Single replacement and double replacement difference between mitosis
Meiosis is required for genetic variation and continuity of all living organisms. Mitosis, on the other hand, is focused on the growth and development of cells. Meiosis also plays an important role in the repair of genetic defects in germline cells.
Frequently Asked Questions 1. What is mitosis? Mitosis is a form of cell division where the cell splits into two, each identical to the original cell. What is Meiosis? Meiosis is a type of cell division that results in four cells, each having half the number of chromosomes of the original cell. List out the difference between mitosis and meiosis, The difference between mitosis and meiosis are as follows: Mitosis was discovered by Walther Flamming, while meiosis was discovered by Oscar Hertwig.
Cytokinesis occurs only in telophase during mitosis, while it occurs in Telophase 1 and telophase 2 during meiosis. The primary function of mitosis is general growth and repair. It is also used for cell reproduction. Tetrad formation is not observed in mitosis. Tetrad formation is observed in meiosis.
Meiosis, on the other hand, aims to provide genetic diversity through sexual reproduction. Asexual mode of reproduction is observed for mitosis. Sexual mode of reproduction is observed for meiosis. State a few similarities between mitosis and meiosis. The similarities between mitosis and meiosis are as follows: Mitosis and meiosis take place in the cell nuclei. Both involve cell division. In both cycles, the stages are common — prophase, metaphase, anaphase and telophase.
Synthesis of DNA occurs in both. Short Biology Quiz! Q5 Put your understanding of this concept to test by answering a few MCQs. Although cell growth is usually a continuous process, DNA is synthesized during only one phase of the cell cycle, and the replicated chromosomes are then distributed to daughter nuclei by a complex series of events preceding cell division. Progression between these stages of the cell cycle is controlled by a conserved regulatory apparatus, which not only coordinates the different events of the cell cycle but also links the cell cycle with extracellular signals that control cell proliferation.
Phases of the Cell Cycle A typical eukaryotic cell cycle is illustrated by human cells in culture, which divide approximately every 24 hours. As viewed in the microscope, the cell cycle is divided into two basic parts: mitosis and interphase. Mitosis nuclear division is the most dramatic stage of the cell cycle, corresponding to the separation of daughter chromosomes and usually ending with cell division cytokinesis.
During interphase, the chromosomes are decondensed and distributed throughout the nucleus , so the nucleus appears morphologically uniform. At the molecular level, however, interphase is the time during which both cell growth and DNA replication occur in an orderly manner in preparation for cell division. The cell grows at a steady rate throughout interphase , with most dividing cells doubling in size between one mitosis and the next.
In contrast, DNA is synthesized during only a portion of interphase. The timing of DNA synthesis thus divides the cycle of eukaryotic cells into four discrete phases Figure The M phase of the cycle corresponds to mitosis, which is usually followed by cytokinesis. This phase is followed by the G1 phase gap 1 , which corresponds to the interval gap between mitosis and initiation of DNA replication. During G1, the cell is metabolically active and continuously grows but does not replicate its DNA.
G1 is followed by S phase synthesis , during which DNA replication takes place. The completion of DNA synthesis is followed by the G2 phase gap 2 , during which cell growth continues and proteins are synthesized in preparation for mitosis. Figure The division cycle of most eukaryotic cells is divided into four discrete phases: M, G1, S, and G2.
M phase mitosis is usually followed by cytokinesis. S phase is the period during which DNA replication occurs. The cell grows more The duration of these cell cycle phases varies considerably in different kinds of cells.
For a typical rapidly proliferating human cell with a total cycle time of 24 hours, the G1 phase might last about 11 hours, S phase about 8 hours, G2 about 4 hours, and M about 1 hour. Other types of cells, however, can divide much more rapidly. Budding yeasts , for example, can progress through all four stages of the cell cycle in only about 90 minutes. Even shorter cell cycles 30 minutes or less occur in early embryo cells shortly after fertilization of the egg Figure In this case, however, cell growth does not take place.
Instead, these early embryonic cell cycles rapidly divide the egg cytoplasm into smaller cells. There is no G1 or G2 phase , and DNA replication occurs very rapidly in these early embryonic cell cycles, which therefore consist of very short S phases alternating with M phases. Early embryonic cell cycles rapidly divide the cytoplasm of the egg into smaller cells. The cells do not grow during these cycles, which lack G1 and G2 and consist simply of short S phases alternating with M phases.
In contrast to the rapid proliferation of embryonic cells, some cells in adult animals cease division altogether e. Cells of the latter type include skin fibroblasts, as well as the cells of many internal organs, such as the liver, kidney, and lung. As discussed further in the next section, these cells exit G1 to enter a quiescent stage of the cycle called G0, where they remain metabolically active but no longer proliferate unless called on to do so by appropriate extracellular signals.
Analysis of the cell cycle requires identification of cells at the different stages discussed above. Although mitotic cells can be distinguished microscopically, cells in other phases of the cycle G1, S, and G2 must be identified by biochemical criteria. Cells in S phase can be readily identified because they incorporate radioactive thymidine, which is used exclusively for DNA synthesis Figure For example, if a population of rapidly proliferating human cells in culture is exposed to radioactive thymidine for a short period of time e.
The cells were exposed to radioactive thymidine and analyzed by autoradiography. Labeled cells are indicated by arrows. From D. Stacey et al. Cell Biol. Variations of such cell labeling experiments can also be used to determine the length of different stages of the cell cycle. For example, consider an experiment in which cells are exposed to radioactive thymidine for 15 minutes, after which the radioactive thymidine is removed and the cells are cultured for varying lengths of time prior to autoradiography.
Radioactively labeled interphase cells that were in S phase during the time of exposure to radioactive thymidine will be observed for several hours as they progress through the remainder of S and G2. In contrast, radioactively labeled mitotic cells will not be observed until 4 hours after labeling. This 4-hour lag time corresponds to the length of G2—the minimum time required for a cell that incorporated radioactive thymidine at the end of S phase to enter mitosis. Cells at different stages of the cell cycle can also be distinguished by their DNA content Figure For example, animal cells in G1 are diploid containing two copies of each chromosome , so their DNA content is referred to as 2n n designates the haploid DNA content of the genome.
DNA content then remains at 4n for cells in G2 and M, decreasing to 2n after cytokinesis. A population of cells is labeled with a fluorescent dye that binds DNA. The cells are then passed through a flow cytometer, which measures the fluorescence intensity of individual cells. The data are plotted as cell more Regulation of the Cell Cycle by Cell Growth and Extracellular Signals The progression of cells through the division cycle is regulated by extracellular signals from the environment, as well as by internal signals that monitor and coordinate the various processes that take place during different cell cycle phases.
An example of cell cycle regulation by extracellular signals is provided by the effect of growth factors on animal cell proliferation. In addition, different cellular processes, such as cell growth, DNA replication, and mitosis , all must be coordinated during cell cycle progression.
This is accomplished by a series of control points that regulate progression through various phases of the cell cycle. A major cell cycle regulatory point in many types of cells occurs late in G1 and controls progression from G1 to S. However, passage through START is a highly regulated event in the yeast cell cycle, where it is controlled by external signals, such as the availability of nutrients, as well as by cell size.
For example, if yeasts are faced with a shortage of nutrients, they arrest their cell cycle at START and enter a resting state rather than proceeding to S phase. Thus, START represents a decision point at which the cell determines whether sufficient nutrients are available to support progression through the rest of the division cycle. Polypeptide factors that signal yeast mating also arrest the cell cycle at START, allowing haploid yeast cells to fuse with one another instead of progressing to S phase.
In addition to serving as a decision point for monitoring extracellular signals, START is the point at which cell growth is coordinated with DNA replication and cell division. The importance of this regulation is particularly evident in budding yeasts , in which cell division produces progeny cells of very different sizes: a large mother cell and a small daughter cell.
In order for yeast cells to maintain a constant size, the small daughter cell must grow more than the large mother cell does before they divide again. Thus, cell size must be monitored in order to coordinate cell growth with other cell cycle events. This regulation is accomplished by a control mechanism that requires each cell to reach a minimum size before it can pass START. Consequently, the small daughter cell spends a longer time in G1 and grows more than the mother cell.
The proliferation of most animal cells is similarly regulated in the G1 phase of the cell cycle. In particular, a decision point in late G1, called the restriction point in animal cells, functions analogously to START in yeasts Figure
