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.

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.

Chargaff’s DNA Nucleotide Sample Testing Results

ADENINE (A) = THYMINE (T) CYTOSINE (C) = GUANINE (G) In every sample of DNA from every organism, the amount of ADENINE (A) = THYMINE (T) CYTOSINE (C) = GUANINE (G)

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.

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.

Rosalind’s boss, Maurice Wilkins, showed her Photo 51 to other researchers without her knowledge.

. 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.

In 1953 they published a paper in the scientific journal Nature.

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.

Awarded the the Nobel Prize in Medicine 1962 Francis Harry Compton Crick James Dewey Watson Maurice Hugh Frederick Wilkins

Rosalind Franklin died of cancer at the age of 38. Nobel Prize is not awarded posthumously.

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-containing base

Monomer - Nucleotide Polymer – DNA

Each Nucleotide is composed of 3 molecules bonded together 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-containing base

Nucleotides are named for the nitrogen base they contain. 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

Explained Chargaff’s Rule Base Pairing in DNA– One purine (double ring) hydrogen bonds with one pyrimidine (single ring) Explained Chargaff’s Rule C G T A

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 C G T A

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

DNA 2 stranded The sugar-phosphate backbone is connected by covalent bonds. Hydrogen bonds between base pairs hold strands together hydrogen bond covalent bond

RNA Single strand instead of double strand Ribose instead of deoxyribose 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.