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How was DNA discovered?
• There were 3 major experiments that led to the discovery of DNA as the genetic material.– Griffiths Transformations– Avery – The Hershey-Chase Experiment
Griffiths
• Griffith’s injected bacteria into mice.
• He showed that something in the smooth bacteria TRANSFORMED the non-deadly rough.
Avery and DNA• Avery wanted to figure out what exactly
transferred from the smooth strain to the rough.– One at a time he used enzymes to destroy
the:• Proteins• Lipids• Carbohydrates• RNA• Still the smooth bacteria tranferred something
All that was left was the DNA!!!!
Hershey and Chase
• Studied viruses that infect bacteria – called bacteriophage.
• Bacteriophages are simple: just DNA or RNA and a protein coat.
• The labeled the DNA OR protein coat with a a radioactive marker.
• What they found was the DNA entered the cells, not the proteins.
DNA
• Deoxyribonucleic Acid
• Made up of units called nucleotides– Each nucleotide has 3 parts:
• A) 5-Carbon sugar• B) Phosphate group• C) nitrogenous base
Nuclotides – differ in the Nitrogen Base!• Nucleotides belong to 1 of 2 groups:
• Purines: Have 2 rings in their structure– Adenine– Guanine
• Pyrimidines: 1 ring– Cytosine– Thymine
Structure of DNA
• These 4 nucleotides are linked in various ways and order – creating DNA.
• There are 2 strands of DNA• The sugar and the
phosphate make up the backbone of the DNA.
• One nucleotide from each strand come bond together to form what looks like ladder rungs.
Chargaff• Chargaff’s Rules
– Erwin Chargaff showed that in most samples of DNA the Percentages of Cytosine and Guanine were equal.
– Likewise, the Adenine and Thymine Percentages were also very close.
Percentages of Bases in Organisms
Organism A T C G
Human 30.9 29.4 19.9 19.8
Chicken 28.8 29.2 20.5 21.5
Yeast 31.3 32.9 18.7 17.1
E. coli 24.7 23.6 26.0 25.7
Shape of DNA
• In the 1950’s Rosalind Franklin looked at the shape of DNA.
• Used technique called X-Ray Diffraction
• Her work showed that the strands of DNA wrap around each other called a Double Helix
• Watson and Crick used Franklin’s X-Ray diffraction to build a model of DNA