Problem 94 - Objective Biology for NEET

Question

Life cycle in plant has generally two distinct phase the sporophytic and the gametophytic that alternate with each other. This phenomenon is called as alternation of generation. (a) diploid, diploid (b) diploid, haploid (c) haploid, diploid (d) haploid, haploid

Step-by-Step Solution

Answer

The correct answer is (b) diploid, haploid.

Step 1: Identifying Sporophytic and Gametophytic Phases

The sporophytic phase in the life cycle of a plant is the phase when the plant is diploid, meaning it has two sets of chromosomes in its cells. This is the phase when the plant is physically mature and is ready to reproduce.

Step 2: Identifying Gametophytic Phase

The gametophytic phase is when the plant is haploid, meaning its cells have a single set of chromosomes. This is the phase where the plant produces gametes, or sex cells, for reproduction. The gametes are produced by a process called meiosis, which halves the number of chromosomes in the cells.

Step 3: Answer Selection

Combining the properties of sporophytic and gametophytic phases i.e., sporophyte is diploid and gametophyte is haploid. Thus, looking at the options given, the pair that consists a diploid phase followed by a haploid phase is the correct answer.

Key Concepts

Sporophytic Phase

In the life cycle of plants, the sporophytic phase is one of the two fundamental stages. During this phase, the plant is in a diploid state. This means each cell carries two sets of chromosomes—one set from each parent. This is why the plant is often robust during the sporophytic phase. The plant is considered mature in this stage and capable of producing spores. This phase generally dominates the life cycles of many plants, such as ferns and mosses.

• Significance: Primary phase of growth and maturation.
• Major role: Responsible for asexual reproduction.
• Outcome: Produces spores through meiosis that lead to the gametophytic phase.

The sporophytic phase reflects the plant's potential to grow and spread, ensuring the continuation of its genetic material.

Gametophytic Phase

The gametophytic phase is the second key phase following the sporophytic phase in a plant’s life cycle. Here, the plant structure is haploid, meaning each cell contains only one set of chromosomes. During this stage, the plant produces gametes—specialized sex cells that are necessary for sexual reproduction.

• Characteristics: Occurs as a smaller or less noticeable part of the plant.
• Purpose: Ensures genetic diversity through the fusion of gametes.
• Transition: Forms after the haploid spores undergo mitosis.

Ultimately, the gametophytic phase is shorter compared to the sporophytic phase but is crucial for introducing genetic variation through sexual reproduction.

Diploid

In the context of plant life cycles, diploid refers to a state where each cell contains two sets of chromosomes—one from each parent. This diploid nature characterizes the sporophytic phase. It's a key concept because the genetic information is complete and helps plants maintain stability and robustness as they grow and develop.

• Chromosome Number: Two sets of chromosomes (2n).
• Significance: Ensures genetic information is preserved across generations.
• Key Processes: Cellular division and growth are prominent during this phase.

This diploid stage not only supports the plant's physical structure but also primes it for the production of haploid spores through meiosis.

Haploid

The haploid phase is essential to the alternation of generations in plant biology. In this phase, each cell carries only one set of chromosomes. This characteristic is due to meiosis, which reduces the chromosome number by half, producing ingredients for genetic variation and fidelity through sexual reproduction.

• Chromosome Number: One set of chromosomes (n).
• Importance: Haploid spores develop into gametophytes, carrying half genetic material.
• Effect: Ensures sexual reproduction creates genetically unique offspring.

The transition to the haploid phase is a cornerstone in ensuring genetic diversity essential for the adaptation and evolution of plant species.

Meiosis

Meiosis is a critical process in the plant life cycle and is responsible for reducing the chromosome number by half. This reduction leads to the formation of haploid cells from diploid ones. It plays a crucial role in the alternation of generations, ensuring that future generations have genetic variation.

• Process: Comprises two rounds of cell division but only one round of DNA replication.
• Outcome: Produces genetically diverse gametes or spores.
• Significance: Key to both variability and stability in plant populations.

Meiosis ensures that each gamete receives a distinct combination of genes, enabling plants to adapt to changing environments and survive over generations.