Main content
Biology library
Course: Biology library > Unit 36
Lesson 1: Crash Course: Biology- Why carbon is everywhere
- Water - Liquid awesome
- Biological molecules - You are what you eat
- Eukaryopolis - The city of animal cells
- In da club - Membranes & transport
- Plant cells
- ATP & respiration
- Photosynthesis
- Heredity
- DNA, hot pockets, & the longest word ever
- Mitosis: Splitting up is complicated
- Meiosis: Where the sex starts
- Natural Selection
- Speciation: Of ligers & men
- Animal development: We're just tubes
- Evolutionary development: Chicken teeth
- Population genetics: When Darwin met Mendel
- Taxonomy: Life's filing system
- Evolution: It's a Thing
- Comparative anatomy: What makes us animals
- Simple animals: Sponges, jellies, & octopuses
- Complex animals: Annelids & arthropods
- Chordates
- Animal behavior
- The nervous system
- Circulatory & respiratory systems
- The digestive system
- The excretory system: From your heart to the toilet
- The skeletal system: It's ALIVE!
- Big Guns: The Muscular System
- Your immune system: Natural born killer
- Great glands - Your endocrine system
- The reproductive system: How gonads go
- Old & Odd: Archaea, Bacteria & Protists
- The sex lives of nonvascular plants
- Vascular plants = Winning!
- The plants & the bees: Plant reproduction
- Fungi: Death Becomes Them
- Ecology - Rules for living on earth
© 2023 Khan AcademyTerms of usePrivacy PolicyCookie Notice
Meiosis: Where the sex starts
Hank gets down to the nitty gritty about meiosis, the special type of cell division that is necessary for sexual reproduction in eukaryotic organisms. Created by EcoGeek.
Want to join the conversation?
- Is meiosis typically for sex cells and mitosis for somatic cells?
Is the following correct: the first stage of meiosis I results in the same endpoint as mitosis. However, in meiosis II, the cells divide once more resulting in haploid cells?(29 votes)- Meiosis is ONLY for sex cells. Mitosis is for somatic cells so they can repair parts of the body, make more cells, etc.
Meiosis I divides the number of chromosomes in half, making the cell haploid. The homologous chromosomes divide. Each cell still has 1 of each type of chromosome (1 #1, 1#2, 1 #3, etc.) but only one of it. Note that 1 chromosome still has 2 sister chromatids, which are identical. Meiosis II divides the sister chromatids apart, still keeping the cells haploid.
The cells need to be haploid so when they pair with a sex cell of the other sex, the offspring will have DIPLOID cells.(16 votes)
- Why does the reproduction does not continue forever?(8 votes)
- Telomeres are special, essential DNA sequences at both ends of each chromosome. Each time chromosomes replicate a small amount of the DNA at both ends is lost, by an uncertain mechanism. Because human telomeres shorten at a much faster rate than many lower organisms, we speculate that this telomere shortening probably has a beneficial effect for humans, namely mortality. The telomere hypothesis of aging postulates that as the telomeres naturally shorten during the lifetime of an individual, a signal or set of signals is given to the cells to cause the cells to cease growing (senesce). At birth, human telomeres are about 10,000 base pairs long, but by 100 years of age this has been reduced to about 5,000 base pairs.
Telomerase is actually an enzyme (a catalytic protein) that is able to arrest or reverse this shortening process. Normally, telomerase is only used to increase the length of telomeres during the formation of sperm and perhaps eggs, thus ensuring that our offspring inherit long "young" telomeres to propagate the species.(10 votes)
- How do scientists know which chromosomes control specific genes?(9 votes)
- Its not really that the chromosomes control specific genes, they just house them.
Chromosomes are just the skeletal structure where genes are located. And a specific genes always "live" on the same chromosome.
Finding out which chromosome contain which genes are done by taking a single chromosome and "reading" all the dna to see if they find genes there.(10 votes)
- what is the difference between diploid and haploid?(6 votes)
- A diploid cell is a cell with chromosomes that come in homologous pairs. Homologous just means similar but not identical. So a diploid cell would have two chromosomes for eye color.
A haploid cell is a cell that has only one representative of each chromosome pair. So a haploid cell would only have one chromosome for eye color.(6 votes)
- I am really confused. So when gametes are being formed they exchange genetic information with other gametes? I thought gametes are formed separately from the other specie's gametes until the sperm is ready to meet the egg.(6 votes)
- When the diploid cell starts going through Meiosis, the cell consists of (in humans) 23 pairs of homologous chromosomes i.e 1 chromosome in each pair from mother and father of that person which relate to the same alleles (traits/genes). During Prophase 1, the pairs line up and genetic information is shared between the homologous chromosomes in each pair.
At the conclusion of Meiosis, this basically means that each gamete's chromosomes have genetic information from the mother and the father of that person and are completely unique.
Gametes do not exchange genetic information with other gametes. You could think of it as, during Meiosis, 'mother chromosomes' exchange genetic information with 'father chromosomes' to create unique 'child chromosomes'.(4 votes)
- what if you don't have all 42 chromosomes(0 votes)
- Humans actually have 46 chromosomes, 23 from the mother and 23 from the father.(11 votes)
- , cleavages form during meiosis in animal cells. Plant cells will create cell plates and not create cleavage, right? 8:35(4 votes)
- A cleavage furrow forms when an animal cell begins cytokenesis, while plant cells grow a cell plate that will eventually become the cell wall in the places where the cell split.(6 votes)
- Is it possible to have 2 or more of the same chromosome?(2 votes)
- It is possible to get two or even THREE of the same chromosome. A human being has 46 total chromosomes and 23 pairs (assuming it is a female: If it's a male, there are 22 pairs and 2 leftovers). 23 are from each parent. However, there are some disorders in which people can get more than two chromosomes. One such disorder is called trisomy 21, where there are 3 chromosome 21s. This is called Down's Syndrome. Most trisomys (especially in larger chromosomes) result in death for the recipient.(2 votes)
- At, why are the X chromosomes shown to have crossed over if they can't cross over? 7:17(3 votes)
- Hank explains that since the chromosomes are the same, they can cross over. The XY pair can't cross over because they are different chromosomes.(2 votes)
- Which discovery did Gregor Mendel make?(3 votes)
- By studying pea plants he discovered that each generation acquired their traits and characteristics from their parents (half of which acquired from each parent) by particles passed on to them known as genes.(2 votes)
Video transcript
- Reproduction! Always a popular topic, and
one that I don't mind saying that I am personally interested in. The kind of reproduction
that we're most familiar with is of course sexual reproduction, where sperm meets egg, they
share genetic information and then that fertilized
egg splits in half, and then those halves
split in half, and so on, and so on, and so on
to make a living thing with trillions of cells that
all do specialized things. And if you're not suitably
impressed by the fact that we all come from one single cell, and then we become this, then I don't, I don't know how to impress you. But riddle me this, my
friends, if sexual reproduction begins with sex cells,
the sperm and the egg, where do the sperm and egg come from? Ah, dude. So, how do sex cells form,
so that they each have only half of the genetic
information that the resulting offspring will end up with? And for that matter, why aren't all of our sex cells the same? Like why, are my brother
John and I, different? Sure, we both wear glasses, and we both kind of look
like a tall Doctor Who, but you know, we have different color hair and different noses, and I'm way better at Assassin's Creed than he is,
so why aren't we identical? As far as we know, we both
came from the same two people, with the same two sets of DNA. The answer to these questions, and a lot of other life's
mysteries, is meiosis. (Crash Course intro song plays) In the last episode, we talked
about how most of your cells, your body or somatic cells clone themselves through the process of mitosis. Mitosis replicates a cell with the complete set of 46 chromosomes into two daughter cells that are each identical to each other. But of course, even though the
vast majority of your cells can clone themselves, you
cannot clone yourself. And for good reason, actually reasons. If mitosis were the only
kind of cell division we were capable of that would mean: a) you would be a clone
of one of your parents, which would be awkward to say the least, or possibly b) half of
your cells would be clones from your mom and half would
be clones from your dad, and you would look really weird. But that's not how we roll,
we do things a better way, where all of your body cells
contain the same mix of DNA, 46 chromosomes grow up into 23 pairs. One in each pair from your
mom, and one from your dad. Those pairs of chromosomes
are pretty similar, but they're not identical. They contain versions of
the same genes, or alleles, in the same spot for any given trait. Since they're so similar, we call the pairs
homologous chromosome pairs. Homologous is a word that
comes up a lot in genetics, it just means that two things have the same homo-relation logos, even if they are a little bit different. However there are some very
special cells that you have, that have only one half of
that amount, 23 chromosomes, those are sperm and egg cells. These are the haploid cells, they have half of a
full set of chromosomes. And they need each other to
combine to make the complete 46. Creating those kind of cells
requires a process that's very similar to mitosis, but with a totally
different outcome, meiosis. That's when a specialized diploid cell splits in half, twice,
producing four separate cells, each of which is genetically
distinct from the others. Meiosis is a lot like
mitosis, except twice. It goes through the
same stages as mitosis, prophase, metaphase,
anaphase and telophase, but then it goes through another
round of the stages again, and they have the same
names, conveniently, except with a two after them. They're like sequels. And just as with the
Final Destination movies, the sequels have pretty
much the same plot, just some new actors. The raw materials for this
process, are in your ovaries, or your testes depending on, you know, you know what it depends on. They're diploid cells called
either primary oocytes, or primary spermatocytes depending on what kind of gamete they make. Men produce sperm, you may have heard, and they produce it
throughout their adult lives, whereas women are born with
a certain amount of eggs that they'll release over
many years after puberty. Here you might wanna go
watch the previous episode about mitosis again, because
that's where we go into detail about each stage of the process. Once you're done with that, we can start making some babymakers. Just like with mitosis,
there's a spell between rounds of cell division where
the cell is gearing up for the next big split, and
this is called interphase, when all the key players
are replicating themselves. Long strings of DNA in the
nucleus begin to duplicate, leaving two copies of every strand. To jog your memory
about how DNA does this, we did a whole episode on it, you can watch it and come back. A similar process takes
place with the centrosomes, a set of protein cylinders
next to the nucleus that will regulate how all of
the materials will be moved around along these ropey
proteins called microtubules. And that brings us to the first
round of meiosis prophase I. This is nearly the same as in mitosis, the centrosomes start heading
to their corners of the cell, unspooling the microtubules,
and the DNA clumps up with some proteins in the chromosomes. Each single chromosome is
linked to its duplicate copy to make an X-shaped double chromosome. And keep this in mind, once
attached, each single chromosome is called a chromatid,
one on each side of the X. Each double chromosome,
has two chromatids. Here meiosis prophase I
includes two additional and very important steps, crossover and homologous recombination. Remember that the point here is to end up with four sex cells
that each have just one single chromosome from each
of the homologous pairs. But unlike in mitosis, where
all the copies end up the same, here, every copy is going to
be different from the rest. Each double chromosome lines
up next to its homologue, so there's your mother's
version lined up right next to your father's version
of the same chromosome. Now if you look, you'll see that these two double chromosomes
each with two chromatids, add up to four chromatids. Now watch, one chromatid from each X, gets tangled up with the
other X, that's crossover. And while they're all tangled up, they trade sections of DNA,
that's the recombination. The sections that they're trading
are from the same location on each chromosome, so one
is giving up its genetic code for like hair color or
body odor, and in return, it's getting the other
chromosome's genes for that trait. This is important, what
just happened here, creating new gene combinations
on a single chromosome. It's the whole point of
reproducing this way. Life might be a lot less stressful if we could just clone ourselves, but then we'd also clone all
our bad gene combinations, and we wouldn't be able to change and adapt to our environment. Remember that one of the
pillars of natural selection is variation, and this is a
major source of that variation. What's more, since all
of the four chromatids have swapped some DNA segment at random, that means that all four
chromatids are now different. Later on in the process, each chromatid will end
up in a separate sex cell, and that's why all eggs
produced by the same woman have a slightly different genetic code. Same for sperm and men. And that's why my brother
John and I look different, even though we're made from
the same two sets of DNA. Because of the luck of the genetic draw that happens in recombination. I got this mane of luscious hair, and John was stuck with
his trash, brown puff, and don't forget about my
mad Assassin's Creed skills. But then of course, there is
that one pair of chromosomes that doesn't always go through the crossover or recombination. That's the 23rd pair, and
those are your sex chromosomes. If you're female, you have
two matched, beautiful, fully capable chromosomes
there, your X chromosomes. Since they're the same, they can do the whole crossover
and recombination thing. But if you, like me, are a male, you get one of those X chromosomes, and another from your dad
that's kind of ugly and short, and runted and doesn't have a lot of genetic information on it. During prophase, the X wants nothing to do with the little Y because
they're not homologous. So they don't match up,
and because the XY pairs on these chromosomes will split later into single chromatids, half
of the four resulting sperm will be X, leading to female offspring, and half will be Y,
leading to male offspring. Now what comes next, is another kind of
amazing feat of alignment. This is metaphase I, and
in mitosis you might recall that all of the chromosomes
lined up in a single row, powered by motor proteins,
and were then pulled in half, but not here, in meiosis. Each chromosome lines up next
to its homologous pair partner that it's already
swapped a few genes with. Now the homologous pairs get pulled apart and migrate to either end of the cell, and that's anaphase I. The final phase, of the
first round, telophase I, rolls out in pretty much
the same way as mitosis. The nuclear membrane reforms
a nucleoli form within them, the chromosomes fray
out back into chromatid. A crease forms between the
two new cells called cleavage, and then the two new nuclei
move apart from each other, the cells separate in a
process called cytokinesis, literally again cell movement. And that is the end of round one. We now have two haploid cells, each with 23 double
chromosomes that are new, unique combinations of the
original chromosome pairs. And these new cells, the
chromosomes are still duplicated and still connected at the centromeres, they still look like X's. But remember, the aim is
to end up with four cells, so it's time for those sequels. Here, the process is
exactly the same as mitosis, except that the aim
here isn't to duplicate the double chromosomes, but
instead, to pull them apart into separate, single-strand chromosomes. Because of this, there's
no DNA replication involved in prophase II, instead the
DNA just clumps up again into chromosomes and the
infrastructure for moving them, the microtubules, are put back in place. In metaphase II, the chromosomes
are moved into alignment into the middle of the
cell, and in anaphase II, the chromatids are pulled apart into separate, single chromosomes. The chromosomes uncoil into chromatid, the crease forming cleavage
in the final separation of cytokinesis then mark
the end of telophase II. From one original cell with
46 original chromosomes, we now have four new cells with
23 single chromosomes each. If these are sperm, all
four of the resulting cells are the same size, but they
each have slightly different genetic information, and half
will be for making girls, and half will be for making boys. But if this is the egg-making process, then it goes a little
bit differently here, and the result is only one egg. To rewind a little, during telophase I, more of the inner goodness
of a cell, the cytoplasm, the organelles heads into one of the cells that gets split off,
then to the other one. In telophase II, when
it's time to split again, the same thing happens
with more stuff going into one of the cells than the other. This big ol' fat remaining
cell becomes the egg, with more of the nutrients
and cytoplasm and organelles that it will take to make a new embryo. The other three cells that were produced, the little ones, are called polar bodies. And they're totally useless in people, though they are useful in plants. In plants, those polar bodies actually also get fertilized too, and
they become the endosperm. That's the starchy, proteiny
stuff that we grind into wheat or pop into popcorn, and
it's basically the nutrients that feed the plant embryo, the seed. And that's all there is to it, I know you probably were
really excited when I started talking about reproduction,
but then I rambled on for a long time about
haploid and diploid cells, but now you can't say
that you know more about the miracle of reproduction-- it's not actually a miracle, it's science!