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.