Cell cycle and Cell Divisions MCQs for NEET — Botany Questions with Answers

Meiosis is significant for sexual reproduction because it reduces the chromosome number by half, ensuring that when gametes fuse during fertilization, the resulting zygote has the correct diploid number of chromosomes. This process also introduces genetic variation.

During sexual reproduction in angiosperms, meiosis occurs in microspore and megaspore mother cells. In one meiotic division, one microspore mother cell in the anther forms 4 microspores, each of which develops as a pollen grain. When there are 100 pollen grains, the meiotic divisions which will be required are 100/4= 25. The number of meiotic divisions for 100 pollen grains is 25.

Meiosis is a fundamental process for evolution because it leads to genetic variation in organisms. During meiosis, homologous chromosomes exchange genetic material through a process called crossing over, and then the chromosomes are distributed into different gametes. This shuffling of genetic material is crucial for the evolution of species as it increases genetic diversity, enabling organisms to adapt to changing environments.

During prophase of mitosis, the chromatin condenses into visible chromosomes, and the mitotic spindle begins to form. Asters, which are star-shaped structures consisting of microtubules radiating from the centrioles, also appear around the centrioles in the prophase. These asters play a crucial role in the proper alignment and separation of chromosomes during cell division.

During the G2 (Gap 2) phase of the cell cycle, the cell undergoes final preparations for mitosis. This includes the synthesis of proteins and microtubules that are essential for the mitotic phase. The G2 phase follows the S (Synthesis) phase, where DNA replication occurs, and precedes the M (Mitosis) phase, where cell division takes place.

During interphase, the chromosomes are not visibly distinct as they are in a dispersed form called chromatin. This allows for the necessary processes of DNA replication and gene expression to occur. Thus, both the assertion (A) that chromosomes are recognized as a chromatin network during interphase and the reason (R) that they are highly dispersed during this phase are true, and R correctly explains A.

Meiotic division-I is also known as reduction division because it reduces the chromosome number by half. This is essential for maintaining the chromosome number across generations. During this division, homologous chromosomes are separated into two different cells, reducing the chromosome number by half in each cell. Therefore, both A and R are true, and R correctly explains A.

The G1 phase (often written as Gd1d in some contexts) is the first growth phase, involving cell growth and protein synthesis. The S phase is where DNA replication occurs. The G2 phase is the second growth phase, where synthesis of proteins and microtubules occurs in preparation for mitosis. Thus, the correct matching is (A-2), (B-3), (C-1).

The correct matching for the stages of meiosis with their respective characteristics is as follows:

(A) Prophase-I: Longest phase of meiosis-I (3) (B) Metaphase-II: Chromosomes move toward one plane along with centromere (1) (C) Anaphase-I: Half the number of chromosomes is seen (2) (D) Telophase-I: Two nuclei are seen (4)

Therefore, the correct option is (A-3), (B-1), (C-2), (D-4).

In Meiosis-II:

(3) In anaphase-II, the chromatids with their independent centromeres are called chromosomes. (4) In metaphase-II, centromere of each chromosome becomes attached to the filament of a bipolar spindle.

Statements (1) and (2) are incorrect because chromosomes are arranged on the equatorial plate in metaphase-II, not prophase-II, and the nucleolus reappears during telophase-II, not disappears. Hence, the correct option is only 3 & 4.