Directions: In the following questions, a statement of assertion is followed by a statement of reason. Mark the correct choice as:
(a) If both assertion and reason are true and the reason is the correct explanation of assertion.
(b) If both assertion and reason are true but reason is not the correct explanation of assertion.
(c) If the assertion is true but the reason is false.
(d) If the assertion is false but the reason is true.
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Assertion: Interphase occupies \(75-95 \%\) of the total generation time.
Reason: Interphase (I-phase) is the long non-dividing phase.
(b) The cell cycle is divided into two basic phases : (I) Interphase and (II) M-phase. The M phase represents the phase when the actual cell division or mitosis occurs and the interphase represents the phase between two successive M phases. In the 24 hour average duration of cell cycle of human cell, cell division proper lasts for only about an hour. The interphase lasts more than \(95 \%\) of the duration of cell cycle.
Assertion: Some cells enter \(G_0\) phase leading to inactivation of cell cycle.
Reason: \(G_0\) phase occurs due to non-availability of mitogen and energy rich compounds.
a
Assertion: \(G_1\) phase is the interval between mitosis and initiation of DNA replication.
Reason: The cell is metabolically inactive during \(G_1\) phase.
(c) \(G_1\) phase is also known as first growth phase or post mitotic gap phase. It is the phase between end of mitotic phase of previous cell and initiation of S-phase of next mitotic phase. During \(G_1\) phase the cell is metabolically active and continuously grows. In this phase different types of RNA (mRNA, tRNA and (RNA) and proteins are synthesised.
Assertion: Prophase is the first stage of mitosis which follows \(S\) and \(G_1\) phases of interphase.
Reason: Prophase is marked by the initiation of condensation of chromosomal material.
(d) Prophase which is the first stage of mitosis follows the \(S\) and \(G_2\) phases of interphase. In \(G_2\) phase, the newly formed DNA molecules are not distinct but interwined. Prophase is marked by the initiation of condensation of chromosomal material. The chromosomal material becomes untangled during the process of chromatin condensation.
Assertion: Small disc-shaped structures at the surface of the centromeres are called kinetochores.
Reason: Kinetochores serve as the sites of attachment of spindle fibres to the centromeres.
b
Assertion: During mitotic anaphase, centromere of each chromosome splits and chromatids separate.
Reason: During anaphase, chromatids move to opposite poles.
b
Assertion: Karyokinesis follows cytokinesis.
Reason: Cytokinesis is the division of cytoplasm.
(d) Cytokinesis is the division of cytoplasm. Cytokinesis follows karyokinesis. Karyokinesis is the division of nucleus. The M phase starts with nuclear division, corresponding to the separation of daughter chromosomes and leads to the division of cytoplasm.
Assertion: Cell growth results in disturbing the ratio between the nucleus and cytoplasm.
Reason: Mitosis helps the cell to restore the nucleocytoplasmic ratio.
(b) The growth of multicellular organisms is due to mitosis. Cell’s functions are controlled by nucleus. The size of nucleus does not change, but cytoplasm increases during cell growth. Increase in size disturbs the nucleocytoplasmic ratio. This ratio is restored to efficient level through cell division.
Assertion: The process of pairing of the chromosomes is called synapsis.
Reason: Synapsis occurs during leptotene stage.
(c) Synapsis occurs during the second stage of prophase called zygotene. During this stage chromosomes start pairing together and this process of association is called synapsis. Such paired chromosomes are called homologous chromosomes. Chromosome synapsis is accompanied by the formation of complex structure called synaptonemal complex. The complex formed by a pair of synapsed homologous chromosomes is called a bivalent or a tetrad.
Assertion: Crossing over leads to recombination of genetic material on the two chromosomes.
Reason: Crossing over is the exchange of genetic material between two homologous chromosomes.
(a) Crossing over or recombination occurs during pachytene. Recombination involves mutual exchange of the corresponding segments of non-sister chromatids of homologous chromosomes. It takes place by breakage and reunion of chromatid segments. Breakage, called nicking, is assisted by an enzyme endonuclease and reunion, termed annealing, is aided by an enzyme ligase. Crossing over leads to recombination of genetic material on the two chromosomes.
Assertion: The crossing over is an enzyme-mediated process.
Reason: The enzyme involved in crossing over is lyase.
(c) Crossing over or recombination occurs during pachytene. Recombination involves mutual exchange of the corresponding segments of non-sister chromatids of homologous chromosomes. It takes place by breakage and reunion of chromatid segments. Breakage, called nicking, is assisted by an enzyme endonuclease and reunion, termed annealing, is aided by an enzyme ligase. Crossing over leads to recombination of genetic material on the two chromosomes.
Assertion: The final stage of meiotic prophase I is diplotene.
Reason: The beginning of diplotene is recognised by the dissolution of synaptonemal complex.
(d) The final stage of meiotic prophase I is diakinesis. This is marked by the terminalisation of chiasmata.
Assertion: The stage between the two meiotic divisions is called interkinesis.
Reason: Interkinesis is generally short lived.
b
Assertion: Metaphase II begins with splitting of centromere of each chromosome into two.
Reason: In metaphase II, chromosomes align at the equator.
(d) During metaphase II, the chromosomes align at the equator. The microtubules from opposite poles of the spindle get attached to the kinetochores of sister chromatids. Anaphase II begins with the simultaneous splitting of the centromere of each chromosome into two. Consequently, the daughter chromosomes move towards the opposite poles of the cell.
Assertion: Variations are important for the process of evolution.
Reason: Meiosis increases the genetic variability in the population of organisms from one generation to the next.
b
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