In the previous lecture I have already discussed what is meiosis And I have described different
phases or different stages of meiosis and from that lecture you must be knowing that
prophase one of meiosis is the busiest phase of meiosis many things happen in this phase
I have described that but because it is the busiest phase and it is most important phase
many authors, they love to subdivide it into different stages or different sub phases.
Right, so today we will discuss that what are the sub phases of prophase one. We are
going to discuss phases of prophase one, one by one so let's start. Hello I am Dr Aizaz
from medicovisual.com welcome to this visual lecture. So you know in meiosis there is the
concept of homologous chromosomes we have 23 chromosomes that came from mother, maternal
and 23 chromosome that came from Father. Right, and similar chromosomes from mother and father
they become like a couple they come close to each other and they exchange their parts
that happens in prophase one. Right, there is no demarcation of homologous chromosomes
in mitosis so here we have I have shown a single homologous chromosomes I have removed
others for the sake of simplicity. So in first stage of prophase, first of all what happens
in prophase it is similar to that happens in mitosis that you know before prophase start
before cell division start the genetic material the DNA is in the form of thread like structure
called chromatin. Right, but it then condenses when cell division start when prophase start
it further condenses genetic material or chromatin further condenses to form chromosomes. So
chromosomes is further supercoiled form of chromatin so that is the first stage that
is leptotene that is first phase of prophase one. Leptotene in which homologous chromosomes
rather all the chromosomes are formed from chromatin or you can say chromatin condenses
to form chromosomes now this is the first phase of prophase one how can you remember
that this is the first phase well it is very simple I think of it is that it is a beginning
of love story of a beautiful couple not just a single couple but 23 couples or 23 homologous
chromosomes so when a love story began to form in couple they love to put their head
into lap of each other, righ, and that is the first phase of love Story. And because
they are putting heads in lap that is called Laptotene. Actually, lepto means thin and
we say it lepto because initially the chromatin is thin and they condense to form Chromosome.
Right, but I am abusing this term in order to make you remember this that this is the
first stage. Right, so what happens in laptotene very simple that condensation of chromatin
occur to form chromosomes then what happens then as the love bond in between them becomes
stronger they become close to each other they become close to each other and they pair up
with each other end-to-end pairing and as they pair up with each other one of them lie
on top of the other, right, and when they do so a special bond is formed between them
and that is not just a bond of love but that is the bond of Proteins, a protein complex
is formed between paternal chromosome and maternal chromosome, homologous Chromosomes.
Right, and this, this complex that is formed it is called synaptonemal complex, nemal mean
thread, thread like complex synaptonemal complex and this process of pairing it is called Synapses.
This process of pairing of paternal chromosome with maternal chromosome this is called synapses
and it is demarcated by presence of synaptonemal complex and because they are pairing with
each other we call it Zygotene phase or zygotene stage. Zygo means to pair up with each other
so in zygotene they form, what happens in zygotene they form synapse and that is demarcated
by beginning of formation of synaptonemal Complex. Now what happens when a couple lie
on top of one another? Of course they will do something and while doing something some
material is exchanged in between them actually that is a genetic material a part of this
chromosome it will, it will go here it will strapped with this part of chromosome so you
can see here that this part of paternal chromosome it goes to this maternal chromosome and this
part of maternal chromosome it is, it lies here in paternal chromosomes so there is some
exchange of genetic material and this process of that is what I am calling doing something
is actually exchange, exchange of genetic material and it is called crossing over. So
in this phase in which crossing over occurs it is called Pachytene, they are packed up
with each other in order to do something I call it Pachytene not I call it scientists
call it I am not a scientist so that is Packaging. Then what happens, of course, after a couple
has done something they will have to separate from each other they cannot all the time remain
attached with each other lying on the top of each other so they have to detach from
each other and when they detach from each other this complex is lost and they are separated
from each other. Previously they had become a single entity they had become one. Right,
now they are separated and they are two different things, right, and we call it Diplotene. Di
means two, so they have become from one from being paired up in zygotene and packed up
in Pachytene now they are two and two different entities but remember they are still attacked
here so this phase is called diplotine but remember the part at which the exchange, the
exchange the genetic material at that part they are still attached with each other their
parts are still attached with each other and you can see they have formed a cross like
structure, here you can see this cross like structure is formed and this cross like structure
is called, what it is called chiasma Chiasma means cross so chiasma Formation, chiasmata
formation occurs in Diplotane. There are usually more than one chiasma and that is called chiasmata
formation. Right, so here after crossing over had occurred you can see still there is chiasma
here. Now interesting thing is that homologous chromosomes they are also called bivalent
this is one chromosome and this is other chromosome these are two so we also call them that this
is bivalent arrangement of chromosome this is bivalent arrangement of chromosomes but
how many chromatids are here one two three four so we have four chromatids so if we talk
about in reference to chromatids there are four chromatids so we call it as tetrad. So
homologous chromosomes are bivalent as well as tetrads. Bivalent as far as chromosomes
are concerned and Tetrads as far as chromatids are Concerned. Right, and after Diplotene
they further, they move apart from each other they move away from each other right, and
this is called diakinesis. Diakinesis mean to move through kinesis you know it means
movement kinetic energy you must have heard of that. So this was the phases of prophase
one. Finally in diakinesis what happens that nuclear membrane is also lost, so that in
subsequent phases that is in metaphase and anaphase they could easily separated from,
they could be easily separated from each Other. So here we don't have just a single couple
but actually we have 23 couples out of that I have only shown three couples, right, and
again they have done something and they are attached with each other at chiasma right
so let's review what Happens. In this first you will watch the complete animation of prophase
one then we will review it Synapses, crossing over [Music] Chiasmata formation. Right, do
you think it is difficult to learn these stages? It's very easy. First of all a couple sitting
in lap of each other and not sitting in lap of each other they are putting their heads
in lap of each other right that is laptotene they, they further compress they condense
into laps. Right, [Music] then after that, that stage comes when they come so close to
each other that pair up that they pair up with each other they become one so after laptotene,
they pair up with each other that is zygotene. Right, and that is after that they become
so much packed with each other that they do something, they cross over, their parts, right
and that is Pachytene and then they separate from each other that is Diplotene, they become
two and finally they further move away from each other I think that is a break up phase,
well that is not break up phase break up phase is anaphase when they are separated from each
other. Still they are attached and that is Diakinesis. Here's something important to
mention that when they move further away from each other this cross becomes more prominent
this chiasma becomes more Prominent. Right, this was about prophase one of Meiosis. Thank
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