In this visual lecture we will talk about what is the concept of non-disjunction of
chromosomes and how it leads to trisomy and monosomy and what these terms actually mean.
Hello I am Dr Aizaz from medicovisual.com welcome to this visual lecture. Let's suppose
this is a somatic cell and you know somatic cell consists of 46 chromosomes in human and
these 46 chromosomes they are in the form of pair that is we are having 23 pair of chromosomes
and each pair consists of, of course, two chromosomes one chromosome from maternal side
that is from mother side and the other chromosome that has come from paternal side, right. So,
we have 23 multiplied by two. Right, two sets of 23 chromosomes we are having and let's
suppose this is let's say primary oocyte or primary spermatocyte and this has to undergo
meiosis one and as it undergoes meiosis one, these homologous chromosomes they will separate
out they will disjoin in prophase of meiosis one, they are joined together and they undergo
the process of crossing over that is not shown in this diagram but you know they undergo
the exchange of their genetic material that is crossing over and after the crossing over
as the meiosis one ends they separate, they disjoin, these homologous chromosomes they
disjoint and one set of this, these homologous chromosomes out of these two chromosomes one
set goes to one cell and the other set, it goes to other cell. So, we will have two cells
after the process of meiosis one. That is what happens normally and after that these
cells, these cells will have now 23 chromosomes. So, initially we were having 46 chromosome
that is 22 pair of autosomal chromosomes and one pair of sex chromosomes. Let's suppose,
these are the autosomal chromosomes so we have 22 Autosomal, 22 pair of autosomal chromosome
and one pair of sex chromosomes and here then what will happen that each cell will receive
out of this pair one set of Chromosome. Of course, I cannot show you all the 23 chromosomes
here so I have just represented it with only six chromosomes in this diagram. Right, and
after and during meiosis 2 that is just like mitosis these chromatids will separate. These
chromatids will disjoin, they are joined here but they will then disjoin and each cell will
get one of the chromatid. Right, so that is what happens Normally. So, in the end we will
have 23 chromosomes in each of these gametes. That is what happens Normally. Now, let me
tell you another story that again in a cell it started the process of meiosis one and
now all these chromosomes they successfully disjoined but this chromosome it failed to
disjoin, it failed to disjoin so these two chromosomes they remain attached to each other,
so rather than each of these daughter cells getting one of the chromosome each, one cell
will get both of these chromosomes, one set will Get, one cell will get complete both
sets of Chromosomes, Both sets of homologous chromosomes. While this poor cell it will
not get any of the pair, will not get any of the chromosome out of the pair. So this
poor cell will be having, will be having less chromosomes and this process is called non-disjunction
of homologous chromosome During meiosis one. So, out of 46 chromosome this should be getting
23 chromosome but it has got an extra chromosome so it is now having 24 chromosome and this
poor cell it has got one less chromosome so rather than 23 it has now got 22 chromosomes.
This is, this process is non-disjunction. So, it is having this extra chromosome. Right,
let's look at it in a more diagrammatic and visual way. So, this is that, this cell and
in metaphase spindle fibres will arise and this spindle fibre should pull it here and
this spindle fibre should pull it Here. But, this does not happen here due to some unknown
reason and these two cells they remain joined and this spindle fibre pulls rather than this
single chromosome, it pulls both of these chromosomes. Are you getting the point? It
should be, it should have pulled only this chromosome and this chromosome should have
moved here, right, but here this thing does not happen. So, both of these chromosomes,
it went to this cell. So, this cell is having an extra chromosome. This is actually non-disjunction.
Clear? And, then meiosis 2, it goes on normally as it should happen but damage has already
been done. So, these two gametes, these are not, rather these all four gametes that are
now formed all of these are now faulty. These two gametes having extra chromosomes while
these two gametes having one less chromosome than normal. So, rather than 23 it is having
24 and rather than 23 it is having 20. Now, what problem, what problem could Arise? If
these faulty gametes, they fuse with a normal gamete, let's suppose, what happen, let's
see what happens. So, this is that faulty gamete with extra chromosome and this is a
normal gamete. Let's suppose this is a gamete from female side, that is let's suppose this
is an ovum and let's suppose this is a sperm. Of course, you Know by now if you are following
my lectures that sperm never looks like this. But I have represented it like a usual cell
for the sake of simplicity and ease of understanding. Now, when they combine, when they fuse together
during the process of fertilization, they will then form a zygote and this zygote it
is now having an extra chromatid here. You can see it is having this extra Chromatid.
Right, so rather than having 46 chromosomes it is now having 47 Chromosomes, then in interface
you know there will be Replication, there will be, each of these, each of these chromatids
their copy will be formed in front of it and they will form these complete Chromosomes.
So, now we have an extra Chromatid. Clear? So, here we can see that normally you know
each chromosome should have an other copy of that chromosome, Right, and there should
be two copies of chromosome as I have already discussed that, chromosomes must be in the
form of homologous pair. Right, but here rather than having two copies because of that extra
chromosome this chromosome is having an extra copy. So, rather than two there are three
chromosomes here. This is called Trisomy. Somy comes from chromosomes. So, rather than
normal diasomy or bisomy as in all other chromosomes here we are having Trisomy. Tri means three,
somy means Chromosomes. So, we are having three chromosomes. Normally there should be
disomy or bisomy but here is Trisomy. Now, let's imagine another situation, another scenario
that now this faulty chromosome, not Chromosome I mean this faulty gamete that is having one
less chromosome now this is gonna fertilize a normal gamete, fertilize and mate with a
normal gamete. And let's see the result as you can expect the zygote that will be formed
by fusion of these will be having one less chromosome. Here it is having one less chromosome
and it is now having 45 chromosome rather than normal 46. Now, in during interphase
again there will be DNA replication and if we try to pair these up these two will again
pair up nicely but this chromosome it is now lonely, there is only single chromosome rather
than disomy it is now which somy? It is monosomy. You should also participate in the Lecture.
So, it is monosomy rather Than disomy. So, let's compare these three scenarios. Normally,
when normal 23, 23 chromosomed Gametes, when they fuse, they will be having all these chromosomes,
will be having two Sets, there will be two sets of chromosome and there will be disomy
of each of these chromosomes but if there is an extra chromosome in one of the gamete
then it will, it will lead to trisomy and if there is one less chromosome it leads to
Monosomy. The trisomy could be of any chromosome. For example, if chromosome number 21 came
here, an extra chromosome 21 came here we will call it chromosome number 21 trisomy
or simply trisomy 21 that is down syndrome. Or it could be chromosome number 18. Similarly
Monosomy can also be of any of the chromosomes. These monosomies and trisomy, these are more
common in women and especially the woman of advanced age. Why so? Because you know that
women start preparing for their future generation even when they are in the womb of their mothers,
even before they are born. So, before the female is born they create the primary oocytes
and these primary oocytes they remain arrested in prophase one of meiosis. Right, and when
the puberty start then this female start releasing These, releasing the arrest of few of these
oocytes each month. She starts releasing the arrest of few of these oocyte. Now, let's
suppose out of these oocyte, this is the one oocyte which is destined to be released much
later in the life of that female, and because of This, this oocyte, it remained arrested
in prophase one of meiosis one for much longer duration as compared to the other oocyte that
were released earlier in her life. So, because it remain arrested for Longer, these homologous
chromosomes in the nucleus of the cell, they remain close together for much in longer duration,
right, and they are more accustomed to each other so it is more, there are more chances
that non-disjunction* will occur in such cell? Clear? So, that is why in female specially
of advanced age there is more chance of non-disjunction. Clear? Now, let's suppose that meiosis one,
it went uneventfully and nicely. Right, then what happened that in meiosis 2 this cell
also nicely underwent meiosis 2 but during meiosis 2 of this cell again these two chromatids,
now there is separation of chromatids in meiosis. These two chromatids, they fail to Disjoint.
Non-disjunction occurred here. Right, and one of these cell it got both of these chromatids
but this cell has not got any of the chromatid. But, it is a very interesting scenario you
know we measure, we estimate, we count the number of chromosome by counting the number
of centromere. Now, let me tell you an interesting thing how many centromeres are here? One,
two, three and how many centromeres are here? One, two, three. So, number of chromosome
here is normal in the cell, number of chromosome is normal. That is, it is 23. There is no
extra chromosome here. There is extra chromatid, but there is no extra chromosome. Now my question
is that here is the real mystery that why is that a problem still the number of chromosomes
is normal so why we should consider and we should consider this gamete faulty, but the
question is why we should consider this gamete faulty when number of chromosome is normal?
Let's see what happens, let's try to fuse this apparently normal gamete which is not
normal, which is faulty, let's try to fuse it with a perfectly normal gamete. Now both
of these are having 23 chromosome and when they will fuse, they will form zygote that
is also having 46 chromosomes again here another confusion that it looks like that number of
chromosomes is normal. So, it is a normal situation it is actually not the problem,
the problem arises during a phase of interphase. We have discussed that during a phase of interphase
again what happens that chromatids will copy DNA will replicate and copy of chromatids
will be formed. So another copy, copy, copy and so on but when DNA polymerase will come
here it will see that it is, it has to copy chromatids it has to copy chromatid so when
DNA polymerase will come here it will see that there are two chromatids so it will make
copy of one copy of each of the chromatids. So, one copy of this chromatid and one copy
of this chromatid. Right, so what will happen that we will have two chromatids here that
we will have a complete extra chromosome that will be formed in S phase of interphase of
this zygote. Right, now we are having actually a trisomy here. Let's see how? So, you can
see now that there are three chromosomes here rather than normal two chromosomes we are
having three chromosomes here and That’s situation is trisomy and now we have rather
than 46 we have magically we have got 47 chromosomes. Now, you know it does not happen magically
it happens During S phase of interphase. So, this was about non-disjunction of chromosomes
and how it leads to trisomies and monosomies, this numerical chromosome and I hope you got
the whole concept of this. Thank you so much for watching this video and if at any point
you feel any type of confusion please feel free to ask your question in the comment section.
Thank you so much for watching this Video. Please subscribe to this channel, please give
your comments regarding this Video. Please like this video and please support me on Patreon.