Download (Brainard) How do we know DNA is the genetic material

NL 8:30-2:30 Weather Plan?
Room #
8:30 – 9:45
AP BIOLOGY I
Jolene Jacquet
226
Lab Review 1-6
AP BIOLOGY II
Nancy Bishop
228
DNA & Protein
Synthesis
229
Evolution Major
Concepts &Hardy
Weinberg
(Brainard)
AP BIOLOGY III
Cindy Brainard /
Brian Elliott
How do we know DNA is
the genetic material
What is the genetic material?
• Fredrick Griffith
• How do certain bacteria cause pneumonia?
• 2 strains (types) of Streptococcus pneumonia
– Smooth – when grown in petri dish smooth edged
colonies and shiny
– Rough – when grown in petri dish rough edged
colonies and not shiny
• SMOOTH COLONIES ARE PATHOGENIC –
disease causing
• ROUGH COLONIES ARE USUALLY
DESTROYED BY HOST IMMUNE SYSTME
bacterium
smooth strain –
capsule
rough strain –
no capsule
??????????
Does the capsule cause disease?
• DEAD S cells were mixed with live R cells.
• Bacteria isolated from blood of dead mice
were S strain and produced more S strain
cells
• Change was permanent and heritable –
capsule did not cause disease.
TRANSFORMATION
• Something in dead, disease-causing bacteria
turned harmless bacteria into killers!
• He called this process TRANSFORMATION
• Some factor in dead pathogens turned
harmless bacteria into pathogens!
What is the transforming factor?
“C (somatic) carbohydrate, nucleoproteins, free
nucleic acids of both the yeast [RNA] and thymus
[DNA] types, lipids and other cell constituents. Try
to find in that complex mixture the active
principle..!! Try to isolate and chemically identify
the particular [transforming] substance....!Some
job--full of heartaches and heartbreaks. But at last
perhaps we have it....”
• Oswald Avery
What is the transforming factor?
• Oswald Avery and colleagues
• Made extract of bacteria
• Destroyed proteins, lipids, carbohydrates,
and nucleic acids (RNA and DNA) one-byone with enzymes
• When DNA was destroyed by enzymes the
cells did not become pathogenic
• Conclusion??????
Many people remained unconvinced
that DNA was the genetic material!
• Many thought protein must be the genetic
material.
• Protein is more complicated – 20 amino
acids
• DNA is made of only 4 bases – adenine,
guanine, cytosine and thymine
• Protein had “greater heterogeneity and
specificity of function”
• Little was know about nucleic acids at the
time
EVIDENCE DNA IS GENETIC
MATERIAL
• Alfred Hershey and Martha Chase
• Worked with a simple system: E. coli bacteria
and bacteriophages – viruses that infect
bacteria
EVIDENCE DNA IS GENETIC
MATERIAL
bacteria
VIRUSES are not cells. Consist of nucleic acid
wrapped in a protein coat. Must infect a cell in
order to reproduce.
EVIDENCE DNA IS GENETIC
MATERIAL
• How could they determine whether DNA or
protein from phage entered cell?
• Protein has sulfur, DNA does not – grow
phages in radioactive sulfur 35S
• DNA has phosphorus, protein does not – grow
phages in radioactive phosphorus 32P
• Virus capsid – protein coat labeled with 35S
• Virus core – DNA labeled with 32P
• Infect the bacteria and see which one enters
the cell
EVIDENCE DNA IS GENETIC
MATERIAL
• Radioactive DNA entered the cell but
radioactive protein stayed outside.
• Conclusion?
What does DNA look like?
Watson and Crick
Rosalind
Franklin
HOW IS DNA REPLICATED?
• Meselson and Stahl
• Use radioactive isotopes to track old and new
strands of DNA
• Use centrifugation to separate DNA
molecules by mass
and …
CHASE
HOW DO WE
KNOW WHICH
MODEL IS
CORRECT?
First make the original DNA “HEAVY”
http://www.sumanasinc.com/webcontent/anima
tions/content/meselson.html