Molecular GeneticsDNA: The Genetic Material [12.1]

Discovering Genetic Material

• For a long time there was debate about whether DNA or protein housed the genetic code.

• It took a number of different researchers and experiments to show that it was in fact DNA

• Frederick Griffith became the first of these when he found that he could change one strain of S. pneumoniae to another.

• Two strains:– S-strain (lethal)– R-strain (nonlethal)

• S-strain + mouse = dead mouse

• R-strain + mouse = live mouse

• Dead S-strain + mouse = live mouse

• Dead S-strain + live R-strain + mouse = DEAD MOUSE

• Griffith’s conclusion was that a transforming principle must be passed as coded information from the lethal S-strain to the nonlethal R-strain, making the R-strain lethal

• Oswald Avery later repeated Griffith’s work with the S and R strains

• He modified the experiment by using a substance that breaks down protein prior to exposing the mice to the bacteria

• He was able to show that it was the DNA, not the protein, that was Griffith's transforming principle

• Alfred Hershey and Martha Chase supported Griffith and Avery’s research in their own experiment, using a bacteriophage with labeled DNA and protein.– (a bacteriophage is a virus that attacks

bacteria by injecting its DNA into them and hijacking the bacteria)

– Tagged bacteriophages with 2 labels:• Radioactive 35S protein • Radioactive 32P DNA

– Allowed the labeled bacteriophages to attack a colony of bacteria

– Observed that the radiolabeled DNA was passed to the highjacked bacteria, but not protein.

DNA Structure• DNA (deoxyribonucleic acid) and RNA

(ribonucleic acid) are both nucleic acids that contain the code required to build and maintain all living things.– DNA is the primary source code– RNA is a messenger and assistant

• DNA & RNA are chains constructed of subunits called nucleotides

Nucleotides have 3 parts:1. Phosphate group

2. 5-carbon sugar

3. Nitrogenous base • Adenine (A)• Guanine (G)• Cytosine (C)• Thymine (T)• Uracil (U) (replaces Thymine in RNA only)

Pyrimidine Bases

Purine Bases

• DNA is double stranded, RNA is a single strand

• The beginning of a strand is called the 3-prime (3’) end, and the end of the strand is called the 5’

• DNA is antiparallel: the 2 strands run opposite to each other just like the 2 lanes of a road.

• James Watson and Francis Crick were the first researchers to publish data that confirmed that DNA is a double helix structure– Outside strands are alternating

phosphate and sugar– Bases in the middle

• Erwin Chargaff, an Austrian researcher, discovered that the nitrogenous bases always pair a certain way.

• Chargaff’s rule: C = G and T = A

• Every base pair is between a pyrimidine and a purine

• The order that the base pairing occurs is the actual code itself

• Read the article titled “Discovery of DNA Structure and Function: Watson and Crick”

• On your own sheet of paper, Define the terms listed on the following slide using the article ONLY (use contextual clues as well as direct explanations of the term)

Assignment

• DNA-

• Nucleotide-

• Nitrogenous Base-

• Chargaff’s Rule-

• Base Pairing-

• DNA backbone (what’s it made of)

• Describe the structure of DNA and include the previous terms and more in your description

Terms