Ch. 10: DNA, RNA and Protein Synthesis

The discovery of DNA

1928: Frederick GriffithTried to find a vaccine for

pneumonia.Studied two strains of

Streptococcus pneumoniae, R cells and S cells, by injecting them into mice.

Griffith’s Experiment

“S” strain are virulent (causes disease)

“R” strain are avirulentGriffith injected mice with strains

of live and dead “S” and “R” bacteria.

Griffith’s DataStrepococcus

pneumoniae strainResult on mouse:

Live Live S S (control)(control)

Live Live R R (control)(control)

Heat-killed Heat-killed S S (control)(control)

Heat-killed Heat-killed SS & & Live Live RR

Dies

Dies

Lives

Lives

Griffith’s Conclusion:

The Transforming PrincipleThe dead S bacteria transferred an

inheritable material to the R strain. (heredity: passing on of traits)As a result, the living R bacteria

was transformed into the S strain.

So, what in the bacteria So, what in the bacteria caused them to caused them to “transform” from one “transform” from one strain to another? strain to another?

Protein, DNA, or RNA??Protein, DNA, or RNA??

1940’s: Oswald Avery’s Experiment

Repeated Griffith’s experiments but removed the protein, DNA, or RNA molecules.

Observed which molecule was responsible for “transforming” the R bacteria into S.

Avery’s Data

Dead S strain with…

Result when S strain is mixed with live R strain:

Proteins removed

DNA removed

RNA removed

Mouse dies

Mouse dies

Mouse lives

Avery’s Conclusion:

DNA is the hereditary material that transformed R bacteria into S bacteria.

1952: Alfred Hershey and Martha Chase Experiment

Studied bacteriophages (viruses that infect bacteria).

Hershey and Chase Experiment

1. Used radioactive tags to identify DNA (32P) and protein (35S)

2. Infected bacteria with the virus. 3. Used blender to remove virus from

the surface of the bacteria.4. Observed what molecule was

transferred from the virus into the infected bacteria

Observations

Radioactive protein molecules found outside bacteria.

Radioactive DNA found inside bacteria

Hershey and Chases’ Conclusion:

DNA is the hereditary material that stores the information about building new virus particles.

The race to find DNA’s structure

1950’s: Watson and Crick

Discovered that the structure of DNA is a double helix - two chains that wrap around each other.

Rosalind Franklin and Maurice Wilkins

Took X-ray diffraction photographs of DNA crystals, which led Watson and Crick to their discovery.

3 parts of DNA

1. Sugar (Deoxyribose)

2. Phosphate group

3. Nitrogenous base

These make up the basic unit of DNA, which is called the ___________.nucleotide

4 Nitrogenous Bases

1. Adenine (A)

2. Guanine (G)

3. Thymine (T)

4. Cytosine (C)

Purine2 rings

Pyrimidine1 ring

The structure of DNA is like a spiral ladder

The backbone of alternating _____ and _________ are like the handrails.

sugarphosphate

The__________ ____ are like the rungs/ steps of the ladder.

nitrogenous bases

Draw a picture of DNA

Complementary Chains

Complementary base pairing

Adenine bonds with ________Guanine bonds with________

Bonded by ________ ______. The order of the bases is called the ______ _____________. base sequence

Thymine

Cytosinehydrogen bonds

Practice: Base pairing

EXIT TICKET1. If one strand of DNA has the following

base sequence, what sequence would the complementary strand contain?

ATAGCAGCT

2. What are the two components of the DNA backbone (railing of the staircase)?

3. Name at least two scientists that contributed to deriving the structure of DNA.