Mitosis, Meiosis and Sexual Reproduction


I touched on this a little bit
in the video on how variation can be introduced into a
population, but I think it’s fairly common knowledge that all
of us– when I talk about us I’m talking about human
beings, and frankly, most eukaryotic organisms– we’re
the product of sexual reproduction. So if this is the first cell
that had the potential to become Sal, we know that this
first cell– let me say this is the nucleus of that first
cell so I can draw the whole cell and all that, but let’s
just focus on the nucleus. It has 23 chromosomes. Well, let me put it this way. It has 46 chromosomes, 23 from
my father and 23 from my mother, so that’s 1, 2 3, 4, 3
5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20,
21, 22, 23 from my father. And then let’s say that last
one actually helps to determine my gender, or it fully
determines my gender. That’s my Y chromosome. And let’s say I had 23
homologous chromosomes, or one chromosome that kind of was
the homologue for each of these, but I have 23 of them
from my mother, so 1, 2, 3, 4, 5– oh, you get the idea. I can just draw a bunch of
them, and then have the X chromosome that is essentially
one of the gender-determining chromosomes from my mother. And we learned before that
each of these pairs are homologous chromosomes, that
they essentially code for the same gene, one from my father
and one from my mother. Now, that first cell that had
the potential to become me, it was a product of fertilization,
of an egg from my mother– so an egg
from my mother. I’ll just draw the whole
egg like that. I’ll just focus on the DNA from
now, so my mother’s DNA, it had 23 chromosomes. So it didn’t have pairs,
and this is key. So there’s 1, 2, 3, 4, 5, 6, 7,
8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21,
22, and then 23 was the X chromosome. And so it’s a combination from
my mother, so this is from my mother, and a sperm
from my father. Let me do that here. And I’ll draw the sperm much
larger than it is normally relatively to the egg. This is kind of the nucleus of
the egg, but let’s say that this is the sperm, and it has a
tail that helps it swim, and it has 23 chromosomes. So 1, 2, 3, 4, 5, 6, 7, 8, 9,
10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, and then
it has that Y chromosome. Let me do that Y chromosome
in a separate color. Just as an aside, this
unification, this fertilization that occurred from
this sperm cell to this egg cell, so it essentially
penetrates into this egg cell and it creates this zygote,
which is a fertilized egg cell from my mother, and this
contains all DNA from both my father and my mother. So this very first cell that
was created from this fertilized egg, this
is called a zygote. It’s a product of fertilization between two gametes. So that’s a gamete and
this is a gamete. Both a sperm cell or an
egg cell, they’re both examples of gametes. Now, the whole reason why I’m
doing this is I want to introduce you– and I already
introduced this notion to you when we talked about the
variation of population, that, look, this has my full
chromosome complement. It has 23 pairs, and each pair
is a pair of homologous chromosomes. They essentially code for the
same things, one from my mother, one from my father,
and that is 46 individual chromosomes, 23 from my mother,
23 from my father. These gametes, they each have
only 23 chromosomes, or half the number of a full
complement. Now, everything that I’m talking
about here, the number 46, or 23 pairs, or 23
individual chromosomes, this is unique to human beings. If I talked about other species,
they might have 10 chromosomes or they might
have 5 chromosomes. But one thing that is universal
for all sexually reproducing organisms is that
gametes have half the number of chromosomes as the zygote, or
you can kind of view it as the organism itself, the way we conventionally think about it. So when people talk about half
the number of chromosomees, they say it has a
haploid number. And that literally just means
half the number of chromosomes. It’s very easy to memorize,
because haploid starts with the same two letters as half. Haploid number for humans
is 23 chromosomes. And so, you say, oh, if you say
this is a haploid number, what do you call it when you
have the full complement of chromosomes? Well, that’s called the
diploid number. And I remember that because
di- often is a prefix associated with having two of
something, and so you have twice the number
of chromosomes. So this is haploid, this is
diploid number, and this is for humans, right? For an organism where the
diploid number is N, and you’ll sometimes see this
notation, so I want to make sure you’re comfortable with it,
there’s some organism, or actually any organism. If the diploid number is 2N,
then the haploid number is going to be half of
that, or just N. Now, in the case of humans, the
diploid number is 46, so N is equal to 23. So a fertilized egg or even just
a regular somatic cell or a body cell will have a diploid
number of chromosome, while a sex cell, and I’ll be
a little bit clearer about that in a second, will have a
haploid number of chromosomes. So gametes, which are either a
sperm or an egg, those are both examples of gametes, they
have half the number, they merge, and then you get a
zygote, which is that very first cell that had the
potential to turn into me, that has a diploid number
of chromosomes. And I actually want to do a
little bit of a side here, because it’s fascinating. We talk about natural selection,
and we even wonder today to what degree is it
occurring, because our society, it’s not as tough of an
environment as the natural world would be where we’re being
stalked by predators and we have to live out in
the wild and find food and all of that. But even this process of
fertilization is an incredibly competitive process, because
this sperm that happened to be the one that kind of won the
race from my father to fertilize my mother’s egg, it
was actually the first of roughly 200 million other
sperms. There were 200 million, roughly. There could’ve been 200
to 300 million other sperms in that race. From the moment we’re born,
we’re already the product of an intense competition amongst
these male– I guess we could call them male gametes, or
amongst these sperm cells. Some of them might have had
weird mutations, that they didn’t know which direction to
swim, they happened to go in the wrong direction, maybe some
of them had weird tails that didn’t allow them to swim
as fast, so you’re already on some level selecting for
fitness within this environment. So if you had some weird
mutations from the get go in some of these sperm cells, it
would have been less likely, especially if they affected
their ability to kind of swim, it would’ve been less likely
that they would have been the ones to win this race. So already, you are the product
of a race of 280 million organisms, if you
consider each of these sperm cells an organism, and you are
the product of that winning combination. So, you know, sometimes we
feel lost on this planet. We’re one of 6 billion people
and all that, or just a number, but we already are the
product of a pretty intense accomplishment. But now with some of this
vocabulary thrown out of the way, let’s talk a little bit
about zygotes and how do zygotes turn into people, and
then how do those people essentially produce gametes,
which then can fertilize other people’s gametes to
form more zygotes. So the general idea: So that
very first cell that was essentially my mom’s egg
fertilized by a sperm cell from my father, that was a
zygote, and as soon as it’s successfully fertilized, it has
2N, or it has the diploid number of chromosomes in the
case of humans, which I believe I am one of them. I have 46 chromosomes. And then this cell right here
begins to split and divide over and over and over again. We’ll do a whole series of
videos on the actual mechanics of that, but it splits by a
mechanism called mitosis. And mitosis literally is just
a cell splitting to form copies of itself. So it just starts splitting into
two more cells that are– and actually, let me do it this
way, just because the actual way it works is right
when a cell is split, the cells that it splits into aren’t
that much larger than the original one. But now each of these have 2N
chromosomes, or 46 in the case of humans, and you keep
splitting, and it happens over and over and over again. So eventually– well, let
me just do it this way. This keeps splitting, and then
you have– and I’ll go into the words for some of these
initial collections of cell, but I won’t go into
that right now. 2N, all of these are original
copies from a genetic point of view of that original cell. And then eventually, they start
to really– I start to have tons of them. There’s just a gazillion of them
that are all duplicates of the cell, and they all
contain the 2N number of chromosomes, the diploid
number of chromosomes. They all contain all of my
genetic material, but based on how they relate to each other
and what they see around them, they start differentiating. So all of these have 2N number, so they’re all diploid. And mitosis– this is the
process the whole time– is these divide one cell into two
cells and those two cells into four cells and keep going. And then these begin
to differentiate. Maybe these cells eventually
differentiate into things that’ll turn to my brain. These cells right here
differentiate into things that’ll turn into my heart. These cells here differentiate
into things that will turn into my lungs and so
forth and so on. And eventually, you
get a human being. But it doesn’t have to
be a human being. It could be whatever species we
happen to be talking about. So let me draw the
human being. So I’ll draw my best shot at an
outline of a human being. Now, we’re talking about
gazillions of cells. You have your human being, and
I’ll just draw a very simple diagram, outline of
a human being. When I was in high school, I was
a class artist, so I don’t want to make this representative
of my true artistic ability. I’m doing this here just to
kind of give you an idea. But anyway, eventually, you keep
dividing these cells and you end up with a human being,
and this human being, you know, you wouldn’t even notice
the cells on this scale. Now, most of these cells of this
human being, if this is me or you, these are all the
product of mitosis that started off with that zygote,
and it just kept dividing and dividing and dividing
into mitosis. But it differentiated. I said some of them will
turn into brain cells. Some of them will turn
into heart cells. The whole process of
differentiation is actually fascinating, and we’ll talk a
lot more about that when we talk about stem cells, embryonic
stem cells, and maybe we’ll even talk about
the debate of it. But the question is,
well, how do I then produce those gametes? How do I produce those things
that eventually, if I’m going to reproduce, turn into
these kind of haploid number of cells? And that’s what happens
in your sexual organs. So in a male, you have some germ
cells, so some of these cells turn into germ cells. And the germ cells exist
as part of your reproductive organs. So let’s say those are
the germ cells. In a male, they would be
part of the gonads, so they would be there. In a female, they would be
involved in the ovaries. And these germ cells, they’re
the product of mitosis. So let me draw a germ cell. So a germ cell is the product of
mitosis, so it still has 2N number of chromosomes, so it
still is a diploid cell or has a diploid number. But what’s special about a
germ cell is it has the potential, one, it can either
continue to do mitosis and produce more germ cells that
are identical to it, so it could produce two germ cells
that are identical to it, or it can undergo meiosis. And meiosis is essentially what
a germ cell undergoes to produce gametes. And so if this germ cell
undergoes meiosis, and I’ll do a whole video on the mechanics
of it, instead of two cells, it’ll actually produce four
cells that each have half the number of chromosomes in them,
so these cells are haploid. In the case of a male, these
would be sperm cells. This would be sperm. In the case of a female, these
would be ova, sperm or ova, and these are the gametes. So it’s an interesting thing to
talk about, because in the last several videos, I talked a
lot about mutations and what does that do to a species, but
think about what happens. If I have a mutation in some
cell here, some somatic cell, some body cell, somatic cell,
will that mutation or can that mutation in any way affect
what’s going to happen to my kids? Will that mutation be carried
on to my kids? Well, no. Because in no way will what goes
on in this cell affect what I actually pass on
eventually in the sperm cells. It’ll just be a random
mutation. It could affect my ability
to reproduce. For example, it could be– God
forbid, it could be some type of cancer or something that,
especially if you contract it at a young age, it might be some
type of terminal form so that might affect your ability
to reproduce, but it will not affect the actual DNA that you
pass on to your offspring. So if you have some really bad
mutation here, it could affect how you live or it could turn
cancerous and start reproducing, but it will not
affect what you pass on to your children. The traits that will be passed
on or the changes that will be passed on are those that occur
in the germ cells. So if you have mutations in your
germ cells, or during the process of meiosis, you have
essentially recombination of DNA because of crossovers, and
we saw that in the variation video, then that will introduce
new forms or new variants inside that could be
passed on to your children. And I really want to make that
point there, because we talk about mutations, but there’s
different types of mutations. There’s some mutations that
won’t be passed on to your children, and those are the
ones that occur in your somatic cells. Maybe some of them do nothing
so then it really doesn’t affect your overall function,
but in the mutations that either occur in your germ cells
or the recombination or the variation that is introduced
during meiosis, that will be passed on
to your children. But even there I want
to be careful. Because remember, this is
a severe competition. So out of all of the– let’s say
there’s 280 million sperm cells that at one time are being
competitive for an egg, it’s possible that some of
them have mutations. In order for one of those
mutations– let me do the mutations in different colors. That’s a purple mutation. That’s a blue mutation. But in order for that mutation
to truly be passed on to my offspring, the sperm containing
the mutation is the one that has to win the race. So already you have a selection
going on at kind of this sexual reproduction level
where you’re selecting for things that are at least good
enough– I mean, to some degree, the sperm has to be
good enough to win this is incredibly, incredibly
competitive race. So that mutation that somehow
made the sperm deformed or didn’t allow it to swim or made
it behave in some weird way, it’s very unlikely that
that mutation would go on to be the one or that cell would go
on to be the one that would successfully fertilize an egg. So anyway, I wanted to introduce
you to these ideas. The main idea is really
some of the vocabulary: haploid, diploid. It’s very confusing when you
first learn it, but it literally just means half the
normal group of chromosomes. And in the case of humans,
that would be 23. And the cells that have a
haploid number of chromosomes are our gametes, which are sperm
cells for men, and ova, or egg cells for women. But everything else in our
body, all of our somatic cells, are diploid, which means
that the full complement of chromosomes, they all
have a copy of our DNA. And that’s why DNA testing is so
interesting because you can get any cell from someone
anywhere, and you have their full complement of DNA. You have all of the information
that describes them genetically. Anyway, see you in
the next video.

100 Replies to “Mitosis, Meiosis and Sexual Reproduction”

  1. i love it, but im ocd about how if you just did two more chromosomes from your mother you would've have all 23… just TWO more chromosomes!

  2. This man over here,knows pretty much everything. Thank you so much for putting all of this widespread variation of videos up, helps me and probably LOTS of others a lot, definitely actually!

  3. As a high school english teacher, I can tell you he has a terrible teaching style. It's all route formula, no critical thinking or understanding. My warning to you: if you think you're learning here, you're hopelessly unprepared for college.

  4. To the the "high school teachers" here who believe that they are some how superior to Khan academy in teaching style, shove it up your butt! I am a junior in college, dyslexic and have a 3.9 GPA. Khan academy has tutored me the whole way through. I never learned a thing from a high school instructor because they were all too busy checking their twitter accounts.

  5. Sting ray I feel as if you may have been teaching high school for so long (most likely a lower grade) that you've forgotten how detailed things in college are.

    You NEED the answer to be explained in "route formula" as you put it… There is no critical thinking when teaching someone completely unfamiliar with this (which everyone is unless they've studied it on their own in detail, or have taken a basic biology course).

    I'm to be told things like (i'll make something up) "Interphase is the where the cell spends most of it's time" because even though that is OBVIOUS once you know the process, it's not when you're learning it….. and can actually take time out of progressing just trying to figure out this one stupid thing that could have been easily explained in detail.

    If I misunderstood your criticism then… well… idk… I'm surprised I've taken as much time as I have with this as it is… English teachers are always highly opinionated with bleeding hearts xD (not to say I'm not this way because I am)

  6. To the Non-STEM (Science, Technology, Engineering, Mathematics) teachers criticizing Khan academy;

    It's Biology being discussed here, Not Kafka and his bug man as a metephor for aging into a burden, Not Lewis Carrol and his pedophillia, not Twain and his southern memories, Not Hawthorne and his depressingly glum takedowns of Christian idolatry but the structural layout of organisms.

    Khan academy videos lay out a different version of the map we're given in lectures and text books and brings it closer to 6-7th grade language that makes the latin-based jargon easier to follow and remember. 

    Critical thinking comes later in general pathology and human diseases, where you need to know how one problem (disease, habit, physical trauma, glandular or electrolyte difficiency, viral infection [what virus? where?]) affects one system and how that one system will affect other systems throughout the body.

    To reiterate; it's a map (or engineering flow chart), and the good students will be able to connect this map with a map of the endocrine system and that map with the Digestive system. It's a foundation for what comes later when you have to be critical of what's going on with patient symptoms and what they mean. . .but that's covered in Lecture.

  7. LOL my high school biology teacher is really bad so I've had to watch Khan Academy to learn the entire course.

  8. At around 12 mins the guy is talking about how our cells differentiate. But I don't understand how they do that if they are direct copies or clones of the cell that has split in mitosis. Wouldn't all our cells be the same? I thought that modification in cells or animals occurs in reproduction. Do the cells change after they have split from the previous cell?

  9. Can someone correct if I am wrong, thank you. I thought that after mitosis, each chromosome has one chromatid (1n) instead of two chromatids (2n).

  10. I learned more in these 18 minutes than I did in my biology class all semester. Mitosis, meiosis test tomorrow and this should definitely help. Thank you!

  11. Really appreciate your time and dedication of showcasing your talent in these videos for the use of others. You're a God sent!

  12. Really appreciate your time and dedication of showcasing your talent in these videos for the use of others. You're a God sent!

  13. When science finally makes sense @Khan Academy Thank you so much, I never thought I would ever be able to understand this but now i feel ready for my exam.

  14. Fitness of a sperm has nothing to do with the fitness of the genetic information it carries. You can be as healthy as Bruce Lee and Swatcenegger in one face but your head can be full of total crap. I wonder why sprint winner carries the best DNA if it carries DNA of some another person. Probably we do not need to born the babies and grow up the adult people if we can select the best fit ones at the sperm level. We can accelerate the evolution and make it much more resource-efficient and much less violent if we start selecting at the sperm level. Right?

  15. Hello, thanks for your videos helps a lot. It might be a stupid question, but why cant life start with just 23 chromosomes, what is the reason for having diploid number of chromosomes, since each homologous pair have similar genes that can code for a protein. So why cant an egg cell just start to divide?

  16. Hi, you talked about 280 millions sperm in competition and 4 daughter cells. What do they have to see in common and the differences?

  17. This is a why question, which I know scientists hate. But why is meiosis configured to produce 4 gametes? What is the evolutionary predicate? For example, if the germ cell was to forego DNA replication, not produce sister chromatids, but simply divide up the paternal and maternal chromosomes into two cells, your could still have 2 haploid gametes. So, why did the process evolve the way it did?

    I'm not asking for wild uninformed speculation. But surely some prominent evolutionary biologists must have considered this question over previous decades and come up with feasible scenarios? I just can't seem to find anything out there. Lots of "whats" and no "whys" – which is kind of boring.
    Show less

  18. i always had a hard time learning from khan academy math videos, but these bio videos are godsent

  19. What if both parent's dna don't pair up right with each other, i.e. adenine keeps pairing up with cytosine or guanine with thymine? Is this taken care of by rna polymerase during the transcription and translation stages? please explain.

  20. Sperm cells basically does this battle royale for the egg. remember, you were the one who got the victory royale.

  21. I got a question. How can hereditary diseases be passed over to your child without mutations? Like for example, autism is a hereditary disease, but it's still caused by mutations, right?? Then how does that work?

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