Chapter 9 and some of 8
DNA and the Molecular Structure
of Chromosomes
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
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.
Functions of the Genetic
Material
Genotypic Function: Replication
Phenotypic Function: Gene Expression
Evolutionary: Allows for Mutation
Chromosomes
Genes are located on chromosomes.
Chromosomes contain proteins and
nucleic acids.
The nucleic acids are deoxyribonucleic
acid (DNA) and ribonucleic acid (RNA).
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. Viroids are infectious naked
RNA molecules, and prions are
infectious, heritable proteins (not
TRUE).
DNA Mediates Transformation
Convert IIR
to IIIS
By DNA?
Avery MacLeod and McCarty Experiment Circa 1943
The Genetic Material of
Bacteriophage T2 is DNA
The Genetic Material of
Tobacco Mosaic Virus is RNA
Viroids and Prions
Viroids are infectious, naked RNA
molecules.
Prions are heritable (What do they
mean here?), infectious proteins that do
not contain nucleic acids.
Key Points
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).
Viroids and prions are infectious naked
molecules of RNA and protein, respectively.
The Structures of
DNA and RNA
DNA is usually double-stranded, with
adenine paired with thymine
and guanine paired with
cytosine. RNA is usually singlestranded and contains uracil in
place of thymine.
nucleotide = sugar + base + phosphate
nucleoside = sugar +base
When
Do we
Find RNA
Mixed with
The DNA?
Ribonucleotide Reductase
Pyrimidines have the longer name and smaller base
Purines have the shorter name and bigger base
Structure of a
Polynucleotide
Chain
Why do we
Always talk about
DNA going in the
5’ to 3’ direction?
Deoxyribonucleotides
What is dideoxyadenosine triphosphate (ddATP)?
Why are dideoxynucleotides useful in Mol.Bio.?
One of Chargaff’s Rules
X-ray Diffraction Pattern of DNA
The Double Helix
?
?
?
?
I hate when DNA is not depicted properly!
DNA Structure
Complementary Base Pairs (A with T,
G with C
Antiparallel Strands
Right-handed double helix (B-DNA)
(No obvious reason to me why Righthanded is better than left-handed)
A-DNA, B-DNA and Z-DNA
The Z-DNA helix is left-handed and has a structure that repeats every 2 base pairs.
The major and minor grooves, unlike A- and B-DNA, show little difference in width
A-DNA, B-DNA and Z-DNA
Supercoiling
(higher order structure)
What circular DNA exists in humans?
Key Points
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.
Key Points
The two strands of a DNA double helix
have opposite chemical polarity.
RNA usually exists as a single-stranded
HELIX containing uracil instead of
thymine.
Why Does RNA form a helical structure?
STACKING OF BASES
The DNA of Prokaryotes and
Viruses
Prokaryotes are monoploid.
Most viruses and prokaryotes have a
single set of genes stored in a single
chromosome, which contains a single
molecule of nucleic acid.
The E. coli Chromosome
Key Points
The DNA molecules in prokaryotic and
viral chromosomes are organized into
negatively supercoiled domains.
Bacterial chromosomes contain circular
molecules of DNA segregated into
about 50 domains.
The Genetics of Bacteria
Bacteria contain genes that mutate
to produce altered phenotypes.
Gene transfer in bacteria is
unidirectional—from donor cells to
recipient cells.
Bacteria
One main chromosome with a few
thousand genes.
Variable number of plasmids and
episomes.
Asexual reproduction by simple fission.
Recombination in Bacteria
Does this recombination process interfere
with DNA cloning experiments?
Key Points
 Bacteria usually contain one main chromosome.
 Wild-type bacteria are prototrophs; they can
synthesize everything they need to grow and
reproduce given an energy source and some
inorganic molecules.
Auxotrophic mutant bacteria require
additional metabolites for growth.
Key Points
Gene transfer in bacteria is unidirectional;
genes from a donor cell are transferred to a
recipient cell, with no transfer from recipient
to donor.
Mechanisms of Genetic
Exchange in Bacteria
Bacteria exchange genetic
material through three different
parasexual processes.
Transformation in
Bacillus subtilis (NOT TRUE
for E.Coli)
Streptococcus pneumoniae
Phenotypes
Transformation-Griffith’s Expt
1928