DNA Chromo Replication 2c'
Summary
TLDRThis lecture delves into the intricate structure and replication of DNA, the genetic material. It explains the chromatin structure, highlighting the role of histone proteins in packaging DNA into a compact form. The lecture then transitions into DNA replication, discussing the semiconservative model supported by the Meselson-Stahl experiment. It further explores the process in prokaryotic cells, detailing the initiation at a specific origin, the role of helicases, topoisomerases, and primase in the replication machinery, and the bidirectional nature of the process.
Takeaways
- 🧬 Chromatin structure is composed of DNA and proteins, including histones that form a 'beads on a string' arrangement.
- 🌀 The 30 nm fiber is formed by the coiling of the DNA around histones, which can further organize into loops anchored to the nuclear matrix.
- 🔬 Electron micrographs illustrate the transition from the 30 nm fiber to loop domains, highlighting the role of scaffold proteins.
- 🧪 The Meselson-Stahl experiment provided evidence for semiconservative DNA replication, where each new DNA molecule consists of one old and one new strand.
- 🧵 DNA replication is not random; it initiates at specific origins of replication and proceeds bidirectionally.
- 🔄 Helicases play a crucial role in replication by unwinding the DNA double helix, creating template strands for new strand synthesis.
- 🌀 Topoisomerases relieve the strain caused by the supercoiling of DNA that occurs during replication.
- 🔬 Single-stranded binding proteins prevent reannealing of separated DNA strands and protect them from nucleases.
- 🧬 Primase is essential for replication as it synthesizes RNA primers to which DNA polymerases can add nucleotides, initiating new strand growth.
- 📚 The process of DNA replication is more complex in eukaryotic cells due to larger genomes and involves multiple origins of replication.
Q & A
What is the nucleosome structure described in the lecture?
-The nucleosome structure consists of an octomer of four histones forming a 'beads on a string' configuration, where DNA wraps around the histones 1 and 3/4 turns.
What are the two models that explain the further compaction of the 30 nm fiber into a more condensed chromatin structure?
-The two models are the solenoid model and the zigzag model, which help to form loops of DNA that are anchored to proteins called a scaffold or nuclear matrix.
What are scaffold attachment regions (SARS) and how do they relate to chromatin structure?
-Scaffold attachment regions (SARS), also known as matrix attachment regions (MARs), are the sites where the DNA loops are anchored to the scaffold proteins, contributing to the higher-order folding of chromatin.
How does the removal of histone proteins affect the appearance of DNA as seen under an electron microscope?
-Upon removal of histone proteins with high salt, the DNA appears as loose strands that are no longer organized into the structured chromatin fibers, indicating the role of histones in maintaining chromatin structure.
What is the significance of the Meselson-Stahl experiment in understanding DNA replication?
-The Meselson-Stahl experiment provided evidence supporting the semiconservative model of DNA replication, where each new DNA molecule consists of one original and one newly synthesized strand.
What are the differences between the three models of DNA replication: semiconservative, conservative, and dispersive?
-Semiconservative replication results in each new DNA molecule having one old and one new strand. Conservative replication would result in two old strands staying together and two new strands forming together. Dispersive replication would mix old and new segments in both new DNA molecules.
How does the use of nitrogen isotopes in the Meselson-Stahl experiment help to differentiate between replication models?
-The use of nitrogen-15 (heavy) and nitrogen-14 (light) isotopes allowed for the visualization of DNA strands based on their density after centrifugation, which helped to demonstrate that DNA replication is semiconservative.
What is the role of DNA helicase in the process of DNA replication?
-DNA helicase unwinds the DNA double helix into single strands by breaking hydrogen bonds between the bases, creating a replication bubble with two replication forks.
Why is topoisomerase necessary during DNA replication?
-Topoisomerase is necessary to relieve the strain caused by the supercoiling that occurs as the DNA helicase unwinds the DNA strands, preventing the replication fork from becoming too tight to continue replication.
What is the function of single-stranded binding proteins in DNA replication?
-Single-stranded binding proteins bind to the separated DNA strands during replication to prevent their reannealing and degradation, thus maintaining the integrity of the single-stranded DNA template.
Why can't DNA polymerase initiate DNA synthesis on its own, and what is the solution to this limitation?
-DNA polymerase cannot initiate DNA synthesis de novo because it requires a pre-existing strand to add nucleotides to. Primase solves this by synthesizing a short RNA primer that provides a starting point for DNA polymerase to begin DNA synthesis.
Outlines
🧬 DNA Structure and Chromatin Organization
This segment of the lecture delves into the intricate structure of DNA and its packaging into chromatin. The discussion begins with an exploration of chromatin's protein components, specifically the nucleosome, which is an octomer of histones around which DNA wraps in a 'beads on a string' configuration. The lecturer describes the further coiling of this structure into a 30 nm fiber, either through a solenoid or zigzag model, and the formation of loops that are anchored to the nuclear matrix. These loops, which can be up to 85 kilobases in length, are attached by scaffold attachment regions (SARS). The lecture also touches on the visualization of these structures through electron micrographs and the effects of histone removal on DNA structure. Additionally, the use of DNA enzymes to break down and analyze DNA within chromatin is discussed, illustrating the different sized pieces of DNA resulting from various concentrations of the enzyme.
🔬 DNA Replication: Semiconservative Model and Meselson-Stahl Experiment
The second part of the lecture shifts focus to the process of DNA replication. It emphasizes the importance of accurate replication for genetic material and introduces the concept of semiconservative replication, where each new DNA molecule consists of one original and one newly synthesized strand. The lecture recounts the Meselson-Stahl experiment, which provided evidence for this model. This experiment utilized bacterial cells, nitrogen isotopes (N14 and N15), and equilibrium density gradient centrifugation to track DNA replication over generations. The results, which showed a transition from a 'heavy' DNA band to a 'light' and 'intermediate' band, supported the semiconservative model over conservative or dispersive models. The lecturer also explains the theoretical basis for different replication models and the experimental evidence that led to the acceptance of the semiconservative model as the prevailing mechanism for DNA replication.
🌟 DNA Replication Process and Initiation
In this section, the lecture provides an in-depth look at the initiation of DNA replication, highlighting the role of specific proteins and the unique starting point known as the origin of replication. It contrasts the replication processes in prokaryotic and eukaryotic cells, noting the differences in speed and the number of origins of replication. The lecture describes the replication apparatus, including the initiator protein (DNA A), helicase (DNA B), and the formation of a replication bubble. It also touches on the role of topoisomerases in managing the strain caused by the unwinding of DNA and the formation of supercoils. The importance of single-stranded binding proteins in preventing reannealing and degradation of the separated DNA strands is also discussed. Lastly, the lecture introduces the concept of primer synthesis by primase, an enzyme that lays down a short RNA primer necessary for DNA polymerases to begin DNA strand elongation.
🧪 DNA Polymerase and Primer Synthesis in Replication
This segment continues the discussion on DNA replication, focusing on the role of DNA polymerases and the process of primer synthesis. It explains that DNA polymerases cannot initiate new strands but can only add nucleotides to an existing strand, necessitating the use of RNA primers synthesized by primase. The lecture describes the formation of a primosome, a complex of helicase and primase, which is essential for the synthesis of the RNA primer at the replication fork. The process of how DNA polymerases add nucleotides to the 3' end of the primer in a 5' to 3' direction is detailed, illustrating the mechanism of DNA strand elongation. The lecture also includes visual aids to depict the formation of RNA primers and the subsequent DNA synthesis, providing a clear understanding of the molecular events during the initiation phase of DNA replication.
Mindmap
Keywords
💡Chromatin
💡Histones
💡Nucleosome
💡Scaffold Proteins
💡Semiconservative Replication
💡Meselson-Stahl Experiment
💡DNA Polymerase
💡Primase
💡Replication Fork
💡Topoisomerase
💡Single-Stranded Binding Proteins
Highlights
Chromatin structure involves proteins and DNA forming a bead-on-a-string appearance.
The nucleosome octomer consists of four histones creating a 1 and 3/4 turn around the DNA.
The solenoid or zigzag model of chromatin forms a 30 nm fiber.
DNA loops of about 85 kilobases are anchored to proteins known as the scaffold or nuclear matrix.
Loop domains can further fold to create a 700 nm width chromatid.
Histones help maintain the width of chromatin structure.
DNA can be broken down into different sized pieces using DNase enzymes.
Low concentration DNase results in smaller DNA pieces, while high concentration leaves larger pieces.
DNA replication is semiconservative, with one original strand and one new strand in each daughter molecule.
Meselson and Stahl's experiment provided evidence for semiconservative DNA replication using nitrogen isotopes.
Equilibrium density gradient centrifugation was used to analyze DNA replication models.
The experiment showed that DNA replication is not conservative, as no all-heavy band appeared after one generation.
After two generations, the presence of both intermediate and light bands confirmed semiconservative replication.
DNA replication starts at a specific point on the chromosome known as the origin of replication.
Bacterial chromosomes typically have a single origin of replication, while eukaryotic cells have multiple.
DNA replication is bidirectional, proceeding in both directions from the origin.
Initiation of replication involves DNA helicase unwinding the DNA at the origin, forming a replication bubble.
Topoisomerases relieve the strain caused by the supercoiling of DNA during replication.
Single-stranded binding proteins prevent reannealing and degradation of separated DNA strands.
Primase synthesizes an RNA primer required for DNA polymerase to start DNA synthesis.
DNA polymerases can only add nucleotides to the 3' end of an existing strand, not de novo.
Transcripts
all right this is the third part in the
lecture series on DNA genetic material
structure and
replication all right so we going over
chromatin structure talking about the
proteins involved nucleome is at octomer
of these four histones beads on a string
forming 1 and 3/4 turns around seeing
that here that one and 3/4 turns further
uh joining together and either this um
solenoid model or zigzag model creates
what is a 30 NM um fiber here's a
electron micrograph showing that so this
30 nanm chromogen fiber then can form
these Loops like I said about 85
kilobases of DNA and these are anchored
to um proteins called a scaffold or
nuclear Matrix the loops are attached to
the scaffold by scaffold um attachment
regions or SARS scaffold attachment
regions or Matrix attachment region or s
m RS these Loop domains can f can
further fold to create a 700 nanm width
chromatid um so if we look down here the
chromatid which is half of this so the
width from here to here is about 700 um
nanometers and so here they've sort of
digested away the histone protein so all
the DNA is just loose see DNA strand
here and it just tons of DNA just looped
here so all these loops and then they're
attached to the scaffold proteins so
this red stuff in here is scaffold
proteins and again so high salt removed
the histone protein so that stuff went
away but it was still connected to these
scaffold
proteins and so the uh hison then help
keep it at about this width and you can
see if the histones are gone they sort
of unravels and goes everywhere um so
here we can say um a loop um bound to
nuclear
Matrix filament so here you got this
Loop this 30 nanom nanometer Loop here
and you got these attachment regions
that attach to these Matrix attachment
regions um and here you can see a
closeup a nuclear Matrix just this and
again it's called a matrix because
they're just joined in all these um
different parts and here's the nuclear
Matrix and here joined in all these
different
areas and here's a um way to sort of
break DNA down into different sized
pieces as it's contained in chromatin so
you can use DNA one which is just an
enzyme that chops up DNA you can use a
low concentration DNA Swan up to a high
concentration DNA Swan and then uh run
those out on gel and the idea is that so
before digestion you got a lot of DNA
long pieces join beats on the string
after DNA is use a Cuts between all
these nucleosomes so you get these
different sized pieces the bigger pie
pieces stay close to where they started
smaller pieces go further down so if you
then look at the actual results of this
if you got a low amount of DN some of
the DNA gets cut into really small
pieces but there's big chunks of DNA
because there's not a lot of DNA so some
of that DNA isn't cut so you get big
long pieces medium amount you get more
smaller pieces and you're missing larger
pieces and a high amount of DNA Cuts
almost all the DNA into then um smaller
pieces that can be seen here so just a
summary of you know what we're looking
at here so DNA without any um protein
DNA in um those histone octomer
nucleosomes wrapped around um those
rolling up so that's 10 10 net wide or
11 um rolling up to 30 NM fiber solonoid
structure or a zigzag model so you get
this 30 big Loops then contained or
joined to these matrices or M
in the um nucleus and then those even
further Loop up so that here we go the
uh width here is 700 nimet so one of
these right here that's 700 net so both
of them together
is400 um
together all right so that's uh keriton
structure now we're going to move on and
talk to DNA replication or how DNA makes
an identical copy of itself so the
double helix model of DNA that Watt and
Crick proposed made it immediately
evident to them a mechanism for DNA
replication in fact they included a
quote in their paper saying as much so
this comes directly um from their paper
their study um talking about the
structured DNA and they say quote it has
not escaped our notice that the specific
pairing we have postulated immediately
suggests a possible copying mechanism
for the genetic material unquote so they
sort of slightly state that you know
this model they came up with really
explains how DNA could replicate itself
quite easily so recall that the ability
to replicate information Faithfully is
one of the requirements for the genetic
material because the two strands are
complimentary to one another one can
imagine the strands separating and each
New Strand is red or replicated from the
information contained on the original
strand this type of replication is
called
semiconservative replication because one
conserved strand stays and and one New
Strand is made there are however other
possible models of DNA replication
conservative and
dispersive the mesel install experiment
provided evidence to support the
semiconservative um semiconservative
replication for DNA so in 1958 mes estal
studied DNA replication using bacterial
cells isotopes of nitrogen nitrogen 14
and nitrogen 15 so these are naturally
occurring Isotopes they are not
radioisotopes they just differ in Mass
so natural isotope 14 and a heavier
isotope N5 and equilibrium density
gradient centrifugation okay so again
Isotopes same atomic number but
different atomic mass making N15
slightly having slightly more mass than
n14 so equilibrium density graded
equilibrium density gradient
centrifugation is centrifugation of
heavy Salt Solutions in this case they
use cium chloride which has a molecul
133 which you don't need to remember um
the heavy
cium during centrifugal forces um tends
to sediment towards the bottom of the
tube however diffusion also takes place
during centrifugation so you have is two
forces that work and after long periods
of centrifugation at high speeds a
linear concentration gradient of cium
chloride is formed in the centrifugation
tube such that you have a higher
concentration at the bottom and then
diffusion creates a gradient As you move
toward the top of the
tube macro molecules then so protein DNA
RNA in the tube will come to rest or
will band in the region of the tube
where their density matches the density
of the cium chloride okay and again the
density of cium chloride goes from high
at the bottom of the tube to lower lower
lower lower lower As you move toward the
top of the tube so chloride is used
for this sucrose can also be used or
sucrose gradients can also be used to um
in this type of um centrifugation so
what was the
experiment um let see oh back up here so
a couple of notes before we get to the
experiment um so this is
semiconservative replication displayed
here so again you start with one strand
of DNA you end up with two strands of
DNA it's semiconservative because this
strand was conserved and this New Strand
was made well this strand on this side
was conserved and this strand was made
so you can see if you could separate
these two strands then make a new strand
that would be a semiconservative model
however you could also Imagine different
models so semiconservative in the middle
which just talked about you could have a
conservative model of replication where
two strands of DNA stay together and two
brand new strands are made together or
you could have a dispersive model where
you have two strands of DNA where parts
of both sides of the strands are new and
old so you can see mixed here so you get
a mix in both of the strands rather than
one purple one blue you get a mix of
purple and blue on both sides here so
those are the models of replication so
what did meselson install do then to
determine what was going on so cells
were grown in a medium containing heavy
nitrogen Okay so in this case suspension
of bacterial cells labeled with
N15 okay so the bacterial cells were
grow grown in N5 for several Generations
so that all the nitrogen in the DNA
contained heavy nitrogen cells were
washed to remove N15 and transfer to
Media containing
n14 okay so it's added to
n14 cells were allowed to grow in the
n14 for various time periods either one
or two cell
divisions and then their DNA was
extracted and analyzed by cium chloride
equilibrium density centrifugation so
again here all Heavy added to n14
allowed to divide either once or allowed
to divide twice and they went further
than that but we don't need to to
explain what's happening and then they
would lice okay or break apart the cells
they would get the DNA out and then they
would run it on this so they put on the
CM chloride gradient here get this and
they would spin it and the DNA then
would run you can see barely see there's
bands here so you either get heavier or
lighter or lighter size bands
as the DNA went along so here's our
actual um data from their paper you can
see 1958 M
install
and generations after adding in14 so
actually move some right to left here so
here's one generation so half heavy
heavy so what they do is they start with
all Heavy because they're all in n14 and
you get a band here after one generation
here all the DNA is half heavy or a mix
of n14 and N15 if you then jump to this
is about two generations what you have
is half heavy or a mix of n14 and15 and
light or about equal amounts of mix and
all light and you can see here as the
keep going Generations further and
further out you get more and more of
this light but you never lose this band
here of the half heavy but you do lose
the heavy right you never get another
Heavy band which immediately after one
generation this says that that
conservative model was not um the model
by which T replicates because you would
expect a band oops out to one here you'd
expect a band but it's not there
um the half Heavy after one generation
explains the um cons uh semiconservative
model but it also explains possibly the
dispersive model but if you go out two
generations here dispersive model you
should only get one band right about the
middle of here but instead you get two
bands one with the half heavy and one
with the light and so by going out two
generations that tells you that is the
semiconservative model of replication um
is how DNA replicates
itself all right so that explains what
happens but um if it's not completely
clear you probably want to take this
model here label both these purple
strands and5 and H blue strand that
forms and14 and make sure that um you
can sort out what would happen if you
centrifuge each of these bands of DNA
where would they band so what ends up
happening is this is The Heavy band this
is the intermediate band there's an
intermediate band there's an
intermediate band these then are the
light bands so that's why after one
generation you see nothing but
intermediate at two generations you see
an equal amount of intermediate
intermediate and light because there's
two light complete lights and two
intermediates on the end all right so we
know that DNA replication is
semi-conservative now we'll talk about
how DNA actually replicates itself and
we'll start with just a few General
statements um so while a lot is known a
lot is still unknown um much more um is
known about procaryotic DNA replication
than eukariotic DNA replication so the
discussion of replication we'll center
around the results of procaryotes uh
since they are known earlier and known
better and then we'll discuss um some
issues about replication UK carots
there's lots of things that are similar
and there's just details um that are
different so replication of DNA does not
start at a random Point okay the origin
of replication is a unique point along
the chromosome where replication begins
this is a short sequence recognized by
the replication apparatus as the point
to begin rarely does replication begin
at the end of a chromosome and of course
bacterial chromosomes are circle and
have or circular and have no free ends
bacterial chromosomes usually around up
to a million base Bears have a single
origin of
replication the rate of replication in
these organisms is about 2,000 nuclear
Tides per second um an ecoli cell can
replicate its DNA between 20 to 40
minutes eukariotic replication is much
slower between 2,000 to 10,000 base
pairs per minute and remember 2,000
nucleotides per second for bacterial
cells 2,000 to 10,000 base Bears per
minute for eukariotic cells due to the
slow speed of replication and larger
number of base pairs in ukar ukar have
several origins of replication each
around 60,000 base pairs apart a yeast
cell can replicate its DNA in about 1.4
hours and animal cells and culture may
take up to 24 hours to replicate their
DNA a Replicon is the origin of
replication Replicon is the origin
replication and the DNA that is
synthesized from the origin DNA
replication is nearly always bio sorry
bidirectional um meaning that
replication proceeds in both directions
along the nucleotide strands so what
that would look like here so if there's
an origin replication here DNA
replication goes to the left and to the
right okay so bir directional moving at
moving in both directions and so here's
some evidence of that started here moves
this way so it's making conservative
semiconservative um there and also in
these little Loops all right and so
here's an electrom micrograph sort of
showing that same
thing um here's chromosome after one
round of replication here chromosome
undergoing its second round
replication there
oops um so it's moving in both
directions going to the left and to the
right all right so details of
replication from E coli or um bacterial
cells initiation replication begins at
the origin replication an initiator
protein called DNA
a is required to recognize and bind the
sequence of nucleotides at the origin of
replication this recruits DNA
b a protein containing two helicases so
a helicase is an enzyme that unwinds DNA
remember DNA is a helix a double helix
so a helicase unwinds the DNA in
um at the origin to denat the DNA double
helix into single strands and form
what's called a replication bubble
untwisted single strands from which new
strands are made are called template
strands okay so when the he case comes
in it separates the double strand into
individual strands those individual
strands then are known as template
strands all right so here's or C or
origin replication E coli and um you can
see it has this specific region here and
region here this then the sequences are
what are recognized by that DNA a uh
protein and you can see here this is
called the DNA a box often times um
scientists you know geneticist call
sequences of DNA that are bound by
protein boxes okay so this is a box or
it's a sequence of DNA and it's a DNA a
box because DNA a is the protein that
binds to that particular sequence so the
origin replication you don't need to
know any of these sequences it's just
the spot where DNA a will bind that
origin
replication all right so so you have
these template strands the rest of the
replication apparatus which contains
several parts in one multi-subunit
apparatus is called the
reome um now we'll join that replication
bubble so initiator protein recognizes
recruits helic cas which um opens up
single strands that creates that
replication bubble or two replication
forks all the other components then that
join all together then known as the
repone all right so then here's just
another figure those DNA boxes here so
DNA protein can bind there it's going to
recruit them the DNA helicases or DNA B
that can come in and those are going to
open so this purple double stranded open
into single strand Brands creating this
replication bubble or two replication
forks so to the right and then to the
left a by directional replication that's
going to take place um this is sort of
advanced we haven't gotten to these
details but same sort of thing so this
now is a replication fork it's Bas
basically if you if you cut this in half
down where this arrow is and you just
looked at one side of it you'd have a
replication fork rather than a bubble so
two replication forks make up a bubble
so here's replication Fork the fork is
moving to the left the purple on the top
new blue being made purple on the bottom
new blue also being made so
semiconservative replication we'll get
to these details here in a
second okay so let's talk about the
details of the initiation or the start
of DNA replication so as that DNA
helicase unwinds the DNA at the growing
Fork it does so by breaking apart the
hydrogen bonds between the bases this
requires ATP so it requires some energy
to do the job this unwinding the DNA by
DNA hel case also causes extra twisting
in other regions and coils called super
coils form to release the strain these
super coils then that form are relieved
or removed by topoisomerases so
topoisomerases are enzymes that control
twisting of DNA and they are needed to
break phosphodiester bonds and twist the
DNA Helix back back to what's called its
relaxed State and join them DNA gyas is
one example which can induce super
coiling of DNA so take a look at
this here so as these strands open up so
D hel cases separating these two strands
what ends up happening is there's extra
twisting that happens here and you get
these super coils well if this was
allowed to continue and this opened up
further and further this super coiling
region would be so tight that the
replication fork would not be able to
open up any further so you need these
top isomerases to cut and unwind these
regions and so if you look then don't
you don't have to look at this one here
I don't think it's going to show up but
if you look at these YouTube videos here
they do a really good job of showing how
these Topo isomerases work to relieve
this um the super coiling and the
structural pressure that's um being
created through this
process okay so the strands are being uh
pulled apart or opened up by the
helicases the top isomerases are taking
care of the super coiling single
stranded binding proteins um bind single
stranded DNA preventing Its reannealing
So once um those strands are separated
you don't want them to anneal or come
back together you also don't want them
to um be chewed apart by any nucleases
within the cell so single strand binding
protein binds to them prevents them from
binding back together and they also
prevent them from being degraded so the
next thing then that happens is primer
synthesis so it turns out that DNA
polymerases and we haven't got that part
yet they're part of the replisome
polymerases polymerize or make and put
DNA together um DNA polymerases that are
responsible for DNA synthesis cannot
catalyze the addition of
nucleotides denovo or all by themselves
they can only add nucleotides to the
exist to a pre-existing strand so there
has to be a strand of nucleotides there
for DNA Pates to add nucleotides to to
grow they can't put that first
nucleotide in brand new they're unable
to do that so therefore how the cell
deals with this is there's an enzyme
called
primase right here that synthesizes a
short RNA primer which is complementary
to the DNA DNA template strand DNA plas
can then add nucleotides to the existing
primer so primase is activated by
helicase so again helicase is already
opening the strands up and they form a
complex with the DNA called a primosome
all right so the primosome then is the
helicase plus primase helps form the RNA
primer so let's take a look at what that
looks like all right so here the DNA is
replication fork is being separated and
as I stated DNA plase can't put in this
first nucleotide all on its own so what
has ends up happening is the primer gets
made by primase and it's complimentary
to the DNA it's there now DNA plyar can
come in and put this blue nucleotide and
it can add a nucleotide to the end of an
existing DNA strand all right and what
we're going to talk about in a few
minutes but since we have the SL up is
DNA plase adds nucleotides to the thre
Prime end of an existing strand so this
strand is five Prime to three prime it's
going to add nucleotides to the three
prime end it cannot DNA polymerases
cannot add nucleotides to the five Prime
end it's just how it works and it can't
do that so
um polymerises DNA pises always create
or grow a DNA strand in a direction five
Prime to three prime it can't do so in
the other direction all right so here's
a slide here here in red is a primer
actually you can see it here better an
RNA primer and this blue is new DNA that
was made here the red is primary here's
a blue and here's the enzymes making it
making it this point here's a new RNA
primer that was put in and we'll start
um DNA plas will be made from that here
again primase making an RNA primer and
then the DNA plases will make the DNA so
here's an RNA primer in red DNA's been
made joins up here's another RNA primer
you see the end of it in red going to be
used to make that so we're going to end
here and then we'll pick up right at
this point to continue talking about how
uh DNA replication works
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