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Chapter 12: Molecular
Genetics
• What You’ll Learn
• How DNA was
discovered to be
the genetic
material & know its
structure
• DNA replication
• Protein synthesis
• Gene regulation &
mutations
Section 12.1:
DNA: The Genetic Material
• Section Objectives:
• Summarize the
experiments
leading to the
discovery of DNA
as the gentic
material
• Analyze the
structure of DNA
• Describe the basic
structure of the
eukaryotic
chromosome
What is DNA?
• Although the environment influences how
an organism develops, the genetic
information that is held in the molecules
of DNA ultimately determines an
organism’s traits.
• DNA achieves its control by determining
the structure of proteins.
• Within the structure of DNA is the
information for life—the complete
instructions for manufacturing all the
proteins for an organism.
Bodies  Cells  DNA
 Bodies are made up of cells
 All cells run on a set of
instructions spelled out in DNA
QuickTi me™ and a
TIFF ( Uncompressed) decompressor
are needed to see thi s pi ctur e.
1928
The “Transforming Principle”
• Frederick Griffith
– Streptococcus pneumonia
bacteria
• was working to find cure for pneumonia
– harmless live bacteria (“rough”)
mixed with heat-killed
pathogenic bacteria (“smooth”)
causes fatal disease in mice
– a substance passed from dead
bacteria to live bacteria to
change their phenotype
• “Transforming Principle”
The “Transforming Principle”
mix heat-killed
pathogenic &
non-pathogenic
live pathogenic live non-pathogenicheat-killed
strain of bacteriastrain of bacteria pathogenic bacteriabacteria
A.
B.
D.
C.
mice die
mice live
mice live
mice die
Transformation = change in phenotype
something in heat-killed bacteria could still
transmit disease-causing properties
DNA is the “Transforming
Principle”
1944
• Avery, McCarty & MacLeod
– purified both DNA & proteins separately from
Streptococcus pneumonia bacteria
• which will transform non-pathogenic bacteria?
– injected protein into bacteria
• no effect
– injected DNA into bacteria
• transformed harmless bacteria into
virulent bacteria
mice die
What’s the
conclusion?
1952 | 1969
Confirmation of DNA
• Hershey & Chase
– classic “blender” experiment
– worked with bacteriophage
• viruses that infect bacteria
– grew phage viruses in 2 media,
radioactively labeled with either
Why use
Sulfur
vs.
Phosphorus?
•
•
35S
in their proteins
32P in their DNA
– infected bacteria with
labeled phages
Hershey
Protein coat labeled
DNA labeled with
with 35S
T2 bacteriophages
are labeled with
radioactive isotopes
S vs. P
Hershey
& Chase
Which
radioactive
marker is
found inside
the cell?
Which
molecule
carries viral
genetic info?
32P
bacteriophages infect
bacterial cells
bacterial cells are agitated
to remove viral protein coats
35S
radioactivity
found in the medium
32P
radioactivity found
in the bacterial cells
Blender experiment
• Radioactive phage & bacteria in
blender
–
35S
phage
• radioactive proteins stayed in supernatant
• therefore viral protein did NOT enter bacteria
–
32P
phage
• radioactive DNA stayed in pellet
• therefore viral DNA did enter bacteria
– Confirmed DNA is “transforming factor”
Taaa-Daaa!
1952 | 1969
Hershey & Chase
Hershey
Martha Chase Alfred Hershey
The structure of nucleotides
• DNA is a polymer made of repeating
subunits called nucleotides.(the monomer)
• Nucleotides have three parts: a simple
sugar, a phosphate group, and a
nitrogenous base.
Nitrogenous base
Phosphate group
Sugar
(deoxyribose)
The structure of nucleotides
• in DNA there are four possible nucleotides,
each containing one of these four bases.
• The phosphate groups and deoxyribose
molecules form the backbone of the chain, and
the nitrogenous bases stick out like the teeth
of a zipper.
Phosphate
group
Nitrogenous base
(A, G, C, or T)
Thymine (T)
Nucleotide
Sugar
(deoxyribose)
Sugar-phosphate backbone
DNA nucleotide
Chargaff
1947
• DNA composition: “Chargaff’s rules”
– varies from species to species
– all 4 bases not in equal quantity
– bases present in characteristic ratio
• humans:
A
T
G
C
=
=
=
=
30.9%
29.4%
19.9%
19.8%
That’s interesting!
What do you notice?
Rules
A = T
C = G
Paired bases
• DNA structure
– double helix
• 2 sides like a ladder
• Bases match
together (
– A pairs with T
•A:T
– C pairs with G
•C:G
1953 | 1962
Structure of DNA
• James Watson and
Francis Crick
worked out the
three-dimensional
structure of DNA,
based on work by
Rosalind Franklin
and Maurice
Wilkens
Wilkins
Rosalind Franklin (1920-1958)
DNA is a double-stranded
helix
• Watson and Crick
also proposed that
DNA is shaped like
a long zipper that
is twisted into a
coil like a spring.
• Because DNA is
composed of two
strands twisted
together, its shape
is called double
helix.
The structure of DNA
Hydrogen bond
Base
pair
Partial chemical structure
Ribbon model
Computer model
Anti-parallel strands
– DNA molecule
has “direction”
– complementary
strand runs in
opposite
direction
5
3
3
5
The importance of nucleotide sequences
Chromosome
The sequence of nucleotides
forms the unique genetic
information of an
organism. The closer the
relationship is between
two organisms, the more
similar their DNA
nucleotide sequences will
be.
• Scientists use nucleotide
sequences to determine
evolutionary relationships
among organisms, to
determine whether two
people are related, and to
identify bodies of crime
victims.
Organizing & packaging DNA
DNA
cell
nucleus
DNA has been
“wound up”
DNA in chromosomes in
everyday “working” cell
cell
nucleus
4 chromosomes
in this organism
DNA in chromosomes in cell
getting ready to divide
DNA Packing
DNA
double
helix
(2-nm
diamete
r
Histones
“Beads on
a string”
Nucleosome
(10-nm diameter)
Tight helical fiber
(30-nm diameter) Supercoil
(200-nm diameter)
700
nm
Metaphase
chromosome
Nucleosomes
• “Beads on a string”
– 1st level of DNA packing
– histone proteins
• 8 protein molecules
• positively charged amino acids
• bind tightly to negatively charged
DNA
8 histone
molecules