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DNA – THE CODE
OF LIFE
Unit 4 – The Cell Cycle
I. The discovery of the structure
and function of DNA
A. Friedrich Miescher – extracted DNA from
the nucleus of white blood cells.
B. Griffith’s Bacterial Transformation (1928)
• This is an experiment used disease
causing and harmless bacteria that were
injected into mice.
• Griffith wanted to find out if the harmless
bacteria can become harmful.
• Observations: Harmless bacteria can
become harmful if they are mixed with
dead harmful bacteria. The harmful
characteristic was inherited from one
generation to the next.
Conclusion: Some chemical component in
bacteria act as a transforming factor that
cause a heritable change.
 Bacterial transformation: When bacteria
pick up DNA from the environment or
other bacteria to use it for their own
benefit.

http://www.youtube.com/watch?v=MRBdbKFisgI and
http://nortonbooks.com/college/biology/animations/ch12a01.htm -- tutorial
C. The Hershey and Chase Experiment
(1952)
This experiment used bacteria and viruses
(bacteriophage) that only infect bacteria
 They also used radioactive isotopes to
label proteins (radioactive sulfur) and DNA
(radioactive phosphorous). These
radioactive isotopes were moved into two
groups of viruses.
 They wanted to find out if proteins or DNA
are responsible for inheritance.


Virus inserts its DNA into bacteria: http://www.youtube.com/watch?v=9hzUjx_oD8E&feature=related

Conclusion: Bacteriophages inject DNA
into the host cell, while proteins remain
outside. So DNA must be responsible for
determining inheritance.

http://highered.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120076/bio21.swf::Hershey%20and%20Chase%20Experim
ent
http://www.dnaftb.org/15/ -- animation on all scientists

D. Watson and Crick
They were the scientists who proposed a
model to describe the structure of DNA
 They also predicted how DNA doubles
itself.

II. The Structure of DNA (Review)
Deoxyribonucleic acid (DNA) is a
macromolecule that is built up of
nucleotide monomers.
 The nucleotide contains:

 Deoxyribose
 Phosphate
 Nitrogen-containing

base (A, T, C, G)
Two long polynucleotide chains face each
other by their complementary base pairs.
The two long chains are antiparallel to
each other and are held together by
hydrogen bonds. The 5’ end of one chain
binds with the 3’ end of the other chain.
 Base pairing rule: A binds with T; C binds
with G

Adenine and guanine are called purine
bases because they are made up of two
rings.
 Cytosine and thymine are pyrimidine
bases because they are made up of one
ring.
 1 purine base binds with a pyrimidine base
with hydrogen bonds.


The nucleotides bind together by covalent
bonds to form the sugar-phosphate
backbone of the polynucleotide chain.
http://www.pbslearningmedia.org/resource/biot11.sci.life.gen.structureofdna/the-structure-of-dna/
III. DNA REPLICATION
Replication – the process in which one
DNA molecule produces two identical DNA
molecules
 This process is semiconservative – one
chain of the DNA molecule is parental
while the other half is a new chain.
 The parental chain is called a template
while the new chain is the
complementary chain.


The steps of DNA replication:
DNA unwinds and the hydrogen bonds are
separated
2. The new chain will be built next to the
separated old chain by DNA polymerase
enzyme. (This enzyme cannot read in the
5’3’ direction and cannot start assembling a
chain)
3. An other enzyme will mark the DNA
molecule by inserting RNA nucleotides in a
3’ – 5’ direction so the DNA polymerase
enzyme can match up the missing
nucleotide and build the leading strand
1.
4. The replication continues until the entire DNA
segment is copied from the 3’ end.
5. A different DNA polymerase replaces the
RNA primer with DNA nucleotides.
6. The lagging strand is the other side of the
DNA molecule that grows away from the
replication fork in smaller chunks
(discontinuously), this is the 5’ end
7. First an enzyme marks the beginning of a
section, than DNA polymerase begins to
synthesize the new section
8. The DNA needs to unwind more before a
new lagging strand piece can be built –
Okazaki fragments – discontinuous
segments of the lagging strand of the DNA
molecule.
9. An other DNA polymerase changes the RNA
primers to DNA and ligase attaches the
separate fragments to each other.
10. The two new DNA molecules are exact
copies of the original DNA. If any mistakes
occurred in the copying, new enzymes will
correct those.



http://www.wiley.com/college/pratt/0471393878/student/animations/dna_replication/index.html -- more detailed
http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter14/animations.html -- many things on
DNA replication and the experiments
http://ideastream.pbslearningmedia.org/resource/tdc02.sci.life.gen.dnaanimation/how-dna-replicates/