नमस्कार मित्रांनो इयत्ता बारावी Biology या Subject च्या Practical मध्ये PART - B: Demonstration Experiments (Spotting) हा भाग Practical Exam च्या दृष्टीने फार महत्त्वाचा आहे. या Blog Post मध्ये आपण Spotting Practical Number 7: Study of meiosis in onion flower bud with the help of permanent slides ह्या Practical Experiment चे Answer पाहणार आहोत. खाली दिलेल्या उत्तरांमध्ये काही अडचण असल्यास आम्हाला comment करा किंवा तुमच्या संबंधीत विषय शिक्षकांशी चर्चा करा.
B. DEMONSTRATION EXPERIMENTS (Spotting)
7. Study of meiosis in onion flower bud with the help of permanent slides.
Aim :- To study and identify different stages of meiosis.
Principle :-
Meiosis is a special type of cell division that occurs in the diploid reproductive or gem 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 can be studied with the help of stained permanent slides of T. S. of anther.
Permanent slides should be observed under the high power of the compound microscope.
Meiosis as a process occurs in two steps viz. A) Meiosis -I B) Meiosis -II
1. Meiosis-I : is a reduction division in which the chromosome number is reduced to half of the number of chromosomes present in the parent cell. It is completed into two stages- A) Karyokinesis I and B) Cytokinesis 1.
A. Karyokinesis I: It takes place in four sub stages.
1. Prophase I :- It is an initial and very lengthy phase, further divided into five sub-stages
i. Leptotene :- Chromosomes appear like long, thin, beaded threads.
ii. Zygotene :- It is characterised by pairing of homologous chromosomes called 'Synapsis' Paired chromosomes are called bivalents or tetrad.
iii. Pachytene :- Each bivalent or tetrad consists of four chromatids (two pairs) and each pair is united by a centromere. In this stage crossing over (X shaped chiasmata) occurs in between two non-sister chromatids of bivalent. Exchange of genetic material or chromatid segments 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 at the chiasmata.
v. Diakinesis :- Terminalisation i.e. shifting of chiasmata toward the end of the chromatid takes place Nucleolus and the nuclear membrane completely dis organize and disappear.
2. Metaphase I :-
i. An imaginary line (Equatorial plane) develops at the centre of the cell.
ii. The pair of chromosomes (Bivalents) arranged at the plane in such a way that they lie equidistant on either 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 I :-
i. It is an important stage of meiosis-I, as reduction division takes place in this stage i.e. chromosome number is reduced to half of the total number.
ii. Tactile (spindle) fibers 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 reappears.
iii. Nuclear membrane gets developed around such set of chromosomes, forming two daughter nuclei. This is how Karyokinesis-I comes to an end.
B) Cytokinesis I :-
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-I, two daughter cells are formed which are qualitatively different from each other but are quantitatively identical. Both the cells now undergo meiosis-II simultaneously.
The interphase following meiosis-I does not involve duplication of genetic material.
II. Meiosis - II : It is also divided into A) Karyokinesis II and B) Cytokinesis II
A) Karyokinesis II :- It consists of four sub stages
1) Prophase II :-
Events occuring in prophase-II are like mitotic prophase, however the chromosomes are recombinants.
i. It is the initial phase.
ji. 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) Anaphase II :-
i. Centromere divides and sister chromatids get separated which are now called as daughter chromosomes.
ii. Tactile (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, a single diploid cell gives rise to four daughter cells which later metamorphose to form either spores or gametes.
*Sometimes but rarely anaphase-I is directly followed by prophase-II so that telophase-1and cytokinesis-I are skipped.
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.
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| Fig. (a) Stages of Meiosis I |
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| Fig. (b) Stages of Meiosis II |
Sketch Diagrams :-
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| Fig 1. |
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| Fig 2. |
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| Fig 3. |
Questions
1. Differentiate :
a. Mitosis and meiosis
Ans :- Mitosis is an equational division that occurs in somatic cells. Meiosis is reductional division that occurs in germinal cells, mitosis results in two daughter cells while meiosis produces 4 daughter cells.
b. Anaphase I and Anaphase II
Ans :- In Anaphase I, homologous chromosomes separate, while in Anaphase II, sister chromatids separate.
c. Metaphase I and mitotic metaphase
Ans :- In Metaphase I, homologous chromosomes align as pairs on the metaphase plate, while in mitotic Metaphase, individual chromosomes align.
d. Zygotene and pachytene
Ans :- In Zygotene, homologous chromosomes pair (synapsis) to form bivalents, while in Pachytene, crossing over occurs between chromatids of homologous chromosomes.
2. Write significance of Meiosis.
Ans :- Meiosis ensures genetic variation through recombination and independent assortment, creating unique gametes. It also halves the chromosome number, maintaining stability across generations in sexually reproducing organisms.
3. Why should meiosis II occur, when a reduction in the chromosome number has already occurred in meiosis I?
Ans :- Meiosis II is necessary to separate sister chromatids, ensuring each gamete receives a single copy of each chromosome. This maintains genetic integrity and produces haploid cells with the correct chromosome composition.
4. What is crossing over?
Ans :- Crossing over is the exchange of genetic material between non-sister chromatids of homologous chromosomes during pachytene of prophase I in meiosis. It creates genetic variation by producing new combinations of alleles.
5. Enlist the different stages of prophase I
Ans :- i) Leptotene
ii) Zygotene
iii) Pachytene
iv) Diplotene
v) Dikinesis
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