DNA

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.

• Only the DNA entered the cell

• Therefore DNA must be the genetic material

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

DNA• DNA is a long chain of

repeating nucleotides.

• DNA has 2 strands to it

• The phosphate group of one nucleotide is bonded to a sugar group of the next nucleotide.

• The nitrogenous bases of one strand extends out and bonds with a complementary nitrogenous base on the other DNA strand.