explain why crossing over between nonsister chromatids of homologous chromosomes cannot occur during mitosis?
463039
In order for crossing over to occur, two different cells are needed, one maternal and one paternal. In mitosis a cell produces two identical daughter cells (no other chromatid to cross with). In meiosis, after DNA replication the two chromatids recombined or cross over.
930754
Mitosis is the reproduction of somatic (body) cells, while meiosis is the production of gametes (sex cells).\n\nThe normal cell has a chromosome number of "2n".  For example, a fruit fly has a chromose count of 8, so 8=2n.\n\nFollowing so far? Good.\n\nMitosis:\nMitotic division begins with a cell in Interphase, with a chromosome count of 2n.\nThe chromosomes are not visible right now under the microscope, they just look like a mass of tangled string called chromatin.\n\nIn Prophase, the chromosomes replicate, but remain attatched to their partner by a centromere (kinda look like an x).  Spindle fibres form and the centrioles migrate to the ends of the cell, opposite each other.  Nuclear membrane disappears.\n\nIn Metaphase, the chromosomes line up on the equatorial line (a bunch of x in a row along the middle of the cell), and spindle fibres attatch to each chromatid in the pair (right now, the two parts to the x are individually chromatids, but are together a chromosome).\n\nDuring Anaphase, the spindle fibres pull each daughter chromosome so that the pair of chromatids splits apart and half of them go to one end of the cell, and half to the other.\n\nDuring Telophase, the nucleolus forms and the nuclear membrane forms around the two sets of chromosomes.  A clevage furrow separates the cell into two daughter cells, each having a 2n number of chromosomes.\n\nPhew.  Still following?  Good- that was just Mitosis!\n\nNow for the fun one.  Meiosis.\nMeiosis takes the 2n cell and turns it into FOUR  (4) gametes.\nIn your body, each individual chromosome is part of a homologous pair.  That is, each has a match.  This is why we represent the chromosome number as 2n- There are two sets of n chromosomes.  In our fruit fly, the 2n number is 8, so there are 4 sets of homologous pairs.  Following? Good.\n\nMeiosis in the fruit fly begins with a single cell with 8 chromosomes, and ends with 4 cells with 4 chromosomes each.\n\nWhy does it end with "n" instead of "2n"?  Each gamete provides half the genetic information needed to create the new organism.  You get half your genes from your mother and half from your father.\n\nAnyway, meiosis takes place in two stages: aptly called meiosis 1 and meiosis 2.\n\nMeiosis 1:\nIn Interphase, the cell has roughly the same appearance as it did in Interphase of Mitosis.\n\nIn Prophase 1: The chromosomes duplicate and find their homologous pairs.  Now, the little groups istead of looking like an x, look like an x with four little lines joining, because there are 4 chromatids attached together, called a tetrad.  Understand?  I hope so!\nAt this point, the two chromatids nearest together in the tetrad may "cross over".  In other words, they swap genetic information.  Homologous pairs carry similar, but not identical coding (remember one was from your father, one from your mother).  In this way, you now have 4 chromatids that are each unique in their coding, which allows for genetic variation!  \nCentromers go to opposite ends, and spindle fibres begin to form.  Nuclear membrane disappears\n\nIn Metaphase 1: The tetrads line up on the equatorial line (remember this?)  The spindle fibres attatch to each side of the tetrad.\n\nIn Anaphase 1: The spindle fibres split the tetrads in half, so that 2 chromatids (1 chromosome now) go to one pole of the cell, and 2 chromatids (1 chromosome) go to the other.\n\nIn Telophase 1: The nuclear envelope/membrane reappears around each set of chromosomes, and a cleavage furrow splits the cell into two daughter cells.\n\nAt this point, we now have two daughter cells with "n" for the chromosome count, because there is half the amount of chromosomes we had when we started!\n\nOk, now each daughter cell undergoes meiosis 2, so that at the end we have 4 gametes.\n\nProphase 2:  Nuclear envelope dissappears.  Centroles migrate to the opposite sides of the cells, spindle fibres form.  Remember that the chromosomes are composed of
824582
Fertilization.\nOr are you asking about what happens in Prophase 1 of Meiosis? Homologous chromosomes, one from each parent pair.
166727
This may seem like a silly suggestion, but try representing the elements mentioned in your textbook (which I'm sure has some illustrations) with pieces of paper, matches, beans or something like that, so you can understand the events and the outcome. \nNow, if you _really_ want an explanation, here it comes (I've ommited the names of the phases, they'll be in your textbook).\n\nMeiosis includes 2 cell divisions with different characteristics. \nWe start with a diploid (2 n) cell, in which DNA duplication has occurred so that each DNA strand is double. These strands do not yet appear as chromosomes (condensed bodies); in fact, the chromatin strands have to be "relaxed" in order to duplicate. \n\n***Meiosis I: (in which the nº of chromosomes decreases to half)\n1. The chromatin (DNA strands) starts to condense and appears as small dense dots (the chromosomes) within the cell nucleus. These chromosomes will each have 2 "arms" (they look like an X or a V) because the DNA in each has been duplicated.\n2. Now all these chromosomes align in homologous pairs, with the aid of a transparent structure formed by tiny intracellular fibers (the spindle) that has formed and will help them to move in orderly fashion within the nucleus.\n3. While they keep condensing, each homologous chromosome contacts its "brother", and they exchange some fragments of their DNA ("crossing over"). Usually only one "arm" of each  chromosome participates in crossing over. \n4. In the meantime, the nuclear membrane is disintegrating...\n5. The pairs of homologues, still united, align at the middle (equator) of the spindle... \n6... and then each pair of homologues is divided, one chromosome of each pair goes toward each "pole" of the spindle. \n7. New nuclear membranes form around the 2 groups of chromosomes. Each of these groups will only have ONE of each pair of homologues. Essentially the information in each new nucleus is the same, BUT has some differences because, as you recall, there has been crossing over i.e. partial exchange of DNA between homologues. \n8. The original cell with the 2 nuclei divides and forms 2 cells, with half the nº of chromosomes of the original cell (haploid or n). Each of the chromosomes still has a duplicate DNA strand.\n\n... the cell takes a short break (you can take a break too!)... but the chromatin does not duplicate this time (it's already done that)\n\n***Meiosis II: (in which each double DNA strand becomes single again, this phase is similar to a normal mitosis).\n1. The chromatin condenses again, so that it appears as chromosomes.\n2. This time the spindle aligns the center of each chromosome at its equator.\n3. The central part of the chromosome is duplicated and the sister "arms" (chromatids) separate, each migrating to opposite poles.\n4. Now you have two groups of chromosomes, each group still has the haploid (n) nº of chromosomes (the same amount with which they started this 2nd meiosis) but now, each chromosome has a single DNA strand. \nA new nuclear membrane forms around each of these groups of chromosomes.\n5. The rest of the cell divides.\n\nSo, to sum up: you started with ONE diploid cell. You end with FOUR haploid cells. In animals, this process only happens during the production of gametes. \nIf we're talking about the formation of female gametes, then only ONE of the 4 resulting cells will become an ovule (and that only after more maturation), the other 3 cells form "polar bodies".\nIn contrast, when it's spermatozoids being produced, all FOUR resulting cells will become spermatozoids. \nThat's one of the reasons sperm is cheaper than ovules! There's just more of them in the entire animal kingdom.\n\nNow, what is the purpose of this extremely complicated (the above is a very simplified version) process? The result will be cells that have half the chromosomes of the normal body cells. These cells
535424
a chromatid forms one part of a chromosome after it has coalesced for the process of mitosis or meiosis. during either process, the word "chromosome" indicates a pair of two exactly identical or "sister" chromatids joined at the central point of each chromatid, called the centromere.\n\na chromosome, by definition, has exactly one centromere. thus when sister chromatids are pulled apart by the mitotic spindle during anaphase - that is, when the centromere joining two sister chromatids splits into two centromeres - then each sister chromatid becomes an independent chromosome.\n\nin non-gametic, non-dividing human cells, there are 23 pairs of chromosomes and thus 46 chromatids.
938356
Cells reproduce by cell division. This is called a-sexual reproduction or binary fission or Mitosis. This type of reproduction only requires one induvidual. The cell divides in order two create two daugter cells that are identical to the original cell. First Interphase, when the cell is making copies of DNA. In Prophase, the nucleolus fades and the chromatins condense into chromosomes. In Metaphase, all the chromosomes are aligned on spindle fibers. In Anaphase, the spindle fibers dissaspear. In Telophase, the daughter chromosomes move to the poles. In Cytokinesis, not truly part of Mitosis, there are now two finallized daughter cells.\n______________________\nHope this helps!!! :) :) \nHere's for more information:  http://www.cellsalive.com/mitosis.htm
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
84693
So in short, sister chromatid are duplicates of the homologous pairs. I hope this helped If so 333 (I) would like the 10 points. :)\n\nThe normal chromosome number in human cells is 46. This number is called a diploid or a 2n number of chromosomes. The chromosomes consist of 23 pairs. Each pair of chromosomes is called a homologous pair. One chromosome of each homologous pair is from the male parent, and the other is from the female parent. The chromosomes of each homologous pair look alike, and they contain genes for the same traits. \n\nSister chromatids are two identical copies of a single chromosome that are connected by a centromere. \n\nFirst, consider mitosis (typical cell division) in a normal diploid cell, with two copies of each chromosome. The chromosomes are duplicated during DNA replication, and the duplicates, called sister chromatids, remain attached to one another until chromosome segregation. Each daughter cell then receives one of the sisters from each homologue pair. Importantly, this means that the daughter cells are exact copies of the mother cell, with two copies of each chromosome, so they can go through the same process again. Thus, we call this process the cell cycle.
957887
Each chromosome in your body has an "identical pair"- there are two chromosomes that code for the same functions- one from your mother and one from your father.\n\nBecause they come from two sources, they are coding for the same functions, but may code differently for them (ie blue eyes as opposed to brown eyes).\n\nDuring prophase one, each chromosome duplicates into two sister chromatids joined by a centromere, and then the double stranded chromosome finds it's partner which is coding for the same thing (just came from the opposite parent), and is also double stranded.  Now you have a 4 stranded tetrad.\n\nGenetic variability comes from crossing over.  This is where the two chromatids closest together in the tetrad exchange some genetic information.\n\nFor example, two of the chromatids code for blue eyes, brown hair and shortsightedness.\nThe other two chromatids code for brown eyes, red hair and normal vision.\n\nDuring crossing over one chromatid from each pair forming the tetrad literally have ends "crossing over" and bits of information are exchanged.\n\nAfter crossing over, you have 4 chromatids with unique codings.  Using the example above, we may now have:\n-blue eyes, brown hair, shortsightedness\n-blue eyes, red hair, normal vision\n-brown eyes, brown hair, shortsightedness\n-brown eyes, red hair, normal vision\n\nWe now have genetic variability in each gamete produced from this meiotic division.
283484
ur question should be under biology but still, mitosis is the process of when the cells split to create 4 indentical cells during mitosis, one copy of the dna is distributed into each of the tow daughter cells mitosis id divided into 4 interphases: prophase, metaphase, anaphase, and telophase. in prophase, the chromatin in the nucleus condenses to form chorosomes. structures celled spindle fibers form a bridge between the ends of the cell. the nuclear membrane breaks down. in metaphase, the chromosomes line up across the center of the cell. each chromosome attaches to a spindle fiber at its centromere, which still holds the chromatids together. in anaphase, the centromeres split. the two chromatids separate. one chromatid moves along the spindle fiber to one end of the cell. the other chromatid moves to the opposite end. the cell becomes stretched out as the opposite ends pull apart. in telophase, the chromosomes begin to stretch out and lose their rodlike appearance. this occurs in the two regions at the ends of the cell. a new nuclear membrane forms around each region of chromosomes. o yeah and btw mitosis is on stage of the cell cycle there are like 2 before it and 1 after it
152438
#1)  Primarily by mutation.  A mutation must occur a germ cell (cells that produce sperm and eggs).  This can produce progeny with novel traits.  This is fairly rare event (one out of every billion nucleotides copied during meiosis).  Our body has ways of fixing these errors, but this is the biological basis of evolution.\n#2)  By crossing over.  During meiosis, parental chromosomes will swap pieces with each other.  For example, I used pieces of my mother's chromosome #1 and my father's chromosome #1 when I made the chromosome #1 that I passed to my daughter.  I passed a different combination of pieces to construct a chromosome #1 for my second daughter.\n#3)  By independent assortment during meiosis.  So not only did I make novel chromosomes out of pieces of my parents chromosomes, but I created a gamete using a unique combination of my mother and father's chromosomes.  For example I might have used chromosomes 1-11 of my father's (with pieces of my mother's chromosomes 1-11) and numbers 12-22 of my mother's (with pieces of my father's chromosomes 12-22).  For my second daughter it could have been totally different.  It is a unique situation each time!  No two gametes are the same.
327006
You're trying to say you have a synthetic chromosome. I would check all the experiments for contamination, and check the cells for endogenous expression before jumping to that conclusion. There are a lot of hurdles to leap, as mentioned in the other posts.
698789
it is NOT in mitosis!!!!\nmitosis happens AFTER G2\ncytokinesis is on of the final stages of mitosis!\n\neverything else is right :)
971621
"A" is the answer.  It happens in prophase of meiosis 1 and it does exactly what choice "A" states.  The rest of the choices happen during meiosis as well, but do not explain the importance of crossing over.
538881
Prophase I\nIn the leptotene stage, the cell's genetic material, which is normally in a loosely arranged pile known as chromatin, condenses into visible threadlike structures. Along the thread, centromeres are visible as small beads of tightly coiled chromatin. Recall that centromeres are connection sites between sister chromatids, which are not yet distinguishable. As the chromatin becomes progressively ordered and visible, homologous chromosomes find each other and bind together. In this process, called synapsis, a protein structure called the synaptonemal complex attaches the homologous chromosomes tightly together all along their lengths.\n\nThe zygotene stage sees the completion of synapsis. The paired homologous chromosomes are said to be bivalent. They may also be referred to as a tetrad, a reference to the four sister chromatids. During this stage, one percent of DNA that wasn't replicated during S phase is replicated. The significance of this cleanup act is unclear.\n\nThe pachytene stage heralds crossing over. Nonsister chromatids of homologous chromosomes exchange segments of genetic information. Because the chromosomes cannot be distinguished in the synaptonemal complex, the actual act of crossing over is not perceivable through the microscope.\n\nDuring the diplotene stage, the synaptonemal complex degrades. Homologous chromosomes fall apart and begin to repel each other. The chromosomes themselves uncoil a bit, allowing some transcription of DNA. They are held together by virtue of recombination nodules, betraying the sites of previous crossing over, the chiasmata.\n\nChromosomes recondense during the diakinesis stage. Sites of crossing over entangle together, effectively overlapping, making chiasmata clearly visible. In general, every chromosome will have crossed over at least once. The nucleoli disappears and the nuclear membrane disintegrates into vesicles.\n\nDuring these stages, centrioles are migrating to the two poles of the cell. These centrioles, which were duplicated during interphase, function as microtubule coordinating centers. Centrioles sprout microtubules, essentially cellular ropes and poles, during crossing over. They invade the nuclear membrane after it disintegrates, attaching to the chromosomes at the kinetochore. The kinetochore functions as a motor, pulling the chromosome along the attached microtubule toward the originating centriole, like a train on a track. There are two kinetochores on each tetrad, one for each centrosome. Prophase I is the longest phase in meiosis.\n\nMicrotubules that attach to the kinetochores are known as kinetochore microtubules. Other microtubules will interact with microtubules from the opposite centriole. These are called nonkinetochore microtubules
1136702
Meiosis Tutorial \nDNA replication precedes the start of meiosis I. During prophase I, homologous chromosomes pair and form synapses, a step unique to meiosis. The paired chromosomes are called bivalents, and the formation of chiasmata caused by genetic recombination becomes apparent. Chromosomal condensation allows these to be viewed in the microscope. Note that the bivalent has two chromosomes and four chromatids, with one chromosome coming from each parent. The nuclear membrane disappears. One kinetochore forms per chromosome rather than one per chromatid, and the chromosomes attached to spindle fibers begin to move. Bivalents, each composed of two chromosomes (four chromatids) align at the metaphase plate. The orientation is random, with either parental homologue on a side. This means that there is a 50-50 chance for the daughter cells to get either the mother's or father's homologue for each chromosome.Chiasmata separate. Chromosomes, each with two chromatids, move to separate poles. Each of the daughter cells is now haploid (23 chromosomes), but each chromosome has two chromatids. Meiosis Tutorial \nA review of Meiosis \nComparing Meiosis and Mitosis\nChromosome behavior \nMitosis: Homologous chromosomes independent \nMeiosis: Homologous chromosomes pair forming bivalents until anaphase I \nChromosome number- reduction in meiosis \nmitosis- identical daughter cells \nmeiosis- daughter cells haploid \nGenetic identity of progeny: \nMitosis: identical daughter cells \nMeiosis: daughter cells have new assortment of parental chromosomes \nMeiosis: chromatids not identical, crossing ove \nMeiotic errors\nNondisjunction- homologues don't separate in meiosis 1 \nresults in aneuploidy \nusually embryo lethal \nTrisomy 21, exception leading to Downs syndrome \nSex chromosomes \nTurner syndrome: monosomy X \nKlinefelter syndroms: XXY \nTranslocation and deletion: transfer of a piece of one chromosome to another or loss of fragment of a chromosome. \n\nMitosis, Meiosis, and Ploidy\nMitosis can proceed independent of ploidy of cell, homologous chromosomes behave independently \nMeiosis can only proceed if the nucleus contains an even number of chromosomes (diploid, tetraploid). \nTrisomy 21 does not prevent meiosis
128581
I'm pretty sure that its random because in meiosis there is crossing over. The chiasmata are where the chromosomes cross, which is done randomly. Since the codons are part of the DNA, they would be crossing at random too. The only exception that I can think of is when genes are located at least within 50 map units of eachother on the chromosomes, which makes it more likely that the genes will cross over together.\nHope this helps!