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Class Notes
Discovering DNA
Questions/Main
Idea:
Briefly describe Frederick
Griffith’s pneumonia
experiment.
Name: _______________________________________
Period:_______________________________________
Date: _______________________________________
Notes: pgs 287-294
 1928 – Frederick Griffith conducted experiments with mice to study
the effects of pneumonia bacteria.

Griffith isolated two strains of pneumonia bacteria – one caused
the lung disease (pneumonia) and the other did not.
 Mice injected with the disease causing bacteria died.
 Mice injected with the harmless bacteria lived.
 Mice injected with the heat-killed disease causing bacteria also lived.
 Griffith then mixed the live harmless bacteria with heat-killed disease
causing bacteria.
 Mice injected with this combination died.
How did Griffith explain
the results from his
experiment?
Based on his results, Griffith hypothesized that when the harmless and
heat-killed bacteria where mixed, some factor was exchanged between
them, making the live harmless bacteria deadly.
What is transformation?
Is the process in which one strain of bacteria is changed by the gene(s) of
another bacteria.
Describe how did Oswald  In 1943, Avery, with fellow scientists, conducted an experiment
Avery’s modify Griffith’s
similar to Griffith’s, except they used enzymes to selectively destroy
experiment.
molecules one at a time.
 When they injected harmless bacteria with only lipids, carbs, or
proteins: transformation did not occur.
 When they used the nucleic acids (DNA): transformation did occur,
the bacteria became lethal.
 This helped to determine that DNA stores and transmits genetic
information.
Briefly describe Alfred
 Hershey and Chase (1950) conducted experiments with bacteriophages
Hershey & Martha
(viruses that attack bacteria) to determine if genetic information is
Chase’s experiment.
carried on proteins or DNA.
 They used radioactive elements to ‘mark’ DNA and protein.
 Only the radioactively-labeled DNA was found in bacteria cells.
What did their Hershey & These findings further supported the conclusions of Avery’s experiment,
Chase conclude from their that genetic material is DNA and NOT protein.
experiment?
Explain Chargaff’s rules.  Chargaff determined that in any sample of DNA:
 The # of adenines (A) = the # of thymines (T)
 The # of cytosines (C) = the # of guanines (G)
 Thus in DNA, the bases A and T pair together, and C and G pair
together.
How did Rosalind
Franklin used x-ray diffraction to create pictures of DNA’s molecular
Franklin contribute to the structure
discovery of the DNA
structure?
Who discovered the
 James Watson and Frances Crick determined the structure of DNA in
structure of DNA?
1953 using their data and the work of previous scientists.
Describe the structure of
DNA is shaped like a double helix, with strands held together by the weak
DNA.
hydrogen bonds formed between the bases A-T and C-G.
 On the diagram:
 Circle and label a
nucleotide.
 Label the sugar and
phosphate molecules.
 Label the bases that
are not already labeled
 Label a base pair.
 Label the sugarphosphate backbones.
 Label the hydrogen
bonds.
Why and when does DNA  DNA must be replicated (copied) in order to insure that during cell
replication occur?
division, each daughter cell receives a complete copy.
 DNA replication occurs in the nucleus during S phase of the cell cycle,
before chromatin (DNA wrapped around proteins) condenses into
chromosomes.
Describe the 4 steps of
 Step 1 – Helicase unzips the strand of DNA by breaking the weak
DNA replication.
hydrogen bonds between base pairs.
 Step 2 – DNA polymerase inserts the appropriate bases.
 Step 3 – A new sugar-phosphate backbone is built.
 Step 4 – The sequence is proofread by DNA polymerase.
Why is DNA replication
considered semiconservative?
Because each side of the parent strand serves as a template for the 2 new
DNA strands.
Summary:
Base Pair Practice: For each example below, please give the correct complementary strand of DNA.
1. T T G C T A G
3. A C C G T C A
AACGATC
TGGCAGT
2. T A G C G C T
ATCGCGA
4. G C T A T G T
CGATACA
DNA Replication:
For DNA to replicate, an enzyme, helicase must first ‘unzip’ the hydrogen bonds between
complementary base pairs. The site where the unzipping begins is referred to as a replication fork.
The two template strands of DNA are kept separate by single stranded binding proteins. New
strands of DNA are built from the template strand, using DNA polymerase (enzyme) to bring in
the nucleotides. Nucleotides on the leading strands are brought to the template strand in a
continuous fashion. Nucleotides on the lagging strand are brought in segments known as Okazaki
fragments. The Okazaki fragments must then be bonded together using the enzyme DNA ligase.
After replication is complete, DNA polymerase then acts a spellchecker, checking for any
mutations in the new strands of DNA.