DNA Deoxyribonucleic Acid

Identifying the Genetic Material 1. Frederick Griffith (1928) – was working with S. pneumonia in mice when he showed that harmless bacteria could turn virulent (able to cause disease) when mixed with heat killed bacteria that cause disease; this showed that genetic material could be transferred between dead bacteria and living bacteria

Griffith’s Experiment: Griffith showed that genetic material was transferred from the heat-killed cells of the S strain into the live cells of the R strain changing the harmless bacteria into a disease-causing bacteria. He called this process transformation because one type of bacteria was changed (or transformed) into another.

2. Avery’s Experiment (1944) – he demonstrated that the genetic material was composed of DNA (deoxyribonucleic acid) 3. Hershey and Chase (1952) – used a bacteriophage and radioactive labels to show that viral genes are made of DNA, not protein

The Structure of DNA The discovery of the structure of DNA can be attributed to the following scientists: Erwin Chargaff (1949) Maurice Wilkins & Rosalind Franklin (1952) James Watson & Francis Crick (1953)

X-ray Diffraction Maurice Wilkins and Rosalind Franklin developed X-ray diffraction photographs of strands of DNA that suggested that the DNA molecule resembled a tightly coiled helix and was composed of at least two or three chains of nucleotides.

The DNA Model: Watson & Crick determined that the structure of the DNA molecule was a “double helix”. It was made of two strands of nucleotides, running in opposite directions, and they were held together by hydrogen bonds.

Each strand is made of linked nucleotides Each strand is made of linked nucleotides. Nucleotides are the subunits that make up DNA. Each nucleotide is made of three parts: a phosphate group, a 5-carbon sugar (deoxyribose), and a nitrogen-base.

Four Possible Nucleotides: Adenine Purines Guanine (two rings of carbon) Thymine Pyrimidines Cytosine (one ring of carbon)

Complimentary Base Pairs: Watson and Crick determined that a purine on one strand always paired with a pyrimidine on the opposite strand Ex: T – A C – G G – C A – T

Finding the Complimentary Strand Add the complimentary nucleotide according to the base pairing rules T G C T A

Erwin Chargaff– showed that different species have different amounts of the four nucleotides that make up DNA, but that the amount of A (adenine) always equaled the amount of T (thymine), and likewise, the amount of C (cytosine) always equaled the amount of G (guanine).

The Double Helix Model DNA resembles a twisted “ladder” “Rails” or sides are made up of a sugar- phosphate backbone “Rungs” or steps are nitrogenous bases