Problem 111 - Objective Biology for NEET

Question

The longest phase of meiosis I is (a) Metaphase I (b) Prophase I (c) Anaphase I (d) Telophase I

Step-by-Step Solution

Answer

b) Prophase I

Step 1: Understand the Phases of Meiosis I

There are four phases of meiosis I: Prophase I, Metaphase I, Anaphase I, and Telophase I. They all comprise different steps in the process of cell division.

Step 2: Identify the Longest Phase

In the prophase I of meiosis I, chromosomes pair up to form tetrads, which is a long and complex process. Therefore, Prophase I is often considered the longest phase in meiosis I.

Key Concepts

Prophase I

Prophase I is the first and most intricate stage of meiosis I, a specialized cell division leading to the production of germ cells—sperm in males and eggs in females. This phase is characterized by several critical events essential for genetic diversity.

First, the chromatin condenses into visible chromosomes. Each chromosome consists of two sister chromatids joined at the centromere. The pairing of homologous chromosomes, meaning chromosomes of the same type but from different parents, occurs during this phase, a process known as synapsis. As they pair up, crossing over or recombination takes place; this exchange of genetic material between the homologous chromosomes is a source of genetic variation in the offspring.

The nuclear envelope breaks down during Prophase I, enabling the spindle fibers to contact chromosomes. These changes set the stage for aligned chromosomes to separate accurately in the subsequent phases of meiosis. The complexity and duration of these events make Prophase I the most extended phase in meiosis I.

Meiosis Cell Division

Meiosis is a type of cell division that reduces the chromosome number by half, resulting in the formation of four genetically distinct haploid cells from a single diploid cell. This reduction is vital for sexual reproduction and maintaining the species' chromosome number across generations.

The meiotic process consists of two consecutive divisions: meiosis I and meiosis II. Meiosis I includes Prophase I, Metaphase I, Anaphase I, and Telophase I, with Prophase I being particularly significant for its role in genetic recombination and chromosome pairing. While meiosis II resembles a typical mitosis, dividing each cell produced in meiosis I into two, and thereby focusing on the separation of sister chromatids. Unlike mitosis, which maintains the diploid chromosome number and creates identical daughter cells, meiosis produces cells with half the chromosome count of the original cell—haploid cells.

Tetrads Formation

One of the key processes during Prophase I of meiosis is the formation of tetrads, critical for genetic variation. Tetrads are formations that consist of four chromatids or two homologous chromosomes that come together.

Each chromosome lines up next to its corresponding homolog, creating a pair that consists of four chromatids—this structure is the tetrad. The process of tetrad formation is vital as it facilitates crossing over, where chromatids within a tetrad exchange genetic material. This genetic exchange increases genetic diversity in the offspring by producing new combinations of alleles. Understanding tetrads and their role in genetic diversity is important in grasping the significance of meiosis in evolution and inheritance.