DNA and
Replication
1
History of DNA
• Early scientists thought protein
was the hereditary material
– Protein is more complex than
DNA
– Proteins are composed of 20
different amino acids in long
polypeptide chains
2
Transformation
• Fred Griffith worked with virulent “S”
and nonvirulent “R” strains of
Pneumoccocus bacteria
• He found
– R strain becomes virulent when it absorbs
DNA from killed S strain bacteria
– Transformation- bacteria gets DNA from
a dead bacteria and aquires new traits
• Study suggested that DNA was
probably the genetic material
3
Griffith Experiment
4
History of DNA
• Viruses are made of DNA
and protein
• Hershey & Chase used
radioactively tagged
bacteriophage viruses to
prove DNA is the genetic material
– Tag DNA with 32P
– Tag protein with 35S
– 32P shows up in the bacteria,
35S
doesn’t
5
DNA
Replication
6
Replication Facts
• DNA must be copied before a
cell divides
• DNA is copied during S
(synthesis) phase of interphase
• New cells need identical DNA
strands
7
Synthesis Phase (S phase)
• S phase during interphase of the
cell cycle
• Nucleus of eukaryotes
S
DNA replication takes
place in the S phase.
phase
G1
interphase
G2
Mitosis
-prophase
-metaphase
-anaphase
-telophase
8
DNA Replication
• Enzyme Helicase unwinds& separates
the 2 DNA strands by breaking weak
hydrogen bonds
• Begins at Origins of Replication
• Two strands open forming Replication
3’
Forks (Y-shaped region)
5’ Parental DNA Molecule
3’
Replication
Fork
9
5’
DNA Replication
• DNA polymerase adds new nucleotides
• Replication Bubbles form as the DNA
strands open at the origin
• Eukaryotic chromosomes have Multiple
bubbles
• Prokaryotes (bacteria) have one Single
bubble
Bubbles
Bubbles
10
Proofreading New DNA
• DNA polymerase makes about 1 in
10,000 base pairing errors
• Enzymes proofread and correct
mistakes
• Error rate for proofread DNA is 1
in 1 billion base pairing errors
11
DNA Damage & Repair
• Chemicals & ultraviolet radiation damage
DNA in our cells
• Cells continuously repair DAMAGED DNA
• Excision repair- one of over 50
different repair enzymes removes
damaged DNA
• DNA polymerase and DNA ligase replace
and tie in (bond) new, correct
nucleotides
12
Replication is Semiconservative
Watson & Crick suggested:
• The parental DNA acts a template for a
new complementary strand
• Final, new DNA molecules consist of 1
Parental (original) strand and 1 New
strand of DNA
DNA Template
Parental DNA
New DNA
13
Question:
• What would be the
complementary DNA
strand for the following
DNA sequence?
DNA 5’-CGTATG-3’
14
Answer:
DNA 5’-GCGTATG-3’
DNA 3’-CGCATAC-5’
15