Question

Which type of life cycle has both a haploid and diploid multicellular stage? a. an asexual life cycle b. diploid-dominant c. haploid-dominant d. alternation of generations

Short answer

The type of life cycle that has both a haploid and diploid multicellular stage is: d. alternation of generations

Step-by-step solution

Definition of Haploid and Diploid Stages

Haploid cells contain a single set of chromosomes, represented as (n). Diploid cells, on the other hand, contain two sets of chromosomes, represented as (2n). In a life cycle, organisms may have stages with either haploid (n) or diploid (2n) cells.

Option A: Asexual life cycle

An asexual life cycle involves the production of offspring through mitotic cell division without any exchange of genetic material. It mainly contains a diploid stage (2n). Therefore, this option cannot be the correct answer as it does not have both haploid and diploid stages.

Option B: Diploid-dominant

In a diploid-dominant life cycle, the majority of the life cycle is spent in the diploid (2n) stage. Haploid cells (n) are produced during the process of meiosis, and they quickly fuse to form a diploid zygote, which then goes through mitosis to produce a multicellular diploid organism. The haploid stage is brief and not multicellular, so option B is not the correct answer.

Option C: Haploid-dominant

In a haploid-dominant life cycle, the majority of the life cycle is spent in the haploid (n) stage. The only diploid phase occurs when two haploid cells fuse to form a zygote, which then immediately undergoes meiosis to produce new haploid cells. The diploid stage is brief and not multicellular, so option C is not the correct answer.

Option D: Alternation of generations

In the alternation of generations life cycle, both haploid and diploid stages are multicellular. This life cycle is typically found in plants, algae, and some fungi. In this cycle, both the haploid (gametophyte) and diploid (sporophyte) stages form multicellular organisms. Since this option involves both haploid and diploid multicellular stages, option D is the correct answer. The type of life cycle that has both a haploid and diploid multicellular stage is: d. alternation of generations

Key concepts

Haploid Stage

The haploid stage is a phase in a life cycle where the cells contain a single set of chromosomes. This stage is represented by the symbol \( n \). In organisms, haploid cells are typically the gametes, such as sperm and egg cells in animals, or spores in plants.

• In animals, the haploid stage is usually limited to the gametes themselves. These cells will combine during fertilization to create a diploid zygote.
• In plants and fungi, the haploid stage can be more predominant. For example, mosses spend a greater part of their life in the haploid stage, with the entire visible plant being haploid.

Understanding the role of haploid stages is crucial because it explains how genetic diversity is passed and maintained across generations. Without the haploid stage, sexual reproduction wouldn't result in genetically diverse offspring.

Diploid Stage

The diploid stage is when cells have two sets of chromosomes, symbolized as \( 2n \). This stage is predominant in many organisms, especially those that are diploid-dominant.

• Most animals, including humans, primarily exist as multicellular diploid organisms. Here, the diploid structure contains paired chromosomes – one set inherited from each parent.
• In plants, the diploid stage is highly prominent in the form of the sporophyte. It produces spores through meiosis that develop into the haploid gametophyte stage.

During this stage, organisms have increased genetic material relatively, which may result in greater evolutionary advantages, such as increased size and complexity. Diploid stages thus play a significant part in genetic stability and diversity through their role in sexual reproduction.

Life Cycle Stages

The concept of life cycle stages revolves around understanding how an organism grows, develops, and reproduces. There are typically three types of life cycles based on whether haploid or diploid stages are predominant:

• **Diploid-Dominant**: Seen in animals, where the diploid stage is long-lasting, and the haploid stage is only the gametes. After fertilization, the resulting zygote is diploid and undergoes mitotic growth.
• **Haploid-Dominant**: Most of the life cycle is spent in the haploid stage, common in fungi and some algae. The diploid stage occurs only temporarily during sexual reproduction.
• **Alternation of Generations**: Found in plants and some algae, this cycle alternates between multicellular haploid (gametophyte) and diploid (sporophyte) stages, both substantially contributing to the organism's life cycle.

Each life cycle stage is significant as it provides an understanding of how genetic material is organized and passed on, offering insights into evolutionary strategies adopted by different life forms.