Prepared by Zafar Ali Abro
zafar.abro1@gmail.com
Cell Division
The process in which the cell divided and replicate, on the basis for growth
and replication
There are two main types
mitosis or Karyokinesis
meiosis
MITOSIS
It is a type of cell division which involves the splitting of the chromosomes,
in this parent cell divided into two daughter cells, the numbers of
chromosomes are same as in the parent cell
Stages of mitosis
Prophase
First phase of mitosis, in which the DNA molecules of the chromosomes
condense and outer boundary of the cell is weak.
Characteristics
The nucleus of cell is disappears
The nucleus membrane disappears
Metaphase
second stage of mitosis, during this phase the chromosomes line up in the
center of the cell , become a pair of identical chromosomes , the
chromatids become un coiled and apart from each other
Anaphase
Third phase during this phase each set of chromosomes moves towards the
opposite end of the cell
Talophase
Fourth phase during this stage following events happen
· The spindle fiber disappear
· The nucleus membrane appears
· The cells divided into two daughter cells.
· Nucleus re-appears
· The chromosomes re appear and no longer visible
Significance of mitosis
1. it is main source of reproduction
2. mitosis is very important for wound healing which needs rapid mitosis
3. it is important for restoring the worn out cells of blood and skin, etc
4. It is the process in which a single zygote is converted into large number
of cells, these cells latter on form tissues and again tissues form organs
and organs form a body.
Meiosis
The cell division in which the number of chromosomes are reduced to half
of the number of the mother cell
This type of reduction takes place in sexual reproduction at the time of
formation of male and female gametes
Mother cells has 2N or diploid number of chromosomes, one set from
mother side one set from father side
Two haploid 1N gametes will unite to farm the same diploid 2N zygote or
embryo
Here are two consecutive division takes place
reductional or meiotic
ordinary or vegetative or mitosis
Hence at the end of the meiosis we get four haploid cells
Following steps are present
Prophase 1
Mere lengthy than ordinary prophase of mitosis, it has five stages
1. leptotene
Nucleus increases in size due to absorption of water (hydrogen)
@ Chromosomes become long, uncoiled threads
@chromosomes become more visible and large
2. Zygotene
Similar chromosomes attract each other and form a pair, as the
chromosomes have two chromatids, therefore in synapses there are four
chromatids
3. Patehytene
(1) The chromosomes become condense due to widening of coil
(2) They farm chaismata i-e cross each other in double nature or bivalents
4. Diplotene
Homologous chromosomes go apart from each other except at chaismata
Chromosomes become more short and thicker
Nucleus and nuclear membrane are still present
5. dikinisis
The bivant become more apart
Chromosomes become deeply stained
Nucleus and N membrane disappear and spindles become distinct
Metaphase
The chromosomes now rearrange themselves in an equatorial line, spindles
to the Centro some of the chromosome
Anaphase 1
Now spindles start to contract and try to split the tetrahedral chromosome
into chromatides towards each pole due to repulsion
Talophase 1
Now on each pole chromosomes of double nature come to daughter nuclei.
Nuclear propane and nucleus reappear, it will act as prophase 2 and second
consecutive division starts
prophase 2
Here the chromatin network breaks into double (bivalent) chromosome.
Nuclear membrane and nucleus disappear
metaphase 2
Double nature chromosome rearrange in equatorial line, two poles and
spindles are formed
anaphase 2
Spindle starts to contract, they try to split the chromosomes longitudinally
into two chromatids, and each chromatid will travel towards the pole
Talaohase 2
Each chromatids now called as daughter chromosome reach up to poles,
spindles disappear, and nuclear membrane and nucleolus reappear; now 4
Nuclei are formed as a result of meiosis
Importance of meiosis
It has great significance in those plants and animals which reproduce
sexually
The process of meiosis creates the genetic variation in species.
It creates hybrid vigour.
That it brings reduction in number of chromosomes i-e from diploid 2N to
haploid 1N
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