Question

A pollen grain with 120 chromosomes undergoes normal development. What will be the chromosome number in each male gamete formed later?

• A. 120
• B. 240
• C. 60
• D. 30

Hint:

Male gametes are produced by mitosis of the generative cell, so chromosome number remains unchanged.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
This question tests the understanding of ploidy levels and the type of cell division involved in the formation of male gametes in plants.
Pollen grains are formed through meiosis, making them haploid ($n$).
Any further division of a haploid cell to produce more cells of the same generation must be mitotic.
Key Formula or Approach:
1. Identify the starting ploidy: Pollen grain = Haploid ($n$).
2. Identify the process: Development into gametes = Mitotic division of the generative cell.
3. Apply the rule of mitosis: In mitosis, the chromosome number ($n$) remains constant from parent to daughter cell.
Step 2: Detailed Explanation:
In the life cycle of an angiosperm, the diploid ($2n$) sporophyte produces haploid ($n$) microspores through meiosis.
Each microspore develops into a pollen grain. Therefore, the pollen grain is already haploid.
In this question, the chromosome number of the pollen grain is given as 120. This means \(n = 120\).
Inside the pollen grain, the nucleus divides to form a large vegetative cell and a small generative cell.
To form the final male gametes, the generative cell undergoes mitosis.
Mitosis is an equational division. It does not reduce the chromosome number (which would be meiosis) and it does not double the chromosome number (which would be fertilization).
Consequently, when the generative cell (with 120 chromosomes) divides mitotically, it produces two male gametes, each of which will also contain 120 chromosomes.
Option (B) is wrong because doubling only occurs during fertilization (syngamy).
Option (C) and (D) are wrong because meiosis has already occurred prior to the formation of the pollen grain.
Step 3: Final Answer:
The chromosome number remains 120 in each male gamete because they are formed by mitosis of a haploid cell.