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 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 Assertion is true but Reason is false.
(d) If both Assertion and Reason are false.
Q.1. Assertion : Interphase is resting stage.
Reason : The interphase cell is metabolically inactive.
AnswerAnswer: (c) Previously interphase was called resting stage because there is no apparent activity related to cell division. The interphase cell is metabolically quite active. Interphase consist of three sub-phases (G1 , G2 and S). Synthesis of DNA occurs in S phase. G1 is the period between the end of mitosis and the start of S phase. G2 is the interval between S phase and start of mitosis. As the synthesis of DNA occurs in S phase so, it is considered as metabolically active phase.
Q.2. Assertion : Histones are basic proteins of major importance in packaging of eukaryotic DNA.
DNA and histones comprise chromatin, forming the bulk of eukaryotic chromosome.
Reason : Histones are 5 major types H1, H2A, H2B, H3 and H4.
AnswerAnswer: (b) Chromosome contains equal amounts of DNA and histone. The DNA and histone octamer forms a nucleosome. Histone is the protein found in eukaryotic chromosomes. There are 5 types of histone namely H1,H2A,H2B,H3 and H4.
Q.3. Assertion : DNA synthesis occurs in G1 and G2 periods of cell cycle.
Reason : During G1 and G2 phase, the DNA content become double.
AnswerAnswer: (d) The synthesis of DNA occurs only in a restricted portion of the interphase during S period, which is preceded and followed by two “gap” periods of interphase (G1 and G2) in which there is no DNA synthesis. G1 is the period between the end of mitosis I and the start of DNA synthesis. S is the period of DNA synthesis and G2 is the interval between the end of DNA synthesis and the start of mitosis. During G1, a cell contains two times (4C) the amount of DNA present in the original diploid cell (2C). Following mitosis, the daughter cell again enter is the G1 period and have a DNA content equivalent to 2C.
Q.4. Assertion: Due to inactivation of the cell cycle, some cells undergo G0 phase.
Reason : G0 phase occurs due to non-availability of mitogen and energy rich compounds.
AnswerAnswer: (a) The phase in which cells do not undergo S-phase after G1-phase is known as G0 phase or quiescent stage. It occurs due to non-availability of mitogen and energy rich compounds. The cells remain metabolically active, but no longer proliferate unless called on to do so depending on the requirement of the organisms.
Q.5. Assertion: G1 phase is the interval between mitosis and initiation of DNA replication.
Reason : The cell is metabolically inactive during G1 phase.
AnswerAnswer: (c) G1 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 G1 phase, the cell is metabolically active and continuously grows. In this phase, different types of RNA (mRNA, tRNA and rRNA) and proteins are synthesised.
Q.6. Assertion: Cell growth results in disturbing the ratio between the nucleus and cytoplasm.
Reason : Mitosis helps the cell to restore the nucleocytoplasmic ratio.
AnswerAnswer: (a) Mitosis causes the growth of multicellular organism. The nucleus controls the functions of cell. The size of nucleus does not change, but cytoplasm increases during cell growth. Increase in size disturbs the nucleo –cytoplasmic ratio. This ratio is restored to efficient level through cell division.
Q.7. Assertion : Cell growth is a continuous process in terms of cytoplasmic increase.
Reason : DNA synthesis occurs only during two specific stages in the cell cycle.
AnswerAnswer: (c) DNA synthesis takes place only during one specific stage in cell cycle (interphase).
Q.8. Assertion : Every chromosome, during metaphase has two chromatids.
Reason : Synthesis of DNA takes places in the S-phase of interphase.
AnswerAnswer: (a) In S phase or synthetic phase chromosomes replicates, DNA content doubles, i.e., 1C to 2C for haploid cells and 2C to 4C for diploid cells. Along with replication of DNA, new chromatin fibres are formed which remain attached in pairs and the number of chromosomes does not increase i.e. 1n to 1n and 2n to 2n. As the chromatin fibres are elongated chromosomes, each chromosomes comes to have two chromatin threads which are attached at centromere. During metaphase, every chromosome have two chromatids. The chromosomes separate from each other during anaphase.
Q.9. Assertion : Mitosis maintains the genetic similarity of somatic cells.
Reason : Chromosomes do not undergo crossing over.
AnswerAnswer: (a) Mitosis keeps all the somatic cells of an organism genetically similar, resembling the fertilized egg. Mitosis involves replication and equitable distribution of all the chromosomes so that all the cells of a multicellular organism have the same number and type of chromosomes. This helps in proper co-ordination among different cells.
Q.10. Assertion : Karyokinesis occurs in M-phase.
Reason : Cell division stops in M-phase.
AnswerAnswer: (d) M-phase represents the phase of actual division. It consists of karyokinesis (the division of nucleus) followed by cytokinesis (the division of cytoplasm). Cell divisions stop after M-phase.
Q.11. Assertion: Prophase is the first stage of mitosis which follows S and G1 phases of interphase.
Reason : Prophase is marked by the initiation of clusters of chromosomes.
AnswerAnswer: (d) Prophase which is the first stage of mitosis follows the S and G2 phases of interphase. In the S and G2 phases, the new DNA molecules formed 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.
Q.12. Assertion: During anaphase, centromere of each chromosomes splits and chromatids separate.
Reason : Chromatids move to opposite poles.
AnswerAnswer: (b) In anaphase, centromeres of chromosomes start to divide into two, forming daughter chromatids with centromere in each. Daughter chromosomes are repulsive so, migrate towards opposite poles. Spindle fibres attached to the centromeres shorten and pull the chromosomes, the centromeres lead the path while the limbs trail behind. So, anaphasic chromosomes appear as V, L, J and L shaped.
Q.13. Assertion : Mitosis is often called indirect division.
Reason : Mitosis divides a parent cell into two daughter cells.
AnswerAnswer: (b) Mitosis is often said as an indirect division. It is the method of equal division of nuclei. It is an elaborate process which includes a series of important changes in the nucleus as well as in cytoplasm. Therefore, it is called indirect method of division. In direct division, the nucleus becomes constricted at the center, assuming an hour glass shape, and then divides into two. This is followed by cleavage or division of cytoplasm to form 2 daughter cells. It does not involve doubling of chromosomes. Also called amitosis.
Q.14. Assertion : Mitosis is important in the life of an organism, especially in the growth of a multicellular organism.
Reason : Mitosis restores the nucleocytoplasmic ratio.
AnswerAnswer: (a) Mitosis results in the production of diploid daughter cells with identical genetic complement. In multicellular organism, growth is due to mitosis. Cell growth results in disturbing the ratio between the nucleus and the cytoplasm. It therefore, becomes essential for the cell to divide and to restore the nucleocytoplasmic ratio.
Q.15. Assertion : In animal cells, the cytokinesis is marked by the appearance of a furrow in plasma membrane.
Reason : In plant cells, the formation of the new cell wall starts with the formation of simple precursor called cell plate.
AnswerAnswer: (b) In an animal cell, cytokinesis is marked by the appearance of a furrow in the plasma membrane. The furrow slowly deepens and ultimately joins in the centre dividing the cell cytoplasm into two. Plants cells however, are enclosed by a relatively inextensible cell wall, therefore they undergo cytokinesis by a different mechanism. In plant cells, wall formation starts in the centre of the cell and grows outward to meet the existing lateral walls. The formation of the new cell wall begins with the formation of a simple precursor called the cell-plate that represents the middle lamella between the walls of two adjacent cells.
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