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DNA
The Genetic Code
Chapter 8 in your textbook
Discovery of DNA
•
Experiments that identified
DNA as the genetic
material & determined its
structure:
1.
2.
3.
4.
5.
6.
Griffith
Avery
Hershey – Chase
Chargaff
Franklin & Wilkins
Watson & Crick
Griffith’s Experiment
• Used mice and pneumonia
bacteria
– One strain, “S”, was harmful
(pathogenic)
– Other strain, “R”, was not
harmful (non-pathogenic)
• Discovered transformation –
one bacteria was genetically
changed by another bacteria
Griffith, cont’d
Animation
• The harmless strain (R) mixed
with the heat-killed pathogenic
strain (S) killed the mice.
Griffith, cont’d
• Some “factor” was transferred
from the S to the R bacteria,
turning the harmless R into
something deadly
• At this point, scientists weren’t
sure whether the transforming
factor was DNA or protein
Avery’s Experiment
• Piggy-backed off of Griffith’s
experiment to figure out what
the “transforming factor” was
• Used enzymes to isolate
various components of the
bacteria: DNA, proteins, lipids,
and carbohydrates
• Transformation only took place
when the harmless bacteria
was treated with DNA from the
harmful bacteria
Avery, cont’d
• Discovered that DNA stores and
transmits genetic information
Hershey & Chase’s Experiment
• Used viruses that infect bacteria
(bacteriophages)
• Used radioactive phosphorus to
mark the DNA
• Used radioactive sulfur to mark
the proteins
• This allowed them to trace where
these molecules went during the
viral infection process
• Demonstrated that DNA was the
genetic material -- video (2:05)
Hershey / Chase, cont’d
Chargaff
• Chargaff determined that the
amount of adenine is the same
as the amount of thymine in all
the organisms he studied (and
that C = G)
Franklin & Wilkins
• Franklin & Wilkins did X-ray
crystallography pictures of the
DNA molecule that showed it to
be a helical structure with at
least 2 helices
Watson & Crick
• Using Chargaff’s rules, they
knew that A=T and C=G
• Using the X-ray pictures from
Franklin & Wilkins, they knew
it was a helical shape
• They knew the structure had
to provide a way to replicate
very accurately
• Video (4:04)
Watson & Crick, cont’d
• Determined the structure of DNA in 1953
– Double helix with bases on the inside and sugar
and phosphate alternating on the outside
DNA Basics
• Deoxyribonucleic Acid
• Double helix (twisted ladder)
• Located in the nucleus of
eukaryotic cells
– Prokaryotes lack a nucleus, but
still have DNA in their cells;
prokaryotic (bacteria) DNA
hangs out in the nucleoid
region
DNA is a Nucleic Acid
• Organic compound made up
of carbon, hydrogen, nitrogen,
oxygen, and phosphorus.
• Made of repeating subunits
(monomers) called
nucleotides
The Nucleotide
• Made of a sugar (deoxyribose),
a phosphate group and one of
4 nitrogen bases
Types of Nitrogen Bases
Pyrimidines (1 ring)
• Cytosine
• Thymine
Purines (2 rings)
• Adenine
• Guanine
DNA Structure
• Sides of the ladder are the
sugar and phosphate group
• The rungs (part you’d step on)
are 2 of the nitrogen bases
• Each rung is one purine
paired with one pyrimidine –
always C-G and A-T
DNA Structure
DNA Structure
DNA Replication
• The process of making
identical copies of DNA
• It is semiconservative –
in each molecule, there is
one old and one new
strand
• Occurs during the S
phase of the cell cycle
• It is necessary for cell
division
Steps of DNA Replication
• Step 1: DNA helicase (an
enzyme) unwinds the
molecule
• Step 2: DNA polymerase
binds free-floating
nucleotides to the exposed
bases on the original strands
• Step 3: The 2 new molecules
wind up again
• Animation