(A) The connecting link between Meiosis-I and Meiosis-II is ...............
a. interphase-I b. interphase-II
c. interkinesis d. anaphase-I
Sub-phases of interphase:
(c) interkinesis
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(B) Synapsis is pairing of .............
a. any two chromosomes
b. non-homologous chromosomes
c. sister chromatids
d. homologous chromosomes
Sub-phases of interphase:
(d) homologous chromosomes
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(C) Spindle apparatus is formed during which stage of mitosis?
a. Prophase. b. Metaphase.
c. Anaphase. d. Telophase.
Sub-phases of interphase:
(b) Metaphase.
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(D) Chromosome number of a cell is almost doubled up during ...............
a. G1-phase b. S-phase
c. G2-phase d. G0-phase
Sub-phases of interphase:
(b) S-phase
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(E) How many meiotic divisions are necessary for formation of 80 sperms?
a. 80 b. 40 c. 20 d. 10
Sub-phases of interphase:
(c) 20
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(F) How many chromatides are present in anaphase-I of meiosis-I of a diploid cell having 20 chromosomes?
a. 4 b. 6 c. 20 d. 40
Sub-phases of interphase:
(d) 40
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(G) In which of the following phase of mitosis chromosomes are arranged at equatorial plane?
a. Prophase b. Metaphase
c. Anaphase d. Telophase
Sub-phases of interphase:
(b) Metaphase
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(H) Find incorrect statement -
a. Condensation of chromatin material occurs in prophase.
b. Daughter chromatids are formed in anaphase.
c. Daughter nuclei are formed at metaphase.
d. Nuclear membrane reappears in telophase.
Sub-phases of interphase:
(c) Daughter nuclei are formed at metaphase.
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(I) Histone proteins are synthesized during ......
a. G1 phase b. S-phase
c. G2 phase d. Interphase
Sub-phases of interphase:
(b) S-phase
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Question 2.
Answer the following questions
(A) While observing a slide, student observed many cells with nuclei. But some of the nuclei were bigger as compared to others but their nuclear membrane was not so clear. Teacher inferred it as one of the phase in the cell division. Which phase may be inferred by teacher?
Sub-phases of interphase:
Prophase
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(B) Students prepared a slide of onion root tip. There were many cells seen under microscope. There was a cell with two groups of chromosomes at opposite ends of the cell. This cell is in which phase of mitosis?
Sub-phases of interphase:
Anaphase
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(C) Students were shown some slides of cancerous cells. Teacher made a comment as if there would have been a control at one of its cell cycle phase, there wouldn’t have been a condition like this. Which phase the teacher was referring to?
Sub-phases of interphase:
The phase teacher was referring would be G1 phase.
[Note: Cells actively control only three checkpoints in the cycle. These checkpoints (G1/S, G2/M, the Spindle checkpoint) allow the cell cycle to be delayed or halted when necessary. ]
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(D) Some Mendelian hybridization experimental results were shown to the students. Teacher informed that there are two genes located on the same chromosome. He enquired if they will be ever separated from each other?
Sub-phases of interphase:
Genes are located on chromosomes at specific distance and position.
The greater this distance, the greater the chance that a crossover can occur between the genes and the greater the chances of recombination. I
The chances of recombination are less between the genes that are_ placed closed to each other on the chromosome.
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(E) Students were observing a film on Paramoecium. It underwent a process of reproduction. Teacher said it is due to cell division. But students objected and said that there was no disappearance of nuclear membrane and no spindle formation, how can it be cell division? Can you clarify?
Sub-phases of interphase:
Paramoecium is a unicellular organism. The division in Paramoecium occurs by amitosis.
It is the simplest mode of cell division.
In amitosis, nucleus elongates and a constriction appears. This constriction deepens and divides the nucleus in two daughter nuclei followed by the division of cytoplasm.
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(F) Is the meiosis responsible for evolution? Justify your answer.
Sub-phases of interphase:
Meiosis ensures that organisms produced by sexual reproduction contain correct number of chromosomes.
Meiosis exhibits genetic variation by the process of recombination.
Variations increase further after union of gametes during fertilization creating offspring with unique characteristics.
Thus, it creates diversity of life and is responsible for evolution.
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(G) Why mitosis and meiosis-II are called as homotypic division?
Sub-phases of interphase:
In mitosis, the chromosome number and genetic material of daughter cells remain same as that of the parent cell
In meiosis — II, two haploid cells formed during first meiotic division divide further into four haploid cells.
This division is similar to mitosis. The daughter cells fonned in second meiotic division are similar to their parent cells with respect to the chromosome number formed in meiosis — I.
Hence mitosis and meiosis — II are called homotypic division.
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(H) Write the significance of mitosis.
Sub-phases of interphase:
As mitosis is equational division, the chromosome number is maintained constant.
It ensures equal distribution of the nuclear and the cytoplasmic content between the daughter cells, both quantitatively and qualitatively. Therefore, the process of mitosis also maintains the nucleo-cytoplasmic ratio.
The DNA is also equally distributed.
It helps in growth and development of organisms.
Old and worn-out cells are replaced through mitosis.
It helps in the asexual reproduction of organisms and vegetative propagation in plants.
Through mitotic divisions, meristematic tissues such as the apical and lateral cambium show continuous growth of plants throughout their life.
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(I) Enlist the different stages of prophase-I.
Sub-phases of interphase:
Prophase -I:
It is the most complicated and longest phase of meiotic division
It is further divided into five sub-phases viz. leptotene, Zygotene, pachytene, diplotene and diakinesis.
Leptotene :
The volume of the nucleus increases.
The chromosomes become long distinct and coiled.
They orient themselves in a specific form known as bouquet stage. This is characterized with the ends of chromosomes converged towards the side of nucleus where the centrosome lies.
The centriole divides into two and migrate to opposite poles,
Zygotene :
Pairing of non-sister chromatids of homologous chromosomes takes place by formation of synaptonemal complex. This pairing is called synapsis.
Each pair consist of a maternal chromosome and a paternal chromosome.
Chromosomal pairs are called bivalents or tetrads.
Pachytene:
Each individual chromosome begins to split longitudinally into two similar chromatids.
Therefore, each bivalent now appears as a tetrad consisting of four chromatids.
The homologous chromosomes begin to separate but they do not separate completely and remain attached to one or more points.
These points are called chiasmata (Appear like a cross-X).
Chromatids break at these points and broken segments are exchanged between non-sister chromatids of homologous chromosomes resulting in recombination.
Diplotene :
The chiasmata become clearly visible in diplotene due to beginning of repulsion between synapsed homologous chromosomes. This is known as desynapsis.
Synaptonemal complex also starts to disappear.
Diakinesis :
The chiasmata beings to move along the length of chromosomes from the centromere towards the ends of chromosomes. The displacement of‘ chiasmata is termed as terminalization.
The terminal chiasmata exist till the metaphase.
The nucleolus and nuclear membrane completely disappear and spindle fibres begin to appear. .
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Question 3.
Draw labelled digrams and write explanation
(A) With the help of suitable diagram, describe the cell cycle.
Sub-phases of interphase:
Cell cycle :
Sequential events occurring in the life of a cell is called cell cycle.
Interphase and M - phase are the two phases of cell cycle.
Cell undergoes growth or rest during interphase and divides during M — phase
Click here to View Figure-1
Interphase: It is the stage between two successive cell divisions. It is the longest phase of a cell cycle during which the cell is highly active and prepares itself for cell division.
The interphase is subdivided into three sub-phases as G1-phase, S-phase and G2-phase.
G1- phase (First gap period/First Gap Phase):
It begins immediately after cell division.
RNA (mRNA, rRNA and tRNA) synthesis, protein synthesis and synthesis of membranes take place during this phase.
S - phase (Synthesis phase):
In this phase DNA is synthesized (replicated), so that amount of DNA per cell doubles.
Synthesis of histone proteins takes place in this phase.
G2- phase (Second growth phase/Second Gap Phase):
Metabolic activities essential for cell division occur during this phase.
Various proteins which are necessary for the cell division are also synthesized in this phase.
Apart from this, RNA synthesis also occur during this phase.
In animal cells, a daughter pair of centrioles appear near the pre-existing pair.
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(B) Distinguish between mitosis and meiosis.
Sub-phases of interphase:
Difference Between mitosis and meiosis :
Mitosis
Meiosis
1-It occurs in somatic cells and stem cells.
2-In this nucleus divides only once.
3-In these two daughter cells are formed.
4-Daughter cells formed by mitotic division are diploid (2n).
5-Mitosis plays an important role in growth, repair, healing and development.
1-It occurs in germ cells.
2-In this nucleus divides twice (Meiosis I and Meiosis II).
3-In these four daughter cells are formed.
4-Daughter cells formed by meiotic division are haploid
5-Meiosis 1s important for formation of haploid gametes and spores.
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(C) Draw the diagram of metaphase.
Sub-phases of interphase:
Metaphase :
Click here to View Figure-2
Chromosomes are completely condensed and appear short.
Centromere and sister chromatids become very prominent.
All the chromosomes are arranged at equatorial plane of cell. This is called metaphase plate.
Mitotic spindle is fully formed in this phase.
Centromere of each chromosome divides into two, each being associated with a chromatid.
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Question 4.
Match the following column-A with column-B
Column-A
(phases)
Column-B
(Their events)
a. Leptotene
b. Zygotene
c. Pachytene
d. Diplotene
1. Crossing over
2. Desynapsis
3. Synapsis
4. Bouquet stage
Sub-phases of interphase:
Column-A
(phases)
Column-B
(Their events)
a. Leptotene
b. Zygotene
c. Pachytene
d. Diplotene
4. Bouquet stage
3. Synapsis
1. Crossing over
2. Desynapsis
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Question 5.
Is a given figure correct? why?
Sub-phases of interphase:
The given figure is incorrect as the spindle fibres are not attached to centromere of the chromosomes
During metaphase, chromosomes are attached to spindle fibres with the help of centromeres.
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Question 6.
If an onion has 16 chromosomes in its leaf cell, how many chromosomes will be there in its root cell and pollen grain.
Sub-phases of interphase:
The chromosomes in root cell will be 16 as root cell is a diploid cell.
The chromosomes in pollen grain will be 8 as pollen grain is a haploid cell
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Question 7.
Identify the following phases of mitosis and label the 'A' and 'B' given in diagrams
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