1. CH 16: The Molecular Basis of
Inheritance
Francis Crick
James Watson

2. Major Concepts
• The discovery of DNA as the
genetic material
• How DNA is replicated and
repaired
To assist in your note taking, key
concepts have been underlined and
vocab terms are in orange

3. Life’s Operating Instructions
DNA is an example of a nucleic acid
Of all the macromolecules, nucleic
acids are the only ones that can
replicate themselves
DNA is the chemical language that is responsible
for biochemical, anatomical, physiological,
& even some behavioral traits

4. DNA is the Genetic Material…
But How Do We Know???
The Debate: DNA vs. Protein
In the 1940’s….
Proteins
Nucleic Acids: DNA
•A very wide variety
was known
•Little was known
about them
•They were known to •They seemed too
be very specific
simple to account for
the great diversity of
life

5. Building a Case for DNA:
DNA Can Transform Bacteria
Frederick Griffith’s Experiment, 1928
Transformation:
The uptake &
assimilation of
external DNA by a
bacterial cell that virulent
results in a change
in genotype and
phenotype
Concl: Living R bacteria
were transformed into S
bacteria by a heritable
substance from the dead
S cells
harmless

6. Quick Think
Describe Griffith’s experiment & the conclusion
that he reached
What was the change in phenotype that the
transformed Streptococcus pnumoniae underwent?

7. Follow-up to Griffith’s Experiment
Griffith new that something from the heat-killed S bacteria was
transforming the harmless R strain, but he didn’t know what…
Oswald Avery was determined to pick up where Griffith left off….
How would YOU design an experiment to figure out which part of
the pathogenic bacteria was transforming the harmless bacteria???

8. Avery performed Griffith’s experiments, but treated the nonpathogenic bacteria with isolated molecules from the heat-killed
pathogenic strain
Lysed cells
DNA was the only
macromolecule to
successfully transform the
R strain

9. When Avery concluded that
DNA was the transforming agent,
few took interest! Most were hung
up on the idea that protein was the
genetic material…..plus not much
was known about DNA at that
point……

10. Building a Case for DNA:
Viral DNA Can Program Cells
Background Info
•The T2 virus infects
the bacteria E. coli
•The T2 virus is made
mostly of DNA &
protein
Viral DNA injected into cell
•A virus must infect a cell in order to reproduce
•A Bacteriophage ( AKA phage) is a virus that infects bacterial
cells

11. Building a Case for DNA:
Viral DNA Can Program Cells
In 1952, the
research duo
Alfred
Hershey &
Martha Chase
wanted to
figure out
whether it
was viral
protein or its
DNA that
caused a cell
to become a
virus factory

12. Building a Case for DNA:
Chargaff’s Investigations, 1947
•Chargaff found diversity in
the DNA molecules of
different organisms: Various
organisms had different
percentages of the 4 bases
nucleotide
Each nucleotide
of DNA has 3
parts-
•Chargaff’s Rule: In any
given organism, the % of
A in DNA = the % of T. In
the same way, % G = % C

13. Quick Think
Human DNA is composed of 30% thymine.
What would be the percent composition for
the other three bases??

14. Building a Case for DNA:
Additional Evidence
Prior to cell
division,
eukaryotic cells
make exact
copies of their
DNA
and then the
DNA is
distributed
evenly
among the
daughter
cells.

15. DNA as the Genetic Material:
How Does Form Fit Function?
By the early 1950’s, the race was on to find the 3-D structure of
DNA
In 1950 Maurice
Wilkins &
Rosalind Franklin
were hard at work
using X-ray
diffraction
technique to figure
out DNA structure
They planned to use mathematical equations derived from the
diffraction images to determine the 3-D structure of DNA.

16. DNA as the Genetic Material:
How Does Form Fit Function?
Watson & Crick used Franklin’s x-ray diffraction images
to deduce the following:
•DNA is a double helix withwith T and C
Base-pairing rules: A pairs a sugar-phosphate backbone on the
•Watson & Crick’s model suggested the basic mechanismon the
pairs with G. “ladder” and relatively hydrophobic bases for
outside of theIn other words, purines always
DNA with bases are the rungs of the ladder.
bond replication
inside; the pyrimidines.

17. Quick Think
How did Watson & Crick’s model
explain the basis for Chargaff’s rule?