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AP Biology
AA
A
A
TC
G
CG
TG
C
T
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AP Biology
Macromolecules: Nucleic Acids Examples:
RNA (ribonucleic acid) single helix
DNA (deoxyribonucleic acid) double helix
Structure: monomers = nucleotides
RNADNA
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AP Biology
Nucleotides 3 parts
nitrogen base (C-N ring) pentose sugar (5C)
ribose in RNA deoxyribose in DNA
phosphate (PO4) group
Are nucleic acidscharged molecules?
Nitrogen baseI’m the
A,T,C,G or Upart!
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AP Biology
Types of nucleotides 2 types of nucleotides
different nitrogen bases purines
double ring N base adenine (A) guanine (G)
pyrimidines single ring N base cytosine (C) thymine (T) uracil (U)
Purine = AGPure silver!
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AP Biology
Nucleic polymer Backbone
sugar to PO4 bond phosphodiester bond
new base added to sugar of previous base
polymer grows in one direction N bases hang off the
sugar-phosphate backbone
Dangling bases?Why is this important?
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AP Biology
Pairing of nucleotides Nucleotides bond between
DNA strands H bonds purine :: pyrimidine A :: T
2 H bonds G :: C
3 H bonds
Matching bases?Why is this important?
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AP Biology
DNA molecule Double helix
H bonds between bases join the 2 strands A :: T C :: G
H bonds?Why is this important?
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AP Biology
Copying DNA Replication
2 strands of DNA helix are complementary have one, can build other have one, can rebuild the
whole
Matching halves?Why is this
a good system?
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AP Biology
When does a cell copy DNA? When in the life of a cell does DNA have
to be copied? cell reproduction
mitosis gamete production
meiosis
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AP Biology
Learning Check Use the candy and instructions to build
a DNA model that follows Chargaff’s rules of base pairing
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AP Biology
But how is DNA copied? Replication of DNA
base pairing suggests that it will allow each side to serve as a template for a new strand
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AP Biology
Models of DNA Replication Alternative models
become experimental predictions
conservative semiconservative
Can you designa nifty experiment
to verify?
dispersive
1
2
P
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AP Biology
Semiconservative replication Meselson & Stahl
label “parent” nucleotides in DNA strands with heavy nitrogen = 15N
label new nucleotides with lighter isotope = 14N
1958
parent replicationMake predictions…
15N parent strands
15N/15N
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AP Biology
Semiconservative replication Make predictions…
15N strands replicated in 14N medium 1st round of replication? 2nd round?
1958
where should the bands be?
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AP Biology
Franklin Stahl
Matthew Meselson
Matthew Meselson Franklin Stahl
Meselson & Stahl
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AP Biology
DNA Replication Origin(s) of replication
specific sequence of nucleotides recognized by replication enzymes
Prokaryotes – Single sequence Bidirectional Synthesis
Replication proceeds in both directions
Eukaryotes – hundreds/thousands of origin sites
per chromosome Replication forks
Bubbles elongate as DNA is replicated and eventually fuse
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AP Biology
Bidirectional Synthesis In prokaryotes, the circular DNA is opened
up, and synthesis occurs in both directions
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AP Biology
In eukaryotes, the linear DNA has many replication forks
Replication forks
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AP Biology
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AP Biology
Learning Check Break the toothpicks at the center of
your models and replicate your DNA strand You should end up with 2 complete
strands of DNA
Keep in mind Chargaff’s rules and Meselson & Stahl’s semi-conservative model
Animation
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AP Biology
Replication- Create a diagram that shows how the following components interact with each other (15 min)
Lagging strand Helicase DNA polymerase Single stranded
binding protein Topoisomerase
Replication fork RNA primer Leading strand DNA ligase RNA primase Okazaki fragments
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AP Biology
DNA Replication Issues1. DNA strands must be
unwound during replication DNA helicase
unwinds the strands Single stranded binding
proteins (SSB) prevent immediate
reformation of the double helix
Topoisomerases “untying” the knots that
form
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AP Biology
Replication Issues2. A new DNA strand can
only elongate in the 5’ 3’ direction
DNA polymerase can add only at the 3’ end
Replication is continuous on one strand Leading Strand
discontinuous on the other Lagging strand Okazaki fragments
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AP Biology
Okazaki fragments
Synthesis of the leading strand is continuous
The lagging strand (discontinuous) is synthesized in pieces called Okazaki fragments
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AP Biology
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AP Biology
Replication Issues3. DNA polymerase cannot initiate
synthesis because it can only add nucleotides to end of an existing chain
Requires a “primer” to get the chain started
RNA Primase can start an RNA chain from a single
template strand DNA polymerase can begin its chain
after a few RNA nucleotides have been added
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AP Biology
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AP Biology
Summary At the replication fork, the leading strand is
copied continuously into the fork from a single primer
Lagging strand is copied away from the fork in short okazaki fragments, each requiring a new primer
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AP Biology
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AP Biology
Learning Check
1. What is the purpose of DNA replication?
2. How is the new strand ensured to be identical to the original strand?
3. How is replication on one side of the strand different from the other side?
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AP Biology
Replication Issues4. Presence of RNA primer
on the 5’ ends of daughter DNA leading strand leaves a gap of uncopied DNA
Repeated rounds of replication produce shorter and shorter DNA molecules
Telomeres protect genes from being
eroded through multiple rounds of DNA replication
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AP Biology
Telomeres Ends of eukaryotic
chromosomes, the telomeres, have special nucleotide sequences Humans - this sequence is
typically TTAGGG, repeated 100 - 1,000 times
Telomerase adds a short molecule of RNA as a template to extend the 3’ end
Room for primase & DNA pol to extend 5’ end
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AP Biology
Summary Explain how the cell overcomes each of the
following issues in DNA replication
1. DNA strands must be unwound during replication
2. A new DNA strand can only elongate in the 5’ 3’ direction
3. DNA polymerase cannot initiate synthesis and can only add nucleotides to end of an existing chain
4. Presence of RNA primer on the 5’ ends of daughter DNA leading strand leaves a gap of uncopied DNA