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From Wikipedia

Binary asteroid

A binary asteroid is a system of two asteroids orbiting their common center of mass, in analogy with binary stars. 243 Ida was the first binary asteroid to be identified when the Galileo spacecraft did a flyby in 1993. Since then numerous binary asteroids have been detected.

When both binary asteroids are similar in size, they are sometimes called "binary companions", "double asteroids" or "doublet asteroids". A good example of a true binary companion is the 90 Antiope system. Binary asteroids with a small satellite, called a "moonlet", have been more commonly observed (see 22 Kalliope, 45 Eugenia, 87 Sylvia, 107 Camilla, 121 Hermione, 130 Elektra, 243 Ida, 283 Emma, 379 Huenna, etc.). They are also called high-size ratio binary asteroid systems.

Paired impact craters, such as the Clearwater Lakes in Canada, may have been formed by binary asteroids.

Several theories have been posited to explain the formation of binary asteroid systems. Recent work suggests that most of them have a significant macro-porosity (a "rubble-pile" interior). The satellite orbiting around large main-belt asteroids such as 22 Kalliope, 45 Eugenia or 87 Sylvia could have formed by disruption of a parent body after impact or fission after an oblique impact. Transneptunian binary asteroids may have formed during the formation of the solar system by mutual capture or three-body interaction. Near-Earth asteroids which orbit in the inner part of our solar system may have split by tidal disruption after a close encounter with a terrestrial planet. A possible explanation for the relatively greater occurrence of binary asteroids near or inside of Earth's orbit was described in the journal Nature (10 June 2008): this theory states that when solar energy (see YORP effect) spins a “rubble pile� asteroid to a sufficiently fast rate, material is thrown from the asteroid’s equator. This process also exposes fresh material at the poles of the asteroid.


Cytokinesis

Cytokinesis, from the greek cyto- (cell) and kinesis (motion, movement), is the process in which the cytoplasm of a single eukaryotic cell is divided to form two daughter cells. It usually initiates during the late stages of mitosis, and sometimes meiosis, splitting a binucleate cell in two, to ensure that chromosome number is maintained from one generation to the next. In animal cells, one notable exception to the normal process of cytokinesis is oogenesis (the creation of an ovum in the ovarian follicle of the ovary), where the ovum takes almost all the cytoplasm and organelles, leaving very little for the resulting polar bodies, which then die. In plant cells, a dividing structure known as the cell plate forms across the centre of the cytoplasm and a new cell wall forms between the two daughter cells.

Cytokinesis is distinguished from the prokaryotic process of binary fission.

Contractile ring positioning

During different proliferative divisions,barnacles and animal cell cytokinesis begins shortly after the onset of sister chromatid separation in the anaphase of mitosis. A contractile ring, made of non-muscle myosin II and actin filaments, assembles equatorially (in the middle of the cell) at the cell cortex (adjacent to the cell membrane). Myosin II uses the free energy released when ATP is hydrolysed to move along these actin filaments, constricting the cell membrane to form a cleavage furrow. Continued hydrolysis causes this cleavage furrow to ingress (move inwards), a striking process that is clearly visible through a light microscope. Ingression continues until a so-called midbody structure (composed of electron-dense, proteinaceous material) is formed and the process of abscission then physically cleaves this midbody into two. Abscission depends on septin filaments beneath the cleavage furrow, which provide a structural basis to ensure completion of cytokinesis. After cytokinesis, non-kinetochore microtubules reorganize and disappear into a new cytoskeleton as the cell cycle returns to interphase (see also cell cycle).

The position at which the contractile ring assembles is dictated by the mitotic spindle. This seems to depend upon the GTPaseRhoA, which influences several downstream effectors (such as the protein kinases ROCK and citron) to promote myosin activation (by influencing the phosphorylation of Myosin regulatory light chain (rMLC)) and actin filament assembly (by regulating formin protein) at a particular region of the cell cortex.

Simultaneous with contractile ring assembly during prophase, a microtubule based structure termed the central spindle (or spindle midzone) forms when non-kinetochore microtubule fibres are bundled between the spindle poles. A number of different species including H. sapiens,D. melanogasterandC. elegansrequire the central spindle in order to efficiently undergo cytokinesis, although the specificphenotype described when it is absent varies from one species to the next (for example, certain Drosophila cell types are incapable of forming a cleavage furrow without the central spindle, whereas in both C. elegansembryos and human tissue culture cells a cleavage furrow is observed to form and ingress, but then regress before cytokinesis is complete). Seemingly vital for the formation of the central spindle (and therefore efficient cytokinesis) is a heterotetrameric protein complex called centralspindlin. Along with associated factors (such as SPD-1 in C. elegans), centralspindlin plays a role in bundling microtubules to form the spindle midzone during anaphase.

Timing cytokinesis

Cytokinesis must be temporally controlled to ensure that it occurs only after sister anaphase separation during normal proliferative cell divisions. To achieve this, many components of the cytokinesis machinery are highly regulated to ensure that they are able to perform a particular function at only a particular stage of the cell cycle.

Plant cell cytokinesis

Due to the presence of a cell wall, cytokinesis in plant cells is significantly different from that in animal cells. Rather than forming a contractile ring, plant cells construct a cell plate in the middle of the cell. The stages of cell plate formation include (1) creation of the phragmoplast, an array of microtubules that guides and supports the formation of the cell plate; (2) trafficking of vesicles


From Yahoo Answers

Question:i have to do a report for biology about amoebas and i need facts. help please!

Answers:or amoeba (both: symbolm bsymbol) , common name for certain one-celled organisms belonging to the phylum Sarcodina of the kingdom Protista. Amebas were previously classified as members of the animal kingdom. Most amebas are very small (from 5 to 20 microns in diameter) and contain a single nucleus. A. proteus averages 0.25 mm in length. Members of the genus Pelomyxa, however, may be well over a millimeter (up to 8 mm) in diameter and may contain hundreds of nuclei. Amebas constantly change the shape of their bodies as a result of the phenomenon known as ameboid movement, involving the formation of temporary extensions (pseudopodia, or false feet) of the body. Pseudopodia, used in locomotion and feeding, may be rounded at the tip (lobopodia), pointed (filopodia), branched and fused together (rhizopodia), or somewhat rigid and pointed (axopodia). Although simple in form, amebas are very successful organisms and are found abundantly in a variety of habitats all over the world. Amebas live in freshwater, the oceans, and in the upper layers of the soil, and many have adapted to a parasitic life on the body surface of aquatic animals or in the internal organs of both aquatic and terrestrial animals. Few animals escape invasion by some type of ameba. Some are harmless, but others are pathogenic and cause serious diseases; e.g., Entamoeba histolytica causes amebic dysentery, which is fatal if untreated. The many genera of amebas were given their common name because of their resemblance to the genus Amoeba (order Amoebida), which includes several large, common species of which the freshwater Amoeba proteus is the most familiar. The term ameba is sometimes also used to refer to other unicellular protists (e.g., slime molds) that have ameboid features such as pseudopodia. Other ameboid protozoans of the phylum Sarcodina include the marine radiolarians, which form silicate skeletons; their freshwater counterparts, the heliozoans; and the shell-bearing foraminiferans. Digestion and Respiration In a process known as phagocytosis, amebas engulf their prey, or particles of appropriate size, with their pseudopodia, forming food vacuoles. Digestive enzymes, manufactured and secreted by the organism, are then poured into these vacuoles, and the particles are digested. Useful compounds are subsequently absorbed into the ameba's body. Useless residues remain in the vacuoles and are ultimately expelled (egested) as the vacuole comes in contact with the membrane at the body surface. Amebas can distinguish food (e.g., algae, diatoms, bacteria, and other protozoans) from other material and use different tactics in approaching different food. Freshwater amebas take up water constantly through the process of osmosis, and water content is regulated with a pulsating contractile vacuole. Marine amebas lack a contractile vacuole. Respiration is by diffusion of gases through the cell membrane. Reproduction Under favorable conditions amebas divide by binary fission (splitting) to produce two daughter amebas, the nucleus dividing by mitosis. When an ameba is divided artificially, the portion containing the nucleus forms a new cell membrane and continues as a whole animal, while the other portion lives only as long as its present food supply lasts, ultimately dying, since it cannot ingest food or reproduce. If conditions are unfavorable, e.g., in the absence of food and water, amebas secrete a firm protective covering and encyst until conditions are again favorable to active division.

Question:I'm talking about Fission, the nuclear reaction, not binary fission or anything We learned about 4 types of radiation: Radioactive decay, artificial transmutation, fission and fusion I get the other 3 but fission makes no sense What is it exactly?

Answers:When you bombard an element such as for example Uranium with a free Neutron the Neutron splits the Uranium atom up into pieces, as it breaks apart it creates even more free neutrons which hit other Uranium atoms and creates a fission chain reaction. Where Fusion is the joining/fusing of atoms fission is the splitting/breaking apart of atoms.

Question:1.what is binary fission?How does binary fission differ from mitotic cell division 2.describe three ways fungi can reproduce asexually 3. describe how an animal reproduces by budding 4. what is one advantage of this type of reproduction 5.explain why covering and refrigerating food helps to reduce spoilage from molds

Answers:1.what is binary fission? this occurs in simple unicellular organisms, like bacteria and protozoans such as amoeba. the organism divides into 2 parts, each of which develops into a new organism. the chromosomes replicate first, then the nucleus divides into 2, followed by the cytoplasm. 2 identical organisms are formed. 2.describe three ways fungi can reproduce asexually. a)binary fission- In its simplest form asexual reproduction is by budding or binary fission b)fragmentation- Many fungi can reproduce by fragmentation.Fragmentation is a form of asexual reproduction where a new organism grows from a fragment of the parent. Each fragment develops into a mature, fully grown individual c)spore formation- Asexual reproduction is extremely important to fungi. It is responsible for the production of large numbers of spores throughout the year. These asexual spores are formed on a phase of the fungal life cycle termed in some texts as the mitosporic, or anamorphic phase 3.Some cells split via budding (for example baker's yeast), resulting in a larger 'mother' cell and smaller 'daughter' cell. Offspring is larger than parent. Budding is also known on a multicellular level. An animal example is hydra, which reproduces by budding. The buds grow into fully matured individuals which eventually break away from the parent organism. 4.Hydra rely on a budding form of asexual reproduction, where it develops an outgrowth, which when detached from the parent, becomes a new individual. This is perfect for the Hydra, which doesn't move around much, so its chances of finding a partner are slim. well dats all i got..hope it helps :P

Question:But Prokaryote - bacteria do not contain a cell nucleus. I also understand that they reproduce via binary fission or budding. However, if they do not have a cell nucleus, how is the activities within the cell controlled? Please advise. Thank you so much!

Answers:The basic answer is that activity is less controlled than it is in a eukaryotic cell. There's a number of interesting tricks that not having a nuclear envelope allows prokaryotes to do (for example translating protein while the RNA is still being transcribed), but overall the system of organelles in eukaryotes allows a more fine-grained control of gene expression, and this is simply absent in prokaryotes.

From Youtube

Mitosis :Check us out at www.tutorvista.com Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets in two nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly equal shares of these cellular components. Mitosis and cytokinesis together define the mitotic (M) phase of the cell cycle - the division of the mother cell into two daughter cells, genetically identical to each other and to their parent cell. This accounts for approximately 10% of the cell cycle. Mitosis occurs exclusively in eukaryotic cells, but occurs in different ways in different species. For example, animals undergo an "open" mitosis, where the nuclear envelope breaks down before the chromosomes separate, while fungi such as Aspergillus nidulans and Saccharomyces cerevisiae (yeast) undergo a "closed" mitosis, where chromosomes divide within an intact cell nucleus. Prokaryotic cells, which lack a nucleus, divide by a process called binary fission. The process of mitosis is complex and highly regulated. The sequence of events is divided into phases, corresponding to the completion of one set of activities and the start of the next. These stages are prophase, prometaphase, metaphase, anaphase and telophase. During the process of mitosis the pairs of chromosomes condense and attach to fibers that pull the sister chromatids to opposite sides of the cell ...

mitosis :Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets in two nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly equal shares of these cellular components. Mitosis and cytokinesis together define the mitotic (M) phase of the cell cycle the division of the mother cell into two daughter cells, genetically identical to each other and to their parent cell. This accounts for approximately 10% of the cell cycle. Mitosis occurs exclusively in eukaryotic cells, but the process varies in different species. For example, animals undergo an "open" mitosis, where the nuclear envelope breaks down before the chromosomes separate, while fungi such as Aspergillus nidulans and Saccharomyces cerevisiae (yeast) undergo a "closed" mitosis, where chromosomes divide within an intact cell nucleus.[1] Prokaryotic cells, which lack a nucleus, divide by a process called binary fission. The process of mitosis is complex and highly regulated. The sequence of events is divided into phases, corresponding to the completion of one set of activities and the start of the next. These stages are interphase, prophase, prometaphase, metaphase, anaphase and telophase. During mitosis the pairs of chromosomes condense and attach to fibers that pull the sister chromatids to opposite sides of the cell. The cell then divides in cytokinesis, to ...