Section 1
Molecular Genetics
DNA: The Genetic Material
Griffith
 Performed the first major experiment that led to
the discovery of DNA as the genetic material
Section 1
Molecular Genetics
DNA: The Genetic Material
Avery
 Identified the molecule that transformed the
R strain of bacteria into the S strain
 Concluded that when the S cells were killed,
DNA was released
 R bacteria incorporated this DNA into their
cells and changed into S cells.
Section 1
Molecular Genetics
DNA: The Genetic Material
Hershey and Chase
 Used radioactive labeling to
trace the DNA and protein
 Concluded that the viral
DNA was injected into the
cell and provided the
genetic information needed
to produce new viruses
Section 1
Molecular Genetics
DNA: The Genetic Material
DNA Structure
 Nucleotides
 Consist of a five-carbon sugar, a phosphate
group, and a nitrogenous base
Section 1
Molecular Genetics
DNA: The Genetic Material
Chargaff rule: for
every adenine
there is a
thymine. For
every guanine
there is a
cytosine,
 Chargaff’s rule:
C = G and T = A
One team
Section 1
Molecular Genetics
DNA: The Genetic Material
X-ray Diffraction
 X-ray diffraction data helped solve the
structure of DNA
 Indicated that DNA was a double helix
James Watson (Left) and Francis
Crick (right)
Section 1
Molecular Genetics
DNA: The Genetic Material
Watson and Crick
 Built a model of the double helix that
conformed to the others’ research
1. two outside strands consist of alternating
deoxyribose and phosphate
2. cytosine and guanine bases pair to each
other by three hydrogen bonds
3. thymine and adenine bases pair to each
other by two hydrogen bonds
Section 1
Molecular Genetics
DNA: The Genetic Material
DNA Structure
 DNA often is compared to a twisted ladder.
 Rails of the ladder are represented by the
alternating deoxyribose and phosphate.
 The pairs of bases (cytosine–guanine or
thymine–adenine) form the steps.
Section 1
Molecular Genetics
DNA: The Genetic Material
Orientation
 On the top rail, the strand is said to be oriented 5′ to 3′.
 The strand on the bottom runs in the opposite
direction and is oriented 3′ to 5′.
Section 1
Molecular Genetics
DNA: The Genetic Material
Chromosome Structure
 DNA coils around histones to form nucleosomes,
which coil to form chromatin fibers.
 The chromatin fibers supercoil to form chromosomes
that are visible in the metaphase stage of mitosis.
Section 2
Molecular Genetics
Replication of DNA
Semiconservative Replication
 Parental strands of DNA separate. They serve
as templates, and produce DNA molecules
that have one strand of parental DNA and one
strand of new DNA.
Let’s draw
Section 2
Molecular Genetics
Replication of DNA
Steps. Of DNA Replication
1. DNA helicase, an enzyme, is responsible for unwinding and
unzipping the double helix.
2. DNA polymerase, another enzyme, adds appropriate
nucleotides using the original DNA as a templete.
3. The leading strand is build continuously, the lagging strand
is build discontinuously in Okazaki fragments.
4. DNA ligase, another enzyme, closes up any gaps in the new
DNA strand.
Section 2
Molecular Genetics
Replication of DNA
Comparing DNA Replication in Eukaryotes
and Prokaryotes
 Eukaryotic DNA unwinds in multiple areas
as DNA is replicated.
 In prokaryotes, the circular DNA strand is
opened at one origin of replication.
Section 3
Molecular Genetics
DNA, RNA, and Protein
 RNA
 Contains the sugar ribose and the
base uracil instead of thymine
 A-U
G-C
 Usually is single stranded
Section 3
Molecular Genetics
DNA, RNA, and Protein
Messenger RNA (mRNA)
 Long strands of RNA nucleotides that are
formed complementary to one strand of DNA
Ribosomal RNA (rRNA)
 Associates with proteins to form ribosomes
in the cytoplasm
Transfer RNA (tRNA)
 Smaller segments of RNA nucleotides that
transport amino acids to the ribosome
Section 3
Molecular Genetics
DNA, RNA, and Protein
Transcription
 Through transcription,
the DNA code is
transferred to mRNA
in the nucleus.
 DNA is unzipped in
the nucleus and RNA
polymerase binds to a specific section where an
mRNA will be synthesized.
Section 3
Molecular Genetics
DNA, RNA, and Protein
RNA Processing
 The code on the DNA is interrupted
periodically by sequences that are not in the
final mRNA.
 Intervening sequences are called introns.
 Remaining pieces of DNA that serve as the
coding sequences are called exons.
DNA and Genes
Section 3
Molecular Genetics
DNA, RNA, and Protein
The Code
 Experiments during the 1960s demonstrated
that the DNA code was a three-base code.
 The three-base code in DNA or mRNA is
called a codon.
Section 3
Molecular Genetics
DNA, RNA, and Protein
Translation
 In translation, tRNA
molecules act as the
interpreters of the mRNA
codon sequence.
 At the middle of the folded
strand, there is a three-base
coding sequence called the
anticodon.
 Each anticodon is
complementary to a codon
on the mRNA.
Section 3
Molecular Genetics
DNA, RNA, and Protein
Section 4
Molecular Genetics
Gene Regulation and Mutation
Mutations
 A permanent change that occurs in a cell’s
DNA is called a mutation.
 Types of mutations
 Point mutation
 Insertion
 Deletion
Section 4
Molecular Genetics
Gene Regulation and Mutation
Section 4
Molecular Genetics
Gene Regulation and Mutation
Protein Folding and Stability
 Substitutions also can lead to genetic
disorders.
 Can change both the folding and stability
of the protein
Section 4
Molecular Genetics
Gene Regulation and Mutation
Causes of Mutation
 Can occur spontaneously
 Chemicals and radiation also can
damage DNA.
 High-energy forms of radiation, such as X rays
and gamma rays, are highly mutagenic.
Section 4
Molecular Genetics
Gene Regulation and Mutation
Body-cell v. Sex-cell Mutation
 Somatic cell mutations are not passed on
to the next generation.
 Mutations that occur in sex cells are passed
on to the organism’s offspring and will be
present in every cell of the offspring.