Problem 78 - Objective Biology for NEET

Question

Which of the following statements is correct for meiosis? (a) First division is equational and the second is reductional (b) First division is reductional and the second is equational (c) Both divisions are equational (d) Both divisions are reductional

Step-by-Step Solution

Answer

The correct statement is option (b): First division is reductional and the second is equational.

Step 1: Evaluate the Statements

Consider each statement about meiosis given in the exercise and evaluate it according to the understanding of the two divisions of meiosis. This would involve understanding that in the first division, the homologous chromosomes separate which is reductional, hence reducing the number of chromosomes by half. While the second division is equational because here, sister chromatids separate, without changing the number of chromosomes.

Step 2: Matching the Correct Statement

After evaluating the statements, match them with the key concepts of meiosis. In this case, it is found that the first division of meiosis is reductional and the second division is equational. This statement corresponds to option (b).

Key Concepts

Reductional Division

During meiosis, the first division is known as the reductional division. This step plays a crucial role in halving the chromosome number in the resulting daughter cells. Unlike mitosis, where the chromosome number remains unchanged, reductional division specifically targets homologous chromosomes to ensure this decrease occurs. As homologous chromosomes are paired sets of chromosomes containing similar genetic sequences, they are unique to meiotic processes.

Here's what happens in reductional division:

• Homologous chromosomes pair up in a process called synapsis.
• These paired chromosomes then line up in the metaphase plate during metaphase I.
• In anaphase I, homologous chromosomes are pulled apart to opposite poles of the cell.
• This results in each daughter cell receiving half the original number of chromosomes, marking it as haploid.

This halving is vital for maintaining genome stability when gametes fuse during sexual reproduction.

Equational Division

Following reductional division, meiosis enters the equational division phase. Often likened to mitosis, this second division in meiosis ensures the equal distribution of genetic material among daughter cells. What makes equational division distinct is its focus on the separation of sister chromatids rather than reducing chromosome number.

Here's how equational division unfolds:

• The division proceeds through stages remarkably similar to mitosis: prophase II, metaphase II, anaphase II, and telophase II.
• Chromosomes, each already consisting of two sister chromatids, line up at the metaphase plate during metaphase II.
• In anaphase II, sister chromatids are separated and pulled to opposite poles of the cells.
• This results in four cells, each containing the same number of chromosomes as the cells at the end of meiosis I.

While the chromosome count remains unchanged, the result is genetically unique daughter cells due to earlier genetic shuffling.

Homologous Chromosome Separation

One of the hallmark processes in meiosis is the separation of homologous chromosomes. Occurring during reductional division, this step ensures genetic diversity and proper chromosome distribution. Homologous chromosomes, while similar, contain variations in alleles inherited from each parent.

Here's what happens during this process:

• Homologous chromosomes pair up and exchange segments through a process known as crossing over, enhancing genetic diversity.
• After crossing over, paired homologous chromosomes align at the metaphase plate in metaphase I.
• During anaphase I, homologous chromosomes are separated and pulled to different poles of the cell, each moving as an intact chromosome.
• This separation reduces the chromosome number in each resulting daughter cell, marking the completion of reductional division.

This essential mechanism is key to generating genetic variation and, thus, contributes greatly to evolutionary processes.

Sister Chromatid Separation

Sister chromatid separation is a pivotal element of equational division. After the reductional division, cells enter this phase where each chromosome, composed of two sister chromatids, plays a critical role in distributing the genetic material equally.

During sister chromatid separation, the following processes occur:

• Sister chromatids, once attached at the centromere, are aligned at the cell equator during metaphase II.
• In anaphase II, these chromatids are separated as the centromeres divide, pulling individual chromatids toward opposite cell poles.
• Each chromatid becomes an independent chromosome in this phase, ensuring that each daughter cell inherits one complete set of chromosomes.
• This results in four haploid daughter cells, each genetically distinct due to previous crossing over events.

This step ensures the maintenance of genetic consistency across cell divisions while allowing the diversity introduced in meiosis to be preserved.