8.2 Structure of DNA
And the RACE BEGINS!
• Once DNA was identified as the genetic molecule the
race was on to determine its structure. The combined
work of different researchers led to its discovery in
February 1953.
8.2 Structure of DNA
Important Experiments of the 1950’s
Erwin Chargaff
• Analyzed the DNA of
many living
organisms. In every
sample he found
equal amounts of
certain nitrogen
bases.
8.2 Structure of DNA
Chargaff’s DNA Nucleotide Sample Testing Results
8.2 Structure of DNA
• In every sample of DNA from every
organism, the amount of
• ADENINE (A) = THYMINE (T)
• CYTOSINE (C) = GUANINE (G)
8.2 Structure of DNA
Chargaff’s Research Implications
• Chargaff finding explained how all organisms have
the same universal DNA but still be so different.
• All organisms contain the same nitrogen bases in
their DNA but the amount of each base is different
for each organism.
8.2 Structure of DNA
2. Rosalind Franklin and Maurice Wilkins
Used a technique called x-ray crystallography to capture the
first images of DNA which suggested that it had a spiral or
helical shape and an even width.
8.2 Structure of DNA
Rosalind’s boss, Maurice Wilkins, showed
her Photo 51 to other researchers without
her knowledge.
8.2 Structure of DNA
• .
3. James Watson and Francis Crick
Based their model on other’s research.
Used Franklin’s photograph of DNA and
Chargaff’s rules of base pairing to
develop a model of DNA.
8.2 Structure of DNA
• In 1953 they
published a
paper in the
scientific journal
Nature.
8.2 Structure of DNA
Watson and Crick’s findings
• DNA’s structure is:
• double stranded
• two strands complementary to
each other
• shaped like a double helix
• structure would likely allow it to
replicate /copy itself.
8.2 Structure of DNA
Awarded the the Nobel Prize in Medicine 1962
Francis Harry Compton Crick
James Dewey Watson
Maurice Hugh Frederick Wilkins
8.2 Structure of DNA
Rosalind
Franklin died of
cancer at the
age of 38.
Nobel Prize is
not awarded
posthumously.
8.2 Structure of DNA
KEY CONCEPT
DNA structure is the same in all organisms.
Nucleic Acid Structure
• DNA and RNA are polymers made up of billions
of repeating monomer units called nucleotides.
Nitrogen-containing
base
Phosphate
Sugar
• Monomer - Nucleotide
• Polymer – DNA
Nucleotide Structure
• Each Nucleotide is composed of 3 molecules bonded together
1. Phosphate (P)
2. 5 Carbon Sugar (S)
3. Nitrogen-containing base (5 types)
Nitrogen-containing
base
Phosphate
Sugar
Types of Nucleotides
• There are 5 types of nucleotides but each
nucleic acid only contains 4 types.
• Nucleotides are named for the nitrogen base
they contain.
• Nitrogen bases are grouped based on their
ring structure.
Types of Nucleotides depends on base
1. Pyrimidines – Single ring structure
*Cytosine -- C
*Thymine – T (Only in DNA)
*Uracil
– U (Only in RNA)
2. Purines – Double ring structure
*Guanine - G
*Adenine - A
.
Types of Nucleotides depend on their nitrogen base
8.2 Structure of DNA
Base Pairing in DNA–
One purine (double ring)
hydrogen bonds with one
pyrimidine (single ring)
G
Explained Chargaff’s Rule
C
A T
Hydrogen bonds between the base pairs hold the two
strands of DNA together in the double helix
Base pairing allows the
DNA molecule to
maintain a uniform width
along its entire length
G
C
A T
What makes each individual organism
unique is the order of their base pairs
– the base sequence - in their DNA.
LET’S COMPARE DNA TO RNA
Comparison of DNA and RNA
DNA
Name Deoxyribonucleic Acid
Sugar type
Deoxyribose
RNA
Ribonucleic Acid
Ribose
Nitrogen Bases A,T,G,C
A,U,G,C
# of Strands Double stranded
Single Stranded
DNA
• 2 stranded
• Hydrogen bonds between base pairs hold strands together
• The sugar-phosphate backbone is connected by covalent bonds.
hydrogen bond
covalent bond
8.2 Structure of DNA
RNA
1.
Single strand instead of
double strand
2.
Ribose instead of
deoxyribose
3.
Uracil instead of thymine
Functions of Nucleic Acids
• DNA – Serves as the cell’s “blueprint” for
making proteins
• RNA – 3 types.
Each type has a specific function in
making proteins.