why must DNA be duplicated during the S phase of the cell cylce prior to mitosis?
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S phase is the period of synthesis.  Mitosis is the nuclear division phase. If mitosis occur before DNA duplication during  anaphase or nuclear division results imbalance chromosome number. There is also some regulatory mechanism that copletion of DNA synthesis stimulates the mitosis to occur.
909375
Cells that are undergoing constant mitosis such as hematopoetic stem cells (cells in your bone marrow that give rise to blood cells) and keratinocytes (skin cells) generally replicate in an ongoing manner about once a day, or faster.  I should say that it is only the skin cells of the bottom two layers of epidermis that undergo replication (stratum basale, and stratum spinosum).  Most of the replication occurs in the bottom layer of the epidermis (basale).\n\nMost of the time will be spent in the pre-mitotic cell cycle stages, G1, S, and G2, giving the cell ample time to replicate its DNA, and prepare for the division.  These stages are collectively referred to as 'interphase'.  The G1 phase varies (it will be at least 4 hours), the G2 phase lasts aroud 4-5 hours, and the S phase is usually around 6-8 hours (this is when the DNA is replicating).  So the entire interphase will be anywhere from 14-24 hours.\n\nThe mitotic process (the M phase) only takes about an hour.  Prophase is usually the longest phase of mitosis, and is in itself often broken down into a few different substages.  Once the metaphase stage begins, things move along pretty swiftly.  \n\nHope that helps!
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the correct order:\n\n1) Chromosomes copy themselves (interphase).\n2) Chromosome copies fasten together as the nuclear... (prophase).\n3) Chromosomes line up in the center of the cell (metaphase).\n4) Chromosome copies separate (anaphase & telophase).\n5) Cell splits (cytokinesis).\n\nThis is correct. Disregard the former answers.
847979
During mitosis one of our cells (such as a skin cell for instance) divides to form two cells each of which is identical to the single parent cell, each new cell has the same ploidy, if the originating cell is diploid then the two progeny cells are diploid and identical to the parent cell. Mitosis is the kind of cell division that occurs when body cells replace each other during normal aging and wear and tear bodily growth, in cells such as those of the skin, digestive tract etc. Since this kind of cell division is non sexual it is also often described as vegetative division, and since it produces exact copies of the parent cell it is sometimes called clonal division. The function, then, of mitosis, is to ensure that each of the two new cells generated from one receives one of the sister chromatids from each chromosome in the original cell.
246213
Those organells have their own genome, and it replication is not involved in the chromosome crossing over and other variation mechanisms on cell splitting. During the cell splitting they just stay a few on each side and thats how each new cell has chloroplasts
281036
It is not possible to divide a cell without replicating its DNA. This is how it works: DNA replicates during interphase. So now we have two copies of the same genetic material. These two copies then split apart into two cells in mitosis and cytokinesis. The result is two identical cells with the same genetic material as the parent cell.\n\ndid it help, or make it more confusing?
1066390
The purpose of both mitosis and meiosis is to take one cell and make two cells. Before the cells do this though, they have to make sure that a few things are in order. Obviously, there has to be enough stuff for the 2 cells to each be able to live independently. so, there has to be duplication of a lot of stuff before the first cell (let's call it the parent) divides. One of these things is the genome. These are the chromosomes which are made up of DNA. Once the DNA replicates (makes 2 copies of everything from what the parent originally had), the cell can divide. There are now 4 chromosomes of each type, we started out with our usual 2 of each (most organisms have 2 of each chromosome and they differ slightly from each other-this becomes important for meiosis), then when everything duplicated, we got 4! \n\nIn MITOSIS, the 2 cells that come out of the division (lets call them the daughters), are EXACTLY the same in genetic makeup as the original mother cell. They have the very same DNA and therefore make the same proteins and therefore the cells behave the same. \n\n In MEIOSIS on the other hand, the cells are not the same at all. And in fact, this is the point of meiosis - to make new types of cells. You may know that eggs and sperm are products of meiosis, but you not only get a mixing of genes from your mother (from the egg) and father (from the sperm), but the genes inside the egg and sperm are pre-mixed because of meiosis. Thus, genetic diversity!, and an explanation for why you have traits given to you from your grandmother through your father, even though your father may not have that trait (he carries it in his genes for later mixing). This may be a complicated issue in itself but since we have 2 of every chromosome and therefore 2 of every gene (even before the DNA replication that preceeds mitosis), there's a chance that the 2 copies of the same gene are different, one of which shows up in your father and one which you have that you got from your grandmother throuh your father. The mixing, by the way comes about because after the DNA replicates in preparation for cell division. This is neat and involves chromosomes breaking apart and joining back together. Take a look at the animated movie of meiosis at http://www.biology.yale.edu/animatedMeiosis.nclk - make a point to notice the red and green chromosomes at the beginning and the mixed ones at the end.
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Meiosis, as the name implies is a reduction division. That is at the end of it the chromosome number is reduced to half the original number; from diploid to haploid.\nThus, it can occur only in diploid cells, haploid can never undergo meiosis.\nIt occurs during gamete formation, spore formation or zygote germination. It is the nature’s mechanism by means of which the chromosome number and the quantity of DNA remains constant from one generation to the next.\n What happens before meiosis? – Chromosome duplication occurs before the cell begins to divide; the DNA content of the nucleus doubles.\nWhat is produced after the first division? – Two daughter nuclei are produced at the end of it. The chromosome number is reduced to half in each one of them; i.e. they are, now, haploid. But the DNA content is not exactly halved. \nWhat is produced after the second division? - The second meiotic division is the same as mitosis. From two daughter nuclei in the earlier phase four daughter nuclei are produced. The chromosome number is already reduced, now the DNA quantity is also reduced to half of that present in the original parental cell.\nThus the four cells at the end of meiosis are haploid and DNA is also half of the original parental cell. Thus the products are different than the one at the end of mitosis. Besides, the genes are also different and not identical on all the four cells.  \n What can go wrong? – Many things can go wrong and it will be difficult to explain them here.\n1) Chromosome pairs may fail to separate properly, leading to chromosomal imbalance in the resulting cell. This in turn can produce genetic defects or variations in the next generation.
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to separate the sister chromatids, dragging one to each side of the cell on either side of the metaphase plate.  This allows replicated chromosomes to be divided between the original cell and the newly forming cell.
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At the time of cell division only the chromatid number doubles.Inter phase is the resting phase intht phase no doubling tales place .It takes place at the early meta phase stage\nIn anaphase the polar fibers or spindle fibers get attached to chromosome near the centromere.Then while they are moving towars poles,each centromere splits and carries half chromosome or one  chromatid.The chromatid in telophase becomes a full chromosome\nSo 46 chromosoes become 92 and 92become 46at the end of telophase stage.Hope you got the point.
806915
Telophase (sometimes spelled telephase) is a stage in either meiosis or mitosis in a eukaryotic cell reversing the effects of prophase and prometaphase events. During those events, the nucleus was dissolved and the chromatin in the cell was condensed into chromosomes. Telophase thus "cleans up" the secondary after effects of mitosis.\n\nMitosis is the process by which a cell separates its duplicated genome into two identical halves. It is generally followed immediately by cytokinesis which divides the cytoplasm and cell membrane. This results in two identical daughter cells with a roughly equal distribution of organelles and other cellular components. Mitosis and cytokinesis together is defined as the mitotic (M) phase of the cell cycle, the division of the mother cell into two daughter cells, each the genetic equivalent of the parent cell.\n\nMitosis occurs exclusively in eukaryotic cells. In multicellular organisms, the somatic cells undergo mitosis, while germ cells — cells destined to become sperm in males or ova in females — divide by a related process called meiosis. Prokaryotic cells, which lack a nucleus, divide by a process called binary fission.\n\n1.The S phase, short for synthesis phase, is a period in the cell cycle during interphase, between G1 phase and the G2 phase. Following G1, the cell enters the S stage, when DNA synthesis or replication occurs. At the beginning of the S stage, each chromosome is composed of one DNA double helix molecule, which is called a chromatid. At the end of this stage, each chromosome has two identical DNA double helix molecules, and therefore is composed of two sister chromatids. During S phase, the centrosome is also duplicated. These two events are unconnected, but require many of the same factors to progress. The end result is the existence of duplicated genetic material in the cell, which will eventually be divided into two.\n\nDamage to DNA often takes place during this phase, and DNA repair is initiated following the completion of replication. Incompletion of DNA repair may flag cell cycle checkpoints, which halts the cell cycle.
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In biology, meiosis is the process that allows one diploid cell to divide in a special way to generate haploid cells in eukaryotes. The word "meiosis" comes from the Greek meioun, meaning "to make smaller," since it results in a reduction in chromosome number.\n\nMeiosis is essential for sexual reproduction. It therefore occurs in most eukaryotes, including single-celled organisms. A few eukaryotes, notably the Bdelloid rotifers, have lost the ability to carry out meiosis and acquired the ability to reproduce by parthenogenesis. Meiosis does not occur in archaea or prokaryotes, which reproduce by asexual cell division processes.\n\nDuring meiosis, the genome of a diploid germ cell, which is composed of long segments of DNA called chromosomes, undergoes DNA replication followed by two rounds of division, resulting in haploid cells called gametes. Each gamete contains one complete set of chromosomes, or half of the genetic content of the original cell. These resultant haploid cells can fuse with other haploid cells of the opposite gender or mating type during fertilization to create a new diploid cell, or zygote. Thus, the division mechanism of meiosis is a reciprocal process to the joining of two genomes that occurs at fertilization. Because the chromosomes of each parent undergo genetic recombination during meiosis, each gamete, and thus each zygote, will have a unique genetic blueprint encoded in its DNA. In other words, meiosis is the process that produces genetic variation.\n\nBiochemically, meiosis uses some of the same mechanisms employed during mitosis to accomplish the redistribution of chromosomes. There are several features unique to meiosis, most importantly the pairing and recombination between homologous chromosomes, which enable them to separate from each other.\n\nMeiosis facilitates stable sexual reproduction. Without the halving of ploidy, or chromosome count, fertilization would result in zygotes that have twice the number of chromosomes than the zygotes from the previous generation. Successive generations would have an exponential increase in chromosome count, resulting in an unwieldy genome that would cripple the reproductive fitness of the species. Polyploidy, the state of having three or more sets of chromosomes, also results in developmental abnormalities or lethality.\n\nMost importantly, however, meiosis produces genetic variety in gametes that propagate to offspring. Recombination and independent assortment allow for a greater diversity of genotypes in the population. As a system of creating diversity, meiosis allows a species to maintain stability under environmental changes.
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Meiosis reduces a cell from a diploid cell to a haploid cell. This is important because if it didn't reduce a diploid to a haploid cell, when fertilization would occur, chromosomes would double and that is bad. Chromosome number is species based.\n\nSexual reproduction requires meiosis.\n\nMeiosis in stages....\n\nMeiosis1\n\nInterphase-same as interphase usually is. \n\nMeiosis I\nProphase I-crossing over (homologous chromosomes switch nonsister chromatids). Also the mitotic spindle fibers start forming from the aster and the nuclear envelope disappears.\n\nMetaphase I-The chromosomes meet together at the metaphasic plate, longest phase outside of interphase.\n\nAnaphase I-the chromosomes stay together (aka no break at centromere) they just don't pair up anymore. The pairs split, one chromosome going one way, the other going the other way.\n\nTelophase I and Cytokinesis-Nuclear envelope may or may not reform once the cleavage furrow or cell plate occur. Understand cytoplasmic inheritance with this as well.\n\nMeiosis II\n\nProphase (No interphase) II-no duplicaiton of chromosomes. The mitotic spindles form again\n\nMetaphase II-Everything goes towards the mitotic plate\n\nAnaphase II-The kinetochore spindles, or mitotic spindles, attach to the kinetochores and separate the chromosomes, bringing one chromatid to one end of the cell, another to the other end of the cell.\n\nTelophase II and cytokinesis-four haploid daughter cells are created. Think about it, meiosis I produces two cells, second division creates 4 because each diploid cell produces two haploids.
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Before a cell divides (that is, undergoes mitosis or meiosis), the DNA strands in the cell nucleus replicate, so that each chromatin strand is double. Chromatin condenses before cell division and becomes visible as chromosomes; each chromosome has "twin" strands that appear as arms, these identical copies of the DNA molecule are the chromatids, united to each other at the centromere. \nDuring cell division, they will become separated (due to centromere duplication) and each sister chromatid will go to one of the descendant cells, so that the same genetic information from each chromosome is passed on to the descendant cells.
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two copies of chromosomes ion the same cell
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...DNA through a process called mitosis.
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remember the phases in meiosis (in order) :Prophase, Metaphase, anaphase, telephase (pmat)\nMitosis\nI  INTRODUCTION \n\nMitosis, process in which a cell’s nucleus replicates and divides in preparation for division of the cell. Mitosis results in two cells that are genetically identical, a necessary condition for the normal functioning of virtually all cells. Mitosis is vital for growth; for repair and replacement of damaged or worn out cells; and for asexual reproduction, or reproduction without eggs and sperm.\n\nAll multicellular animals, plants, fungi, and protists, which begin life as single cells, carry out mitosis to develop into complex organisms containing billions of cells. Mitosis continues in full-grown organisms as a means of maintaining the organism—replacing dying skin cells, for example, or repairing damaged muscle cells. In the cells of the adult human body, mitosis occurs about 25 million times per second. Multicellular organisms such as sea stars, sea anemones, fungi, and certain plants rely on mitosis for asexual reproduction at particular stages in their life cycles, and mitosis is the sole mode of reproduction for many single-celled organisms.\n\nII  INTERPHASE \n\nThe life cycle of eukaryotic cells, or cells containing a nucleus, is a continuous process typically divided into three phases for ease of understanding: interphase, mitosis, and cytokinesis. Interphase includes three stages, referred to as G1, S and G2. In G1, a newly formed cell synthesizes materials needed for cell growth. In the S stage, deoxyribonucleic acid (DNA), the genetic material of the cell, is replicated. At this stage, DNA consists of long, thin strands called chromatin. As each strand is replicated, it is linked to its duplicate by a structure known as a centromere. When the S stage is complete, the cell enters a brief stage known as G2, when specialized enzymes correct any errors in the newly synthesized DNA, and proteins involved with the next phase, mitosis, are synthesized. \n\nIII  MITOSIS \n\nMitosis occurs in four steps. In prophase the replicated, linked DNA strands slowly wrap around proteins that in turn coil and condense into two short, thick, rodlike structures called chromatids, attached by the centromere. Two structures called centrioles, both located on one side of the nucleus, separate and move toward opposite poles of the cell. As the centrioles move apart, they begin to radiate thin, hollow, proteins called microtubules. The microtubules arrange themselves in the shape of a football, or spindle, that spans the cell, with the widest part at the center of the cell and the narrower ends at opposite poles. As the spindle forms, the nuclear membrane breaks down into tiny sacs or vesicles that are dispersed in the cytoplasm. Final disintegration of the membrane marks the beginning of metaphase. \n\nIn metaphase, the spindle fibers attach to the chromatids near the centromeres, and tug and push the chromatids so that they line up in the equatorial plane of the cell halfway between the poles. Like two individuals standing back to back at the equator, one chromatid faces one pole of the cell, and its linked partner faces the opposite pole. This precise orientation is necessary in order for the next step, anaphase, to occur. Anaphase begins when the centromeres split, separating the identical chromatids into single chromosomes, which then move along the spindle fibers to opposite poles of the cell. \n\nAs these two identical groups of single chromosomes gather at opposite poles of the cell, telophase begins. A new nuclear membrane forms around each new group of chromosomes. The spindle fibers break down and the newly formed chromosomes begin to unwind. If viewed under a light microscope, the chromosomes appear to fade away. They exist, however, in the form of chromatin, the extended, thin strands of DNA too fine to be seen except with electron microscopes. Mitosis accomplishes replication and division of the nucleus,