1. Watson and Crick: Discovery of DNA structure
and DNA Properties
Brooke Noble and Serena Patel

2. Who are Watson and Crick?
James Watson and Francis
Crick collaborated to
uncover DNA structure
James Watson-Born
in 1928, Earned a PhD
in Zoology, then
pursued his research
in molecular biology at
the Cavendish
Laboratory in
Cambridge.
Francis Crick-(1916-
2004) Earned a
degree in Physics and
later a PhD in
molecular biology

3. Previous research that contributed to Watson
and Crick’s Discovery
 Friedrich Miescher- (1869) Swiss Physiological
Chemist
Discovered the Nucleic Acid

4.  Phoebus Levene- (1905) Russian Biochemist
Discovered the order of a
Nucleotide: Phosphate-
sugar-base
Previous research that contributed to Watson and
Crick’s Discovery

5. Previous research that contributed to Watson
and Crick’s discovery
 Erwin Chargaff- (1950) Austrian Biochemist
Discovered the Total amount
of Purines and Total Amount
of Pyrimidines are equal
Purines: Adenine and Guanine
Pyrimidines: Thymine and
Cytosine

6. Base Composition
If one DNA strand is made up of 23%A, 17%T, 40%G,
and 20%C, what is the other strand’s base
composition?

7. Base Composition
Answer: 17% A, 23% T, 40%C, and 20% G

8. Previous research that contributed to Watson
and Crick’s Discovery
 Rosalind Franklin- Biophysicist and X-ray
Crystallographer
X-Ray diffraction images of DNA structure

9. Watson and Crick in 1953 uncovered the three dimensional,
double helical structure of DNA
Watson and Crick’s Discovery: Helical DNA
Structure

10. X-Ray Crystallography
 X-Ray Crystallography is an experimental technique
that exploits the fact that X-rays are diffracted from
crystals
 A 3-D structure can be determined by examining the
diffraction patterns created by X-rays beamed
through a crystal
 DNA= two strands wound together
The diffracted (X)
pattern that Rosalind
Franklin found in her
research

11. Overview of Major Findings
Watson and Crick’s predicted structure. The twisted
ribbons are the sugar-phosphate backbone. They
are held by the horizontal bars which are the bases.
Two helical chains coiled around axis
Each chain made up of phospodiester
bonds
Phosphate groups are on outside of
helix and bases are inside
Two chains held together by purine and
pyrimidine pairs
Chains run in opposite directions and
are complementary

12. Building Blocks of Deoxyribonucleic Acids
Nucleotides are made
up of 3 main
components:
 Five-carbon sugar
 Phosphate groups
 Nitrogen containing
base (adenine,
thymine, cytosine, or
guanine)

13. What Holds Bases Together?
Two hydrogen
bonds form
between A-T
pairs
Three
hydrogen
bonds form
between C-G
pairs

14. Which DNA Helix will require more energy to
separate?
A. 5’-ATGATCATCATTA-3’
3’-TACTAGTAGTAAT-5’
OR
B. 5’-CCGAGCTGCCAT-3’
3’-GGCTCGACGGTA-5’
http://www.basic.northwestern.edu/biotools
/OligoCalc.html

15. Which DNA Helix will require more energy to
separate?
A. 5’-ATGATCATCATTA-3’
3’-TACTAGTAGTAAT-5’
OR
B. 5’-CCGAGCTGCCAT-3’
3’-GGCTCGACGGTA-5’

16. Complementary Base Pairing
 Purines are made up of two
rings (adenine and guanine)
 Pyrimidines consist of one
ring (thymine and cytosine)
 Two chains held together by
pairs of purines and
pyrimidines

17. If we know the base sequence of this strand,
what is the sequence of the other strand?
DNA strand: 5’-ATCGTACCATAGC-3’

18. Answer
DNA strand: 5’-ATCGTACCATAGC-3’
Complementary strand: 3’-TAGCATGGTATCG-5’

19. Alternating Sugar Phosphate Backbone
 Remember the
phosphates are
negatively charged…
 Hydrophilic, surrounded
by water
 Can the sugar-phosphate
backbone be on the
inside while the bases are
on the outside?
 NO! The negative
charges will repel each
other

20. Antiparallel Strands and Polarity
 Nitrogenous base pairing requires two DNA strands to
run in opposite directions
 The phosphate group is the 5’ end and the –OH group is
the 3’ end

21. What is the significance of complementary DNA
base pairing and polarity?
 There must be a way to
replicate (copy) DNA
 If strands can be pulled apart,
then each original strand may
be copied knowing which
bases go together and the
direction of the strand
 Enzymes yet to be
discovered…
 DNA polymerase
 DNA topoisomerase
 DNA helicase

22. What’s next??
 DNA Replication
 DNARNA
 RNAProtein