The Genetic Material Mrs. Wolfe

Griffith and Avery (1950’s) Experimented to identify DNA as the genetic material. Griffith’s experiments: S. pneumoniae bacteria Two types of bacteria: S – type: disease R – type: no disease

Griffith found that Live S-type bacteria cause the disease. Live R-type bacteria do not cause the disease. Heat-killed S-type bacteria do not cause the disease. A combination of Heat-killed S-type bacteria and Live R-type bacteria do cause the disease. The R-type bacteria take up some genetic material from the dead S-type bacteria and become lethal.

Transformation A change in the phenotype caused when bacterial cells take up foreign genetic material. The S-type cells were enclosed in a capsule that protected it from the body’s defense system. The R-type cells didn’t have a capsule. When the R-type cells engulfed S-type genetic material, it had the tools to create its own capsule, thus protecting it and allowing it to live to cause infection.

How would R-type cells know how to make a capsule? The instructions for creating a capsule, must be contained within the genetic material. What is genetic material? Chromosomes! Chromosomes are made of DNA and proteins. So is it the DNA or the proteins that carry the instructions? Avery performed a series of experiments. Avery found that transformation still occurred when a protein- destroying enzyme was introduced. However, transformation stopped when a DNA- destroying enzyme was introduced.

So the DNA-destroying enzyme… Worked! It destroyed the DNA and the R-type cells were not able to make a capsule. The DNA must carry the information for making a capsule! The protein is not the genetic material. DNA IS THE GENETIC MATERIAL!!

Hershey and Chase Support Avery and Griffith’s Findings Hershey and Chase do a similar experiment with virus DNA to prove DNA is the genetic material instead of protein. Bacteriophage - virus Protein contains sulfur. DNA contains phosphorus.

Experiment A Experiment B

What were their findings? Radioactive sulfur was not taken up by the bacterial cell. Radioactive phosphorus was taken up by the bacterial cell. The infected cell only took up the DNA. So, DNA is the genetic material!

Structure of DNA Rosalind Franklin – credited with showing an X-ray diffraction pattern of DNA

Rosalind was cheated! Watson and Crick visited Franklin’s lab and saw this x-ray diffraction image. Going with her idea, they studied the x-ray and found that DNA is shaped like a double-helix. Then, they built a model and got all of the credit!

DNA Structure Double helix structure DNA has a sugar-phosphate backbone. Made up of Nucleotides Nucleotide – –Phosphate group –Deoxyribose sugar –Nitrogenous base

A Closer Look at Nucleotides… Nucleotides have a sugar and a phosphate always. But, the nucleotides can have different nitrogenous bases. Two types of nitrogenous bases: –Purines – Adenine (A) and Guanine (G) –Pyrimidines – Thymine (T) and Cytosine (C)

Purines

DNA is double-helix in shape. DNA has two strands that are twisted together. Bases of one strand are connected to the bases of the other strand.

Certain bases connect This is called base- pairing. Adenine – Thymine Guanine – Cytosine

Double helix is held together by Hydrogen Bonds The bases are paired together. A Hydrogen bond is formed between bases. This holds the double helix together!

Sequencing DNA DNA is sequenced by writing the nitrogenous bases in order. What is the DNA sequence for the strand of DNA on the right?

Sequencing DNA What is the DNA sequence for the other strand of DNA? Compare the two sequences. ACGT TGCA Do you notice anything?

Complementary strands The second sequence is the same as the first sequence, but backwards. We call this complementary. So, the two strands of DNA are complementary.