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.