Download Junior Inter Botony Questions English Medium

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Junior Inter Botany
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Cell Cycle & Cell Division
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Short Answer Questions
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Q. Explain Prophase - I of Meiosis.
A: It is more complex and divided into 5 phases based on chromosomal behaviour.
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1) Leptotene: Chromosomes appear like long, thread like structures (say for
example Maize microspore mother cells 2n = 20).
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2) Zygotene: Homologous Chromosomes pair with each other under the
influence of a protein called synaptinemal complex. Pairing is called
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synapsis. Each pair thus formed is called bivalent (Maize: 10 bivalents).
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3) Pachylenes: Each chromosome has 2 chromatids. Thus a bivalent is now
called tetrad of chromatids (Maize 10 tetrads). Exchange of genetic materi-
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al occurs between non sister chromatids of homologous chromosomes. It is
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enzyme (Recombinase) mediated process. Chiasmata (X - Shape) are formed
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at the place of recombination nodule (Where non-sister chromatids meet).
Quality of the chromosomes changes. Recombinants are formed.
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4) Diplotene: Synaptonemal complex dissolves. Chromosomes of the bivalents
repel or separate except in the region of X - ma. It lasts long time in oocytes
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of vertebrates.
5) Diakinesis: Chromosomes become more distinct. Chiasmata show
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terminalisation. Nuclear membrane and necleolus dissolve. Spindle apparatus
begins to form.
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Q. Mention the key features of Meiosis.
A: Meiosis, a reductional cell division occurs in diploid cells (Concerned with
reproduction) only.
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e.g.: Zygote in Algae and Fungi: Spore mother cells (of anther and ovule in
angiosperms). In higher plants the meiocyte (2n) divides by Meiosis to form
4 haploid of cells (n), which in turn produce gametes by mitosis. It is
divided into 2 parts.
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Meiosis - I (Hetero type division): Meiocyte divides into 2 haploid daughter
cells.
★ Quality of chromosomes changes in Pachentene of Prophase - I due to
crossing over. Recombinants are formed.
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★ Quantity of chromosomes is reduced to half in Anaphase - I where
centromeres separate and disjunction of homologous chromosomes occurs.
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Inter Kinesis: 2 daughter cells (n) enter into short Interphase where S - stage is
absent.
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Meiosis - II (Homotype division): The 2 daughter cells (n) participate in it.
Each one produces 2 daughter cells similar to their immediate mother cell.
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Key features:
1. Number of chromosomes is reduced to half.
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2. Recombinants are formed due to crossing over in Pachytene. It helps in
evolution.
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Meiocyte
Meiosis - I
(heterotype division)
Meiosis - II
(= Mitosis)
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4 haploid daughter cells
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3. It is essential to keep the chromosome number constant for any number of
generations.
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Q. Which division is necessary to maintain constant chromosome number in all
body cells of multicellular organisms and why?
A: Mitotic cell division is necessary to maintain the chromosome number in all the
cells of an organism. The mother cell in mitotic cell division produces 2
daughter cells which are equal to their mother cell both in quantity and quality of
chromosomes. Every sexually reproducing diploid organism starts its life with a
single cell called Zygote. It under goes numerous mitotic divisions and forms an
organism which consists the same chromosome number in all its cells.
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★ The spore also produces a gametophyte by mitotic divisions due to which
multicellular the chromosome number is maintained constant in all the cells.
Q. Though redundantly described as a resting phase, interphase does not really involve rest. Comment.
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A: Interphase represents the phase between two successive M phases. Interphase
lasts more than 95% of duration of cycle. Thus it is the largest stage. It is a
preparatory stage or resting phase. It is also called phase of non - apparent
division. Though it is called resting phase, it is actually the most active phase as
it involves the following activities. It has 3 phases.
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1) G1 (G = Gap): Cytokinesis is followed by G1
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★ The cell is active and shows normal metabolism. Organelle duplication
occurs. The amount of DNA in the cell is 2C.
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2) S (S = Synthesis):
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★ It is between G1 and G2.
★ It is the largest phase of Interphase.
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★ DNA synthesis (replication) occurs in the nucleus 2C DNA becomes 4C. But
there is no increase in chromosome number. Parallel to this in animal cells
centriole duplicales in cytoplasm.
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3) G2 (G = Gap)
★ Cytoplasm increases. The amount of DNA is 4C. Protein synthesis, ATP
synthesis occurs. The cells are ready to participate in Prophase.
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Writer: Dr. M. Chandraiah
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