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DNA Experiments
Discovering the Genetic Material
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Learning Targets“I Can…”
-Summarize the experiments of Griffith, Avery, Hershey and Chase and their conclusions about the genetic material in cells.-Define “transformation.”-Describe a bacteriophage.-Summarize the contributions of Franklin, Chargaff, and Watson and Crick.
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Frederick Griffith’s Experiment
• Griffith was the first to show that the genetic material can be taken up by a cell; this is now called bacteria transformation(1928)
• How did he do this?
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Oswald Avery’s Experiment
• Avery discovered that DNA was responsible for transforming cells (1944)
• How did he do it?
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Hershey-Chase Experiment
• Hershey and Chase discovered that DNA was the genetics material in ALL types of cells
• How did they do it?
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What is the role of DNA in heredity?
1. Storing2. Copying3. Transmitting
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What is DNA made of?
• Long chains of nucleotides• How do we know this?– Chargaff’s Rule– Franklin’s Xrays– Watson and Crick’s models
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Chargaff’s Rule
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Rosalind Franklin’s X-Rays
Her experiments showed that…1. DNA is twisted like the coils
of a spring2. DNA may have two strands3. The nitrogen bases are
near the center of DNA
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James Watson and Francis Crick
With the help of Franklin’s x-rays, they built the first model of DNA that showed two strands of nucleotide sequences wound around each other.
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What does the double helix model tell us about the structure of DNA?
1. Antiparallel strands2. Hydrogen Bonding3. Base Pairing: explains Chargaff’s Rule
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Antiparallel Strands
• DNA is always read in the 5’ to 3’ direction
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Hydrogen Bonding
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Base Pairing
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How is DNA copied?
• Complementarity– DNA Polymerase: an enzyme that joins individual
nucleotides to produce a new strand of DNA
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How is DNA copied?
• Complementarity– Replication Forks
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How is DNA copied?
• Complementarity– Telomeres/Telomerase: telomeres are the tips of
chromosomes where replication is difficult. – An enzyme called telomerase adds short, repeated
DNA sequences to prevent genes from being damaged or lost during replication.
– Telomerase is especially important in rapidly dividing cells, such as stem cells or embryonic cells.
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Prokaryotic Versus Eukaryotic Replication
• Prokaryotic: starts at a single point, proceeds in both direction
• Eukaryotic: starts at several points, proceeds in both directions
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Prokaryotic Versus Eukaryotic Replication
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References
• www.Quia.com• http://
www.visionlearning.com/en/library/Biology/2/DNA-I/149
• http://study.com/academy/lesson/oswald-avery-experiment-discovery-quiz.html
• http://en.wikipedia.org/wiki/Hershey%E2%80%93Chase_experiment
• http://biology.clc.uc.edu/courses/bio104/hershey-chase%20js.htm