Title Ex-3 Cell Cycle and Cell Division (Qs+sol) (1).pdf ✅

30 CELL CYCLE AND CELL DIVISION EXERCISE – 3: Advance Objective Questions Cell Cycle: Introduction 1. Assertion: Interphase occupies 75-95% of the total generation time. Reason: Interphase (I-phase) is the long non-dividing phase. (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. Ans. (a) Sol. The cell cycle is divided into two basic phases: (I) interphase, (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 hours, the average duration of the cell cycle of human cells, cell division properly lasts for only about an hour. The interphase lasts more than 95% of the duration of the cell cycle. 2. How many chromosomes will the cell have at G1, after S, and after M-phase respectively if it has 14 chromosomes at interphase? (a) 7, 14, 14 (b) 14, 14, 14 (c) 14, 14, 7 (d) 7, 7, 7 Ans. (b) Sol. The cell will have 14, 14, 14 chromosomes at G1, after S, and after M-phase respectively if it has 14 chromosomes at interphase. In interphase, the cell grows in size and prepares itself for the next division but replication of the cell doesn’t take place in this phase. S phase is the DNA synthesis phase. In this phase, the replication of nuclear DNA and the synthesis of histone protein takes place. In the S phase, the amount of DNA doubles but the number of chromosomes remains the same in each cell. The division phase or M-phase is also known as the division phase or mitotic phase. In this phase, the division of the nucleus occurs and the division of the cytoplasm occurs but a number of chromosomes remain the same. 3. The graph given shows the change in DNA content during various phases (A to D) in a typical mitotic cell cycle. Identify the phases and select the correct option. A B C D (a) G2 G1 S M (b) G S G2 M (c) G1 S G2 M (d) M G1 S G2 Ans. (c) Sol. The Gl phase or pre DNA synthesis phase is the 1 st Gap phase. S phase is a DNA synthesis phase. The replication of nuclear DNA and the synthesis of histone protein takes place in the S phase. G2 phase is the 2 nd Gap phase or post DNA synthesis phase. The final preparation of cell division occurs during this phase where the amount of DNA content is doubled while the chromosome number is the same. The division phase or M-phase is also known as the division phase or mitotic phase. In this phase, the division of the nucleus and the division of the cytoplasm occurs. Mitosis 4. If a tissue at a given time has 1024 cells, how many cycles of mitosis had the original parental single-cell undergone? (a) 512 (b) 10 (c) 1024 (d) 256 Ans. (b) Sol. In mitotic division, the single parental cell divides into two daughter cells which are identical. This division mainly occurs in eukaryotic cells, also called equational division. This phase has a time period of 1 hour. Each cycle produces 2 daughter cells. 10 cycles of mitotic divisions will give rise to 1024 cells. 5. A cell’s division time is 1 minute. In 20 minutes, a culture tube (culture medium) is 1/8 th filled with cells. When the tube will be fully filled? (a) 21 minutes (b) 23 minutes (c) 60 minutes (d) 160 minutes Ans. (b)
CELL CYCLE AND CELL DIVISION 31 Sol. Generation time is 1 minute means after every min, the number of cells doubles its initial value. In 20 mins, the culture tube is 1/8 th filled with cells. Each cell divides every single minute producing two daughter cells. Therefore, in 21 min = 1/8×2 =1⁄4. In 22 min=1/4×2 =1⁄2. In 23 min = 1/2×2=1, that means in 23 minutes, the culture tube will be fully filled. 6. Assertion: Prophase is the first stage of mitosis which follows the S and G1 phases of interphase. Reason: Prophase is marked by the initiation of clusters of chromosomes. (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. Ans. (d) Sol. 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 intertwined. Prophase is marked by the initiation of condensation of chromosomal material. The chromosomal material becomes untangled during the process of chromatin condensation. 7. Centrioles undergo duplication during (i) of (ii) and begin to move towards opposite poles of the cell during (iii) stage of (iv). (a) (i)-S phase, (ii)-Interphase, (iii)-Prophase, (iv)- Mitosis (b) (i)-S phase, (ii)-Interphase, (iii)-Anaphase, (iv)- Mitosis (c) (i)-Prophase, (ii)- Mitosis, (iii)-Metaphase, (iv)- Mitosis (d) (i)-Prophase, (ii)-Mitosis, (iii)-Anaphase, (iv)- Mitosis Ans. (a) Sol. In animal cells, during the S phase, DNA replication begins in the nucleus and the centriole duplicates itself in the cytoplasm. And these duplicated centrioles move to the opposite poles during the prophase. So, the centrosome undergoes duplication during the S phase of interphase and begins to move towards opposite poles of the cell during the prophase stage of mitosis. 8. Recognise the figure and find out the correct matching. (a) a-Early prophase, b- late prophase, c-transition to metaphase (b) b-Early prophase, c- late prophase, a-transition to metaphase (c) c-Early prophase, a-late prophase, b-transition to metaphase (d) b-Early prophase, a-late prophase, c-transition to metaphase Ans. (a) Sol. Early prophase Late prophase Transition to metaphase Early prophase-Mitotic spindle starts to form, the chromosome starts to condense, and the nucleolus disappears. The late prophase-Mitotic spindle begins to capture and organize the chromosomes. The chromosomes become even more condensed and very compact. Transition to metaphase-Genetic material gets condensed into chromosomes. The disappearance of the nucleus and the appearance of the chromosome takes place in the cytoplasm of the cell. 9. The two chromatids of a metaphase chromosome represent (a) Replicated chromosomes to be separated at anaphase (b) Homologous chromosomes of a diploid set (c) Non-homologous chromosomes joined at the centromere (d) Maternal and paternal chromosomes joined at the centromere Ans. (a) Sol. The two chromatids of a metaphase chromosome represent replicated chromosomes to be separated at anaphase. In anaphase, centromeres split, and the two sister chromatids move to opposite poles. The cell cycle has two basic phases-interphase and the mitotic phase. In the mitotic phase, division of the nucleus and cytoplasm takes place. It consists of
32 CELL CYCLE AND CELL DIVISION prophase, metaphase, anaphase, and telophase. In interphase, the cell prepares itself for division. Interphase is further divided into the Gap1 phase, S phase, and Gap2 phase. 10. Identify the structures indicated by labels (i), (ii), (iii), and (iv) and select the correct option. (a) (i)-Chromatid, (ii)-Centriole, (iii)-Centromere, (iv)-Chromosome (b) (i)-Chromosome, (ii)-Centriole, (iii)-Centromere, (iv)-Chromatid (c) (i)-Chromatid, (ii)-Centromere, (iii)-Centriole, (iv)-Chromosome (d) (i)-Chromosome, (ii)-Centromere, (iii)-Centriole, (iv)-Chromatid Ans. (a) Sol. A chromatid is an identical half of a replicated chromosome. A centriole is a small set of microtubules that are arranged in a specific way and are made from tubulin protein. Centromeres are the fundamental chromosomal structure where proteins nucleate to form kinetochores that bind to spindle microtubules and mediate chromosome segregation during cell division. The DNA molecule is packaged into thread-like structures called chromosomes that are made up of tightly coiled DNA around proteins called histones that support its structure. 11. Assertion: During anaphase, the centromere of each chromosome splits, and chromatids separate. Reason: During anaphase, chromatids move to opposite poles. (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. Ans. (b) Sol. In anaphase, chromosomes are arranged on the equatorial plate for a short period. 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 fibers attached to the centromeres shorten and pull the chromosomes to the poles. In the anaphasic movement of chromosomes, the centromeres lead the path while the limbs trail behind. So, anaphasic chromosomes appear as V-, L-, J- and I-shaped. 12. Mitotic anaphase differs from metaphase in possessing (a) Same number of chromosomes and the same number of chromatids (b) Half number of chromosomes and half number of chromatids (c) Half the number of chromosomes and the same number of chromatids (d) Same number of chromosomes and half number of chromatids Ans. (d) Sol. Mitotic anaphase differs from metaphase in processing the same number of chromosomes and a different number of chromatids. During metaphase, the microtubules of the spindle get attached and the chromosomes get aligned along with the metaphase plate in the equator of the cell whereas, during anaphase, the sister chromatids are pulled apart and move towards the opposite poles of the cells. 13. In the below sequence of figures showing different stages of cell division, the missing stage (ii) is (a) (b) Chromatid Centriole Centromere Chromosome
CELL CYCLE AND CELL DIVISION 33 (c) (d) Ans. (a) Sol. Anaphase In early anaphase interzonal fibers (small and contracted) appear at the equator of the cell. In this stage, the centromere of each chromosome splits lengthwise (a division of centromere) and chromatids separate and move to opposite poles. These changes occur after metaphase (i). Hence, (a) option is correct. Option (b) is wrong as whole chromosomes are moving to opposite poles. Option (c) represents prophase and (d) represents anaphase but this option is wrong as there should be 4 chromatids (two moves to one pole and the remaining moves to the opposite pole). 14. Identify the given stages of mitosis and select the correct option. A B C D (a) Prophase Metaphase Telophase Anaphase (b) Metaphase Anaphase Prophase Telophase (c) Anaphase Metaphase Prophase Telophase (d) Prophase Metaphase Anaphase Telophase Ans. (c) Sol. Anaphase- During this, chromosomes break at centromeres, and sister chromatids move to opposite ends of the cell. Metaphase- Chromosomes get aligned at the metaphase plate. In prophase, chromatin condenses into chromosomes and the nucleolus disappears. Telophase- Nuclear membrane reforms, nucleoli reappear and chromosomes decondensed. Cytokinesis 15. Read the following statements and find out the incorrect statements. (a) In an animal cell, cytokinesis is achieved by the appearance of a furrow in the plasma membrane (b) Furrow formation starts in the center of the cell and grows outward to meet the existing lateral walls (c) Cell plate represents the middle lamella between the walls of two adjacent plant cells (d) During cytokinesis, organelles like mitochondria and plastids get distributed between the two daughter cells Ans. (b) Sol. Furrow formation during cell division is caused by a ring of actin microfilaments called the contractile ring which forms during the early anaphase. This furrow will then advances inwards to separate the two daughter cells. A cleavage furrow appears on a cell surface when the cell gets prepared to divide. Furrow formation marks the beginning of the cell pinching of its cell membrane and the cytoplasm down in the middle of the cell. 16. Assertion: Cell growth results in disturbing the ratio between the nucleus and cytoplasm. Reason: Mitosis helps the cell to restore the nucleo- cytoplasmic ratio. (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. Ans. (a) Sol. The growth of multicellular organisms is due to mitosis. The function of cells is controlled by the nucleus. The size of the nucleus does not change, but the cytoplasm increases during cell growth. An increase in size disturbs the nucleo-cytoplasmic ratio. This ratio is restored to an efficient level through cell division. Meiosis 17. What do (i) and (ii) represent in the given flowchart? (i) Parent cell 2 Daughter cells 4 Daughter cells M-I M-II (2n) (ii) (a) (i) = 2n, (ii) = n (b) (i) = n, (ii) = n