Chapter 16

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Chapter 16 - Molecular Basis of Inheritance
How was DNA determined to be the genetic
(hereditary / information carrying) material of life?
Chapter 16 - Molecular Basis of Inheritance
Once Morgan showed that genes are located on
chromosomes, the two chemical components of
DNA and Protein
chromosomes, ________________________,
became the candidates for the genetic
(hereditary) material of life…the race was on.
AIM: How was DNA determined to be the genetic (hereditary)
material of life?
Streptococcus
pneumonia
Year: 1928
Streptococcus
pneumonia
Smooth strain is pathogenic (can cause infection) due to the presence of a
capsule that protects the bacterium from the host’s immune system.
AIM: How was DNA determined to be the genetic (hereditary)
material of life?
Streptococcus
pneumonia
Year: 1928
Streptococcus
pneumonia
AIM: How was DNA determined to be the genetic (hereditary)
material of life?
Streptococcus
pneumonia
Year: 1928
Streptococcus
pneumonia
AIM: How was DNA determined to be the genetic (hereditary)
material of life?
Streptococcus
pneumonia
Year: 1928
Streptococcus
pneumonia
The harmless rough strain and harmless dead smooth strain were mixed together
and incubated before being injected into the mouse…
AIM: How was DNA determined to be the genetic (hereditary)
material of life?
Streptococcus
pneumonia
Year: 1928
Streptococcus
pneumonia
What the?
AIM: How was DNA determined to be the genetic (hereditary)
material of life?
Conclusion: There is some “factor” that carries information that is released by
the Smooth strain upon heating and transforms the Rough strain into the
smooth strain.
In 1928, researchers had no idea what was doing this, but this set the stage for what
would be a 14 year search by three scientists (Avery, MacLeod and McCarty) to figure
it out…
AIM: How was DNA determined to be the genetic (hereditary)
material of life?
You now know that something is transforming the Rough strain
that is present in the Smooth strain. What would you do next?
AIM: How was DNA determined to be the genetic (hereditary)
material of life?
Avery, MacLeod and
McCarty’s experiment:
They took the Smooth
strain and heat killed it just
like Griffith.
Year: 1944
(heat kill)
AIM: How was DNA determined to be the genetic (hereditary)
material of life?
Avery, MacLeod and
McCarty’s experiment:
They then separated out the
four major classes of
macromolecules from the
mixture.
What do you think they did
next?
Year: 1944
(heat kill)
AIM: How was DNA determined to be the genetic (hereditary)
material of life?
Avery, MacLeod and
McCarty’s experiment:
They mixed each class
separately with the Rough
Strain and injected it into
mice…
What do you hypothesize their
results were?
Year: 1944
(heat kill)
AIM: How was DNA determined to be the genetic (hereditary)
material of life?
Avery, MacLeod and
McCarty’s experiment:
The mice receiving the Rough
strain mixed with nucleic acids
from the smooth strain died,
while the other mice were fine.
CONCLUSION:
The transforming or information
carrying factor is DNA!!
Year: 1944
(heat kill)
AIM: How was DNA determined to be the genetic (hereditary)
material of life?
Skepticism
It is the nature of critical thinkers like
scientists to be skeptical of
everything…and this finding was no
exception.
Most scientists believed (a dangerous
notion) that protein was a better
candidate due to their great
heterogeneity (diversity) in contrast to
DNA, which was highly uniform in
nature.
Year: 1944
Avery, MacLeod and
McCarty’s experiment:
AIM: How was DNA determined to be the genetic (hereditary)
material of life?
Fig. 10.1A
Bacteriophage
This bring us to 1952, to the VIROLOGY Laboratory of Alfred Hershey and Martha
Chase.
AIM: How was DNA determined to be the genetic (hereditary)
material of life?
Fig. 10.1A
Bacteriophage
In order to figure out the type of nucleic acid, Hershey and Chase (1952) performed an
experiment using viruses that infect bacteria called bacteriophages shown above. They
obviously didn’t know what they looked like, but they knew that they were made solely of
protein and DNA, and that they were able take over a bacterial cell and transform it into a
virus-producing
factory…
The question was,
was the protein or the DNA carrying the information to transform the cells?
AIM: How was DNA determined to be the genetic (hereditary)
material of life?
Fig. 10.1A
Bacteriophage
BEFORE getting into the experiment, let’s compare the structure
of a phage to an animal virus and then look at how phages work
(how they infect bacterial cells)…
AIM: How was DNA determined to be the genetic (hereditary)
material of life?
I. Animals virus (influenza – virus that causes the flu)
Capsid – composed of protein, encloses nucleic
acid (there are DNA as well as RNA viruses)
Envelope – similar to a plasma membrane – composed
of phospholipids and integral membrane proteins that
will act as ligands and bind to cell receptors to gain
access
the cell.more than packaged nucleic acid.
Viruses
are to
nothing
II. Bacterial virus (right; a bacteriophage)
All parts made of protein except DNA of cou
Tail fibres – bind to surface of bacterium
acting as ligands
See video and figures
Know the anatomy (structure) of these viruses…be able to draw and label them.
AIM: How was DNA determined to be the genetic (hereditary)
material of life?
Reproductive cycle of a
AIM: How was DNA determined to be the genetic (hereditary)
material of life?
Bacteriophage life cycle
AIM: How was DNA determined to be the genetic (hereditary)
material of life?
Let’s now get back to the Hershey-Chase Experiment
Is it the protein or the DNA that is the information molecule
responsible for entering the bacterium and transforming it
into a virus producing factory?
Year: 1952
AIM: How was DNA determined to be the genetic (hereditary)
material of life?
The experiment was performed using radioactive isotopes to be
able to follow the protein or DNA.
Isotopes of what elements would you use for protein/DNA
labeling?
AIM: How was DNA determined to be the genetic (hereditary)
material of life?
Let’s follow the protein first:
Phages were initially prepared for the experiment by growing
them in the
presence of radioactive sulfur (35S) making the ONLY proteins
radioactive
as protein have sulfur (the amino acids methionine and cysteine),
AIM: How was DNA determined to be the genetic (hereditary)
material of life?
Let’s follow the protein first:
1. INFECTION: The phage was mixed with bacteria as shown
above to allow them to infect the prokaryote.…
AIM: How was DNA determined to be the genetic (hereditary)
material of life?
2. AGITATE: After the bacteria were infected, the cells were
banged around (agitated) using a blender so that anything stuck
to the surface from the phage would fall off. We are only
interested in what goes inside to transform the cell…
AIM: How was DNA determined to be the genetic (hereditary)
material of life?
3. CENTRIFUGE: The mixture was then centrifuged, which will
pellet the bacterial cells (more dense) on the bottom of the tube.
What do you do next?
AIM: How was DNA determined to be the genetic (hereditary)
material of life?
4. RADIOACTIVITY ASSAY: Is the cell pellet radioactive or is the
(The supernatant is the liquid portion of a centrifuged samp
supernatant radioactive?
Result – the supernatant was radioactive, the protein did not go in the
cell.
Does that mean the DNA went in for certain?
How do we test if the DNA went in the cell?
AIM: How was DNA determined to be the genetic (hereditary)
material of life?
You do the same experiment except
instead of making the protein radioactive,
you make the DNA radioactive in the
beginning by growing the virus in
Because
protein does
have P.
radioactive
phosphorus
(32not
P)…
Why 32P?
AIM: How was DNA determined to be the genetic (hereditary)
material of life?
AIM: How was DNA determined to be the genetic (hereditary)
material of life?
AIM: How was DNA determined to be the genetic (hereditary)
material of life?
AIM: How was DNA determined to be the genetic (hereditary)
material of life?
Conclusion
DNA carries the information to transform the cell.
DNA is the hereditary/information carrying material of life.
Chapter 16 - Molecular Basis of Inheritance
1866 - Mendel’s work published never
gaining popularity.
Timeline Recap:
1875 - Mitosis figured out
1890 - Meiosis figured out
1900 – Mendel’s work rediscovered
1902 – Chromosomal Theory of Inheritance gains
popularity
1910 – Morgan and co-workers show genes are on
chromosomes – are genes protein or DNA?.
1928 – Griffith shows transformation – substance in S
strain transferred to R, turning R into S
1944 – Avery, Macleod, and McCarty show this
substance to be DNA
1952 – Hershey and Chase confirm concluding DNA to
be the hereditary material of life.
Very little was known about DNA at this point. How does this molecule work?
How does it store hereditary information? The findings of Hershey and Chase
catalyzed one of the most controversial stories in science…
The race for the structure of DNA.
Chapter 16 - Molecular Basis of Inheritance
Movie: The
secret of Photo
51
The race for the structure of DNA.
Chapter 16 - Molecular Basis of Inheritance
The road to the structure of DNA
(but first a quick review of the structure)
Chapter 3 - The Molecules of Cells
AIM: Describe the structure of DNA and RNA?
A nucleotide
Chapter 3 - The Molecules of Cells
AIM: Describe the structure of DNA and RNA?
Chapter 3 - The Molecules of Cells
AIM: Describe the structure of DNA and RNA?
Pyrimidines vs Purines
Chapter 3 - The Molecules of Cells
AIM: Describe the structure of DNA and RNA?
Chapter 3 - The Molecules of Cells
AIM: Describe the structure of DNA and RNA?
Nomenclature
Chapter 3 - The Molecules of Cells
AIM: Describe the structure of DNA and RNA?
Draw a nucleotide found in RNA that contains
a pyrimidine base and identify the base
possibilities.
Chapter 3 - The Molecules of Cells
AIM: Describe the structure of DNA and RNA?
Draw a nucleotide found in DNA that
contains a purine base and identify the base
possibilities.
Chapter 3 - The Molecules of Cells
AIM: Describe the structure of DNA and RNA?
Draw CMP. Is CMP found in DNA or RNA?
Chapter 3 - The Molecules of Cells
AIM: Describe the structure of DNA and RNA?
DNA
vs
RNA
Chapter 3 - The Molecules of Cells
AIM: Describe the structure of DNA and RNA?
What do we do
with monomers
?
Chapter 3 - The Molecules of Cells
AIM: Describe the structure of DNA and RNA?
Be able to draw and
number the carbons
Chapter 3 - The Molecules of Cells
AIM: Describe the structure of DNA and RNA?
Chapter 3 - The Molecules of Cells
AIM: Describe the structure of DNA and RNA?
Chapter 3 - The Molecules of Cells
AIM: Describe the structure of DNA and RNA?
The phosphate of one
binds to the #3 C of the
next.
Will this be DNA or RNA?
Chapter 3 - The Molecules of Cells
AIM: Describe the structure of DNA and RNA?
The Paper and the Ink
DNA or RNA?
Chapter 3 - The Molecules of Cells
AIM: Describe the structure of DNA and RNA?
Chapter 3 - The Molecules of Cells
AIM: Describe the structure of DNA and RNA?
Chargaff’s
Rule
He did NOT figure out base-pairing rule
Chapter 3 - The Molecules of Cells
AIM: Describe the structure of DNA and RNA?
Chapter 3 - The Molecules of Cells
AIM: Describe the structure of DNA and RNA?
Chapter 3 - The Molecules of Cells
AIM: Describe the structure of DNA and RNA?
Chapter 3 - The Molecules of Cells
AIM: Describe the structure of DNA and RNA?
Chapter 3 - The Molecules of Cells
AIM: Describe the structure of DNA and RNA?
Base Pairing
Chapter 3 - The Molecules of Cells
AIM: Describe the structure of DNA and RNA?
Base Pairing
Chapter 3 - The Molecules of Cells
The anti-parallel strand
AIM: Describe the structure of DNA and RNA?
Chapter 3 - The Molecules of Cells
AIM: Describe the structure of DNA and RNA?
How many base
pairs (bp) are in
this DNA molecule?
How many base
pairs (bp) are in the
entire human
genome?
The Double Helix
Chapter 3 - The Molecules of Cells
AIM: Describe the structure of DNA and RNA?
3 billion base pairs
Requires 200 volumes the size
of a Manhattan telephone book
(at 1000 pages each) to hold it
all.
It would take about 9.5 years to
read out loud (without stopping)
the 3 billion bases in a person's
genome sequence.
Chapter 16 - Molecular Basis of Inheritance
AIM: The Road to the Structure of DNA
The road to the structure of DNA
Chapter 16 - Molecular Basis of Inheritance
AIM: The Road to the Structure of DNA
Miescher Discovers DNA 1869
- Swiss chemist Friedrich Miescher
identified what he called “nuclein”
inside the nuclei of white blood cells
- Identified the substance to contain
phosphorous
(Mendel published his work in 1866)
Chapter 16 - Molecular Basis of Inheritance
AIM: The Road to the Structure of DNA
Phoebus Levene exposes the nucleotide
(1919)
1. Russian Biochemist
2. Discovered the individual unit of the
nucleotide – ribose/deoxyribose,
nitrogenous bases, and the phosphate
3. Discovered the order of the
components of the nucleotide
Phosphate – sugar - base
4. He proposed that nucleic
acids were polymers of
nucleotides in a circular
arrangement (tetranucleotide)
Chapter 16 - Molecular Basis of Inheritance
AIM: The Road to the Structure of DNA
Phoebus Levene (1919)
If this model were correct, what
should we observe concerning
the amount of dGMP relative to
dCMP, dTMP and dAMP in every
species?
They should be present in an
equal amount or a 1:1:1:1 ratio.
He did not believe DNA to be the
genetic material as it was too
simple…
Chapter 16 - Molecular Basis of Inheritance
AIM: The Road to the Structure of DNA
Chargaff formulates his rule (1950)
1. Austrian Biochemist
2. Discovered that the nucleotide ratios
were not 1:1:1:1 and that the ratios of
different species varied
(Ex. One species might have 20% C while another
has 28% C)
3. Discovered that in the DNA of all species he
looked at the amount of adenine is equal to the
amount of thymine, and that the amount of
cytosine is equal to the amount of guanine
(Chargaff’s Rule)
Chapter 16 - Molecular Basis of Inheritance
AIM: The Road to the Structure of DNA
Rosalind Franklin and photo 51
1. English Physicist (x-ray crystallography)
X-ray Diffraction
a. A technique used to take pictures of molecules too small for any
type of microscope to really observe.
b. These small molecules can be proteins, DNA, nucleotides, water,
NAD+, heme or any other molecule
X-rays are shot at the sample
and “bounce” off of it. They
are measured and can be
used to determine the
structure.
Chapter 16 - Molecular Basis of Inheritance
AIM: The Road to the Structure of DNA
Rosalind Franklin and photo 51 –
(1952)
1. English Physicist (x-ray crystallography)
2. She shot very pure DNA samples with
X-rays and took hundreds of pictures.
One of these was the incredibly clear
image called “Photo 51”.
3. If you were trained in x-ray
crystallography, this image shows
that DNA is a double helix…
If she knew it was a double helix, why
didn’t Dr. Franklin determine the
structure of DNA?
Photo 51
Chapter 16 - Molecular Basis of Inheritance
AIM: The Road to the Structure of DNA
The controversy behind photo 51
1. Wilkins worked in a lab alongside Franklin’s
lab (both a King’s College in London) and was
her colleague and boss in some respect…
Franklin
2. Without going into the full story, Wilkins got a
hold of “photo 51” and showed it either on
purpose or inadvertently to James Watson
(worked 50 miles away in Cambridge).
James Watson (left)
Francis Crick (right)
Chapter 16 - Molecular Basis of Inheritance
AIM: The Road to the Structure of DNA
The controversy behind photo 51
3. Watson ran back to Cambridge
where he worked with Francis
Crick. Neither ever actually did an
experiment on DNA. They used
other peoples data to try and
figure out the structure (Chargaff,
Levene,
Meischer,
Avery,
4. They now
had the
final Griffith,
piece to
etc…)
the puzzle – Franklin’s photo 51
Franklin
Chapter 16 - Molecular Basis of Inheritance
AIM: The Road to the Structure of DNA
The controversy behind photo 51
In 1953, Watson and Crick
published the structure of DNA in
the journal Nature. In the same
issue, Wilkins and Franklin
photo51.
Inpublished
1958, Rosalind
Franklin died from
cancer.
In 1962, the Nobel prize was awarded
to the scientists involved in
determining the structure of DNA:
Watson, Crick, and Wilkins
Franklin
Chapter 16 - Molecular Basis of Inheritance
AIM: The Road to the Structure of DNA
The controversy behind photo 51
Most scientists hypothesize that
had Watson not gotten a view of
photo 51, Franklin would have
figured out the structure of DNA…
Franklin
Chapter 16 - Molecular Basis of Inheritance
AIM: The Road to the Structure of DNA
Chapter 16 - Molecular Basis of Inheritance
AIM: The Road to the Structure of DNA
A Double Helix is simply a twisted
Chapter 16 - Molecular Basis of Inheritance
AIM: The Road to the Structure of DNA
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Chapter 16 - Molecular Basis of Inheritance
We are now going to move back down to the
molecular level (molecular biology) to understand
precisely how DNA…
1. …is replicated during S phase so that the information it
encodes needed to build/maintain organisms can be passed to
the next generation.
2. …stores this information that will be used to make all the
RNA/polypeptides that will directly build/maintain the
organism.
molecular biology- the study of biology at the molecular level (overlaps biochemistry and
genetics in particular). Much of what we have done thus far is molecular biology – cell resp,
photosyn, membrane transport, endomembrane system, central dogma, etc… Mendelian
genetics is not because you never discuss the molecular level, but chromosomal genetics is.
Chapter 16 - Molecular Basis of Inheritance
NEW AIM: How is DNA replicated
What is the next question you would ask about DNA now that
you “know” it to be the hereditary molecule of life?
How is the DNA (chromosomes) replicated (copied) during
mitosis and meiosis so that it can be passed to the
offspring/new cells?
How are the
chromosomes
replicated during S
phase?
Chapter 16 - Molecular Basis of Inheritance
NEW AIM: How is DNA replicated
DNA REPLICATION
Immediately after determining the structure of DNA (1953), Watson and
Crick proposed what is known as the semi-conservative model of DNA
replication, and they happened to be correct although they would now
know this until experiments done by American geneticists Meselson and
Stahl in 1958…
Chapter 16 - Molecular Basis of Inheritance
NEW AIM: How is DNA replicated
Before the answer in 1958, three models were proposed concerning how DNA
replicated shown above – the conservative model, the dispersive model and the
now known to be correct semiconservative model.
Chapter 16 - Molecular Basis of Inheritance
NEW AIM: How is DNA replicated
Each model after two
replications:
What experiment could you do to determine
which was correct?
Chapter 16 - Molecular Basis of Inheritance
NEW AIM: How is DNA replicated
Once again, use radioactive
isotopes:
Meselson and Stahl cultured
(grew) E. coli in the presence
of 15N-labeled nucleotide
precursors (molecules that will
be converted into nucleotides
by enzymes).
Technique is called analytical
centrifugation as is quite popular still
Chapter 16 - Molecular Basis of Inheritance
AIM: How is DNA replicated – The semi-conservative model
GENERAL OVERVIEW
What must happen first?
The DNA strands must separate. An enzyme known as DNA helicase
does this (an enzyme that unwinds and opens a helix is called a
helicase – get it?)…
Fig. 10.4A
AIM:
How16
is DNA
replicated?
Chapter
- Molecular
Basis
of Inheritance
AIM: How is DNA replicated – The semi-conservative model
GENERAL OVERVIEW
Now what must happen?
-The two strands called template or parent strands will be used as a
template to fill in the new strands.
-The template is what you look at to make a new copy. It is a pattern you
follow.
…and
lastly?
AIM:
How16
is DNA
replicated?
Chapter
- Molecular
Basis
of Inheritance
AIM: How is DNA replicated – The semi-conservative model
GENERAL OVERVIEW
Nucleotides, which are in high concentration and randomly diffusing
around the cell / in the nucleus of eukaryotes, are correctly paired and
polymerase
attached to each other (dehydration DNA
synthesis)
by the enzyme…
Fig. 10.4A
AIM:
How16
is DNA
replicated?
Chapter
- Molecular
Basis
of Inheritance
AIM: How is DNA replicated – The semi-conservative model
GENERAL OVERVIEW
Parent or template
strands
Daughter or
complementary
strands
The result is two identical daughter chromosomes, each containing one
strand from the original parent molecule and one newly synthesized
strand called the daughter strand, which is complementary to the parent
strand
(semi-conservative).
Fig. 10.4A
AIM: How is DNA replicated?
Double-Stranded DNA
Molecule
Human chromosomes are millions of basepairs (bps) long. DNA polymerase
can only move at most 50 base pairs per second in mammals and 500 per
second in bacteria. How long would it take to replicate chromosome number 1
in humans, which is 246 million base pairs?
Almost 57 days – TOO
AIM: How is DNA replicated?
How would you hypothesize the cell gets around this?
There must be many DNA polymerases working at the same time
starting at different points along the DNA called ORIGINS OF
REPLICATION (arrows above).
AIM: How is DNA replicated?
There is a specific sequence in the DNA that makes it an origin. A single
chromosome in humans can have upwards of 2,000 origins of replication
to speed up the process (only three are shown above).
What needs to happen to the double-stranded DNA in order to start this
process?
AIM: How is DNA replicated?
At each origin the double-stranded parental (template) DNA is opened up
(the strands are separated by a protein called DNA helicase).
The regions of separated DNA are called REPLICATION BUBBLES
because they look like little bubbles in the DNA…
AIM: How is DNA replicated?
The daughter strands are synthesized IN BOTH DIRECTIONS FROM EACH
BUBBLE via complementary base pairing with the parental strand by DNA
polymerases until they all meet up…
AIM: How is DNA replicated?
The result is two new semi-conserved (one parent strand and one newly
synthesized daughter strand), identical (if no mistakes are made, which is
AIM: How is DNA replicated?
AIM: How is DNA replicated?
Now let’s look at the players that make this process possible.
Players = proteins, but you know that.
AIM: How is POOP replicated?
DNA polymerase
- The enzyme that catalyzes the polymerization of DNA (extension - it is extending
the polynucleotide), adding the appropriate nucleotide (dATP, dTTP, dCTP, CGTP)
across from the complementary base in the parent strand.
AIM: How is DNA replicated?
Cannot add nucleotides here
Can add
nucleotides here
3’
DNA polymerase
**Can only make the new daughter strand FROM the 5’ end toward the 3’
end. In other words, it can only add nucleotides to the 3’-OH of a
nucleotide and not to the phosphate.
In other words, it can only add nucleotides to the 3’-OH of a nucleotide and
not to the phosphate end.
AIM: How is DNA replicated?
3’
DNA polymerase
DNA Polymerase cannot bind to single stranded DNA.
5’
AIM: How is DNA replicated?
5’
U G
3’
C C U G
3’
5’
DNA polymerase
DNA Polymerase cannot bind to single stranded DNA.
It will require a short piece of RNA called a primer, which will be
added by the enzyme RNA primase.
The primer “primes” the DNA or gets it ready for DNA
polymerase .
AIM: How is DNA replicated?
3’
DNA polymerase
Where does the “energy” come from to synthesize DNA (Where does DNA
polymerase get the ability to polymerize DNA by making phosphodiester bonds
between nucleotides)?
It comes from the nucleotides themselves as they are triphosphates! – dATP, dCTP,
dGTP
and dTTP.
Think affinity
- would AMP “rather” be attached to the sugar of another nucleotide or to
two negativity charged phosphates?
AIM: How is DNA replicated?
DNA polymerase
Reaction Rate:
- Can catalyze 50 bases/sec in mammals and 500 bases/sec in prokaryotes
Error Rate:
- Error Rate of 1 mistake in every 100,000 nucleotides…If not repaired this mutation can
result in a new allele if in the gametes, can cause cancer, can have no effect, etc…a roll of
the genetic dice.
AIM: How is DNA replicated?
An dATP by chance diffused
into the active site and was
incorporated across from a C.
Typically it will randomly
bounce in and then bounce
out.
DNA polymerase
Proofreading:
-DNA polymerase has a cool 3’-5’ exonuclease capability (exo
– exit, take out; nuclease – hydrolyze nucleic acid; 3’-5’ – the
reverese direction).
-Therefore, when an incorrect base is added, the polymerase
can take a step backwards in the 3’ to 5’ direction, and cut it
out followed by insertion of the correct base = proofreading
AIM: How is DNA replicated?
DNA polymerase Review
- Catalyzes synthesis of daughter strand of DNA
from 5’ to 3’ using complementary base pairing rules
against template strand
- Can proofread and fix errors
- Makes mistakes 1 in 100,000 bases
- Catalyzes between 50 and 500 bases/second
- Cannot bind single stranded DNA (ssDNA),
requires the presence of a primer to make is doublestranded (dsDNA)
- Synthesis is endergonic and is coupled to the
dephosphorylation of the nucleotide (dTTP, dCTP,
dATP, or dGTP) as it is added
AIM: How is DNA replicated?
Each Replication bubble can be viewed as two identical replication forks,
just rotated by 180 degrees.
replication forks
replication fork
Fork in the road
AIM: How is DNA replicated?
It is time to get into the details.
We will focus on one fork since both work the same way (the process is
the same).
AIM: How is DNA replicated?
5’
3’
3’
5’
DNA is traditionally drawn with the top strand going 5’ to 3’ (or 5’ is in the top
left corner).
AIM: How is DNA replicated?
GENERAL OVERVIEW:
5’
3’
3’
5’
Origin of replication
Replication bubbles form and DNA polymerase starts catalyzing the polymerization of
DNA at the origins of replication…
AIM: How is DNA replicated?
Both forks of every bubble are being replicated at the same time…The animation you
just watched was of only one fork, but the same thing is happening at the other fork.
AIM: How is DNA replicated?
THE DETAILS:
http://www.wiley.com/legacy/college/boyer/0470003790/animations/replication/replicat
on.htm
(Try to put it into words as you watch )
AIM: How is DNA replicated?
AIM: How is DNA replicated?
AIM: How is DNA replicated?
AIM: How is DNA replicated?
THE PLAYERS:
1. DNA Polymerase
- Catalyzes the polymerization of
deoxynucleotides in 5’ to 3’ direction using
the energy in the nucleotides. Removes
2. primers
Helicaseand fill in DNA nucleotides.
- Unwinds and pulls apart the strands
using the energy in ATP
3. RNA Primase
- Adds the RNA primer for DNA
polymerase using energy from the RNA
nucleotides themselves.
4. DNA Ligase
- Seals together the Okazaki
fragments using ATP for energy
5. Single-Stranded Binding Proteins
- Bind to the single-stranded DNA,
preventing it from reannealing or
coming back together after helicase
them. (gyrase in bacteria)
6. separates
Topoisomerases
- Ride ahead of the helicase and unwind the
DNA coils caused by helicase’s activity using
AIM: How is DNA replicated?
This simple process allows for the
information stored in DNA to be
replicated so that it can be passed
from cell to cell, generation after
generation, in all life ever observed by
humans for billions of years…
AIM: How is DNA replicated?
http://www.hhmi.org/biointeractive/dna/index.
html
Chapter 16 - Molecular Basis of Inheritance
Chapter 16 - Molecular Basis of Inheritance
What happens when the replication fork arrives at the
end of a chromosome?
Chapter 16 - Molecular Basis of Inheritance
Chapter 16 - Molecular Basis of Inheritance
DNA Repair
ALL cells have proteins whose job it is
to identify and repair DNA damage.
A number of different mechanisms
have evolved to do this each requiring
a different set of proteins :
1. Base Excision Repair
2. Nucleotide Excision Repair
3. Mismatch Repair
4. Direct Reversal
5. Recombinational Repair
All mechanisms are found together in a cell as each is specialized to
handle a different type of damage. You can think of them as different
Chapter 16 - Molecular Basis of Inheritance
DNA Repair
To the right is one of these
mechanisms in a bit of
detail…Nucleotide Excision Repair.
This mechanism recognizes large
helical distortions (changes in shape)
resulting in one strand of this section
being excised (cut out) and replaced.
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