Cytokinesis

SIGNIFICANCE: Cytokinesis is a process, usually occurring concurrent with mitosis, in which the cytoplasm and organelles are divided into two new cells. In eukaryotes, mitosis and meiosis involve division of the nucleus, while cytokinesis is the division of the cytoplasm.

Events Leading to Cytokinesis

Cytokinesis is the division or partitioning of the cytoplasm during the equal division of genetic material into the daughter cells. Before a cell can divide, its genetic material, DNA, has to be duplicated through DNA replication. The identical copies of DNA are then separated into one of the two through a multistep process, which varies among prokaryotes, plants, and animals. With a single chromosome and no nucleus, prokaryotes (such as bacteria) use a simple method of cell division called (meaning “splitting in two”). The single circular DNA molecule is replicated rapidly and split into two. Each of the two circular DNAs then migrates to the opposite pole of the bacterial cell. Eventually, one bacterial cell splits into two through binary fission. On average, a bacterial cell can go through the whole process of cell division within twenty minutes.

In eukaryotes, cell division is a more complex process given the presence of a nucleus and multiple DNA molecules (chromosomes). Each chromosome needs to be replicated in preparation for the division. The process is completed during the interphase. Once replicated, the copies of each chromosome, called sister chromatids, are connected together in a region called the centromere. The chromosomes then go through a process of shortening, condensing, and packing with proteins that make them visible using a light microscope. Chromosomes then migrate and line up at the equator of the parent cell. Then the sister chromatids are separated and pulled to opposite poles. These multiple steps include (cell growth and DNA replication), prophase (disintegration of nuclear envelope, formation of spindle fibers, condensation of chromosomes), metaphase (lining up of chromosomes at equator plate), anaphase (split of two sister chromatids), and telophase (completion of migration of chromatids to opposite poles). Although animal and plant cells share many common features in and mitosis, some noticeable differences in interphase and exist. Even within the animal kingdom, cytokinesis may vary with the type of cell division. Particularly during oogenesis (the process of forming an egg), both I and meiosis II engage in unequal partitioning of cytoplasm that is distinct from normal of animal and plant cells. In some cases, a cell will complete mitosis without cytokinesis, resulting in a multinucleate cell.

Cytokinesis in Animals

In animal cells, cytokinesis normally begins during or and is completed following the completion of chromosome segregation. First, microfilaments attached to the plasma membrane and form a ring around the equator of the cell. This ring then contracts and constricts the cell’s equator, forming a cleavage furrow, much like pulling the drawstring around the waist of a pair of sweatpants. Eventually the “waist” is pinched through and contracts down to nothing, partitioning the cytoplasm equally into two daughter cells. Partitioning the cytoplasm includes distributing cellular organelles so each daughter cell has what is needed for cellular processes.

Cytokinesis in Plants

Cytokinesis in plant cells is different from that in animal cells. The presence of a tough cell wall (made up of cellulose and other materials) makes it nearly impossible to divide plant cells in the same manner as animal cells. Instead, it begins with formation of a cell plate. In early telophase, an initially barrel-shaped system of microtubules called a phragmoplast forms between the two daughter nuclei. The cell plate is then initiated as a disk suspended in the phragmoplast.

The cell plate is formed by fusion of secretory vesicles derived from the Golgi apparatus. Apparently, the carbohydrate-filled vesicles are directed to the division plane by the phragmoplast microtubules, possibly with the help of motor proteins. The vesicles contain matrix molecules, hemicelluloses, and/or pectins. As the vesicles fuse, their membranes contribute to the formation of the plasma membrane on either side of the cell plate. When enough vesicles have fused, the edges of the cell plate merge with the original plasma membrane around the circumference of the cell, completing the separation of the two daughter cells. In between the two plasma membranes is the middle lamella. Each of the two daughter cells then deposits a primary wall next to the middle lamella and a new layer of primary wall around the entire protoplast. This new wall is continuous with the wall at the cell plate. The original wall of the parent cell stretches and ruptures as the daughter cells grow and expand.

Cytokinesis in Sexual Reproduction

In animal oogenesis, the formation of ova, or eggs, occurs in the ovaries. Although the daughter cells resulting from the two meiotic divisions receive equal amounts of genetic material, they do not receive equal amounts of cytoplasm. Instead, during each division, almost all the cytoplasm is concentrated in one of the two daughter cells. In meiosis I, unequal partitioning of cytoplasm during cytokinesis produces the first almost void of cytoplasm, and the secondary oocyte with almost all cytoplasm from the mother cell. During meiosis II, cytokinesis again partitions almost all cytoplasm to one of the two daughter cells, which will eventually grow and differentiate into a mature ovum, or egg. Another daughter cell, the secondary polar body, receives almost no cytoplasm. This concentration of cytoplasm is necessary for the success of because a major function of the mature ovum is to nourish the developing embryo following fertilization.

Key Terms

• binary fissioncell division in prokaryotes in which the plasma membrane and cell wall grow inward and divide the cell in two
• cell cyclea regular and repeated sequence of events during the life of a cell; it ends when a cell completes dividing
• daughter cellscells that result from cell division
• interphasethe phase that precedes mitosis in the cell cycle, a period of intense cellular activities that include DNA replication
• meiosisa type of cell division that leads to production of gametes (sperm and egg) during sexual reproduction
• mitosisnuclear division, a process of allotting a complete set of chromosomes to two daughter nuclei

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