Meselson Stahl Experiment Video Lecture | Biology Class 12 - NEET

one those Theo's muchachos today we're
going to be taking a closer look at the
mechanisms behind DNA replication well
to put it more precisely we're going to
have a look at an experiment conducted
by two brainbox biologists Matthew
meselson and Franklin Stahl in 1958 the
experiment widely meselson Stoll
experiment of course
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a parent cell divides to create two
daughter cells what's important about
this is both cells are provided with one
complete copy of the genetic information
after division in other words the DNA
must have copied or reproduced itself
make sure to take a look at the videos
on mitosis and meiosis to make sure you
understand the essentials on the subject
matter so this means previously one
parent cell and one set of parents DNA
now two daughter cells with two sets of
daughter DNA why we're four for what
reason and above all how the parent DNA
is divided over both daughter cells was
a complete mystery to the scientific
community at the time the two gentlemen
conducted their experiment biologists
posited three hypotheses the first
option conservative replication the
parent DNA is retained in one of the two
daughter cells and the other daughter
cell has to make do with an identical
copy the second option semiconservative
replication the daughter cells split the
spoils evenly both created cells get
half of the parent DNA with the other
halves being created anew the third and
last theoretical possibility is
dispersive replication which translates
into simple English as scattered or
distributed replication in this theory
each daughter cell would also retain
half the parent DNA however each
individual strand would contain a mix of
parent DNA and newly formed DNA right
that's enough theory for now let's move
on to the experiment but wait there's
still one thing you need to know
nitrogen isotopes were used for the
Maysles instal experiment nitrogen or
simply n is a vital component of our DNA
a nitrogen 15 isotope is heavier and
denser than a nitrogen 14 isotope easy
as pie the researchers first grew a
strain of e.coli bacteria on a culture
medium that exclusively contained the
nitrogen 15 isotope a strain is just how
we describe a group of the little guys
and a culture medium is a small flat
plastic dish with some goo at the bottom
so the e.coli bacteria was now starting
to grow on it and they integrated the
heavy nitrogen into their DNA the next
step involved transferring the bacteria
onto a culture medium with the lighter
night
14 isotope cell division set in after 20
minutes have you been paying attention
if you have then you should now know
that the parent DNA must contain the
heavier nitrogen 15 isotope and the
daughter DNA has to contain the lighter
nitrogen 14 isotope the researchers then
determine the density of the genetic
information of the first successor
generation the filial generation f1 to
do so they got themselves a centrifuge
through in the DNA and started spinning
just for a second let's summarize the
hypothesis of the conservative
replication if it is correct the density
gradient centrifugation would separate
the nitrogen 15 isotope and nitrogen 14
isotope DNA leaving the DNA visible on
two different levels
however the centrifuge treatment showed
quite different results so as anyone
realized what they might be the
bacterial DNA of f1 was retrieved at the
exact border of these two levels what
does this mean guys well it tells us
that the heavy parent DNA and lighter
daughter DNA did not split up which in
turn means that they are connected this
proves that the idea behind the
conservative replication was wrong so
let's not waste any more time worrying
about it this leaves two more theories
so let's just wait and see after another
20 minutes and a successive generation
things were once again looking
completely different to separate bands
could be identified with the upper band
showing the DNA that only contains
nitrogen 14 isotope or rather new DNA
the band at the bottom shows the DNA
that is lighter than the nitrogen 15
isotope but still heavier than the
nitrogen 14 isotope so this has to
contain both new and old DNA now let's
shift our focus to the DNA strands the
yellow 50% are completely new DNA
strands the sections shown in red is a
mix that consists of equal parts old and
new DNA semi-conservative replication
would then give us the following result
a completely new strand in the f2
generation and a double-stranded DNA
that has an old and a new sign when it
comes to dispersive replication we would
actually only see one band as the DNA
double strands of the new generation
would always look the same why because
the single strands of DNA would still
all
consists of both new and old fragments
of DNA the pieces of DNA would just
continue to pile up which is why
everything is turning evermore yellow
however the biologists were still not
entirely sure that's why the dynamic duo
decided to heat up the DNA of the first
daughter generation again for 30 minutes
at 100 degrees Celsius
the heating process split the
double-stranded DNA into two individual
strands then back into the centrifuge at
when and voila they could now also see
the nitrogen 15 isotope and nitrogen 14
isotope DNA in the individual bands and
one part that only contained nitrogen 14
isotope and one that only contained
nitrogen 15 isotope the two scientists
had thus proven that the DNA of daughter
cells receive one strand of the parent
DNA and one new strand after cell
division what are the most important
points to remember about this experiment
at the beginning there were three
hypotheses on the makeup of daughter
cell DNA after reproduction conservative
semi conservative and dispersive
replication the conservative hypothesis
postulated that one daughter cell
receives all the parent DNA while the
other gets coupled together patchwork
the semi conservative hypothesis
asserted that the daughter cells split
their parent DNA fairly while dispersive
replication predicted a colorful mix of
DNA
Mason installed took two nitrogen
isotopes of varying densities and let
ecoli bacteria take up the heavier ones
after the bacteria divided on a surface
that contained the lighter isotope they
managed to first disprove the
conservative replication hypothesis they
heated the bacterial broth to
subsequently rule out dispersive
replication you got all that if you
spent the entire video wondering what
exactly the deal is with replication
then boy do we have a real treat for you
just search for the next video see you
next time
you
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