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Meiosis review

Key terms

TermMeaning
GameteA sex cell (in humans: sperm for males, and eggs for females)
MeiosisA two-step process of cell division that is used to make gametes (sex cells)
Crossing overProcess in which homologous chromosomes trade parts
InterphasePhase of the cell cycle where the cell grows and makes a copy of its DNA
Homologous chromosomesSet of chromosomes (one from each parent), that are very similar to one another and have the same size/shape
Sister chromatidsTwo halves of a duplicated chromosome
Diploid (2n)Cell that contains two sets of homologous chromosomes
Haploid (n)Cell that contains only a single set of genes

Meiosis

The purpose of meiosis is to produce gametes, or sex cells. During meiosis, four daughter cells are produced, each of which are haploid (containing half as many chromosomes as the parent cell).

Stages of meiosis

Meiosis contains two separate cell divisions, meaning that one parent cell can produce four gametes (eggs in females, sperm in males). In each round of division, cells go through four stages: prophase, metaphase, anaphase, and telophase.
Before entering meiosis I, a cell must first go through interphase. This is the same interphase that occurs before mitosis. The cell grows, copies its chromosomes and prepares for division during the G1​​ phase, S phase, and G2 phase of interphase.

Meiosis I

Meiosis I is the first round of cell division, in which the goal is to separate homologous pairs.

Meiosis II

The second round of cell division is meiosis II, in which the goal is to separate sister chromatids.

Common mistakes and misconceptions

  • Interphase is not part of meiosis. Although a cell needs to undergo interphase before entering meiosis, interphase is technically not part of meiosis.
  • Crossing over occurs only during prophase I. The complex that temporarily forms between homologous chromosomes is only present in prophase I, making this the only opportunity the cell has to move DNA segments between the homologous pair.
  • Meiosis does not occur in all cells. Meiosis only occurs in reproductive cells, as the goal is to create haploid gametes that will be used in fertilization.
  • Meiosis is important to, but not the same as, sexual reproduction. Meiosis is necessary for sexual reproduction to occur, as it results in the formation of gametes (sperm and eggs). However, sexual reproduction includes fertilization (the fusion between gametes), which is not part of the meiotic process.

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  • blobby green style avatar for user jennifer komen
    In telophase 1 and telophase 2, I am confused about the use of the word "haploid." They are both labeled "haploid," but they do not look the same.
    (23 votes)
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  • blobby green style avatar for user Hoi Ki Cheng
    I am confused about the number of sister chromatids between mitosis and meiosis. For example, human. After replication, there are 46 chromosomes (92 sister chromatids in total) in a cell, right?

    During mitosis, they are split into 2 cells, each of which has 46 chromosomes (46 sister chromatids in total).

    On the other side, during meiosis I, same as mitosis, they are split into 2 cells, each of which has 46 chromosomes (46 sister chromatids in total).

    So, during meiosis II, these 2 cells was split again into 4 cells, each of which has 23 chromosomes (23 sister chromatids in total).

    Am I right?
    (6 votes)
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    • mr pants purple style avatar for user Kim Kelly
      Your thought is correct but plz notice, after splitting 2 sister chromosomes in mitosis, each single one is no longer a sister chromatid but a chromosome. So you cannot say 46 chromatids in total in your second graph, it is only 46 chromosomes or 23 pairs of homologous chromosomes:)
      Meanwhile, in meiosis1, after splitting into two cells, each one contains 46 sister chromatids but only 23 chromosomes (notice that two sister chromatids is a chromosome).
      (9 votes)
  • leaf orange style avatar for user amaan_zafar
    what is the difference between crossing over and synapsis?
    (4 votes)
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    • aqualine seed style avatar for user Anagha Tiwari
      Synapsis is when the homologous chromosomes of the same size and length pair up. Crossing over is when bits of DNA are exchanged from each chromosome to produce genetically unique chromosomes. Though both happen in Prophase I, synapsis happens before the chromosomes can cross over. Hope that helps!
      (7 votes)
  • leaf orange style avatar for user Lia Naqi
    What does Homologous mean?
    (4 votes)
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    • hopper jumping style avatar for user James Best
      In biology it means similar in position, structure, and evolutionary origin but not necessarily in function for organs, but for chromosomes it means similar in position, structure, and evolutionary origin but not necessarily in function. hope this helps:)
      (7 votes)
  • piceratops ultimate style avatar for user Daniyal Bilal
    what happens to cells ( what is their fate or role) that have completed the mitosis cycle and the meiosis process??
    (3 votes)
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    • blobby green style avatar for user Manan Upadhyaya
      after mitosis, cells go back to the interphase stage of the cell cycle and carry on the cell cycle. for human somatic cells, mitosis can only occur about 20-50 times before it undergoes apoptosis, a number known as the Hayflick limit. scientifically, this is beneficial because of the accumulation of mutations that may potentially be dangerous. an exception is cancerous cells such as with the TP53 mutation, where mitosis keeps going on continuously even if there is insufficient energy/organelles available, often resulting in tumours. note that some cells are stuck in a phase of the cell cycle known as G_0, where they do not go through mitosis till reverted back to the G_1 stage.
      as for meiosis, cells are usually nourished so that they grow. in humans, the sperm is nourished by the sertoli cells in the testes to ensure that they grow and mature. the ova are developed inside the follicles of the ovary and typically the one ovum that matures the most is released from its follicle during ovulation. their fate is either fertilization, in which case the sperm and ovum form an embryo that develops in the fallopian tubes and then in the uterus. in case that this does not occur, the ovum is shed off with the uterine lining in a process known as menstruation. as for the sperm, they are usually just left in the testes, and if not released through a process such as ejaculation they may start to die in the testes.
      (5 votes)
  • blobby green style avatar for user nddaksh23
    How can n divide to give 2n
    Telophase 1 has 2 chromosomes each and they are n and Telophase 2 has 1 chromosomes in 1 cell and is also n how??
    (6 votes)
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  • blobby green style avatar for user doraozioma
    There is something confusing in meosis 1. A cell with 46 chromosomes duplicates and just like in Mitosis divides... But the sister chromatids didn't split but remained together and went to opposite poles. Yet we say the end product of meosis 1 are haploid cells. Where did the duplicated chromosomes go? But in mitosis we say the end products are diploid...yet they divided and even splitted. Is this maths not wrong?
    How man chromosomes are present after meosis 1? That's quite confusing in my opinion
    (2 votes)
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    • duskpin seed style avatar for user Leia
      This confuses me too, but maybe I can offer some help. So I think it is not until Meiosis 2 that cells become haploid. Because first, the cell goes through interphase where it duplicates its 46 chromosomes into 92 chromosomes. At this stage there are now 92 Xs. However, each X is homologous to another, and so is in a homologous pair. That means that there are 46 pairs. (23 pairs duplicated is 46 pairs). You were right that in Meiosis 1 that it is only the pairs that separate. Therefore it is not yet haploid. I like my biology book’s definition of Haploid and Diploid.
      Diploid Cell - “A cell with chromosomes that come in homologous pairs or two sets of chromosomes”
      Haploid Cell - “A cell that has only one representative of each chromosome pair or half the total number of chromosomes.”
      This seems like it would be haploid at the end of Meiosis 1, but since it duplicated beforehand, it is only back to the original number of 46 chromosomes. Then in Meiosis 2 it cuts the actual chromosomes into chromatids and makes 23. It seems though that since the pairs are separated in Meiosis 1 then it would be haploid, but I think it has to do with the total number of chromosomes. A single chromatid can be called a chromosome, and a duplicated 2 sister chromatids joined together can also be a chromosome. So, in Mitosis the cell has 46 individual chromosomes, duplicates to have 92 chromosomes making Xs, and back to having 46 single chromosomes again. This means that the original number was never cut in half despite the X pairs being cut in half. In meiosis 1 they go from 46 to 92 and back to 46, while in meiosis 2 it does not duplicate in between and goes from 46 to 23. Idk. This is honestly the best I can think of. Every definition I find is slightly different than another and I remain confused. But I think that is how it is not yet haploid in meiosis 1. If it were haploid in meiosis 1(going from 46 to 92 to 46) then you could consider mitosis to make haploid as well (also going from 46 to 92 to 46) since it cuts in half the number and splits the chromosomes. But mitosis makes diploid cells from diploid cells. I may have this wrong and maybe I just made things more confusing. But I think the key is the total number that determines if it is haploid or not.
      Oh and also, the definition for haploid states, “A cell that has only one representative of each chromosome pair…”. If you think about it, even though the pairs are separated in meiosis 1, it has the pairs still together in each cell. Kind of.
      } } -> }{ }{-> }{ | }{
      / /-> X X -> X | X
      Ok, so you go from 4 homologous chromosomes not yet duplicated in chromatin form, and so you have 2 pairs. 1 pair: } } and 2nd pair / /. Then you duplicate them and have 2 pairs, but 4 chromosomes counting the whole Xs and 8 chromosomes counting the individual chromosomes. Each X is made of 2 sister chromatids, so count each one. (So I can say that there are 4 chromosomes for counting whole Xs or 8 chromosomes counting the chromosomes individually. You can see that they were doubled though.). Then you split it in half and actually still have the } } and / /. They will have been mixed, but you should have directly returned back to the original because the original pair was between the individual chromosomes. We just don’t show them outside Xs because you can’t see chromatin well in a microscope. I Believe this might also help to explain and show that the cells are not made haploid until meiosis 2. In meiosis 2 there are only one representative of each chromosome pair. Remember, Xs are the duplicated form of the single chromosomes before synthesis. And also that it becomes haploid when the cell has just } and / by themselves.
      And for the amount of chromosomes present after meiosis 1, there are (using the example of an organism with just 2 chromosomes) 2 chromosomes. It duplicated first: 2 chromosomes (1 pair) -> 4 chromosomes (2 pairs) -> 2 chromosomes. This means that there are 2 chromosomes present in each cell (two Xs but the same number of individual chromosomes) after meiosis 1. I hope this wasn’t all a mess of an answer and made at least some sense…
      (7 votes)
  • blobby green style avatar for user Deontae Baker
    Is Meiosis a continues process or does it stop and begin whenever the body wants it to?
    (4 votes)
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    • blobby green style avatar for user amina.mcdiarmid
      Meiosis occurs in the testes in males and in the ovaries in females. In males, sperm production occurs in the testes almost continuously to replenish the supply. In females, egg cells are made from oocytes which are produce when the female is a fetus. Oocytes are cells arrested in meiosis I. Meiosis resumes when the oocyte is activated ready for ovulation and then arrests again in metaphase II. It is during fertilisation that the cell undergoes anaphase II so that the sister chromatids are finally separated
      (2 votes)
  • aqualine ultimate style avatar for user Ammar Hasan
    so are the sister chromatids diploids?
    (3 votes)
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  • blobby green style avatar for user shellyjpix
    In the Starting Cell of Meiosis I, you say that it is the homologous chromosomes from mother and father that cross over... but how can this be if the cell has not been fertilized yet and Meiosis describes how a gamete cell is produced. Therefore, an egg would only have the chromosomes from the mother and a sperm would have only the chromosomes from the father.
    (3 votes)
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    • duskpin ultimate style avatar for user Mary
      You received homologous chromosomes from your mother and some from your father, that is your genetic make up. I think the process of meiosis separates them so that the traits you pass on to your children can seem as if they are coming from grandparents or from you. I think it why I have blue like my dad and not brown ones like my mom. My maternal grandmother had blue eyes so she gave my mom the blue eyed chromosome. I was the one child of 4 with blue eyes. My eye color chromosomes from both my parents were different than for my siblings. I think meiosis has a role in that.
      (4 votes)