Watson and Crick discovered the double helix structure of DNA in 1953. Their model showed that DNA consists of two polynucleotide chains coiled around each other in a right-handed spiral. The bases of each chain bond with the other chain via hydrogen bonds in a base-pairing pattern where adenine pairs with thymine and cytosine pairs with guanine. This discovery helped explain how genetic information is stored and copied in organisms.

2. Watson and Crick (1953)

3. The Discovery of the DNA Double Helix
• James Watson and Francis Crick published the famous double-helix structure
in 1953.
• Two additional sources of data assisted Watson and Crick with their model:
a. Erwin Chargaff’s ratios obtained for DNA derived from a variety of
sources showed that the amount of purine always equals the amount of
pyrimidine, and further, that the amount of G equals C, and the amount
of A equals T.
b. Rosalind Franklin’s X ray diffraction images of DNA showed a helical
structure with regularities at 0.34 nm and 3.4 nm along the axis of the
molecule.

4. Watson and Crick’s three-dimensional
model has these main features:
a. It is two polynucleotide chains
wound around each other in a
right-handed helix.
b. The two chains are antiparallel.
c. The sugar-phosphate backbones
are on the outside of the helix,
and the bases are on the inside,
stacked perpendicularly to the
long axis like the steps of a spiral
staircase.

6. • The 1962 Nobel Prize in Physiology
or Medicine was awarded to Francis
Crick, James Watson and Maurice
Wilkins (the head of the lab in which
Franklin worked). Franklin had
already died, and so was not eligible.

7. Molecular structure of DNA

8. DNA Structure
 DNA consists of two molecules that are arranged into a ladder-like
structure called a Double Helix.
 A molecule of DNA is made up of millions of tiny subunits called
Nucleotides.
 Each nucleotide consists of:
1. Phosphate group
2. Pentose sugar
3. Nitrogenous base

9. Nucleotides
Phosphate
Pentose
Sugar
Nitrogenous
Base

10. • Nucleotides are linked together by covalent bonds called
phosphodiester linkage
Base

11. Nucleotides
 The phosphate and sugar form the backbone of the DNA molecule,
whereas the bases form the “rungs”.
 There are four types of nitrogenous bases.

12. Nucleotides
A
Adenine
T
Thymine
G
Guanine
C
Cytosine

13. There are two classes of nitrogenous bases:
a. Purines (double-ring, nine-membered structures) include
adenine (A) and guanine (G).
b. Pyrimidines (one-ring, six-membered structures) include
cytosine (C), thymine (T) in DNA and uracil (U) in RNA.
Structures of the nitrogenous bases in DNA and RNA

14. Nucleotides
 Each base will only bond with one other specific base.
Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)
Form a base pair.
Form a base pair.

15. DNA Structure
 Because of this complementary base pairing, the order of the bases
in one strand determines the order of the bases in the other strand.

16. Two hydrogen bonds between A:T pairs
Three hydrogen bonds between C: G paired

17. G
G
A
T
T
A
A
C
T
G
C
A
T
C

18. DNA Double Helix and Hydrogen Bonding
• Made of two strands of nucleotides that are joined together by
hydrogen bonding
• Hydrogen bonding occurs as a result of complimentary base pairing
• Adenine and thymine pair up
• Cytosine and guanine pair up
• Each pair is connected through hydrogen bonding
• Hydrogen bonding always occurs between one pyrimidine and one purine

19. • Complimentary base pairing of pyrimidines and purines
DNA Double Helix and Hydrogen Bonding

20. DNA Double Helix and Hydrogen Bonding

21. •Adenine always pairs
with thymine because
they form two H bonds
with each other
•Cytosine always pairs
with guanine because
they form three
hydrogen bonds with
each other
DNA Double Helix and Hydrogen Bonding

22. DNA Double Helix
• The ‘backbones’ of DNA molecules are made of alternating sugar and
phosphates
• The ‘rungs on the ladder’ are made of bases that are hydrogen bonded
to each other

23. Antiparallel strands
The strands
run opposite
of each
other.
The 5’ end
always has
the
phosphate
attached.
5’ 3’
3’ 5’

25. Nucleosome structure
•Nucleosome are the basic unit of chromatin
organization
•In eukaryotes DNA is associated with proteins
•(in prokaryotes the DNA is naked)
•Nucleosomes = basic beadlike unit of DNA packing
•Made of a segment of DNA wound around a
protein core that is composed of 2 copies of each
of 4 types of histones

26. Nucleosome structure
• Nucleosomes have:
• 8 histones in the core
• DNA wrapped twice around the core
• One histone holding the nucleosome
together
• A DNA ‘linker’ continuing towards
the next nucleosome

27. Nucleosome structure
• The DNA has a negatively charged backbone (because of the
phosphate groups)
• The proteins (the histones) are positively charged
• The DNA and proteins are electromagnetically attracted to each
other to form chromatin

29. Assignment (in your notebook)
• 1. Draw the structure of ribose and number the carbons
• 2. Draw a schematic representation of a nucleotide. Label the sugar, base and
phosphate.
• 3. What are the complimentary base pairs to a DNA strand that has the
following order A T A C C T G A A T?
• 4. Draw a schematic representation of an unwound DNA double helix using
the base pairs from your answer in question 3.
• Include the number of hydrogen bonds between each base pair. Be sure to label all
of the bases and the 5’ and 3’ ends of the structure.