History, experiments to prove DNA is the genetic material

1
Chapter 10

• History, experiments to prove DNA is the genetic
  material

2
Protein was initially thought to be the genetic
material

• Protein was abundant in cells
• DNA was thought to be too simple to be the
  genetic material
• only four types of nucleotides as compared to the
  20 different amino acids

3
Griffiths experimentUsed bacteria (Diplococcus
pneumoniae)

• What does a capsule do?
• Virulent (IIIS)
• Capsule
• Form smooth, shiny colonies when grown on agar
• Avirulent (IIR)
• No capsule
• Form rough colonies when grown on agar

4
Griffiths experiment

• Inject IIIS (virulent) into mice
• death
• Inject IIR (avirulent) into mice
• no death
• Inject heat killed IIIS into mice
• no death
• Inject IIR heat killed IIIS into mice ???

5
Griffith showed DNA was the Genetic Material

• Griffith showed that avirulent strains of
  Diplococcus pneumoniae could be transformed to
  virulence.
• Living IIR (avirulent) heat killed
  IIIS(virulent) results in living IIIS
• Heat killed IIIS converted live avirulent IIR
  into virulent IIIS
• Some chemical substance in the heat killed IIIS
  was responsible for the transformation

6
Avery, MacLeod, and McCarty molecule
responsible for transformation was DNA

• Took virulent cells (IIIS)
• Removed protein, RNA or DNA
• Add this substance to IIR cells
• If remove RNA or protein, still have
• transformation
• If remove DNA, no
• transformation
• DNA is active
• component

7
Hershey and Chase used bacteriophage

• What are bacteriophage?
• How are viruses different from other cells?
• How do viruses get into other cells?

8
Hershey and Chase used bacteriophage to determine
DNA is genetic material

• Virus or phage attach to bacterial cell
• Virus takes over cellular machinery of the host
  and directs host cell to make more viruses
• Bacterial cell lysed and release new virus
  particles

9
Hershey and Chase labeled viruses with
radioactive tags

• What is the purpose of using a radioactive tag?
• In what kind of a medical test or research
  procedure would you use a radioactive tag?

10
Hershey and Chase labeled viruses with
radioactive tags

• 32P and 35S are radioactive tags used in science
• P phosphorous
• S sulfur
• Which tag would be used for DNA, which for RNA
  and which for proteins?
• What is the basic structure of DNA?
• What is the basic structure of amino acids?

11
Hershey and Chase showed that DNA directs viral
production

• Mix labeled bacteria with unlabeled viruses
• Viruses become labeled
• Then, let labeled virus infect unlabeled bacteria
• P32 ended up in infected viruses
• S35 did not end up in infected viruses
• DNA directed new viral production

12
Genetic material should be found where it
functions - nucleus

• DNA found in nucleus, mitochondria and
  chloroplasts
• Protein also found in cytoplasm DNA is not in
  cytoplasm
• Twice as much DNA found in somatic cells as
  compared to haploid germ cells
• No correlation between protein content in somatic
  cells and germ cells

13
Nucleic acid is the genetic material

• UV light causes mutations in DNA
• UV light is most mutagenic at a wavelength of 260
  nm.
• DNA and RNA absorb UV light most strongly at 260
  nm
• Protein absorbs most strongly at 280 nm, a
  wavelength at which no significant mutagenic
  effects are observed.

14
RNA Serves as the Genetic Material in Some Viruses

• Some viruses have an RNA core rather than a DNA
  core. This RNA serves as the genetic material for
  these viruses

15
RNA is the genetic material in some viruses not
proteins

• Make hybrid virus
• RNA from TMV virus
• Protein from HR virus
• Put on tobacco leaf
• Produced TMV lesions
• RNA determines the outcome

16
RNA viruses

• Replication of the viral RNA is dependent on RNA
  replicase
• This enzyme needs to be packaged with the virus
  and is made by the virus not by the host cell
• Eukaryotic cells make RNA from DNA and not RNA
  from RNA
• Retroviruses
• RNA is converted into DNA by reverse
  transcriptase.