DNA History and Replication
Scientific History
• The march to understanding that DNA is the
genetic material
• Mendel
• Miescher
• Griffith
• Rosalind Franklin
• Watson & Crick
• Chagraff
Rags to Riches
• Mendel – Father of Genetics
• Discovered Genes by experimenting with pea plants
• Miescher –Discovered “Nuclein”
• Collected rags from infected war wounds.
The “Transforming Factor”
• Frederick Griffith
• Streptococcus pneumonia
bacteria
• was working to find cure for
pneumonia
• harmless live bacteria mixed
with heat-killed infectious
bacteria causes disease in mice
• substance passed from dead
bacteria to live bacteria =
“Transforming Factor”
1928
Confirmation of DNA
1952 | 1969
• Hershey & Chase
• classic “blender” experiment
• worked with bacteriophage
• viruses that infect bacteria
• grew phage viruses in 2 media,
Why use radioactively labeled with either
Sulfur
35S in their proteins
•
vs.
Phosphorus?
• 32P in their DNA
• infected bacteria with
labeled phages
http://highered.mcgraw-hill.com/olc/dl/120076/bio21.swf
Hershey
Protein coat labeled
with 35S
DNA labeled with 32P
T2 bacteriophages
are labeled with
radioactive isotopes
S vs. P
Hershey
& Chase
bacteriophages infect
bacterial cells
bacterial cells are agitated
to remove viral protein coats
Which
radioactive
marker is found
inside the cell?
Which molecule
carries viral
genetic info?
35S
radioactivity
found in the medium
32P
radioactivity found
in the bacterial cells
Rosalind Franklin (1920-1958)
1953 | 1962
Structure of DNA
• Watson & Crick
• developed double helix model of DNA
• other scientists working on question:
• Rosalind Franklin
• Maurice Wilkins
• Linus Pauling
Franklin
Wilkins
Pauling
1947
Chargaff
• DNA composition: “Chargaff’s rules”
• varies from species to species
• all 4 bases not in equal quantity
• bases present in characteristic ratio
• humans:
A = 30.9%
T = 29.4%
G = 19.9%
C = 19.8%
That’s interesting!
What do you notice?
Nucleotides and DNA structure
DNA Strand – Double Helix
Directionality
• You need to
number the
carbons!
• it matters!
nucleotide
PO4
N base
5 CH2
This will be
IMPORTANT!!
O
4
1
ribose
3
OH
2
DNA Replication
• Replicate means to copy.
• DNA Replication – Copying of DNA
Steps of DNA Replication
1. The DNA strands separate (H
bonds break)
2. New nucleotides are added to the
separated strands
3. 2 identical strands are created
14
Step 1: Unzipping
DNA Helicase latches on at the
ORIGIN OF REPLICATION.
DNA Helicase then UNZIPS the DNA
Strand.
At each ORIGIN OF REPLICATION a
REPLICATION BUBBLE IS FORMED.
Step 2: Transcription
DNA Polymerase attaches to the TEMPLATE and adds complimentary bases
to form a new DNA strand.
DNA Polymerase works 5’ to 3’ know as the Leading Strand
It cannot go 3’ to 5’. So the 3’ to 5’ strand has to be done in chunks called
OKAZAKI fragments.
Step 3: Termination
Ligase comes behind and
closes the gap between
Okazaki fragments.
There is then two
“Daughter” strands of
DNA.
DNA Replication Summary
1. The DNA strands separate
(H bonds break)
2. New nucleotides are
added to the separated
strands
3. 2 identical strands are
created