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AP Biology
Discussion Notes
Thursday 01/26/2017
Goals for Today
• Be able to say what the central dogma of
biology is.
• Be able to describe the structure of DNA
and its associated terms
• Be able to describe the process of DNA
replication & Terms involved
• Be able to “build” a strand of DNA
1/26 Question of the Day
• List the 2 groups of Nitrogen bases and
which bases belong to each group. Then
star the group with the larger (2-ring) bases.
(Hint we are not talking about pairing.)
*Bases always pair Pyrimidine to _________
Review – Discuss with a partner
1. Identify and describe what 2 things
prevent “mismatches” in DNA base pairing.
Figure 16.UN01
Purine  purine: too wide
Pyrimidine  pyrimidine: too narrow
Purine  pyrimidine: width
consistent with X-ray data
Figure 16.8
Sugar
Sugar
Adenine (A)
Thymine (T)
Sugar
Sugar
Guanine (G)
Cytosine (C)
Review – Discuss with a partner
1. Explain what is meant by the term “DNA
replication”
2. Identify what term describe this process and
explain what that means. (Is it liberal?)
Figure 16.9-1
Semiconservative Replication
A
T
C
G
T
A
A
T
G
C
(a) Parent molecule
Figure 16.9-2
Semiconservative Replication
A
T
A
T
C
G
C
G
T
A
T
A
A
T
A
T
G
C
G
C
(a) Parent molecule
(b) Separation of
strands
Figure 16.9-3
Semiconservative Replication
A
T
A
T
A
T
A
T
C
G
C
G
C
G
C
G
T
A
T
A
T
A
T
A
A
T
A
T
A
T
A
T
G
C
G
C
G
C
G
C
(a) Parent molecule
(b) Separation of
strands
(c) “Daughter” DNA molecules,
each consisting of one
parental strand and one
new strand
Chapter 16 in your book
Figure 16.10
(a) Conservative
model
(b) Semiconservative
model
(c) Dispersive model
Parent
cell
First
replication
Second
replication
• Experiments by
Matthew Meselson
and Franklin Stahl
supported the
semiconservative
model
• They labeled the
nucleotides of the old
strands with a heavy
isotope of nitrogen
15N, while any new
nucleotides were
labeled with a lighter
isotope, 14N
Figure 16.11a
EXPERIMENT
2 Bacteria
transferred to
medium with
14N (lighter
isotope)
1 Bacteria
cultured in
medium with
15N (heavy
isotope)
RESULTS
3 DNA sample
centrifuged
after first
replication
?
4 DNA sample
centrifuged
after second ?
replication
Less
dense
More
dense
Figure 16.11b
CONCLUSION
Predictions:
First replication
Conservative
model
Semiconservative
model
Dispersive
model
Second replication
DNA Replication: A Closer Look
• The copying of DNA is remarkable
in its speed and accuracy
–E.coli has about 4.6 Million
nucleotide/base pairs and replicates
DNA, then divides into 2 new cells in
less than an hour!
–Humans have ~6 Billion
nucleotide/base pairs and replicate
their DNA in a few hours
Getting Started
• Replication begins at particular sites called
origins of replication, where the two DNA
strands are separated, opening up a replication
“bubble”
• A eukaryotic chromosome may have hundreds or
even thousands of origins of replication
• Replication proceeds in both directions from each
origin, until the entire* molecule is copied
Figure 16.12a
(a) Origin of replication in an E. coli cell
Origin of
replication
Parental (template) strand
Daughter (new) strand
Doublestranded
DNA molecule
Replication
bubble
Replication fork
Two
daughter
DNA molecules
0.5 m
Synthesizing a New DNA Strand
• Enzymes called DNA polymerases catalyze the
elongation of new DNA at a replication fork
• The rate of elongation is about 500
nucleotides per second in bacteria and
50 per second in human cells
• How do we speed this up in our cells?
Figure 16.12b
(b) Origins of replication in a eukaryotic cell
Double-stranded
Origin of replication DNA molecule
Parental (template)
strand
Bubble
Daughter (new)
strand
Replication fork
Two daughter DNA molecules
0.25 m
DNA Replication: Cast of Characters
• The Enzymes*
– Helicase (“Hacks”)
– DNA Polymerase (“Pastes”)
• READS:
• BUILDS:
– Ligase (“Links”)
Polymerase only builds in 5’ to 3’
Direction!
Remember:
it’s an enzyme - & they’re very picky
Antiparallel?
Antiparallel!
The strands are parallel,
but run in opposite
directions!
DNA Replication: Cast of Characters
• Building Blocks:
– Nucleotides (A,T,C,G)
• Strand Terminology:
– Origin(s) of Replication
– Replication Fork
– Leading Strand
– Lagging Strand
• Okazaki Fragments
• DNA Replication in Action
Questions?
DNA Assignment
• DUE – Wednesday– February 1st!