Main difference – Mitosis vs Cytokinesis
Mitosis and cytokinesis are two different processes that occur in the cell division cycle. The term mitosis refers to the nuclear division stages, prophase, prometaphase, metaphase, anaphase, and telophase of the cell cycle. Nuclear division in mitosis is known as karyokinesis. However, the term cytokinesis refers to the cytoplasmic division that follows the mitosis. Mitosis is more or less similar in all eukaryotic organisms including animals and higher plants. But, cytokinesis strictly depends on the cell type, animal or plant. This can be identified as the main difference between mitosis and cytokinesis. During karyokinesis, cells undergo a series of steps to divide the duplicated chromosomes into two equal sets. On the other hand, during cytokinesis, cells undergo a series of processes to divide the cell content into two equal halves.
What is Mitosis
Mitosis is consists of five stages, prophase, prometaphase, metaphase, anaphase, and telophase.
Prophase
Each chromosome in a prophase cell consists of two sister chromatids attached to one centromere. In this stage, chromosomes become more condensed and can be seen under a light microscope. At this stage, mitotic spindle consisting of microtubules and other proteins forms between the two pairs of centrioles as they drift to opposite poles of the cell. However, this structure can not be seen in some plant cells.
Prometaphase
Prometaphase starts with degeneration of nuclear membrane. Some spindle fibers attach to the centromere regions of the chromosomes. Microtubules attach to the either sides of the sister chromatids, to kinetochores. Then the other end of these microtubules attaches to the centrosome of the opposite poles.
Metaphase
In this phase, chromosomes are arranged along the center of the cell, metaphase plate as a single line.
Anaphase
After the metaphase connection between sister, chromatids break down, and chromatids move to the opposite direction from each other, i.e., towards centrosomes. Molecular motor proteins disassemble tubulin molecules in the spindle and generate force so that chromosomes are pulled towards the opposite poles from each other.
Telophase
Once chromatids moved to the spindle poles, the chromatids are referred as chromosomes. During this stage, nuclear membrane re-forms around each set of chromosomes and produce two distinct nuclei within the cell. Chromosomes also start to relax; hence, the condensation disappears. Usually telophase is followed by the cytokinesis.
What is Cytokinesis
Cytokinesis is the process of division of cytoplasm at the end of the cell division cycle; either mitosis or meiosis. Cytokinesis starts in early stages of mitosis, anaphase and ends in telophase. There are special features of cytokinesis depending on the cell type, prokaryotes, and animal or plant.
Binary Fission in Prokaryotes
Bacterial cells divide by a process called binary fission. The circular DNA molecule replicates, usually at one end of the cell and translocate to opposite end. Cell membrane constricts (invagination) to form new cells. Finally, new cell materials are deposited on new cells to form cell walls.
Cytokinesis in Animal Cells
Cytokinesis in animals or many unicellular eukaryotes can be divided into four stages; initiation, contraction, membrane insertion, and completion. Let’s consider those in detail.
Initiation
Cytokinesis of animal cells is marked by the formation of a cleavage furrow on the cell surface. The structure that is responsible for the cleavage is called contractile ring. The contractile ring consists of actin filaments, myosin II filaments, and many structural and regulatory proteins. Mitotic spindle fibers determine the location of the contractile ring, i.e. the plane of cell division.
Contraction
The contractile ring components accumulate just beneath the plasma membrane and contract to constrict the cell into two.
Membrane Insertion
While contraction takes place, the new membrane is formed adjacent to the contractile ring by the fusion of intracellular vesicles.
Completion
Cytokinesis is completed once the mother cell divides into two daughter cells through the constriction of the contractile ring and formation of new membrane to fill the gap of new cytoplasm.
Cytokinesis in Plant Cells
Plant cell cytokinesis is significantly different from that of animal cells due to the presence of a semi-rigid cell wall (consist of cellulose, hemicellulose, pectin, etc.). Therefore, separation by the formation of contractile ring is not possible in plant cells. Plant cells produce a structure called cell plate, a new cell wall from inside to outside of the cell.
A structure known as preprophase band, an actin filament ring formed during G2 phase initially determine the cell plate position and direction. Cell plate formation starts in anaphase and is guided by a structure called phragmoplast, collection of microtubules of the mitotic spindle at the center of the cell. Small vesicles that contain polysaccharides and glycoproteins necessary for the new cell wall formation are transported through microtubules to the phragmoplast. These vesicles fuse to form disk-like flat bodies. This early cell plate grows by fusion of vesicles until it reaches the original cell wall forming two cells separated from the cell membrane and middle lamella. Finally, cellulose microfibrils are accumulated within the matrix of the cell plate to form the primary cell wall.
Special Features of Cytokinesis
Mitosis can occur without cytokinesis. Some cells undergo several rounds of nuclear divisions without cytokinesis
Drosophila embryo – first 13 rounds of nuclear division without cytokinesis resulting a cell with 6000 nuclei. Then cellularization, formation of cell membrane around each nucleus takes place
Mammalian cells – Osteoclasts (type of bone cells), trophoblasts (forms outer layer of blastocytes), hepatocytes (liver cells), and heart muscle cells
Asymmetrically dividing cells
Most of the time cells divide symmetrically to share nearly equal amounts of cell contents. However, in some instances they divide asymmetrically, differ in size, and cytoplasmic contents.
Formation of the ovum in ovaries – ovum or egg cell take up most of the cytoplasm while leaving a little content to polar bodies.
Difference Between Mitosis and Cytokinesis
Occurrence
Mitosis process is more or less the same for eukaryotic cells.
Cytokinesis process differs according to the cell type. i.e. plant or animal.
Nuclei
Mitosis produces two nuclei.
Cytokinesis produces two cells that enclosed two nuclei.
Karyokinesis
Mitosis includes only the karyokinesis.
Cytokinesis includes cytoplasm division after the karyokinesis.
Order of the Processes
Mitosis occurs before cytokinesis.
Cytokinesis occurs after Mitosis.
Separation
Mitosis separate only the genetic material.
Cytokinesis separate cell organelles and cytoplasm.
Equality in Separation
During Mitosis, two equal sets of chromosomes separate into two cells.
During Cytokinesis, organelle separation is not exactly equal. In some cases, cell size also differs.
References
Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. Cytokinesis. Available from: http://www.ncbi.nlm.nih.gov/books/NBK26831/
Cytokinesis from http://www.gordon.edu/download/pages/Cytokinesis-Salem2003.pdf
Image Courtesy:
“Mitosis diagram” by Marek Kultys – Own work.(CC BY-SA 3.0) via Wikimedia Commons
“Cytokinesis eukaryotic mitosis” by LadyofHats (Public Domain) via Wikimedia Commons
