Cytokinesis

After the organelles in a cell are replicated via mitosis, how do we end up with two completely separate daughter cells? The answer is cytokinesis which completes the mitotic phase of the cell cycle, and which often happens concurrently with mitosis.

In this article, we will discuss the definition of cytokinesis and its role in the cell cycle. We will then look at how cytokinesis takes place in animal cells and plant cells. Finally, we will discuss some ways by which the process of cytokinesis is modified in certain organisms.

Recap: What Is The Cell Cycle?

The cell cycle is a set of events involving cell growth and division and results in the formation of two new daughter cells. The cell cycle can be broadly divided into two major phases:

1. Interphase, in which the cell grows, and its nuclear DNA is replicated.

2. Mitotic phase, in which the cell’s replicated DNA and the other contents in its cytoplasm are separated and distributed into daughter nuclei. The cytoplasm undergoes division as well, resulting in two daughter cells.

What Is The Definition Of Cytokinesis?

Now, let's look at the definition of cytokinesis.

Cytokinesis starts in anaphase and concludes in telophase, finishing as the next interphase begins.

Note that cell division is incomplete until all the cell components have been allocated to and entirely split into the two daughter cells.

Although the steps of mitosis are similar in most eukaryotes, the process of cytokinesis in eukaryotes with cell walls, such as plant cells, is substantially different.

Cytokinesis Diagram

A diagram comparing how cytokinesis takes place in animal cells and plant cells is shown below in Figure 1. Take a moment to review Figure 1. We will continue discussing the process of cytokinesis below.

How Does The Process Of Cytokinesis Occur?

The process of cytokinesis generally results in the physical separation of a parent cell into two daughter cells, but exactly how different organisms conduct cytokinesis varies.

In the following section, we will discuss the major events that take place during cytokinesis in animal cells and plant cells.

In animal cells (as well as other cells that lack cell walls), the process of cytokinesis occurs through a cleavage.

During anaphase, a contractile ring made up of actin filaments from the cytoskeleton will form inside the plasma membrane. The mitotic spindle determines where the contractile ring will form, which is typically at the metaphase plate, perpendicular to the spindle's axis. This ensures that division takes place between the two sets of the separated chromosomes.

As the actin filaments interact with myosin molecules, the contractile ring contracts, pulling the cell's equator inward, thereby forming a fissure or crack. This fissure is called the cleavage furrow.

Simultaneously, new membrane is introduced into the plasma membrane next to the contractile ring through intracellular vesicle fusion. This membrane addition compensates for the increase in surface area caused by cytoplasmic division.

The cleavage furrow deepens until the parent cell is eventually split in two. Finally, the microtubule and membrane intercellular connection linking the two daughter cells are cut off in a process called abscission, resulting in two totally distinct cells, each with its own nucleus, organelles, and other cellular components.

Description Of Cytokinesis In Plant Cells

Cytokinesis in plant cells (which have semirigid cell walls) occurs a little differently. A plant cell undergoing cytokinesis does not form a cleavage furrow through a contractile ring; instead, the plant cell builds a new cell wall that would separate the two newly formed daughter cells.

Preparing the cell wall begins back in interphase as the Golgi apparatus stores enzymes, structural proteins, and glucose. During mitosis, the Golgi forms vesicles that store these structural ingredients.

As the plant cell enters telophase, these Golgi vesicles are transported via microtubules to form a vesicular structure called phragmoplast at the metaphase plate.

Then, the vesicles move from the center of the cell toward the cell walls where they fuse together into a structure called cell plate which determines the plane of cell division.

The orientation of the cell plate affects how the two daughter cells would be positioned in relation to nearby cells. Changing the planes of cell division, accompanied by cell enlargement through expansion or growth, results in diverse cell and tissue morphologies that help in determining the plant's structure.

The cell plate continues to grow until its surrounding membrane merges with the plasma membrane around the cell's perimeter. This splits the cell into two daughter cells, each with its own set of organelles, and eventually enzymes harvest the glucose that has built up between the membrane layers to complete the construction of the new cell wall between the two daughter cells.

What Happens After Cytokinesis?

Cytokinesis marks the end of the cell cycle. The DNA has been separated and the new cells have all the cell structures they need to survive. As the cell division is completed, the daughter cells begin their cell cycle. As they cycle through the stages of interphase, they will accumulate resources, duplicate their DNA into matching sister chromatids, prepare for mitosis and cytokinesis, and eventually have their daughter cells as well, continuing the cell division.

What Are Some Examples Of Modifications Of Cytokinesis In Multicellular Organisms?

While cytokinesis generally takes place in the above-mentioned processes, there are interesting exceptions that can be observed in some organisms. Here, we will discuss the concepts of asymmetric division and incomplete cytokinesis.

What Is Asymmetric Division?

Cell division is typically symmetric in the sense that it results in two daughter cells that are similar in size and content. However, there are some cases in developing organisms wherein asymmetric cell division determines the fate of cells.

Asymmetric cell division has been observed in Caenorhabditis elegans (a species of nematode) zygotes and Drosophila (a genus of flies which includes the common fruit fly) neuroblasts.

What Is Incomplete Cytokinesis?

Oogenesis is the growth process in which germ cells are differentiated into mature female gametes called ova.

The ability to stop cytokinesis is important in the maturation process of Drosophila germ cells. In Drosophila, oogenesis begins with the asymmetric division of a stem cell into a daughter stem cell and a cystoblast, which then goes through four rounds of mitosis in the absence of intervening cytokinesis, yielding a 16-germ-cell syncytium.

Cytokinesis is paused at each possible cleavage site, and specialized cytoskeletal structures called ring canals emerge at cleavage furrows. Ring canals, which are similar to typical cleavage furrows in composition, form intracellular bridges between germline cell chambers.