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
Chapter 16
The Molecular Basis
of Inheritance
Searching for Genetic Material, I
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


Mendel: modes of heredity in pea plants
Morgan: genes located on chromosomes
Griffith: bacterial work; transformation: change in genotype and
phenotype due to assimilation of external substance (DNA) by a
cell
Avery: transformation agent was DNA
Searching for Genetic Material, II

Hershey and Chase
√ bacteriophages (phages)
√ DNA, not protein, is the hereditary material
√ Expt: sulfur(S) is in protein, phosphorus (P) is
in DNA; only P was found in host cell
DNA Structure



Chargaff
ratio of nucleotide
bases (A=T; C=G)
Watson & Crick
(Wilkins, Franklin)
The Double Helix
√ nucleotides:
nitrogenous base (thymine,
adenine, cytosine, guanine);
sugar deoxyribose;
phosphate group
DNA Bonding
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
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Purines: ‘A’ & ‘G’
Pyrimidines: ‘C’ & ‘T’
(Chargaff rules)
‘A’ H+ bonds (2) with ‘T’
and ‘C’ H+ bonds (3)
with ‘G’
Van der Waals
attractions between the
stacked pairs
DNA Replication

Watson & Crick
strands are complementary; nucleotides
line up on template according to base pair rules (Watson)
DNA Replication: a closer look
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
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
Origin of replication (“bubbles”): beginning of replication
Replication fork: ‘Y’-shaped region where new strands of DNA are
elongating
Helicase:catalyzes the untwisting of the DNA at the replication fork
DNA polymerase:catalyzes the elongation of new DNA
DNA Replication, II

Antiparallel nature:
• sugar/phosphate
backbone runs in opposite
directions (Crick);
• one strand runs 5’ to 3’,
while the other runs 3’ to 5’;
• DNA polymerase only adds
nucleotides at the free 3’
end, forming new DNA
strands in the 5’ to 3’
direction only
DNA Replication, III



Leading strand:
synthesis toward the
replication fork (only in a 5’ to 3’
direction from the 3’ to 5’ master
strand)
Lagging strand:
synthesis away from the
replication fork (Okazaki
fragments); joined by DNA
ligase (must wait for 3’ end to
open; again in a 5’ to 3’
direction)
Initiation:
Primer (short RNA
sequence~w/primase enzyme),
begins the replication process
DNA Repair



Mismatch repair:
DNA polymerase
Excision repair:
Nuclease
Telomere ends:
telomerase
As a review, lets go over this
again…
Copying DNA

Replication of DNA
– base pairing allows
each strand to serve as
a template for a new
strand
– new strand is 1/2
parent template &
1/2 new DNA
DNA Replication

Let’s meet
the team…
Large team of enzymes coordinates replication
Unzipping: First Step

The enzyme Helicase unwinds the DNA
strands like a zipper
Replication: 2nd step
 Build daughter DNA
strand
add new complementary
bases
 DNA polymerase III

DNA
Polymerase III
But…
Where’s the
We’re missing
ENERGY
something!
for the bonding!
What?
Energy of Replication
Where does energy for bonding usually come from?
We come
with our own
energy!
You
remember
ATP!
Are there
other ways
energy
to
get energy
nucleotides?
out
it?
You of
bet!
ATP
CTP
TTP
GTP
modified nucleotide
And we
leave behind a
nucleotide!
energy
energy
CMP
TMP
GMP
AMP
ADP
Energy of Replication

The nucleotides arrive as nucleosides
– DNA bases with P–P–P
• P-P-P = energy for bonding
– DNA bases arrive with their own energy source for
bonding
– bonded by enzyme: DNA polymerase III
ATP
GTP
TTP
CTP
5
Replication

Adding bases
– can only add
nucleotides to
3 end of a growing
DNA strand
• need a “starter”
nucleotide to
bond to
– strand only grows
53
B.Y.O. ENERGY!
The energy rules
the process
3
energy
DNA
Polymerase III
energy
DNA
Polymerase III
energy
DNA
Polymerase III
energy
DNA
Polymerase III
3
5
5
3
5
need “primer” bases to add on to
3
energy
no energy to
bond

energy
energy
energy
energy
ligase
energy
energy
3
5
3
5
Okazaki
Leading & Lagging strands
Limits of DNA polymerase III

can only build onto 3 end of an
existing DNA strand
5
3
5
3
5
growing
replication fork
3
5
5
5
Lagging strand
ligase
3
Leading strand
3
Lagging strand

Okazaki fragments

3

5
3
DNA polymerase III
Leading strand

continuous synthesis
Replication fork / Replication bubble
3
5
5
3
DNA polymerase III
leading strand
5
3
3
5
3
5
5
5
3
lagging strand
3
5
5
growing
replication fork
3
5
lagging strand
5
leading strand
growing
replication fork
3
leading strand
3
lagging strand
5 5
5
5
5
3
Starting DNA synthesis: RNA
primers
Limits
of DNA polymerase III

can only build onto 3 end of an
existing DNA strand
5
3
3
5
5
3
5
3
5
growing
replication fork
3
DNA polymerase III
5
3
Houston, we
have a problem!
Chromosome erosion
All DNA polymerases can
only add to 3 end of an
existing DNA strand
DNA polymerase
5
3
3
5
5
growing
replication fork
3
DNA polymerase
RNA
Loss of bases at 5 ends
in every replication


chromosomes get shorter with each replication
limit to number of cell divisions?
5
3
Telomeres
Repeating, non-coding sequences at the end of
chromosomes = protective cap

limit to ~50 cell divisions
5
3
3
5
growing
replication fork
5
3
Telomerase



enzyme extends telomeres
can add DNA bases at 5 end
different level of activity in different cells
 high in stem cells & cancers -- Why?
telomerase
5
TTAAGGGTTAAGGG 3
Replication fork
DNA
polymerase
lagging strand
DNA
polymerase
5’
3’
3’
Okazaki
fragments
5’
ligase
5’
3’
DNA
polymerase
5’
3’
leading strand
direction of replication
helicase