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
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.
Phosphate
Sugar
Nitrogen-containingbase
• 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)
Phosphate
Sugar
Nitrogen-containingbase
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
TAC
G
Base Pairing in DNA–
One purine (double ring) hydrogen bonds with one pyrimidine (single ring)
Explained Chargaff’s Rule
TAC
G
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
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 RNA
Name Deoxyribonucleic Acid Ribonucleic Acid
Sugar type Deoxyribose Ribose
Nitrogen Bases A,T,G,C A,U,G,C
# of Strands Double stranded Single Stranded
• 2 stranded
• The sugar-phosphate backbone is connected by covalent bonds.
hydrogen bond covalent bond
• Hydrogen bonds between base pairs hold strands together
DNA
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.