This Class 12 Biology Practical focuses on studying meiosis in onion bud cells using a permanent slide. Students observe different stages of meiotic cell division, including prophase I, metaphase I, anaphase I, telophase I and the stages of meiosis II. This activity helps learners understand reduction division, formation of haploid cells and its importance in sexual reproduction. By identifying clear slide observations, students strengthen their concepts of genetics and heredity. This practical is essential for CBSE and state board exam preparation, enhancing diagram skills and practical file records. Perfect for Class 12 Biology practical learning.
Aim:- To study meiosis in onion bud cells through permanent slide.
REQUIREMENTS
Permanent slide of different stages of meiosis in onion bud cells, microscope,
PROCEDURE
1. Fix the permanent slide under the microscope.
2. First observe the slide under the low power and then under high power of the microscope.
OBSERVATIONS
Under the high power of microscope following stages of meiosis are distinctly observed:
A. Meiosis I
1. Prophase I. It is of long duration and has five sub stages:
(a) Leptotene
(i) Chromatin fibres condense and form thick thread like structures called chromosomes.
(ii) Nuclear envelop and nucleolus are distinct.
(b) Zygotene
(i) Homologous chromosomes form pairs called bivalent. This pairing is called synapsis.
(ii) The individual of a pair are similar in length and in position of their centromere.
(c) Pachytene
(i) 'The two chromatids of each chromosome become visible, so that a bivalent becomes a tetrad.
(i) Crossing over (exchange of chromatid segments between homologous chromosomes) takes place between non- -sister chromatids of homologous chromosomes.
(d Diplotene
The two chromosomes each bivalent move away and homologues are held together at one or more points called chiasmata.
(e) Diakinesis
(i) Homologous chromosomes appear thick and ring shaped,
(ii) Nucleolus and nuclear envelope disappear and spindle begins to be formed.
2. Metaphase I
(i) The bivalent (homologous chromosomes) arrange themselves at the equator of the
spindle.
(ii) The spindle get attached to the centromere of the chromosome.
3. Anaphase I
(i) 'The two chromosomes of each bivalent move to the opposite pole.
(ii) Each pole has half the number of chromosomes with two chromatids each.
4. Telophase I
(i) The chromosome at each pole uncoil, and nucleolus and nuclear envelope reappear
(ii) Cytokinesis occurs to form two haploid daughter cells.
Interkinesis
A very short interphase may intervene between meiosis I and meiosis II.
B. Meiosis II
It includes following four stages:
1. Prophase II
(i) The chromosomes of daughter cell begin to condense and become thick
(ii) Nuclear envelope and nucleolus begin to disappear.
2 Metaphase II
(i) The chromosomes are aranged on the equator of the spindle.
(ii) Each chromosome held by the spindle at the centromere to both the poles.
3. Anaphase II
(i) The sister chromatids (daughter chromosomes) of each chromosomes separate and
migrate towards the opposite poles
(ii) Each pole, thus receives haploid number of chromosomes,
Telophase II
(i) The chromosomes begin to uncoil and become thin,
(ii) The nuclear envelope and nucleolus are reconstituted.
Cytokinesis occurs and four daughter cells are formed, each with haploid number of
chromosomes.
PRECAUTIONS
1. Floral buds should be fixed between 8 to 10 A.M.
2. Slide should be warmed gently to avoid over heating.
Class 12 Biology Lab Manual – Final Practicals | Complete Lab Experiment File
A. List of Experiments
1. Prepare a temporary mount to observe pollen germination.
2. Study the plant population density by quadrat method.
3. Study the plant population frequency by quadrat method.
4. Prepare a temporary mount of onion root tip to study mitosis.
5. Isolate DNA from available plant material such as spinach, green
pea seeds, papaya, banana etc.
B. Study and observe the following (Spotting):
1. Flowers adapted to pollination by different agencies (wind, insects, birds).
2. Pollen germination on stigma through a permanent slide or scanning electron
3. Identification of stages of gamete development, i.e., T.S. of testis and T.S. of
ovary through permanent slides (from grasshopper/mice).
4. Meiosis in onion bud cell or grasshopper testis through permanent slides.
5. T.S. of blastula through permanent slides (Mammalian).
6. Mendelian inheritance using seeds of different colour/sizes of any plant.
7. Prepared pedigree charts of any one of the genetic traits such as rolling of tongue,
blood groups, ear lobes, widow's peak and colour blindness.
8. Controlled pollination - emasculation, tagging and bagging.
9. Common disease causing organisms like Ascaris, Entamoeba, Plasmodium, any
fungus causing ringworm through permanent slides, models or virtual images or
specimens. Comment on symptoms of diseases that they cause.
10. Models specimens showing symbiotic association in lichens, root nodules of
leguminous plants, and parasitic mode of nutrition shown by Cuscuta on host.
11. Flash cards / models showing examples of homologous and analogous organs.
Class
Biology
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