Problem 13 - Campbell Biology Concepts & Connections

Question

An organism called a plasmodial slime mold is one large cytoplasmic mass with many nuclei. Explain how such a "megacell" could form.

Step-by-Step Solution

Answer

A 'megacell' forms through repeated nucleus division without cytokinesis and potential cell fusion, resulting in a large cytoplasmic mass with many nuclei.

Step 1: Understanding the Definition

A plasmodial slime mold is described as a large cytoplasmic mass containing many nuclei. This structure is often referred to as a 'megacell.' To understand how this megacell forms, we need to look at the lifecycle and cellular processes involved.

Step 1 - Nucleus Division without Cytokinesis

During the growth phase, the nuclei of the slime mold undergo repeated divisions. Unlike typical cells, these divisions do not include cytokinesis, the process where the cell splits into two separate cells. Thus, multiple nuclei share the same cytoplasm.

Step 2 - Cell Fusion

In some cases, individual cells can fuse together to form a larger cell structure. This fusion combines the cytoplasms and nuclei of the cells, contributing to the formation of a large cell, or 'megacell.'

Step 3 - Prevention of Cytoplasmic Division

The slime mold's cellular processes are regulated to prevent the cytoplasm from dividing even when the nuclei divide. This leads to continuous growth of the cytoplasmic mass containing many nuclei.

Step 4 - Resulting Megacell Formation

As a result of repeated nuclear divisions without cytokinesis and the potential fusion of individual cells, a 'megacell' or plasmodial slime mold with multiple nuclei forms. This structure allows the slime mold to cover large areas and efficiently search for food.

Key Concepts

cytoplasmic mass

A cytoplasmic mass refers to the large, fluid component within a cell, excluding the nucleus. In plasmodial slime molds, the cytoplasmic mass is unique because it hosts many nuclei. These molds do not divide their cytoplasm during nucleus division, creating a huge, shared space for multiple nuclei. This setup allows for better nutrient flow and energy distribution.

In typical cells, cytokinesis divides the cytoplasmic mass, resulting in two separate cells. However, in plasmodial slime molds, the cytoplasm remains intact, leading to the formation of a larger structure. Think of it as a jelly-like substance where all the important activities occur, just with many command centers (nuclei) instead of one.

nucleus division

Nucleus division is the process where the genetic material within a nucleus is duplicated and divided. In plasmodial slime molds, this process occurs repeatedly without the cell undergoing cytokinesis. This means that the nucleus splits, but the cell itself does not.

In normal eukaryotic cells, after the nucleus divides, the cell follows with cytokinesis. This results in each daughter cell receiving its own nucleus. However, in the case of slime molds, the cellular machinery ensuring cytokinesis is absent or inactive. As a result, the nuclei keep dividing within a single shared cytoplasm, resulting in a multinucleated 'megacell' or large syncytium.

cytokinesis

Cytokinesis is the process where the cell's cytoplasm divides, resulting in two distinct cells. In most organisms, cytokinesis occurs after nucleus division, ensuring that each new cell has its own nucleus and cytoplasm.

In plasmodial slime molds, this stage is skipped. The nuclei continue to divide without the cytoplasm following suit. This lack of cytokinesis is crucial for the formation of a megacell. Multiple nuclei remain within the same cytoplasmic mass. This configuration allows the slime mold to efficiently cover large areas, as it is not restricted to the confines of single, small cells.

The prevention of cytokinesis in plasmodial slime molds is a unique adaptation. It serves ecological advantages, such as enabling the slime mold to consume more resources and rapidly shift its shape and structure in response to environmental conditions.