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How meiosis 1 is able to produce two haploid daughter chromosomes? ...

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pranitingale | Student, Grade 9 | Valedictorian

Posted August 20, 2013 at 3:03 PM via web

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How meiosis 1 is able to produce two haploid daughter chromosomes?  A normal cell is diploid in nature, means it contains 2n

 chromosomes, but before meiosis it replicates during

 interphase. As initially it is 2n ,so why doesn't it become 4n after getting replicated at interphase?

 And because it becomes 4n in genetic nature so why doesn't

it produce diploid daughter cells with 2n and 2n instead of 

haploid daughter cells?

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ncchemist | eNotes Employee

Posted August 21, 2013 at 12:51 AM (Answer #1)

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For some reason the formatting of your question text is behaving strange and I cannot seem to edit it to standard formatting so I had to manually put in line breaks to make all of the text appear in the question box.

Your question is excellent and I can see why you are getting confused by the terminology of meiosis I.  Mitosis takes a diploid cell (2n number of chromosomes) and produces two diploid cells.  Meiosis takes a diploid cell (2n) and produces four haploid (n) daughter cells.  It does this via two separation steps, meiosis I and meiosis II that follow each other sequentially.

During the interphase that precedes meiosis, the chromosomes are replicated to make the parent cell 4n with four complete sets of chromosomes (23*4=92 total for a human cell) that are grouped into 46 pairs of sister chromatids (the familiar X shape with the chromatids joined by a centromere).  The cell then undergoes meiosis I which is referred to as a reductional division.  The parent cell divides the pairs of sister chromatids into two separate groups that are then divided into two new cells.  Even though the two new cells each have 46 total chromosomes, they are considered as haploid (n) since they are composed of sister chromatids that are destined for separation soon.  This is what is most likely confusing you.  Again, even though the two cells produced from meiosis I have 46 chromosomes, they are considered haploid, not diploid, since the 46 chromosomes exist as joined pairs of sister chromatids that will soon be separated.  Think of it as the 46 pairs being split up by the cell.

After the first cell division, the cell then undergoes meiosis II which is called an equatorial division.  The remaining 23 pairs of sister chromatids in each cell (46 total chromosomes in each cell) are then finally separated at the centromeres and the pairs are then separated into two new cells for a total of four cells.  These four new daughter cells contain exactly 23 chromosomes each.  This makes them haploid (n, 23 chromosomes), with exactly half the number of chromosomes of the original diploid parent cell (2n, 23 pairs of chromosomes for a total of 46 chromosomes).  The links below can show you all of this in diagram form and offer more details but this is the basic concept regarding the ploidy of meiosis.

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