Meiosis (Updated)


Captions are on! Click the CC button at bottom right to turn off. Follow us on Twitter (@AmoebaSisters) and Facebook! Have you ever wondered how two siblings can
have the same mom and dad still look so different? Well, today we’re going to talk about a
process that makes that possible—-a process called meiosis —-not to be confused with
mitosis, which sounds unfortunately similar. Mitosis makes identical body cells like your
skin cells and stomach cells. Recall from our mitosis clip that since it
makes identical body cells, mitosis is important for growth and for repair of damage or to
replace worn out cells. But NOT meiosis. Meiosis is a process that contributes to genetic
variety. Meiosis also doesn’t make body cells. It makes sperm and egg cells; otherwise known
as gametes, the fancier word. You might recall that humans have 46 chromosomes. That’s how many chromosomes most body cells
in your body have. But there are some human cells that don’t
have 46 chromosomes. Human sperm cells and egg cells have 23 chromosomes. Why the number difference? Well, if a sperm cell has 23 chromosomes and
an egg cell has 23 chromosomes, when they come together that makes 46 chromosomes. That will allow a newly formed fertilized
egg to develop into a human. Meiosis is what we call a reduction division
because you have a starting cell that has 46 chromosomes and your ending cells—the
sperm and egg cells—have only has 23 chromosomes. Before we start getting into the stages of
meiosis to make gametes, we have to remember what happens before meiosis can start. Actually, this also happens before mitosis. It’s the stage known as interphase. If you remember interphase, it’s when the
cell is growing, it’s replicating its DNA, it’s carrying out cell processes. Just like mitosis, interphase happens before
meiosis is going to start. So the starting cell has 46 chromosomes, and
you have to duplicate those chromosomes in interphase before meiosis starts. That basically means you’re duplicating
your DNA, since chromosomes are made of DNA and protein. Ready for the tricky part? Because we tend to count chromosomes by the
number of centromeres present, when the 46 chromosomes duplicate, we still say there
are 46 chromosomes because the sister chromatids are still attached and we’re counting by
centromeres. So 46 chromosomes here, they replicate in
interphase, and you still have 46 chromosomes in this picture. But you went from 46 to 92 chromatids. Little tricky there. We have a detailed video that explains these
chromosome numbers before and after replicating in interphase that can be useful for understanding
meiosis. Ok so interphase checklist done—now we can
move into meiosis. You might remember the mitosis stages PMAT;
the ‘p’ was for prophase, the ‘m’ for metaphase, the ‘a’ for anaphase, and
the ‘t’ for telophase. The good news is that in meiosis, you still
use those terms, but because meiosis is actually a reduction division, you’re going from
46 chromosomes to 23. Which means you actually divide twice. So instead of mitosis where you divide one
time and do PMAT one time, in meiosis, you’re going to divide twice and therefore do PMAT
twice. Because of this, in meiosis, you put numbers
after the phases to indicate whether you’re in the first division or the second division. So let’s dive right in. So let’s start with the very first step
– prophase I. One thing I like to remember about prophase
is “pro,” this actually means “before.” It kind of helps you remember that it comes
before all the other stages start. This is where the chromosomes are going to
condense and thicken. They are actually going to line up with their
homologous pairs. The word homologous means that the chromosomes
are approximately the same size and that they contain the same types of genes in the same
locations. They are going to match up. It is during this prophase 1 that this amazing
process occurs called crossing over. I know crossing over probably sounds like
something very different, but this is a really awesome process because when these chromosomes
are lined up in homologous pairs, they have a way that they can transfer their genetic
information and exchange it between each other. It’s kind of like these chromosomes flop
over each other and they do a little genetic information exchange here. It makes for what we call—recombinant chromosomes—which
can eventually contribute to the variety that we were mentioning that siblings can have
even when they have the same parents. More about that later. Now we move into metaphase I. In metaphase I, think if the M as standing
for middle. The chromosomes are going to be in the middle
of the cell. It’s a little different, though, from mitosis
because they’re still going to be in pairs in the middle of the cell so it’s not a
single file line; they are in pairs in the middle. During anaphase I, think A for away because
the chromosomes are going to be pulled away by the spindle fibers.Then, we end with telophase
I, where you have two newly formed nuclei and it becomes obvious you will end meiosis
1 with two new cells. Cytokinesis follows with splitting the cytoplasm. But we’re not done yet. On to meiosis 2! The very first step in meiosis II is prophase
II. It’s not going to be nearly as eventful
as it was in prophase I because they are not going to have homologous pairs. They also are not going to have that amazing
process called crossing-over. That doesn’t happen again in prophase II. You have your chromosomes and the spindles
starting to form like in prophase I but prophase II is not nearly as eventful of having that
process of crossing over. In metaphase II, remember think m for middle,
the chromosomes are going to line up in the middle. This time, though, they are in a single file
line. They are not in pairs like they were in metaphase
I. Anaphase II, remember A for away, but this time it’s the chromatids that are getting
pulled away by the spindle fibers. Chromatids are getting pulled away to opposite
sides of the cell. In telophase II, nuclei reform and the 2 cells
are each going to divide so you can see here that 4 cells are going to be formed. Cytokinesis will follow to completely split
the cytoplasm. Now keep in mind that meiosis in males produces
sperm cells and in females, it produces egg cells. Because of independent assortment and also
crossing over, you’re going to have variety. For example, in a male the four sperms cells
that are produced each time, they are all different from each other. They are also different from the starting
cell because the starting cell had 46 chromosomes and the ending cells only have 23. So they are not identical to the original
and they are not identical to each other. This is going to lead to variety. A reason why two siblings with the same parents
can look different from each other—they still developed from an unique egg and unique
sperm cell that came together. One last thing to think about. Scientists are often looking into the process
of meiosis because sometimes the chromosomes don’t separate correctly. It’s called nondisjunction when a cell can
receive too many or too few chromosomes in the separation. This contributes to some genetic disorders,
which is something scientists continue to study. Well that’s it for the Amoeba Sisters, and
we remind you to stay curious.

100 thoughts on “Meiosis (Updated)

  1. Want to see a side by side comparison of this process with *mitosis*? Check out our comparison video here: https://youtu.be/zrKdz93WlVk ! ๐Ÿ˜€

  2. I solely rely on amoeba sisters and crash course for my tests, thanks so much for these videos! Without this id have an f lol

  3. looove you teachers!!!!! ๐Ÿ™‚ Thank you soooo much for your awsome vedios.
    Your vedios are something I always search before and after reading biology related chapters. Keep making vedios for ever and ever.

  4. My biology teacher just opened this vid for us and he thinks this is really good and it made this sub chapter bcms more easier to relate to…. Well, thankyou very much for uploading this vid!

  5. My teacher just puts a bunch of videos and thinks thatโ€™s giving clases
    I wish they were these videos

  6. I wasn't understanding what is going on but You make me understand everything very well thank you very much ๐Ÿ˜ญ๐Ÿ˜ญโ™ฅ๏ธ

  7. I just have no idea how I can thank uโคโคโคโคโคI have an official exam and I did not know how to solve genetics and now it changed! I am really glad. Thank You so much & Keep on goingโคโคโคโคโคโคโคโคโคโคโคโคโค

  8. Just wanted to thank you for making a previously complicated process absolutely understandable, clear, and concise! xx

  9. I really want so bad to pass the regents test itโ€™s based in all your topics I hope a passed it meanwhile Iโ€™m gonna study by watching you vids because you explain very good ๐Ÿ‘๐Ÿผ๐Ÿ‘๐Ÿผ

  10. I am thinking about those dislike who could have been done so!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!very nice and informative great job

  11. Thank you so much for your BEST EXPLANATION. You are just amazing and are very dedicated to your work. I have cleared my concept because of you ๐Ÿ˜ƒ๐Ÿ˜Š๐Ÿ˜Š๐Ÿ˜Škeep adding new and new videos๐Ÿ˜€๐Ÿ˜„

  12. Forget it, i wont remember this in the future anyway.. just the stages i'll remember. Im going to bed. Hope they don't bring in to state the process of meosis and shiz in paper 1

  13. I have a question. After interphase, the chromosomes in two cells are 23 right? And they have to find their homologous pairs, but the number is odd isn't it? So what happens to the last chromosome? Or because of the fact that the 23th chromosome is sex-linked one? Thanks a lot ^^.

  14. Second semester of medschool… and i was so confused as why 46-2×23- 4×23… there was nothing in my books about this. Thanks for clearing it up that the number is about the chomatids, xx you guys are doing a great job!

  15. why do they trasfer their genetic information and exchange it between each other, when they have the same dna? 4:20

  16. Actually i never heard about this information before from my teacher.. And now i really understand all the reason from science…

  17. me: spends one and a half-hour in a bio lesson understanding absolutely nothing
    me: watches a 7.44 min amoeba sisters video
    me: instantly understands

  18. Dear amoeba sisters,

    Thanks for the video, I may be 16 years old now, but 10 years later because of this video that I found I may see a more successful future, thanks!

  19. I've been through biology GCSE, A level and a degree with a significant biology component and it is only now, through watching your video, that I understand why chromosomes and chromatids are counted equally (based on no. of centromeres)! Thank you so much for making it all click into place! Absolute life saver <3

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