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Dr
...


During cell reproduction or cell
division, hereditary
transmission
occurs i
...


 Healing and tissue repair are important function of cell
division

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 For unicellular organisms, cell division reproduce an entire
organism e
...
in case of amoeba
...


IMPORTANT
TERMS

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CELL CYCLE
Cell cycle is sequence of events from the time of formation
of a cell up to the time when this cell enters the dividing
phase is known as cell cycle
...


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



Interphase= It is very
long phase
Mitotic phase =Dividing
phase
Cytokinesis= Short phase

INTERPHASE
 Interphase is quite long and during this phase cell prepares itself for
the cell division
...

 In this phase cell synthesizes various biological molecules like
carbohydrates, proteins, lipids and nucleotides
...
e
...

 Some proteins which are required for the formation of spindle fibers
and astral rays are also synthesized
...

1
...

2
...

3
...


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What is
G0 phase?

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Gap

phase

or

G1

phase S phase (6 to 8 hrs)

G2 phase (3 to 4 hrs)
...


Mitochondria and chloroplast get

because during this phase process as compared to G
1
replication or duplication of
phase
...


place
...


successive completion of S
phase

Enoplasmic reticulum, ribosomes,
centrioles etc
...


G2 phase indicates the

RNAs are also synthesized i
...

t RNA, m RNA
...


Histone

proteins

synthesized
In other words, it can be said that

which

are
are

associated with the chromatin

the growth of the cells occurs during to form nucleosomes
...


In G2 phase cell size
increases in which nucleus
also grow, almost all the
cell organelles increase in
number
...

At the end of this phase some
stimulus is received by the cell and

In G2 phase, RNAs and

G1 phase leads to S-phase
...


proteins that are required
for spindle formation are

In

G1 phase, there is an synthesized
...


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MITOTIC PHASE

 Cell division is a mode of multiplication in unicellular organism and
mode of growth in multicellular organisms
...
These cells are said to
be in G0 phase
...
e
...
g
...

 There are present 3 different modes of cell division
1
...
Mitosis
3
...

 This was first observed by Robert Remek in the RBC of chick
...
e both
organelles and the DNA are not equally distributed among the
daughter cells
...

 Division of the nucleus is known as Karyokinesis
...
In normal division Karyokinesis
occurs earlier cytokinesis
...
Later on another
German biologist Fleming observed the same in 1879
...

 Mitosis is a type of cell division in which two daughter cells are
formed from a single parent cell having equal number of
chromosomes and equal genetic information as present in parent cell
...

 So the daughter cells which are formed from a single parent cell have
almost same amount of genetic information and are identical with
their parents
...

 Mitosis is further divided into 2 steps:
1
...
Cytokinesis =

Division of Cytoplasm

Karyokinesis is a continuous process but for the sake of convenience it is
divided into 4 sub stages
...

1
...
Metaphase
3
...
Telophase
Karyokinesis

Dr
...
Early prophase
2
...
Late prophase

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The detailed process of Prophase is as follow:

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Early prophase

Middle prophase

Late prophase

Cell becomes somewhat rounded

Nucleolus starts disappearing Nuclear membrane starts

due to the depolymerisation of

and

skeleton microtubules
...


and shortens and finally

its

components

get disappearing
...
e star like structure
...


becomes thick and short
...


the cell cycle start moving towards

formed between the pairs of
centrioles
...

Nuclear membrane finally
As the pair of centrioles move apart,

disappears setting the

the microtubules present between

chromosomes free in

these pairs get stretched and

cytoplasm
...


The nucleus contains long, thin
thread like structures called
chromatin material
...

The centriole pairs reach their
respective poles
...

 Chromosome which are present irregularly in the cytoplasm become
more or less regular i
...
all the chromosomes come to lie at the
equator to form the metaphase plate
...
It lasts for 2-3 mins
...


reach their respective

Centromere
...


These sister chromatids start
moving towards their poles
...

 Spindle microtubules get depolymerised and starts disappearing
 Nuclear envelope starts appearing around the daughter chromosomes
 Nucleolus also starts appearing from the nucleolar organizing parts of
chromosomes
...


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After mitotic phase there occurs the Cytokinesis

Cytokinesis is different in plant and animal cells

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Because of thick cell wall,
cleavage or furrowing never
occurs
...
This chain of vesicles is
known as Phragmoplast
...


These vesicles also bring
precursors of polysacchrides i e
...

Cell plate which is actually the
middle lamella
...


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Cytokinesis is different in animal cells :
 In animal cells cytokinesis occurs by means of cleavage or
furrowing
...


 This constriction is brought about the peripheral band of
Actin and Myosin microfilaments
...


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18

Types of Mitosis

Internuclear and
Extranuclear
Mitosis occurs in
unicellular organisms
eg
...
In these
steps of mitosis occurs
within the nuclear
envelope
...

In multicellular
organisms mitosis occur
with the disappearance
of nuclear envelope this
is known as Extra
nuclear ie
...


Anastral and
Amphiastral/Astral mitosis
Plants cell lack
centrioles
...
So
mitosis without the
formation of asters is
called anastral as
compared to amphiastral
in which aster formation
is there eg in animal
cells
...

 This division occurs in certain special cells of the body e
...
sex cells or
gamete cells and daughter cells formed as a result of meiosis are called
Meiocytes
...
These are
Gametocytes (Spermatocytes and Oocytes) and Sporocytes:
 Spermatocytes are sperm forming cells
...

 Sporocytes are spore forming cells in plants
...
Meiosis I
and Meiosis II but the
chromosome duplicate
only once before the
Meiosis I
...

ie
...

But Meiosis II is the
equational division ie
...


Dr
...
It is
further divided into 4 sub stages:

Prophase 1 is quite complicated as compared to prophase of
Mitosis and is further divided into 5 sub divisions
...


2
...


Crossing over
takes place

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4
...


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Prophase 1 of Meiosis
Leptotene

Zygotene

Pachytene

Diplotene

Diakinesis

Centrioles which are

All the

The non sister

Homologus

Nuclear membrane

already duplicated

homologus*

chromatids

chromosomes

finally disappears
...


come to lie

around each

from each other

Spindle microtubules

side by side in

other
...


process is

known as

because of the

arranged around these This pairing of

known as

Chiasma
...


forces which

envelope
...


chromosomes

keep two

Microtubules extend

during

homologus

between the two

Zygotene stage

chromosomes/

Homologus

opposite pairs of

is known as

Non sister

chromosomes come to

centrioles as spindle

Synapsis
...


Microtubules get

microtubules
...


Chromatin material

Each

starts becoming thick

chromosome

and short and

of a

Non sister

becomes visible
...


each other after

These two

exchanging

chromatids of

their genetic

a single

material

chromosome
are known as

Nuclear

Dyad
...


homologus
chromosomes

Nucleolus

form the

finally

Tetrad
...


During
zygotene stage
Nucleolus also
starts
disappearing
and homologus
chromosomes
further become
thick and short

Dr
...


to lie at equator to

towards one pole and its

form two meta phase

homologus moves towards

Nucleolus starts appearing and

plates
...


reach their respective poles
...


spindle microtubules

apart the length of the

appears between the

interzonal spindle

Spindle microtubules start

chromosomes and

microtubules go on

disappearing and finally

the centrioles

increasing and that of

disappears
...


Nuclear membrane and nucleolus
completely appears
...


This is followed by Cytokinesis

Dr
...

In order to reduce
the DNA content
half these cells
undergo Meiosis II
...

During this
interkinesis
duplication of
DNA and
chromosomes
never occur
...


Prophase II

Metaphase II

Anaphase II

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Telophase II

Summary of meiosis

33

Significance of Meiosis

34

35

Third type of Meiosis is Sporogenetic Meiosis
 This type of Meiosis occurs in almost all the higher plants and
some lower plants also
...

 These spores are haploid and can give rise to haploid plants
after germination
...
It helps in formation of gametes i e
...

2
...

3
...

4
...
These variations
ultimately produce evolution of a species
...
Manpreet Kaur

36



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