Meiosis | Cell division | Biology (article) | Khan Academy (2024)

Learn about the intricacies of meiosis: chromosome reduction, crossing over, and more.

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  • Joan D'silva

    8 years agoPosted 8 years ago. Direct link to Joan D'silva's post “In meosis 2 when did the ...”

    In meosis 2 when did the chromosomes duplicate?

    (12 votes)

    • Aditi Rattan

      8 years agoPosted 8 years ago. Direct link to Aditi Rattan's post “there was no chromosomal ...”

      Meiosis | Cell division | Biology (article) | Khan Academy (4)

      Meiosis | Cell division | Biology (article) | Khan Academy (5)

      Meiosis | Cell division | Biology (article) | Khan Academy (6)

      there was no chromosomal duplication in meiosis II only the centrosome duplicated. If there would have been chromosomal duplication cells would never have been able to produce haploid gametes the cell used in meiosis II are the product of meiosis I

      (52 votes)

  • Salisa Sukitjavanich

    6 years agoPosted 6 years ago. Direct link to Salisa Sukitjavanich's post “is there random orientati...”

    is there random orientation in metaphase 2?

    (12 votes)

    • tyersome

      5 years agoPosted 5 years ago. Direct link to tyersome's post “Good question!I think t...”

      Meiosis | Cell division | Biology (article) | Khan Academy (10)

      Good question!

      I think that is assumed to be generally true, but it would be very hard to test in most organisms.

      The only evidence for this being true that I know of comes from the fungus Neuropsora crassa that makes a linear§ ascus (sac containing the meiotic products).

      This allows us to see that in this species independent assortment also occurs in metaphase II.

      §Note: The order of the spores within the ascus reflects the meiotic divisions.

      References:
      http://mcb.berkeley.edu/courses/mcb140/Syllabus/AmacherLecture/Lecture6.pdf
      https://link.springer.com/referenceworkentry/10.1007%2F978-1-4020-6754-9_16853

      (24 votes)

  • Mason Lau

    7 years agoPosted 7 years ago. Direct link to Mason Lau's post “If the starting cell has ...”

    If the starting cell has 46 chromosomes, then how can it produce four cells with 23 chromosomes?

    (6 votes)

    • Jmsmarlowe

      5 years agoPosted 5 years ago. Direct link to Jmsmarlowe's post “Remember that when replic...”

      Meiosis | Cell division | Biology (article) | Khan Academy (14)

      Remember that when replicating in interphase, the chromosome number DOES NOT CHANGE

      in interphase before S (replication phase) we have 46 single stranded chromosomes: 23 are from mom and 23 are from dad (they code for the same things meaning chromosome 1 of mom codes for the same thing as chromosome 1 of dad. Likewise chromosome 5 of dad is similar to chromosome 5 of mom)

      after replication how many chromosomes do we have?
      answer: still 46, but what's different?
      the single strand chromosome (1 chromosome) became two stranded yet attached identical sister chromatids (still 1 chromosome)

      it is only when the sister chromatids separate are they each considered separate chromosomes.

      This means that in meoisis II when we split the sister chromatids:
      the parent cell starts with 23 chromosomes (EACH double stranded=two sister chromatids, so there are 46 chromatids. Anaphase II splits the sister chromatids which now separate (23 chromatids go to one pole and 23 chromatids go to other pole). When the chromatids are separated they are now called chromosomes

      so a haploid parent cell of 23 chromosomes (double strand) just created two haploid daughter cells of 23 chromosomes (now single strand).

      The above is also how a 46 chromosome (double strand) cell in mitosis can result in 2 daughter cells each with 46 chromosomes (single strand).

      Even Sal admits how confusing this is, but he explains all this visually in a separate video differentiating the terms chromatid, chromosome, and chromatin.

      (11 votes)

  • mairaj142

    6 years agoPosted 6 years ago. Direct link to mairaj142's post “Please specify if the num...”

    Please specify if the number of chromosomes becomes haploid in meiosis I or meiosis II? And if does in meiosis I then how? In meiosis I chromatids are not separated then how come chromosome number reduces to half??

    (6 votes)

    • von luger

      6 years agoPosted 6 years ago. Direct link to von luger's post “The number of chromosomes...”

      Meiosis | Cell division | Biology (article) | Khan Academy (18)

      The number of chromosomes becomes haploid in meiosis I, because the actual sister chromatids are not pulled apart by spindle fibers. For example, if a cell was undergoing meiosis, and had a total of 4 chromosomes in it, then 2 of them would go to one daughter cell, and 2 of them would go to the other daughter cell. That makes 2 haploid cells.

      Then, in meiosis II, each of the 2 sister chromatids in the daughter cells would be split apart by spindle fibers, giving each cell 2 chromosomes. As you said, the fact that in meiosis I chromatids are not separated means that the entire chromosome is moved to one cell; if there were 4, then they would be moved to each daughter cell equally. I hope that helps; if you still have trouble please say so!

      (24 votes)

  • Greacus

    7 years agoPosted 7 years ago. Direct link to Greacus's post “When the new nuclear memb...”

    When the new nuclear membrane forms around the chromosomes, how does the cell make sure the centrosomes are outside the nucleus and ALL chromosomes are inside?

    (8 votes)

    • Ivana - Science trainee

      5 years agoPosted 5 years ago. Direct link to Ivana - Science trainee's post “Well, it works based on p...”

      Well, it works based on patterns of nuclear defragmentation. On the places where old fragments of a nucleus are, new form. Also, thanks to cytokinesis, the cell splits exactly half its length.

      (7 votes)

  • datla mayookha reddy

    9 years agoPosted 9 years ago. Direct link to datla mayookha reddy's post “will you please explain m...”

    will you please explain me all the stages of prophase-1 in meiosis

    how can we find the order of stability of covalent compounds by inductive effect

    (1 vote)

    • jackmerf11

      9 years agoPosted 9 years ago. Direct link to jackmerf11's post “1. Chromosomes condense a...”

      Meiosis | Cell division | Biology (article) | Khan Academy (25)

      1. Chromosomes condense and hom*ologs loosely pair along their lengths, aligned by gene.
      2. The paired hom*ologs become physically connected along their lengths through a process called synapsis. This forms a synaptonemal complex.
      3.The random rearrangement of corresponding genes occurs between the non sister chromatids (because at this stage each chromosome consists of two sister chromatids).
      4. Synapsis ends, and the hom*ologs move slightly apart, no longer bonded along their lengths like in the synaptonemal complex.
      5. Some of these hom*ologs have one or more chiasmata, an X shaped region where a genetic rearrangement has occurred. This formation occurs because of sister chromatid cohesion, where a gene that has been given to the hom*ologous pair in synapsis is still bonded to the corresponding part on the sister chromatid of its former chromatid.
      6. Centrosomes move to opposite ends of the cell, and the nuclear envelope dissolves.
      7. Microtubules from one centrosome attach to the kinetochore (protein structures at the centromeres) of one chromosome from each of the hom*ologous pairs, while the other centrosome connects to the kinetochore of the other chromosome in each hom*ologous pair, and each hom*ologous pair moves towards the metaphase plate (where they line up before anaphase).

      (14 votes)

  • 😊

    8 years agoPosted 8 years ago. Direct link to 😊's post “why is interphase not inc...”

    why is interphase not included as a stage of cell-division in both mitosis & meiosis?

    (6 votes)

    • Ivana - Science trainee

      5 years agoPosted 5 years ago. Direct link to Ivana - Science trainee's post “Interphase _is_ stage of ...”

      Interphase is stage of the cell cycle, but not a stage of cell division (meisosis).

      Interphase is that gap phase (exactly G0) where cell cycle stops, DNA and organelles grow and synthesize.

      (4 votes)

  • Aizah Ahmed

    4 years agoPosted 4 years ago. Direct link to Aizah Ahmed's post “So meiosis is just to mak...”

    So meiosis is just to make a zygote? What happens after that? Also, why are there different processes of meiosis for sperms and eggs if they only have to join. Someone help, I'm really confused

    (4 votes)

    • TL The Legend

      3 years agoPosted 3 years ago. Direct link to TL The Legend's post “Yes, meiosis's goal is to...”

      Yes, meiosis's goal is to make a zygote. This zygote will (hopefully) turn into an embryo, then a fetus, which eventually becomes a human if everything works out. Meiosis in sperm and eggs is different because, well, sperm and eggs are different. A spermatocyte needs to split into four cells, while an oocyte needs to split into only one because many sperm are needed to fertilize a single egg. Once a sperm reaches the egg, it is only then that they join.

      (6 votes)

  • Mansha Shah

    10 months agoPosted 10 months ago. Direct link to Mansha Shah's post “The male transfers sperm ...”

    The male transfers sperm to ovaries then sperm is spreading in the ovaries then ultimately it becomes offspring. Am I right?

    (4 votes)

    • Anna Nguyen

      10 months agoPosted 10 months ago. Direct link to Anna Nguyen's post “You're almost correct. Ma...”

      You're almost correct. Males transfer sperm to the female and only one of the many sperm ends up fertilizing the egg. Then the fertilized egg becomes a zygote that ultimately grows into a baby. I hope this helps!

      (4 votes)

  • Satyankar Chandra

    5 years agoPosted 5 years ago. Direct link to Satyankar Chandra's post “Is the only point of Meos...”

    Is the only point of Meosis 2 to regulate the amount of genetic material within a haploid cell?

    What I mean by this is that, in parent diploid cell, each chromosome had 1 chromatid however, at the end of Meosis 1, each chromosome in haploid cell had 2 sister chromatids which renders the amount of genetic material same in daughter and parent cell.

    (3 votes)

    • Ivana - Science trainee

      5 years agoPosted 5 years ago. Direct link to Ivana - Science trainee's post “Correct. Meisosi II is re...”

      Correct. Meisosi II is reduction division.

      Why?

      Because, final products of meiosis, gametes are haploid cells.

      Just remember that ova and spermatozoids are haploid and than it all makes sense.

      Why they are haploid?
      Otherwise,
      within each meiotic cycle number of chromosomes would double, produce polyploidy and polyšloid zygote (incompatible with life). To avoid all of this, Meiosis II is a reduction.

      (5 votes)

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