b, H3, H4
(protamines in sperm)
Structural
(+)
(-)
Nonhistone proteins:
Non structural
Regulation
Nucleosomes: DNA + histones
except H1
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Nucleosomes
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Methyl groups
Acetyl groups
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Structure of the Nucleosome Core
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Human Metaphase
Chromosomes
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The 30 nm Fiber
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Micrococcal nuclease releases individual
nucleosomes from chromatin as particles.
Intermediate
Structures?
Endonucleases
The path of nucleosomes in the chromatin fiber
Both the 10 nm fiber and
the 30 nm fiber were first seen
by electron microscopy.
Higher packing of the
nucleosomes into “inactive
heterochromatin” may involve
Non-histone proteins.
High salts
H1
DNA Around a Scaffold of
Non-histone Proteins
heterochromatin
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Levels of DNA Packaging




2-nm double-stranded DNA molecule
11-nm nucleosomes H1
30 nm chromatin fiber
Organization around a central scaffold
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DNA repeats
Centromeric: specific repeated regions (non-coding DNA sequences=
heterochromatin) of chromosome for attachment of spindle microtubules
( 5000 to 15000 bp).
Satellite sequences:
--Tandemly repeating
--Non-coding DNA
Alpha-------171 ( unit repeat as base pair)
Beta----------68
Satellite 1---48
Satellite 2-----5
Satellite 3-----5
Most satellite DNA is localized to the telomeric or
the centromeric region of© the
chromosome
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Telomeres
Functions of telomeres
– Protect the ends of linear DNA molecules
from deoxyribonucleases
– Prevent fusion of chromosomes
– Facilitate complete replication of the ends
of linear DNA molecules
Most telomeres contain repetitive
sequences and a distinct structure.
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Telomere Structure
-TTAGGG
-500 to 3000 repeats
-G-rich overhang
-T-loop (D-loop)
-Telomeres specific
Proteins ( protection)
POT1
TRF1 and 2
TIN2 and TPP1
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Repeated DNA Sequences
Eukaryotic chromosomes contain repetitive
DNA ( 15 to 80 %), Human (~50%)
--Satellite bands (tandem repeats)
--Transposable genetic elements (transposon)
---Retrotransposon, ---DNA transposon
--Genomic island (G+C)
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The heritable factors that Mendel studied are now known as:
a) Nucleic acids
b) Amino acids
c) Genes
d) Unit factors
e) Peptides
The building blocks of genes are:
a) Proteins
b) Amino acids
c) Nucleic acids
d) Lipids
e) Carbohydrates
Different forms of the same gene are known as:
a) Peptides
b) Amino acids
c) Proteins
d) Alleles
e) Gene differences
The main components of a membrane in a cell are:
a) Lipids
b) Proteins
c) Carbohydrates
d) Lipids and Proteins
e) Lipids and Carbohydrates
Prokaryotes can be characterized by:
1.The lack of a true nucleus or compartment in which
the DNA is located
2. The unique cell walls composed of murein
3. The lack of mitochondria
4. All of these
a) 1
b) 2
c) 3
d) 4
e) 1 and 3
Which large organic molecules are essential chromosome components?
a) Lipids and proteins
b) Proteins and nucleic acids
c) Nucleic acids and polysaccharides
d) Proteins and polysaccharides
Which of the following is a type of nucleic acid?
a) DNA
b) RNA
c) DNAse
d) RNAse
e) DNA and RNA
In Sia and Dawson's 1931 experiment:
a) Mice were required to demonstrate the transforming principle
b) Used serum to precipitate IIIS cells from a mixture of heat-killed IIIS and living IIR cells
c) They showed that mice play no direct role in the transforming principle
d) Heat-killed IIR cells mixed with living IIIS cells gave rise to IIR colonies
e) None of these