a

Spindle, Centrosome, centrioles, chromosomal segregation

in the last few lectures we understood

the terms chromosome chromatid and

sister chromatids we have also covered

the cell cycle and interphase in detail

we now understand that a cell prepares

itself for the cell division this

preparation involves two major events

the cell growth hand duplication of its

chromosomes all this preparation takes

place in the interface of the cell cycle

once the cell is ready it enters the M

phase

recall that mitosis is the process of

cell division in which the duplicated

chromosomes of a cell are separated into

two nuclei suppose this is the cell at

the end of interface for our

illustration we are showing only two

duplicated chromosomes once this cell

enters mitosis the sister chromatids of

each chromosome will separate and get

equally distributed in two nuclei now

the question here is how these sister

chromatids get separated

the answer is mitotic spindle

Kotik spindle is a structure in the cell

that is responsible for organizing and

sorting of chromosomes during cell

division it is made up of microtubules

centrioles and centrosomes in animal

cells we will start our lecture from

finding out what are microtubules

what are centrosomes and centrioles

and how they form the final structure of

mitotic spindle that results in

chromosome sorting during cell division

let's begin every cell has its own

structural framework it is made up of

Manute fibrous protein filaments this

structural framework of the cell is

known as cytoskeleton there are three

families of protein filaments that

together form the cytoskeleton of the

cell these are actin filaments

intermediate filaments and microtubules

actin filaments determine the shape of

the cells surface as we will see later

these filaments play important role in

the cytokinesis of animal cells

intermediate filaments provide

mechanical strength to the cell

microtubules direct intracellular

transport they also determine the

positions of cell organelles and most

importantly they form the mitotic

spindle so we can see that mitotic

spindle is actually made up of

microtubules

let's understand this in more detail

microtubules are hollow tubular

structures that are assembled from the

protein tubulin

they are made up of 13 protofilaments

these protofilaments align side-by-side

in a circular pattern to form a single

microtubule

each proto filament is a linear polymer

of tubulin dimer

and this timer is made up of alpha

tubulin and beta tubulin

the arrangement of the 13 protofilaments

in a microtubule is such that one end of

this microtubule is terminated by a row

of alpha tubulin subunits

and the opposite end is dominated by a

row of beta tubulin subunits

the end of microtubule having beta

tubulin subunits is known as the plus

end

and the other end having alpha tubulin

subunits is called the minus end

now what does the designation plus and

minus means here microtubule can grow in

linked by adding more tubulin dimers

also they can disassemble by losing

tubulin dimers thus the ends of

microtubules can grow and shrink

Plus designation tells that the

accumulation and release of tubulin

dimers at the plus end is much higher

relative to the minus end

microtubules grow out from a specific

intracellular location this

intracellular location is known as the

microtubule organizing Center

microtubule organizing Center is well

defined in animal cells there is a

single such Center in animal cells and

it is located near the nucleus

this microtubule organizing Center in

animal cells is known as centrosome

if we zoom in we will find that the

centrosome consists of an amorphous

matrix of fibrous proteins and this

matrix is organized by a pair of

centrioles

centrioles are cylindrical structures

present or embedded in the centrosome

centrioles are also made up of

microtubules each centriole is composed

of nine sets of triplet microtubules

arranged in a ring

these two centrioles are arranged at

right angles to each other from the

centrosome microtubules grow out

the minus ends of all microtubules are

embedded in the centrosome and the plus

ends point outward the plus ends are

free in the cytoplasm

the centrosome duplicates when the cell

enters the cell cycle

when the cell reaches mitosis there are

two centrosomes

let's understand how this duplication of

centrosomes takes place this is our cell

with single centrosome when the cell

enters cell cycle the two centrioles of

the pair separate during the g1 phase of

interphase centrosome duplication begins

in the S phase the two centrioles

separate and each centriole is

duplicated so we have now two pairs of

centrioles each containing one mother

and one daughter centriole the two pairs

of centrioles remain together at one

side of the nucleus until the cell

enters mitosis

the centriole pair start to separate

when the cell enters their mitosis or M

phase

and as to century-old pay move apart the

spindle begins to form

the microtubules start to grow out of

the separating centrosomes

at the middle of the process of mitosis

the two centrosomes are at the opposite

ends of the cell

these centrosomes now are the two

spindle poles

microtubules radiate outward from

centrosomes in all directions

these microtubules forming the structure

of mitotic spindle are further of three

main types

the first type of microtubules is known

as astral microtubules

astral microtubules radiate outward from

each spindle pole and contact the outer

side or cortex of the cell these

microtubules help to position the

spindle in the cell

the second type of microtubules are

known as interpolar microtubules

as you can see in the image the plus

ends of some microtubules are

overlapping with the plus ends of

microtubules from the other pole these

microtubules overlapping with each other

in the midst own of spindle are called

the interpolar microtubules

the third type of microtubules are known

as they can need to call microtubules

these are most important one because

these are the microtubules whose plus

ends I'll touch two sister chromatids of

a chromosome

the plus ends of these microtubules are

attached to these sister chromatids at

the centromere region to large protein

structures are present at the centromere

at the side of each sister chromatid

these structures are called can needle

course

the microtubules actually get attached

to these canítö cores

therefore these microtubules are known

as can either call microtubules

that's all in today's video lecture

I hope you have now a clear idea about

the mitotic spindle this concept will

help us in understanding the detail

process of mitosis in our next lecture

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