KEY CONCEPT DNA structure is the same in all organisms.

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.

Nucleic Acid Structure DNA and RNA are polymers made up of billions of repeating monomer units called nucleotides. Each Nucleotide is composed of 3 molecules bonded together Phosphate (P) 5 Carbon Sugar (S) Nitrogen-containing base Phosphate Sugar Nitrogen-containing base

Types of Nucleotides

1. Purines – Double ring structure *Guanine - G *Adenine - A 2. Pyrimidines – Single ring structure *Cytosine - C *Thymine – T (Only in DNA) *Uracil – U (Only in RNA)

Comparison of DNA and RNA DNA RNA Sugar type Deoxyribose Ribose Nitrogen Bases A,T,G,C A,U,G,C # of Strands Double stranded Single Stranded

Important Experiments of the 1950’s Erwin Chargaff Analyzed the amounts of nucleotides from the DNA samples of many organisms

Chargaff’s DNA Nucleotide Sample Testing Results

Chargaff’s DNA Nucleotide Sample Testing Results

Chargaff’s Base Pairing Rules These are called base pairs

Chargaff’s Research Implications Chargaff finding explained how all organisms have the same basic DNA but still be so different: Although the same 4 bases are found in the DNA of all living things the amount of each of base pair is different for each organism. The order sequence of base pairs is different in each organism.

2. Rosalind Franklin and Maurice Wilkins Used a technique called x-ray crystallography to capture the first images of DNA which indicated that it had a spiral or helical shape and an even width.

3. James Watson and Francis Crick Determined the three-dimensional structure of DNA by building models. Watson and Crick’s discovery built on the work of Rosalind Franklin and Erwin Chargaff Realized that DNA is a double helix made up of a sugar-phosphate backbone on the outside with bases on the inside.

Nucleotides always pair in the same way. The base-pairing rules show how nucleotides always pair up in DNA. A pairs with T C pairs with G Because a pyrimidine (single ring) pairs with a purine (double ring), the helix has a uniform width. C G T A

The backbone is connected by covalent bonds. The bases are connected by hydrogen bonds. hydrogen bond covalent bond