Cytokinesis: What is it? & What Happens in Plants & Animal Cells?

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Cytokinesis is the process of cytoplasm being divided in eukaryotic cells to produce two distinct daughter cells that are identical to each other. It occurs at the end of the parent cells cycle after meiosis or mitosis when a cleavage furrow or a cell plate is constructed to divide the cell membrane into two new cells. In order to understand the process of cytokinesis, it is important to learn about some common terms such as chromosomes, centromeres, telomeres and cytoplasm located in a cell.

What Are Chromosomes?

Chromosomes are small threadlike structures that are located inside the nucleus of both animal and plant cells. Animal and plant cells are considered eukaryotes and are diploid cells in which the genetic material of DNA in chromosome form is contained in a distinct nucleus.

Each chromosome contains protein and a single molecule of DNA. The DNA makes each organism unique, as it is passed down to daughter cells from the parent cells or parents to offspring. Chromosomes are Greek words for chroma or color and soma or body. They received this name from scientists because the cell structures are stained in bright colors to differentiate them while doing research.

Do All Animals and Plants Have the Same Number of Chromosomes?

Each species of animals and plants have a set number of chromosomes but not always the same amount. For example, humans have 23 pairs of chromosomes from their mother and their father for a grand total of 46 chromosomes in the human body. A dog has 39 pairs of chromosomes, rice plants have 12 pairs of chromosomes and a fruit fly only has four pairs of chromosomes.

What Are Centromeres?

A centromere is the constricted area of a chromosome. Unlike the way it sounds, the centromere is not in the center of a chromosome, and it can actually be near one end of a linear chromosome. The job of a centromere is to keep the chromosomes properly aligned during the process of cell division. The centromere contains the copies of chromosomes to divide into two sister cells as chromotids, one for each sister cell.

What Are Telomeres?

Telomeres are located at the ends of chromosomes as repetitive stretches of DNA that protect each chromosome. Some cells lose a small amount of DNA from the telomeres each time the cells divide. When the telomere is depleted, it will die. White blood cells divide quickly and have an enzyme in the telomeres to prevent the chromosomes from losing any DNA in the telomeres. These types of cells live longer than others.

What Is Cytoplasm?

A cell has a nucleus and an outer membrane that keeps all the content inside the cell. Cytoplasm is the term for all of the content outside of the nucleus but inside of the cell membrane. It is mainly water but also includes salts, enzymes, organic molecules and organelles, which have a specific function within a cell.

The cytoplasm has an important function in a cell to support and suspend the organelles in its liquid. The cytoplasm supports many items such as protein synthesis, cell division of mitosis and meiosis as well as the first stage of cellular respiration. Cytoplasm also moves materials in a cell such as hormones, and it dissolves all cellular waste of a parent cell when it divides into two daughter cells in a diploid cell of an animal or plant.

Cytoplasm has two primary parts called the endoplasm and the ectoplasm. The endoplasm is located at the central area of the cytoplasm, and it contains the organelles that are suspended. The ectoplasm is a thicker gel type liquid on the outer edges of the cell's cytoplasm.

What Is the M Phase?

The M phase in cell division is the mitotic phase in the cell cycle. In this phase, the cell undergoes a major reorganization of almost all of the cell components. The chromosomes condense, the nuclear envelope surrounding the cell as a cell wall breaks down, and the cytoskeleton changes to form a mitotic spindle while the chromosomes move to opposite poles or ends of the cell. The cytokinesis definition is the phase after the M phase that separates the chromosomes into two complete and identical cells from the parent cell, called daughter cells.

What Is the Cell Cycle of Division?

The entire cycle of a cell goes through many changes before the original parent cell is divided into two distinct but identical daughter cells. The actual division of the two daughter cells occurs in the cytokinesis stage, which is the last stage in the cycle. At this point the parent cell dies and is absorbed by the organism of the eukaryotic cell of humans and plants. There are seven distinct stages of the mitosis cell division including interphase, prophase, prometaphase, metaphase, anaphase, telophase and cytokinesis.

Interphase is the stage that a cell stays in for most of its life. The cell is engaging in metabolic activity in order to prepare for mitosis and cell division. In this phase, you cannot easily see the chromosomes in the nucleus, but a dark spot can be seen to show the nucleus.

Prophase is the stage when chromatin in the nucleus starts to condense and become visible as chromosomes. The nucleus itself actually disappears as centrioles start moving to opposite ends or poles of the cell. Centrioles are a tiny cylindrical organelle near the nucleus that occur in pairs and are part of the forming of spindle fibers. Spindle fibers are forming and extending from the centromeres, and some of them cross the cell to form the mitotic spindle of fibers.

The prometaphase is the next stage of mitosis where the nuclear membrane dissolves at the beginning of this phase. Proteins will then attach to the centromeres to create kinetochores. Kinetochores are protein structures on chromatids containing the spindle fibers to pull the sister chromatids apart. Microtubules will then attach at the kinetochores, and the chromosomes begin moving in the cell.

The metaphase stage of cell division is designated as the time that spindle fibers align the chromosomes in the middle of the parent cell's nucleus. This line of chromosomes is called the metaphase plate. The metaphase plate ensures that when the chromosomes are split to form two daughter cells, each new nucleus in the daughter cells will get one copy of each chromosome.

The anaphase stage is next, in which the paired chromosomes separate at the kinetochores and move to opposite poles or ends of the cell. Kinetochore movement among spindle microtubules and the physical interaction of polar microtubules allow the movement of the chromosomes.

The telophase is when the chromatids arrive at the opposite poles of the cell. New cell membranes start to form around the daughter nuclei. The chromosomes will disperse and are no longer visible under a microscope. The spindle fibers also disperse, and cytokinesis or partitioning of the cell may start to begin.

Cytokinesis is the final stage of cell division. In both animal and plant cells, the two daughter cells are partitioned off to form a new membrane and complete the cell division of two identical daughter cells, each with one nucleus.

What Is Mitosis and Meiosis?

Mitosis and meiosis are both forms of cell division in which the parent cell is a diploid cell with two sets of chromosomes, one from each parent cell. In mitosis, the DNA in a cell is duplicated and divided between the two daughter cells. All somatic body cells duplicate by mitosis including fat cells, skin cells, blood cells and all cells that are not sex cells. Mitosis occurs to replace dead or damaged cells or to help an organism grow.

Meiosis is the process of sex cells called gametes when they generate in organisms to reproduce sexually. Gametes are produced in male and female sex cells and have one half the number of chromosomes as the original or parent cell. Through new gene combinations, this process produces four new cells that are genetically different from each other.

What Is the Difference between Cytokinesis in Animal and Plant Cells?

Cell division or cytokinesis in mitosis or meiosis is very similar. The cellular signals tell a cell when it needs to divide and when to stop dividing. There is a region of division to separate the two daughter cells in both processes; however, the division plate is slightly different between animal cells and plant cells.

In animals, the region of division is a division plate. Cytokinesis in animal cells forms a division plate and around this area, the cytokinetic furrow forms and will eventually pinch off the two cells to separate them. The final process in animal cells is called abscission when the actin-myosin contractile ring that created the cytokinetic furrow contracts all around and the outer plasma membranes of each cell undergo fission to separate the two daughter cells completely.

Actin and myosin are the same proteins that cause muscles to contract in muscles cells. The muscle cells are full of actin filaments, and the protein myosin pulls them together with ATP energy. As the actin fibers pull together, it creates a smaller ring. All of the cytoplasm and organelles are eventually excluded from the ring, leaving the midbody structure, which also has to separate through the process of abscission.

In plant cells, the cells are surrounded by a secondary layer as a plant wall, and they are more rigid than animal cells. Cytokinesis in plant cells involves plants using spindle structures called phragmoplasts to carry vesicles of the cell wall material such as cellulose to the new cell plate. The cell wall material forms a complex and strong area. After the plate divides the plant cells into two daughter cells, the plasma membrane seals off and fully separates the two new cells.

What Are Symmetrical and Asymmetrical Cytokinesis?

Symmetrical cytokinesis is when cells divide evenly, such as diploid animal and plant cells in the mitosis process of cell division. During male meiosis when the sex cells are dividing, all four cells at the end of the division have the same size and are close to the number of organelles in each one. This is the process of spermatogenesis to produce millions of small and mostly equal number of organelles in each one in a symmetrical manner.

Asymmetrical cytokinesis occurs when one or more of the cells divide unevenly, and some retain a majority share of the cytoplasm. For example in human oogenesis, or the female's reproductive process, the cells divide through asymmetrical cytokinesis. It produces one very large cell with the addition of three polar bodies. The three polar bodies do not become eggs; however, the eggs that are produced are much larger cells. This process makes only one egg each time the female reproductive cells divide to produce much fewer eggs than the amount of male sperm.

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