Meselson Stahl Experiment - Molecular Basis of Inheritance, Biology, Class 12 Video Lecture - NEET

[Music]
alright friends let's talk about the
last part of the know your experiment
series of video lectures and we've been
talking about the Griffiths experiment
huh Shh his experiment and you reveal an
experiment in the last three videos now
in this video we are going to close it
up with meselson and stahl experiment
done in 1958
so from 1928 to 1958 the Year 30 years
for groundbreaking experiments have been
done that established multiple facts as
truth in the field of life sciences and
meselson and stahl experiment is one of
them
so what meselson and stahl did as an
experiment is to prove the nature of DNA
replication because you know earlier in
the Griffiths experiment with harsh a
cheese we know that DNA is the genetic
material and we also know the idea of
horizontal gene transfer but we never
know how exactly the DNA is duplicated
because we know one thing that if one
generation needs to donate its DNA to
the next generation it needs to make a
copy of that to keep that copy to itself
so definitely whenever the transfer is
being done from one generation to the
next generation
they definitely involve the process of
DNA duplication which is also known as
DNA replication so what happens here in
the DNA replication process in meselson
and stahl proved the idea of DNA
replication and the what type of DNA
replication is actually true and the
example for such earlier than meselson
and stahl it was known there are three
types of model for the DNA replication
because we need to know how exactly
exact same copy of DNA is produced and
the answer to that you know the three
separate versions of the DNA replicants
one is that let's say this is the this
is the parent parental DNA from this DNA
we can produce the new strength of the
DNA so definitely one process says that
this is a version where parent DNA
remains as it is a new
which is a red in color is formed
another situation says that the parental
DNA is remain as it is in both the
stands and a new strand in each case is
formed another theory states that both
parent and both the strands they have
mixed match components so something like
this and also something like this so
it's a mixed version of both the strands
are found so these are the three
separate models prescribed in terms of
explaining the DNA replication process
so this first term is known as the
conservative model because in this case
you can see the parental strands both
blue colored ah the old parental strands
kept as it is in the second type is
known as semi conservative model because
in this case one of the strands kept as
parental the other strand is mu and the
third one is known as dispersive model
because in this case it's mixed or
mosaic model where you know there is no
rule of keeping any strength as parental
every single strand has both mix and
match component from parental strand as
well as a new strength so which among
this three is actually the right process
of DNA replication we never know that
until missiles and installed clear that
out for us in 1958 the experiment
conducted was very clever to find out
which model is true you know to
understand this this process what they
did is that they conducted the
experiment with the help of E coli and
transferring the process and allowing
the DNA replication process in E coli so
what they did is what we normally know
equal I can replicate very fast because
it has the generation time of only 20
minutes in lab conditions so in optimum
conditions so what we can do we can we
can check this one very much out but the
easy way to think is out is that the
first primary thing that they need to
think before doing this pyramid is to
distinguish and differentiate between
parent Ostrander and new strengths so
what they did is that they can easily
tear
the basis because the bases are
nitrogen-rich so what they can do is
they can tag the nitrogen with heavy
nitrogen rate so in 14 is the natural
nitrogen and in 15 is the heavy nitrogen
radioactive form of the nitrogen so that
that's what they they're going to do so
what they did is that they took one set
of e.coli they took this bacteria they
culture it in the heavy nitrogen medium
so they culture it this e.coli for
having an Olivia nitrogen medium for
many generations to make sure that this
e coli receives all the heavy nitrogen
and incorporates all the heavy nitrogen
in the DNA of this e.coli so once all
the heavy nitrogen is incorporated we
make sure that this e coli contain the
DNA material with in 15 or heavy
nitrogen only okay so we call it heavy
nitrogen containing DNA now after this
process is done we know all the genetic
component of this bacteria is tagged
with heavy nitrogen then what we did is
simply we transfer this equally into
light nitrogen containing medium now the
question is how exactly we supply a
medium with light nitrogen or a heavy
nitrogen you meselson and stahl can deep
do that with changing the medium
generally the medium that supplies the
nitrogen ammonium chloride for an
example or any other version of ammonium
salt that can supply nitrogen to the
e.coli and we can tag that medium with
heavy nitrogen for transferring it so
once we do that we transfer this heavy
nitrogen carry ecoli into the light
nitrogen medium so now the medium can
tail light in nitrogen it's being
transferred it's being shifted okay so
now once we put it there then we allow
this eco light to grow to divide now
what happens you know in next
generations this same e coli that will
produce many other cells now you know
all those different eco light cell what
we need to study what we need to figure
out is to find out their genetic
material and to run gel to find out
their exact pattern not
this this can be time-consuming like
looking at their DNA of breaking the DNA
looking at the sequence because at 1958
it was not possible to look for every
single individual of e.coli knowing
their sequence and finding out because
if this experiment has been done this
date it will be far easy but it's it was
not the case so what they did is they
need to find another way to figure out
whether the DNA that that is present in
the e.coli carrying heavy nitrogen or
carrying light nitrogen that was a
biggest question that they had in their
mind because they cannot check their DNA
sequence back then that's why this
experiment is far intriguing so what
they figured out is very simple process
known as cesium chloride gradient
centrifugation
so this centrifugation process is cesium
chloride concentration gradient
centrifugation in the concentration
gradient centrifugation like it depends
on the weight density gradient sorry not
concentration gradient it's density
gradient centrifugation so what happens
in density gradient centrifugation the
one carrying higher density will settle
bottom the one with lower density will
settle on above the the higher density
so what happens you know it you just
take this centrifuge tube and you put
this cesium chloride and cesium chloride
will follow and form what is known as a
gradient well let me show you the
gradient formation looks something like
this so from heavy to low this is the
idea of cesium chloride it can form this
gradient now not only the caesium
chloride forms the gradient but caesium
chloride along with any other molecule
given inside they will also find that
same gradient and will settle
based on their own density in the
caesium chloride gradient centrifugation
okay so that's what meselson and stahl
is going to do so what they did they are
going to extract the DNA simply they
they break the cell open okay they they
agitate the cell they break the cell
open from this medium and what they did
they load it into the gel but before
that why at the very beginning when they
are growing the e.coli
the heavy nitrogen they also take the
DNA from this e.coli and run it with the
gel so let's let's look at the actual
steps of the process the first step
while we are growing while they are
growing the e.coli heavy-light Regine
medium they are also extracting the DNA
from there and checking with density
gradient centrifugation what they found
out is a pattern like this in the heavy
region pattern in the heavy region only
there is only one band in the density
gradient centrifugation that's in the
heavy region now what they will do
because you know this heavy region band
you can also observe in the gel okay so
that's the first thing regarding the
heavy nitrogen then they transferred it
into a like nitrogen medium and I allow
them to grow for next generation so
after that after first generation of
division they also take the DNA from
there they also took the DNA actually
from there and they run again the same
process of density gradient
centrifugation and what they figured out
is that they found two separate band one
in heavy one in some ways somewhere less
somewhere less okay now they allow this
process to continue so they allow this
e.coli to divide and produce multiple
generations and then after two three
more generation they again break the
cell open took the DNA run this
centrifugation again
what did figured out earlier they got a
band here in a heavy they also got a
very faint band in heavy now also then
they have bands in there and now they
are finding a band here so earlier they
got a band in heavy then we saw L so
let's call it m medium and now they got
a banding light nitrogen content so as
we know this is from heavy to low so
same thing we can check with this
changing time of the experiment so at
the first point where we know all the D
means tagged with heavy nitrogen we got
a one band in heavy that's justifiable
fine
so that's justifiable parents this is
heavy all the parties
justifiable now the second time when we
transfer it to the light nitrogen
carrying medium and then after one round
of the division what we found one heavy
one medium so heavy nitrogen band is
there like heavy band is there plus one
bag a little less density so what that
means that mean that either of this is
true either conservative model or semi
conservative model should be true
because in both this model you see 50 50
percent of the band but in dispersive
also 50 50 percent but it dispersive you
will not find any concrete heavy band
because there is no separate concrete
band
all the bands are of medium density got
it all the bands in dispersive model
should give you medium density so he'll
never find this heavy and medium pattern
so this was not true we know that but
still we have a confusion between
conservative and semi conservative now
when once we transfer this and allow
this equal I to divide and grow in that
same light nitrogen medium for two three
four five six generations then we also
extract the DNA we find another band in
the light section and even broader band
beads more and more light strands are
being generated now what about this this
is unique now at this state if it's a
conservative model then you should have
one heavy and then one light because
this is totally here this is totally
light this is totally heavy so if DNA
replication follows conservative model
then after the first transfer you will
find only heavy and only light you
should never find a medium band because
either it will form heavy or it will
form a light
you should never fall medium band but
this is what we are getting here a
medium band so that means conservative
model is also not true that left us with
semiconservative model so what happens
actually in semi conservative
model what we can see first we have a
heavy band this is for the heavy
nitrogen guru this one then a medium
band Y a medium man because afterwards
what we can see that this species when
we have one heavy stand one light stand
this band this particular pattern of one
heavy one light together gives you this
medium band then the more rounds of
division means what will happen new new
and this
so from this red one this one is
generated from this red one this one is
generated from this parent this is
generated from this parent of this soon
every round we will increase the light
that's why the light band gets broader
but we also have this fixed medium band
that should be present that is the only
way to explain the pattern of DNA
replication and that's why we say that
meselson and stahl proved very precisely
that semiconservative model is the
actual model to explain the way DNA
replicates right if you placate where
the same process of one strand one
parental strand acting as a template
producing a daughter strength and that's
how it continues okay so that's an idea
of meselson and stahl experiment now you
can say stall and miss Elson experiment
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