Dispatch
1) When do reviews start?
2) What days of the week are the reviews?
3) Write 5 things you learned from this week’s reading?
4) When is the genetics test?
5) Complete any 7 questions from the study guide.
6) Grades are out in 2 weeks! Do you have an A+?
Chapter 15—Linked Genes
The following is a map of four genes on a chromosome:
a) Between which two genes would you expect the lowest frequency of recombination?
Changes in chromosome structure
• deletion– loss of a chromosomal segment
• duplication– repeat a segment
• inversion– reverses a segment
• translocation– move segment from one chromosome to
another
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You should know the following scientists:• --Alfred Hershey and Martha Chase• -Oswald Avery, Maclyn McCarty, and Colin
MacLeod• -Erwin Chargaff• -Matthew Meselson and Franklin Stahl• Watson, Crick, and Franklin
DNADNA
RNARNA
ProteinProtein
Scientists call this Scientists call this the:the:
Central Central
DogmaDogma of of
Molecular Molecular
Biology!Biology!
How do we know that all of our genetic information
comes from DNA?
• What type of experiment would you design to determine that DNA is the source of all genetic information?
Griffith’s Experiment with Pneumonia and the accidental discovery of Transformation
• Frederick Griffiths was a bacteriologist studying pneumonia
• He discovered two types of bacteria:– Smooth colonies– Rough colonies
CONCLUSION:
The smooth colonies must carry
the disease!
Griffith’s Experiment with Pneumonia and the accidental discovery of Transformation
• When heat was applied to the deadly smooth type…
• And injected into a mouse…
• The mouse lived!
• Griffith injected the heat-killed type and the non-deadly rough type of bacteria.
• The bacteria “transformed” itself from the heated non-deadly type to the deadly type.
Griffith’s Experiment with Pneumonia and the accidental discovery of Transformation
Griffith’s Experiment did not prove that DNA was responsible for
transformationHow would you design an
experiment to prove that DNA was responsible for
transformation?
Avery, McCarty, and MacLeodRepeated Griffith’s Experiment
Oswald AveryOswald Avery Maclyn McCartyMaclyn McCarty Colin MacLeodColin MacLeod
Avery, McCarty, and MacLeodAdded the non-deadly Rough Type of Bacteria to the Heat-Killed Smooth
Type
CarbohydratesCarbohydrates LipidLipidss
ProteinProteinss
RNARNA DNADNA
To the Heat-Killed Smooth Type, To the Heat-Killed Smooth Type, added enzymes that added enzymes that
destroyed…destroyed…
S-Type S-Type Carbohydrates Carbohydrates
DestroyedDestroyed
S-Type S-Type Lipids Lipids
DestroyeDestroyedd
S-Type S-Type Proteins Proteins DestroyeDestroye
dd
S-Type S-Type RNA RNA
DestroyeDestroyedd
S-Type S-Type DNA DNA
DestroyeDestroyedd
Conclusion:Conclusion:
DNA was the DNA was the transforming factor!transforming factor!
The Hershey-Chase Experiment
Alfred Hershey & Martha Chase
worked with a bacteriophage:
A virus that invades
bacteria. It consists of a
DNA core and a protein coat
DNADNA
Protein coatProtein coat
Protein coats of bacteriophages labeled with Protein coats of bacteriophages labeled with Sulfur-35Sulfur-35
DNA of bacteriophages labeled with Phosphorus-32DNA of bacteriophages labeled with Phosphorus-32
BacteriumBacterium
BacteriumBacterium
PhagePhage
PhagePhage1.1. Hershey and Chase Hershey and Chase
mixed the mixed the radioactively-radioactively-labeled viruses labeled viruses with the bacteriawith the bacteria
The viruses infect The viruses infect the bacterial cells.the bacterial cells.
Protein coats of bacteriophages labeled with Protein coats of bacteriophages labeled with Sulfur-35Sulfur-35
DNA of bacteriophages labeled with Phosphorus-DNA of bacteriophages labeled with Phosphorus-3232
2.2. Separated the Separated the viruses from the viruses from the bacteria by bacteria by agitating the virus-agitating the virus-bacteria mixture in bacteria mixture in a blendera blender
Protein coats of bacteriophages labeled with Protein coats of bacteriophages labeled with Sulfur-35Sulfur-35
DNA of bacteriophages labeled with Phosphorus-DNA of bacteriophages labeled with Phosphorus-3232
3.3. Centrifuged the mixture so Centrifuged the mixture so that the bacteria would form a that the bacteria would form a pellet at the bottom of the test pellet at the bottom of the test tubetube
4.4. Measured the radioactivity in Measured the radioactivity in the pellet and in the liquidthe pellet and in the liquid
The Hershey-Chase results reinforced the Avery,
McCarty, and MacLeod conclusion:
DNA carries the genetic code!
However, there were still important details to uncover…
How did DNA:1. Store information?
2. Duplicate itself easily?
These questions would be answered by
discovering DNA’s structure
The Race to Discover DNA’s Structure
The Race to Discover DNA’s Structure
Linus Linus PaulingPauling 1940s1940s
Discovered the Discovered the alpha-helical alpha-helical structure of proteins.structure of proteins.
The Race to Discover DNA’s Structure
19501950
Chargaff’s Rule: Chargaff’s Rule: Equal amounts of Equal amounts of AAdenine and denine and TThymine, and equal hymine, and equal amounts of amounts of GGuanine uanine and and CCytosineytosine
Erwin Erwin ChargaffChargaff
Why do you think Why do you think the bases match up the bases match up
this way?this way?
Purine + Purine = Too widePurine + Purine = Too wide
Pyrimidine + Pyrimidine = Too Pyrimidine + Pyrimidine = Too NarrowNarrow
Purine + Pyrimidine = Perfect Fit from X-ray Purine + Pyrimidine = Perfect Fit from X-ray datadata
The Race to Discover DNA’s Structure
Maurice Maurice WilkinsWilkins
Rosalind Rosalind FranklinFranklin
X-Ray diffraction image of X-Ray diffraction image of DNA taken by Franklin in DNA taken by Franklin in
19511951
The Race to Discover DNA’s Structure
James WatsonJames Watson Francis CrickFrancis Crick
19531953
Compiled data Compiled data from previous from previous scientists to scientists to build a double-build a double-helical model of helical model of DNADNA
The Race to Discover DNA’s Structure was Over
• DNA is made up of:– Four nucleotides: Adenine, Thymine,
Guanine and Cytosine– These follow the rules of base-pairing:
• Adenine bonds with Thymine• Guanine bonds with Cytosine
– A sugar-phosphate backbone
• DNA is arranged in an double-helix
DNA Replication
• The double helix did explain how DNA copies itself
• We will study this process, DNA replication, in more detail
How does DNA replicate?
ConservativeConservative Semi-Semi-ConservativeConservative
DispersiveDispersive
Hypotheses:
1.1.Bacteria cultured in medium Bacteria cultured in medium containing a containing a heavyheavy isotope of isotope of Nitrogen (Nitrogen (1515N)N)
Meselson-Stahl ExperimentMeselson-Stahl Experiment
2.2.Bacteria transferred to a medium Bacteria transferred to a medium containing elemental Nitrogen containing elemental Nitrogen ((1414N)N)
Meselson-Stahl ExperimentMeselson-Stahl Experiment
Meselson-Stahl ExperimentMeselson-Stahl Experiment
3.3.DNA sample centrifuged after First DNA sample centrifuged after First replicationreplication
Meselson-Stahl ExperimentMeselson-Stahl Experiment
4.4.DNA sample centrifuged after DNA sample centrifuged after Second replicationSecond replication
DNA Replication
The “parent” molecule has two complementary The “parent” molecule has two complementary strands of DNA.strands of DNA.
Each is base paired by hydrogen bonding with its Each is base paired by hydrogen bonding with its specific partner:specific partner:
A with TA with T
G with CG with C
DNA Replication
The first step in replication is the separation of the The first step in replication is the separation of the two strands.two strands.
DNA Replication
Each parental strand now serves as a template Each parental strand now serves as a template that determines the order of the bases along a new that determines the order of the bases along a new complementary strand.complementary strand.
DNA Replication
The nucleotides are connected to form the sugar-The nucleotides are connected to form the sugar-phosphate backbones of the new strands.phosphate backbones of the new strands.
Each “daughter” DNA molecule consists of one Each “daughter” DNA molecule consists of one parental strand and one new strand.parental strand and one new strand.
The Race to Replicate DNA
• Two teams: A and B• Individually, each team member
will run to the board to add a nucleotide to the “unzipped” strand of DNA.
• The first team to finish base-pairing their DNA correctly will win the game.