Biology for Engineers and other Non-Biologists

Professor Madhulika Dixit

So hi! Now today we are going to talk about the process of meiosis, and as I had mentioned in
my previous video, we all know that we start our life as a single cell, and this single cell arises
because of the fertilization of egg by a sperm.

(Refer Slide Time: 01:55)

Now, what we have to remember is that when we start our life, we always get a set of
chromosomes from our mother, and a set of chromosomes from our father. So if we have twenty
three pairs; for each pair we are getting one chromosome from the mother, and one chromosome
from the father. So then, we are formed as a child and then as an adult, we end up having forty
six chromosomes.

But, once one attains puberty and is now ready for formation of gametes, we find that certain
specialized cells, called as the germ line cells, are capable of forming these gametes, so that we
can pass on our characteristics to our children. So these germ line cells will contain forty six
chromosomes or twenty-three pairs and then through the process of meiosis, these germ line cells
divide and they give rise to gametes, right, with each gamete having twenty three chromosomes,
wherein each chromosome is now represented only once. So, how does this process of meiosis
takes place is what we are going to study in today’s video.

(Refer Slide Time: 06:19)

But before I get there, I again want to re-emphasize the arrangement of chromosomes. As I had
mentioned during my mitosis video, the DNA normally condenses itself into chromatin with the
help of proteins which we call as histones. And then, each chromosome consists of a further
condensation of this chromatin. And every time a chromosome undergoes duplication through
the process of DNA replication, the duplicated chromosome is attached at the center with the
help of a structure called as ‘centromere’, where each of these then called as ‘sister-chromatids’.

Now what we are going to introduce today is one more term which is called as the homologous
chromosome. Now homologous chromosome is nothing but the pair. So let us assume this yellow
chromosome is what this part, right, this part of the yellow chromosome before the cell entered
S-phase was received from the father. Okay? Now after the process of DNA replication has
happened, the cell has undergone the S-phase, this chromosome that we had received from our
father got duplicated and you had the sister-chromatid form which is now attached with the
centromere.

Now the same chromosome, not identical though, but a chromosome which is coding from
similar features we'll also receive it from our mother, right, so let us say that this green coloured
chromosome was a chromosome that we had received from our mother. So both this one, right,
this part, is from the father, while this part we had received from our mother, and when this green
coloured chromosome which we had received from our mother underwent DNA duplication, it
again gave rise to sister-chromatid which is now the duplicated chromosome attached at the
centromere.

So such chromosomes, which are basically this chromosome from father, and then this
chromosome from the mother is called as the homologous chromosomes, because these
chromosomes contain a set of genes which code for similar characters, say for example hair
colour, where you will have a gene, having a chromosome having a hair colour gene from father,
and a chromosome having a hair colour gene from the mother. So this set is what you call as the
homologous chromosome, and then each of the homologous chromosome will then duplicate
itself during the S-phase of the cell cycle and as a result, each homologous chromosome will
have its own sister-chromatids.

So this is important to know. So what happens in meiosis, is that we are going to separate these
homologous chromosomes and we are going to each cell, each reproductive cell; mind you this
process of meiosis does not happen in all the cells of the body, it happens in those cells which are
capable of undergoing formation of gametes which are the cells found in testes and ovaries in
human beings or animals. So, these cells will undergo the process of reduction division and the
homologous chromosomes will get segregated into the gametes.
(Refer Slide Time: 07:30)

So the idea is, if you start with forty six chromosomes by the end of meiosis, you will have
daughter cells, you will have four daughter cells with each daughter cell containing twenty-three
chromosomes. So this is what happens in meiosis and we have to remember the terms, mainly
the homologous chromosomes and sister-chromatids.

So let me just erase this outer bit again, and then come back to what happens in meiosis a little
more in clarity, so that it clarifies your confusion again. Now, so the homologous chromosomes
are the chromosomes that we are receiving each set, one set receiving from mother, and one set
receiving from father. Now in meiosis, what happens is, there are, as I said, it is a reduction
division. So in what we are going to study in meiosis is that, meiosis itself has got two parts; it is
divided into meiosis one and meiosis two.

Now in meiosis one, the first part, the homologous chromosomes will get separated. So after
meiosis one, you will have daughter cells where these chromosomes would have segregated. And
then, in meiosis two, you will find that there are sister-chromatids which will get separated. So
we will come back to this a little from now.
(Refer Slide Time: 09:18)

So let us go back to what is a diploid cell. As I mentioned, we all start our life through the
process of fertilization where each gamete has one set of chromosomes. So such a cell is called
as a haploid cell. Right?

So a sperm coming from father will have only twenty-three chromosomes, so it is a haploid cell.
The sperm will then end up fertilizing an egg which again has a single set of chromosomes, and
again it is a haploid cell. After fertilization, when these two cells fuse, you end up getting a
diploid cell. So these will get mixed up, right? And then when this diploid cell undergoes DNA
replication at the S-phase of cell cycle, each of the chromosome will then give rise to its copy,
that is a sister-chromatid, it has attached at the centromere. Right? So, this is, this is going to be a
starting point, right?

So after the process of cell cycle has happened, the process of DNA replication has taken place,
we will be starting our process of meiosis from here. Okay? Now, so again meiosis has two
processes; first is meiosis one; in meiosis one, these homologous chromosomes, for example you
take this pair and this pair, they will first get separated and then you will have meiosis two, in
which each of the sister-chromatids will get separated. So it is a reduction division.
(Refer Slide Time: 09:30)

So, what is the importance of meiosis? See it is very important to maintain the chromosome
number, you know if this process of meiosis does not happen, then in the next generation, what
will happen is the mother cell will give rise to forty six chromosome and then the father cells, let
us say have forty six chromosomes and they are fused together, you end up getting ninety two
chromosomes.

Now that is almost increasing the entire characteristic by two-fold and in the subsequent
generation further increases, now that cannot be allowed right? So it has to maintain the same
genetic information, the same number of chromosomes and for that, it is important that before an
organism undergoes a process of sexual reproduction, its copy numbers are reduced to half, and
then, after the process of sexual reproduction, it is restored back. So meiosis plays a very
important role in sexual reproduction and it is this division which is responsible for gamete
formation.

Now, similar to mitosis, in meiosis, each step of meiosis one, or meiosis two is divided into
prophase, metaphase, anaphase, telophase, followed by cytokinesis one; so if once the cell
undergoes meiosis one, let us say it starts with one cell; at the end of meiosis one, it will have
two cells, alright? Then, each of these cells will then further undergo meiosis too, which again
has its stages of prophase, metaphase, anaphase, telophase, followed by cytokinesis, and an each
cell in turn give rise to two daughter cells. Same thing happens here. So at the end of meiosis,
unlike mitosis, you have four daughter cells with half the number of chromosomes. This is what
happens in the process of meiosis. So let us see how it happens.

(Refer Slide Time: 15:28)

Now the first step of meiosis one which is prophase one is the longest phase of meiosis. Now in
this phase, just like in mitosis, the nuclear envelope first disappears, that is why I have drawn this
dashed line representing disappearance of the nuclear envelope. Even the nucleolus, remember
nucleolus is the part of the nucleus which consists of ribosomes and other machinery also starts
disappearing, and then, you find that the pair of centrioles have duplicated and one of them has
reached the other end of the cell and then, these are leading to formation of spindles.

And these spindle formation is important because it will allow and help the chromosomes to
attach. So the chromosomes will start now condensing, as it happened in mitosis, the chromatin
will condense into chromosomes, and the homologous chromosomes, remember one each
coming from mother and one from the father, the pair, the homologous pairs will start coming
together during the process of prophase one. And as they come together, they, so here if you look
at this one, you find that one of them is from one parent and the green one is from the other
parent.

They they start coming together, at the same time, they also, the homologous chromosomes start
attaching themselves to the spindle formation. Here they are still not arranged at the equatorial
plane, and that happens in the metaphase; but in the case of prophase, there is one another
important thing which happens, which does not happen in mitosis, is that, when the homologous
pair of chromosomes come together. So assume this is one chromosome, and this is another
chromosome, when they come together, they tend to undergo a process called as synapse, or
cross over.

So you have a chromosome coming from father, and a chromosome coming from mother, and
they are sitting here together. So this is two sister-chromatids, there are two sister-chromatids,
they are coming together, and when they come together at the stage of synapse, there is an
exchange of genetic material between these chromosomes, and this process of exchange of
material is what is called as the cross over. Now that is important, because remember, I told you
one important reason why there was advantage of sexual reproduction during evolution, is to
bring in variation.

So what happens is because of this cross over, there is a genetic shuffling of some part of the
mother’s chromosome will exchange information with the father’s chromosome, and there is an
interchange of material; this process is also called as DNA recombination. Now what happens is,
because of this, what will happen is the new daughter cells, which will have these chromosomes
will not be an exact copy of the parent cells. So now because of the cross over, some characters
of the father’s chromosome have been exchanged with the mother’s chromosome and vice-versa.
So that kind of a shuffling of characters have happened, and that process is called as the cross
over.

So prophase one involves few things; disappearance of the nuclear envelope, disappearance of
the nucleolus, condensation of the chromatin into chromosome, homologous chromosomes
coming together, attaching to the spindle fibre, and as they come together, they undergo the
process of cross over. Now that is the most important difference from mitosis, because there the
homologous chromosomes are not pairing together for cross over, it happens only in meiosis one.
And thanks to this crossing over, new genetic shuffling takes place and as a result, these
chromosomes have exchanged information in material with each other.
(Refer Slide Time: 20:18)

Then you come to metaphase. Now in metaphase, that each of these homologous pairs, they
arrange at the equatorial plane, and you would find that by this time the cross over has already
happened, the material has been already exchanged. So for example, this one chromosome is
actually, it has already exchanged material with the other chromosome, and it has received some
features of the other chromosome. So this arrangement happens. Now it is not necessary that
every time the chromosomes for one parent will appear at one, will arrange at one half of the
cell, and the other set of chromosome, let us say coming from mother will arrange at the other
half.

It can, it is a random arrangement. So right now, though I have shown in this slide, green
chromosomes on this side and the red chromosomes on that side, it is not necessary. It can just be
that this pair can be flipping over. so if this pair flips over, so this side can flip over and the red
chromosome can be at this end and the green chromosome can be at the other end, it does not
matter, but this arrangement is a random arrangement, and thanks to this random arrangement,
you end up getting independent assortment.

Now let me give this to you with an example, now in this case, we have taken two pairs of
chromosomes; so, each pair of, so you have two chromosomes, so there are 2 2 possibilities by
which these four chromosomes can arrange themselves. There are four different ways in which
these chromosomes can arrange themselves with two red chromosomes on this end, two green
chromosomes on this end, or, this green chromosome arranges here, and the red chromosome
arranges here, while this remains this way, or, it is this green chromosome going up, and then this
red chromosome going down, it does not matter.

The point is, these four chromosomes can arrange themselves in permutations and that itself is
called as independent assortment. Now imagine this we are talking with just two pairs of
chromosomes, if you have twenty-three pairs of chromosomes, you have 2 23 possibilities in
which, these chromosomes can arrange at the equatorial plane, and this is close to about eight
million different combinations, right?

Now that is interesting, and I will come back to this a little later. You have ten to the power six,
eight into ten to power six different possibilities in which after the cross over has taken place, the
homologous pairs can arrange themselves. So that happens in metaphase. Having done that, and
this independent assortment and this multiple combinations is what leads to variations, alright?

And so there are two things which leads to variation; one, as I mentioned, is the cross over, and
this cross over happens during prophase one of meiosis one, and the second one is the
independent assortment of the homologous chromosomes. That in turn gives rise to about eight
million different possibilities in which these chromosomes will arrange themselves along the
equatorial plane. So that happens in metaphase.

After they have arranged themselves at the equatorial plane, just like in mitosis, in anaphase,
these homologous pairs start moving apart. Now you will notice that this chromosome, thanks to
the crossing over has changed, it is not the complete red chromosome, what it was in the
beginning. Similarly, this chromosome has received some information due to the cross over. So
you find that this is introducing variations.
(Refer Slide Time: 21:45)

So during anaphase, the chromosomes will move apart, and by the time they come to telophase,
you start seeing that the nuclear envelope starts reappearing, and, the daughter nuclei are getting
formed, and one pair of centrioles on one end, another pair of centrioles at the other end, and this
would be followed by formation of a cleavage furrow after telophase one, leading to cytokinesis
and two daughter cells.

Now what you will notice is that in these daughter cells, you have received slightly newer
version of the chromosomes, thanks to the process of genetic shuffling or the synapse. So, the
meiosis one ends with the separation of homologous chromosomes, so this is one part, this is
another; so this is one homologue, this is the another homologue, and mind you again, each of
these homologues are slightly different from their starting material because of the cross over.

Now after cytokinesis one, there is a brief period before which the cell again goes back to
meiosis two, and then again, it just loosens up the DNA, which is the chromatin, and then,
immediately, there is no more further DNA replication now. Whatever DNA replication had to
happen has already happened during the S-phase of cell cycle, it is not happening anymore.
(Refer Slide Time: 25:10)

Now, after cytokinesis one, the cell then undergoes the process of meiosis two, and meiosis two
again, as I said, contains prophase, metaphase, anaphase and telophase. Now each of these cells,
so you will notice, here the homologous pairs are separated, Now in this, meiosis again, this cell
again undergoes the process of prophase, where the nuclear envelope disappears each
chromosome, still attached to its centromere. It is attaching to the spindle fibre and it aligns to
the equatorial plane during the metaphase.

So, you find that in prophase, the chromosome start attaching to the spindle fibre through the
centromere, they will arrange at the equatorial plane at the metaphase, and then at the anaphase,
they start separating apart. And, by the telophase, and then, followed by cytokinesis, which have
not shown in this cartoon, but it is like a continuous process. After the metaphase, you have the
anaphase taking place, then you find that the two chromatids have separated.

So this chromatid, so what has happened after the anaphase has taken place, after the telophase
has taken place; this chromatid has gone onto this side, and this chromatid has gone onto the
other side, and then the telophase happens, cytokinesis takes place, and now you have the new
daughter cell with the chromosomes.

Now you will notice that the chromosomes are different; for example this daughter cell has
received a different version of the chromosome than the starting material. So, you find, same
thing happens with the other cell. This cell also undergoes the process of prophase, where the
nuclear envelope disappears, spindle formation takes place, the chromosomes with the sister-
chromatids is attaching to the spindle fibre; then the metaphase happens where each of these
chromosomes try to arrange at the equatorial plane, followed by anaphase, at which each of these
chromatids will separate, they will reach the polar ends in the telophase, and this will be then
followed by cytokinesis.

So what has happened is after meiosis two, you end up getting four different cells. With half the
number of chromosomes, and most importantly, none of them, none of these gametes are an
exact copy of each other. And why it is so? The reason they are not an exact copy of each other is
thanks to the crossing over which has taken place in prophase one of meiosis one.

And this is one of the reasons why, though we have characters which are similar to our parents,
we are still not an exact carbon copy of our parents. We may receive some features from our
parents, and some features from our mother, and even the features that we receive from our
mother is not an exact carbon copy, it is still a slight variation, because of the process of crossing
over.

So what have we learnt in today’s video is that meiosis is a process which allows for reduction of
chromosome, it happens only in the reproductive cells which are responsible for formation of
gametes, and meiosis is divided into two stages, meiosis one and meiosis two. In meiosis one, the
homologous chromosomes get separated, while in meiosis two, the sister-chromatids get
separated.

In meiosis one, prophase one is the longest step and it is one of the most critical step which sets
it apart from mitosis because it is in prophase one that the homologous pairs come together, they
undergo a process of crossing over, and because of the process of crossing over, you introduce
new variations. And that is the reason why none of us are an exact copy of our parents, though
we have characters which are similar to our parents.

The other important thing to note is that in meiosis one, during the process of metaphase, there is
various, 223 possibilities by which these homologous chromosomes can arrange themselves. Now
that is a huge combination of independent assortment, which further tells you why two different,
two siblings of a given parents are not exact copy of each other. So, meiosis is very important in
the process of cell division because it is meiosis which is responsible for variation, and for
maintaining the chromosome number from one generation to another, because it is meiosis which
is responsible for, for first reducing the chromosome numbers into the gametes, and then its the
sexual reproduction which brings back these chromosome numbers to the constant number for a
given species.

(Refer Slide Time: 27:50)

I hope this video has been helpful; I would also recommend you to go through some of the
interesting videos on YouTube. Some of them have got very good animations; I would like you
to go through them which beautifully explains the process of meiosis. Thank you and see you
later.