Download Chapter 16 Molecular basis of inheritance

AP Biology: Ch. 16
The MOLECULAR BASIS OF
INHERITANCE
DNA can transform bacteria
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Frederick Griffith performed experiments
showing evidence that genetic material was
a specific molecule (1928).
Showed that bacteria had been
transformed; their phenotype changed due
to the assimilation of external genetic
material
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Viral DNA can program cells
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Alfred Hershey and Martha Chase
conducted studies showing that DNA was
the genetic material of a bacteriophage
called T2. (1952)
They concluded that viral proteins stay
outside the host cell, and that viral DNA is
injected into the host cell.
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Chargaff’s Rules
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In 1947, Erwin Chargaff analyzed the DNA
composition of different organisms using
paper chromatography to separate
nitrogenous bases.
He discovered that DNA composition is
species-specific; amounts and ratios of
nitrogenous bases vary among species
Chargaff, cont.
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In a DNA sample, the amount of adenine
(A) = the amount of thymine (T). Guanine
(G) = Cytosine (C)
Watson and Cricks’ structural model for
DNA explained these rules.
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Base-pairing rules:
A—T
G—C
Rosalind Franklin at King’s College in
London produced an X-ray photo of DNA
(1950s)
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Discovering the Double Helix
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Watson and Crick studied Franklin’s work
and suggested that:
DNA is a double helix with uniform width (2
nm)
Purine and pyrimidine bases are stacked
.34 nm apart.
There are 10 layers of nitrogenous base
pairs in each turn of the helix.
To be consistent with a 2 nm width, a purine on
one strand must pair by hydrogen bonding with a
pyrimidine on the other
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DNA Replication and Repair
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Watson and Crick proposed that genes on
the original DNA strand are copied by a
specific pairing of complementary bases.
The complementary strand can then be
used as a template to produce a copy of
the original strand.
This is a semiconservative model of DNA
replication.
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DNA replication, cont.
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Enzymes and other proteins carry out DNA
replication.
The process is complex, extremely rapid,
and accurate.
DNA replication is similar in prokaryotes
and eukaryotes.
Origins of Replication
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DNA replication begins at special sites called
origins of replication that have a specific
sequence of nucleotides.
Specific proteins required to initiate replication
bind to each origin, causing the double helix to
open.
Replication forks spread in both directions away
from the origin creating a replication bubble.
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*Bacterial or viral DNA may have only one replication
origin.
Elongating a new strand
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Enzymes called DNA polymerases catalyze
synthesis of a new DNA strand.
According to base-pairing rules, new nucleotides
align along the template of the old DNA strand.
DNA polymerase links the nucleotides to the
growing strand in the 5’3’ direction.
Hydrolysis of nucleoside phosphates provides the
energy necessary to synthesize the new DNA
strands.
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Antiparallel DNA strands
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The sugar-phosphate backbones of the two
complementary DNA strands run in opposite
directions.
DNA polymerase can only elongate strands in the
5’  3’ direction.
The problem of antiparallel DNA strands is solved
by the continuous synthesis of one strand (leading
strand) and discontinuous synthesis of the
complementary strand (lagging strand).
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Leading vs. Lagging strands
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The leading DNA strand is synthesized as a single
polymer in the 5’ to 3’ direction towards the
replication fork.
The lagging strand is synthesized against the
overall direction of replication. It is produced as a
series of short segments called Okazaki
fragments.
The many fragments are connected by DNA
ligase, which catalzes the formation of a covalent
bond between the 3’ end of each new fragment to
the 5’end of the growing chain.
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Priming DNA synthesis
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Before new DNA strands can form, there must be
small preexisting primers to start the addition of
new nucleotides.
Primers are short RNA segments (linked by
primase enzymes) that are complementary to
DNA segments. Needed to begin DNA replication.
Only one primer is needed for replication of the
leading strand, but many are required to replicate
the lagging strand. An RNA primer must initiate
the synthesis of each Okazaki fragment.
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Other proteins assisting in DNA
replication
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Helicases are enzymes which catalyze
unwinding of the parental double helix to
provide the template.
Single-strand binding proteins are
proteins which keep the separated strands
apart and stabilize the unwound DNA until
a complementary strand can be
synthesized.
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DNA Proofreading and Repair
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DNA replication is highly accurate due to
base-pairing specificity and
proofreading/repair mechanisms.
DNA can be repaired as it is being
synthesized (mismatch repair) or after the
accidental changes in existing DNA
(excision repair)
DNA repair, cont.
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Mismatch repair- DNA polymerase proofreads
each newly added nucleotide against the
template. Incorrectly paired nucleotides are
removed and replaced before synthesis
continues.
Excision repair- Segments damaged by physical
or chemical agents are removed by a repair
enzyme, then the gap is filled in by base-pairing
nucleotides with the undamaged strand. DNA
polymerase and DNA ligase catalyze the filling in
process.
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Telomeres
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DNA polymerase can only add nucleotides to the
3’ end of a preexisting DNA chain.
Repeated replication produces shortened DNA
molecules, potentially deleting some gene
sequences.
Telomeres are special nucleotide sequences (not
containing genes) at the end of eukaryotic
chromosome molecules that prevent this.
Telomerase is an enzyme that catalyzes the
lengthening of telomeres
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Telomeres (The yellow dots!)
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