Std12th Bio Pract Part B Spotting No 7
7. Study of
meiosis in onion flower bud with the help of permanent slides.
Date:
Alm To study
and identify different stuges of meincis.
Principle:-
Meiosis is a
special type of cell division that occurs in the dipkiid reproductive or germ
tissue (cells). It is a reduction division that results in the formation of
four haploid daughter cells each receiving half of the total number of
chromosomes than those present in the mother cell from with it is produced.
Observation-
For the
preparation of meiosis slides, generally anther of Lily or Onion is used
Meiosis cun be studied with the help of stained permanent slides of T. S. of
anther.
Permanent
slides should be observed under high power of compound microsсоре
Meiosis as a
process occurs in two steps viz. A) Meiosis -1 B) Meiosis-II
L
Melods-Iris a reduction division in which chromosome number is reduced to half
of the number of chromosomes present in the parent cell. It is completed into
twostages A) Karyokinesis I and
B)
Cytokinesis L
A.
Karyokinesis E It takes place in four sub stages.
1. Prophase
I:- It is an initial and very lengthy phase, fiurther divided into five
sub-stages
1.
Leptotene: Chromosomes appears like long, thin, beaded threads.
IL Zygotene:
It is characterised by puring of homologous chromosomes called 'Synopsis Puired
chromosomes are called bivalents or te trad.
III.
Pachytene: Each bivalent or tetrud consist of four chromatids (two pairs) and
each pair is united by a centromere. In this stage crossing over (X shaped
chiasmato) occurs in between two non-sister chromatids of bivalent. Exchange of
genetic material or chromatid sogments takes place which result in
recombination that leads to variation and thus the evolution of the organism.
iv.
Diplotene :- After crossing over, homologous chromosomes start repelling from
each other but remain attached of the chinsmoto
v.
Dialdnesis:- Terminalisation Le shifting of chiamata toward end of chromatid
takes place. Nucleolus and nuclear membrane completely disorganize and
disappear.
2. Metaphase
IS
i. An
imaginary line (Equatorial plane) develops at the centre of the cell
it The pair
of chromosomes (Bivalents) arranged at the plane in such a way that they lie
equidistunt
oneither
side of plane, their arms towards the equator and centromere towards the pole
iii. At the
end, spindle formation takes place which helps in arrangement and movements of
chromosomes
iv. Members
of each bivalent are connected to only one of the two poles but opposite poles.
3. Anaphase
l
i. It is an
important stage of meiosis-1, as reduction division takes place in this stage
Le chromosome number is reduced to half of the total number.
i. Tactile
(spindle) fibres start condensing, become shorter and pull chromosomes
(homologues) toward opposite poles.
iii. It
results in the separation and dragging of recombined homologous chromosomes
towards opposite poles
4. Telophase
I :-
i In this
phase chromosomes reach the opposite poles
ii.
Nucleolus gets reorganized and then reuppeurs.
iii. Nuclear
membrane gets developed around each set of chromosomes, forming two daughter
nuclei. This is how Karyokinesis-I comes to an end.
B)
Cytokinesis 1s
1. It occurs
by formation of the cell plate in plant cell, which is formed at the equator of
the dividing cell.
2. Two
daughter cells formed are having single, haploid nucleus each,
This is the
end of meiosis-1. At the end of meiosis-l, two daughter cells are formed which
are qualitatively different from each other but are quantitatively identical.
Both the cells now undergo meiosis-II simultaniously.
The
interphase following meiosis-i does not involve duplication of genetic material.
IL
Melosh-II: It is also divided into A) Karyokinesis II and B) Cytokinesis II
A)
Karyokinesis II > It comists of four sub stages
1) Prophase
II:-
Events
occuring in prophase-II are like mitotic prophase, however the chromosomes are
recombinants
It is the
initial phase
ii. The
nucleus enlarges
iii.
Chromosomes becomes prominent, shorter and thicker.
iv.
Nucleolus and nuclear membrane disappear.
2) Metaphase
II:-
i In this
stage, an imaginary line develops forming the equatorial plane which is at
right angle to the previous division
ii.
Chromosomes get arranged along the equatorial plane in such a way that the
centromere lie on the plane the arms towards the poles.
iii. At the
end spindle formation takes place.
3) Anophane
II-
i Centromere
divides and sister chromatids get separated which are now called as daughter
chromosomes.
ii. Toctile
(spindle fibres) fibres start condensing becoming shorter and shorter pulling
daughter chromosomes up to the opposite poles.
4) Telophase
II-
i. Daughter
chromosomes get collected at the opposite poles
ii.
Reappearance of nucleolus and nuclear membrane results in the formation of two
daughter nuclei
At the end
of Karyonesis II, four nuclei are formed.
B)
Cytokinesis II:
1. In plant,
cell plate formation takes place at the centre of the cell.
2. It forms
four daughter cells called as tetrads.
Thus in the
process of meiosis, single diploid cell gives rise to four daughter cells which
later metamorphose to form either spores or gametes.
Significance
of Meiosis :- Crossing over at pachytene, alignment of bivalents at the equator
and the anaphasic-I separation, eventually lead to variations.
It helps in
restoring the chromosome number of a species that remains constant from
generation to generation.
Questions
Question 1.
Differentiate:
a. Mitosis
and Meiosis
Answer - Meiosis
a type of cell division that results in two daughter cells each having the same
number and kind of chromosomes as the parent nucleus, typical of ordinary
tissue growth.
b. Anaphase
I and Anaphase II
|
Anaphase I |
Anaphase II |
|
1 . Anaphase I occurs in deploid cells |
1. Anaphse If occurs in haploid cells |
|
2.It results in the seperation of
chromosomes |
2. It results in the seperation of
sister chromatids. |
|
3.In this one centromere remains
intact. |
3. In this two centromeres gets split. |
|
4.In this chromosome seperates to
opposite poles and sister chromatid remains together |
4. In this two chomosomes gets
seperate splitting the centromere and sister chromatids. |
c. Metaphase
I and mitotic metaphase Metaphase I of meiosis
|
Metaphase
I |
Metaphase
of mitosis |
|
1. Chromosomes form two parallel
plates in metaphase I |
All chromosomes form a single plate in
metaphase |
|
2.On equatorial plate, chromosomes
appear four threaded in metaphase I |
On the equatorial plate, chromosomes
appear two threaded. |
|
3. Tetrad of chromosomes lie at the
metaphase plate. |
Individual chromosome align at
metaphase plate |
|
4. The chromosomes of two pairs are
put together. Thus, crossing over occurs. |
The chromosome of only one pair are
put together so, no crossing over occur |
|
5. Centromeres of chromosomes remain
undivided. |
Centromeres of chromosomes align at
the equatorial plate. |
|
6. Chromosome forms loop. |
Chromosome do not form loop |
d. Zygotene
and pachytene
|
Zygotene |
Pachytene |
|
1. is the second sub-stage of prophase
during which homologyous chromosomes pair with each other aligning their
corresponding genetic information |
is the sub-stage of prophase-1 during
which the genetic combination takes place between non-sister chromatids of
homologous chromosomes. |
|
2. third sub-stage of prophase 1 |
second sub-stage of prophase-1 |
|
3. crossing over of genetic material
occures between non-sister chromaids |
synapsis begins with a synaptonemal
complex, forming between homologous chromosomes. |
Question 2.
Write significance of Meiosis.
Answer - Meiosis
is responsible for the formation of sex cells or gametes that are responsible
for sexual reproduction. It activates the genetic information for the
development of sex cells and deactivates the sporophytic information. It
maintains the constant number of chromosomes by halving the same.
Question 3. Why should meiosis II occur, when a
reduction in the chromosome number has already occurred in meiosis I?
Answer - Meiosis comprises of two divisions- meiosis I
and meiosis II. The meiosis I is reductional division in which the number of
chromosomes are reduced to one half. Grossing over occurs during meiosis I. The
meiosis II is simple division like mitosis and does not involves crossing over
or reduction in number of chromosomes. The meiosis II serves to increase the
number of daughter cells from 2 to 4.
Question 4.
What is crossing over?
Answer - Crossing over is the exchange of genetic
material between non-sister chromatids of homologous chromosomes during
meiosis, which results in new allelic combinations in the daughter cells.
Question 5. Enlist the different stages of prophase I.
Answer - Leptotene
Zygotene
Pachytene
Diplotene
Diakinesis.
4. Study of flowers adapted to pollination by differentpollinating agencies (Wind and Insects)
PART B Question No 04 Spotting Practical Experiment No 3
5. Study of V. S. of anatropus ovule through a permanent slide/ relevant chart.
6. Study of T. S. testis, T. S. ovary and V. S. of blastula, through permanent slides.
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