DNA Experiments

Discovering the Genetic Material

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.

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?

Oswald Avery’s Experiment

• Avery discovered that DNA was responsible for transforming cells (1944)

• How did he do it?

Hershey-Chase Experiment

• Hershey and Chase discovered that DNA was the genetics material in ALL types of cells

• How did they do it?

What is the role of DNA in heredity?

1. Storing2. Copying3. Transmitting

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

Chargaff’s Rule

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

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.

What does the double helix model tell us about the structure of DNA?

1. Antiparallel strands2. Hydrogen Bonding3. Base Pairing: explains Chargaff’s Rule

Antiparallel Strands

• DNA is always read in the 5’ to 3’ direction

Hydrogen Bonding

Base Pairing

How is DNA copied?

• Complementarity– DNA Polymerase: an enzyme that joins individual

nucleotides to produce a new strand of DNA

How is DNA copied?

• Complementarity– Replication Forks

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.

How is DNA copied?

Prokaryotic Versus Eukaryotic Replication

• Prokaryotic: starts at a single point, proceeds in both direction

• Eukaryotic: starts at several points, proceeds in both directions

Prokaryotic Versus Eukaryotic Replication

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