Problem 133 - Objective Biology for NEET

Question

Assertion: The complex formed by a pair of synapsed homologous chromosome is called as tetrad. Reason: Each of the homologous chromosomes in meiotic prophase I consists a two closcly apposed sister chromatids.

Step-by-Step Solution

Answer

Both the assertion and the reason are correct and the reason correctly explains the assertion.

Step 1: Confirmation of Assertion

We are to look at the assertion that the complex formed by a pair of synapsed homologous chromosome is called a 'tetrad'. This is indeed correct since in the process of meiosis, when the homologous chromosomes synapse, a structure is formed that is comprised of four chromatids, thus earning the name tetrad.

Step 2: Confirmation of Reason

We proceed to confirm the reason statement which posits each of the homologous chromosomes in meiotic prophase I is comprised of two closely apposed sister chromatids. This is also correct because at the beginning of meiosis, each chromosome replicates to form two sister chromatids which are held together at the centromere.

Step 3: Comparing Assertion with Reason

Having established the correctness of both the assertion and the reason independently, they can be compared to verify whether the reason is an accurate explanation of the assertion. The tetrad formation is indeed due to each homologous chromosome having two closely apposed sister chromatids. Hence, the reason correctly explains the assertion.

Key Concepts

Synapsed homologous chromosomes

In meiosis, a crucial event is the synapsis, where homologous chromosomes pair up closely. These are pairs of chromosomes containing the same genes but potentially different alleles inherited from each parent. During synapsis, these chromosomes line up side by side.
This alignment allows genetic recombination, a vital process that increases genetic diversity. Because they carry similar genetic content, synapsed homologous chromosomes provide a foundation for crossing over.

• This pairing is essential for forming structures like tetrads.
• Synapsed chromosomes ensure accurate separation during meiosis I.

These stages are part of what's known as meiotic prophase I, setting the stage for genetic variation.

Tetrad formation

Tetrads are structures formed when synapsed homologous chromosomes undergo a process called crossing over. This formation is intrinsic to meiosis, particularly in prophase I, and is essential for genetic variation.
Why is it called a tetrad? The name comes from its structure:

• Each pair of synapsed homologous chromosomes consists of four chromatids.
• This quartet of chromatids appears as a cluster during meiosis.

The tetrad structure supports the exchange of genetic material between chromatids, facilitating genetic recombination. This cross-exchange preserves essential genetic instructions while allowing for variation between generations.

Meiotic prophase I

Meiotic prophase I is a critical and intricate phase in the entire meiosis process. This stage sets the groundwork for reducing the chromosome number by half and for genetic interplay by forming tetrads and promoting genetic recombination.
During this stage:

• Homologous chromosomes pair and synapse.
• Tetrads form as crossing over occurs, leading to genetic diversity.
• Sister chromatids become distinct due to DNA replication.

It ends with the separation of homologous chromosomes in meiosis I, paving the way for the second meiotic division. Each step ensures the proper segregation of genetic material during gamete formation.

Sister chromatids

Sister chromatids are identical copies of a chromosome, connected by a region called the centromere. They arise from the replication of a single chromosome during the synthesis phase preceding meiosis.
In meiosis, sister chromatids play a vital role:

• They ensure proper chromosome alignment and attachment to the spindle apparatus.
• During later stages, they allow for proper genetic crossover within tetrads.

Eventually, sister chromatids are separated during meiosis II, ensuring each gamete receives only one copy of each genetic instruction. The presence of sister chromatids is crucial for distributing genetic material accurately during cell division.

Chromatid structure

Inside each chromosome, chromatids embody the structural unit of genetic replication and subsequent division. Their architecture is designed for efficient packaging of DNA and precise distribution during cell division.
Key features of chromatid structure include:

• Chromatids have identical sequences when replicated, critical for forming homologous pairs.
• Each chromatid contains a DNA double helix, supported by proteins for structural integrity.

During cell division, proper chromatid structure ensures error-free segregation. Their role is indispensable for maintaining genome stability and powering evolutionary dynamics through meiosis.