Cells and cell division
Mitosis, Meiosis and Sexual Reproduction Mitosis, meiosis and sexual reproduction. Understanding gametes, zygotes, and haploid / diploid numbers.
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- 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.
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At 5:31, how is the moon large enough to block the sun? Isn't the sun way larger?
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