Exercise MCQs with Answers and Explanations
1. In which phase of the cell cycle does maximum growth occur in a cell?
Options:
a) M phase
b) S phase
c) G1 phase
d) G2 phase
Correct Answer: c) G1 phase
Explanation:
The G1 phase is the first stage of interphase in the cell cycle. During this phase, the cell grows in size, produces RNA, and synthesizes proteins necessary for DNA replication. This is the phase of maximum growth.
Tip:
Remember “G1 = Growth 1.”
2. In which phase of the cell cycle do chromosomes duplicate?
Options:
a) Mitosis
b) G1 phase
c) G2 phase
d) S phase
Correct Answer: d) S phase
Explanation:
The S phase (Synthesis phase) is when DNA replication occurs, resulting in duplicated chromosomes (each having two sister chromatids).
Tip:
“S for Synthesis and S-phase for duplication of DNA.”
3. At which stage of mitosis do chromosomes line up in the center?
Options:
a) Prophase
b) Metaphase
c) Anaphase
d) Telophase
Correct Answer: b) Metaphase
Explanation:
During metaphase, chromosomes align at the metaphase plate (center of the cell) with their centromeres attached to spindle fibers from opposite poles.
Tip:
“Metaphase = Middle alignment.”
4. If you observe a cell in which the nuclear membrane is reforming around two sets of chromosomes, what stage of the cell cycle is this?
Options:
a) Anaphase
b) Telophase
c) Prophase
d) Metaphase
Correct Answer: b) Telophase
Explanation:
Telophase is the final stage of mitosis, where the nuclear envelope re-forms around each set of chromosomes, and the chromosomes begin to de-condense.
Tip:
“Telophase = Two nuclei reforming.”
5. How does the centrosome contribute to mitosis?
Options:
a) Initiates DNA replication
b) Makes mitotic spindle
c) Forms the nuclear envelope
d) Duplicates organelles
Correct Answer: b) Makes mitotic spindle
Explanation:
Centrosomes organize microtubules to form the mitotic spindle, which is essential for chromosome segregation during mitosis.
Tip:
“Centrosomes are the center of spindle formation.”
6. Centrosomes make the mitotic spindle in:
Options:
a) Animal cells
b) Plant cells
c) Prokaryotic cells
d) All of these
Correct Answer: a) Animal cells
Explanation:
Centrosomes are found only in animal cells and are responsible for forming the mitotic spindle. Plant cells use other mechanisms involving microtubule organizing centers.
Tip:
Remember “Centrosome = Center in animals.”
7. An organism has 4 pairs of chromosomes. After meiosis-II, how many chromosomes and chromatids will be present in each daughter cell?
Options:
a) 8 chromosomes and 16 chromatids
b) 4 chromosomes and 8 chromatids
c) 4 chromosomes and 4 chromatids
d) 8 chromosomes and 8 chromatids
Correct Answer: c) 4 chromosomes and 4 chromatids
Explanation:
During meiosis-II, sister chromatids separate, resulting in daughter cells with half the chromosome number (4 chromosomes) and no duplicate chromatids.
Tip:
Meiosis halves the chromosome number, producing haploid cells.
8. Which one is the feature of mitosis but not of meiosis II?
Options:
a) Daughter cells with the same number of chromosomes
b) Separation of homologous chromosomes
c) Crossing over
d) Separation of sister chromatids
Correct Answer: a) Daughter cells with the same number of chromosomes
Explanation:
Mitosis results in two daughter cells with the same chromosome number as the parent cell. In meiosis-II, chromosome number remains haploid.
Tip:
“Mitosis = identical cells.”
9. Which event is unique to meiosis but not mitosis?
Options:
a) DNA replication
b) Chromosome alignment
c) Crossing over
d) Nuclear division
Correct Answer: c) Crossing over
Explanation:
Crossing over occurs during prophase I of meiosis, where homologous chromosomes exchange genetic material. This does not occur in mitosis.
Tip:
“Meiosis = Mixing through crossing over.”
B. Short Answers
1. Enlist the events that occur during the G1 phase of interphase.
- Cell growth occurs.
- Synthesis of RNA and proteins needed for DNA replication.
- Organelles are duplicated.
- The cell ensures it is ready for DNA synthesis.
2. What is the main purpose of the S phase in the cell cycle?
- DNA replication takes place in the S phase, resulting in two identical sister chromatids for each chromosome.
3. During which phase of mitosis do sister chromatids separate?
- Sister chromatids separate during anaphase of mitosis.
4. How does crossing over contribute to genetic variation in meiosis?
- During prophase I of meiosis, homologous chromosomes exchange genetic material through crossing over, leading to genetic recombination and variation in offspring.
5. What is the role of spindle fibers in mitosis?
- Spindle fibers help align chromosomes at the metaphase plate and separate sister chromatids during anaphase, ensuring proper distribution to daughter cells.
6. How is cytokinesis in animal cells different from plant cells?
- In animal cells, cytokinesis occurs through the formation of a cleavage furrow, where the cell membrane pinches inward.
- In plant cells, cytokinesis occurs by the formation of a cell plate, which develops into a new cell wall.
7. What is the difference between prophase of mitosis and prophase I of meiosis?
- In mitosis prophase, chromosomes condense, and the nuclear membrane breaks down. Homologous chromosomes do not pair up.
- In prophase I of meiosis, homologous chromosomes pair up (synapsis) and undergo crossing over.
8. How does meiosis differ from mitosis in terms of chromosome number?
- Mitosis results in diploid cells (same chromosome number as the parent).
- Meiosis produces haploid cells (half the chromosome number of the parent).
9. What are the key events of anaphase in mitosis?
- Sister chromatids are pulled apart by spindle fibers and move to opposite poles of the cell.
10. What is the function of the centrosome during cell division?
- The centrosome organizes microtubules and forms spindle fibers required for the separation of chromosomes during cell division.
11. What are sister chromatids, and when do they separate in meiosis?
- Sister chromatids are identical copies of a chromosome connected by a centromere. They separate during anaphase II of meiosis.
12. How is mitosis related to the process of regeneration?
- Mitosis allows for the production of new cells, which replace damaged or lost cells, playing a critical role in tissue regeneration.
C. Write Answers in Detail
1. Describe the events that occur during the phases of mitosis.
- Prophase: Chromosomes condense, spindle fibers form, and the nuclear membrane dissolves.
- Metaphase: Chromosomes align at the metaphase plate.
- Anaphase: Sister chromatids are pulled apart to opposite poles.
- Telophase: Nuclear membranes reform around each set of chromosomes, and the cell begins to split.
- Cytokinesis: Cytoplasm divides, resulting in two daughter cells.
2. Describe cytokinesis in animal and plant cells.
- Animal cells: A cleavage furrow forms, pinching the cell into two daughter cells.
- Plant cells: A cell plate forms in the middle of the cell, eventually developing into a new cell wall that separates the two daughter cells.
3. Describe the significance of mitosis.
- Mitosis allows for growth, repair, and replacement of damaged or dead cells.
- It ensures genetic stability by producing two identical daughter cells.
- It is essential for asexual reproduction in some organisms.
4. Describe the events that occur during the phases of meiosis-I.
- Prophase I: Homologous chromosomes pair up (synapsis), and crossing over occurs.
- Metaphase I: Homologous chromosome pairs align at the metaphase plate.
- Anaphase I: Homologous chromosomes separate and move to opposite poles.
- Telophase I: Nuclear membranes may reform, and cytokinesis divides the cell into two haploid cells.
5. Describe the significance of meiosis.
- Meiosis reduces the chromosome number by half, ensuring stability across generations.
- It introduces genetic variation through crossing over and independent assortment.
- Meiosis is essential for sexual reproduction, producing haploid gametes.
