This podcast will deal with meiosis.
And when we talk about meiosis I like to ask what is the advantage of sex and
what is the purpose of meiosis? Now, if you remember mitosis and the purpose of
mitosis is to produce genetically identical daughter cells. Remember that
equation 1 to the 46th power equals 1. On the other hand, the purpose of
meiosis is to produce genetically diverse daughter cells. The daughter
cells here being the gametes. Remember this equation 2 to the 23rd.
8,388,608 possible
combinations from independent assortment of homologous chromosomes at metaphase I.
And this number it gets even way larger because of homologous
recombination or crossing over that occurs in prophase I. This diagram
compares and contrasts mitosis and meiosis and I'm gonna spend a little
time going over it because it might help to put things in perspective. So, let's
look at somatic cells, mitosis on the left and meiosis what's occurring as
gametes are formed. So, in mitosis, one cell division results in two daughter
cells. Chromosome number, humans 2N = 46 and so you have a single cell
division producing two daughter cells 2N = 46. Here if we look at DNA
replication, there's a pre mitotic S. DNA goes from 2C to 4C in S and then
through G2 and M and back down to 2C during mitosis and back to the 2C DNA
content. There is normally no pairing of homologous chromosomes so you don't form
tetrads. There's normally no recombination. This is an important point
during metaphase of mitosis. There's biorientation of sister kinetochore.
There's loss of cohesin between sister chromatids during metaphase and
so the centromeres divided anaphase and each pole gets an identical copy of DNA
from each chromosome. If you look at my meiosis, on the other hand and the cells
that give rise to gametes, there are two cell divisions that are going to result
in four products of meiosis. So, you're going to start with a cell that is 2N,
it's gonna go in the first meiotic division to N, the second meiotic
division also is going to go to N, but here you're going to look at differences
in DNA content. We can look at a pre-mitotic S phase. For both cell divisions, DNA
replicates so you're gonna go from 2C to 4C. First meiotic division
you're going to go 2C and the chromosome number is also reduced. The second
meiotic division, there's no DNA replication. The chromosomes divide a
second time, DNA content is reduced. So, you're going to go from 2C to 1C. The
chromosome division in the second meiotic division is much like a mitotic
division. So, here we look at what's happening in meiosis I. There's full
synopsis of homologous chromosomes. In prophase, a tetrad forms so these are the
homologous chromosomes. Let's say this is for example, maternal chromosome I,
paternal chromosome I. Remember there are 23 pairs in humans. Here shown some
cross and over here the chiasmata, the point where crossing over has occurred.
In the first meiotic division, there's co-orientation of sister
kinetochores towards the same pole. There's maintenance of cohesin between
sister chromatid arms during metaphase I of meiosis, so at the first meiotic
division an entire homologue goes to one pole so here both chromatids of maternal
chromosome I go to the one pole both of paternal chromosome I go to the
opposite pole. This produces variation in gametes. So, here's another
way to look at it comparing mitosis and meiosis in humans. In mitosis there's one
DNA replication, one chromosome division. So, if you look at for humans 2C of
seven picograms 2N is 46 chromosomes, so you go from seven picograms to fourteen
picograms through S , so you're entering at the end of G2, 14 picograms 46
chromosomes. Go to G1, go through division, back to 2C 46 chromosomes. DNA amount
remains the same; chromosome number remains the same. And this is because the
chromosomes are behaving as individual units. Meiosis there's one DNA
replication. There's two chromosome divisions so that a diploid cell
produces four genetically different haploid cells. Notice what happens here,
pre-mitotic DNA synthesis, you go from seven picograms to fourteen picograms,
46 chromosomes. First meiotic division you go to seven picograms of DNA, 23
chromosomes. Second meiotic division three-and-a-half picograms of DNA, still
23 chromosomes and it is because at metaphase 1 the chromosomes are behaving
as homologous pairs, they're synapsed with each other. So, just to remind you,
mitosis 1 to the 46th is 1; meiosis 2 to the 23rd a really big number. Meiosis in
males leads to an even distribution of cytoplasm among sperm. Meiosis and human
males begins at puberty and it can actually continue through the seventh or
eighth decade of life and meiosis in human males reflects a renewing cell
population. We'll talk about this in a little more detail as we talk about the
male and female reproductive systems. But here, you've got a spermatogonium,
which was a diploid cell, it's 2N and 2C. The primary spermatocyte is 2N
4C. At meiosis I, the secondary spermatocytes is 1N and and 2C. At
meiosis II, the spermatid is 1N and 1C. So, the secondary spermatocytes is
haploid with respect to chromosome number. The spermatid is haploid with
respect to chromosome number and with respect to DNA content. And in sperm
maturation there's a series of post mitotic maturation events that occur to
get mature sperm. We'll look at that in much more detail when we talk about the
male reproductive system. The female system is unusual. Meiosis here in
females leads to an uneven distribution of cytoplasm between the ovum and the
polar bodies. As you probably know, meiosis and human females begins at
three months of fetal life and it then arrests and it remains arrested until
puberty and then at puberty meiosis resumes and it continues through
menopause. We talk about meiosis and human females as reflecting a decaying
cell population, and we'll talk about this in more detail as we talk about the
female reproductive system. But here, you've go,t for example, oogonia
dividing by mitosis, they're 2N 2C. Eventually, they're going to enter the
primary oocytes stage at about six to seven months of fetal life, there'll be
2N 4C. The LH surge just prior to ovulation stimulates the first meiotic
division and that will produce a secondary oocytes, which is 1N and 2C and
a polar body. Now these are drawn out of proportion because of course the polar
body is much smaller than the secondary oocytes. Meiosis II in humans will only
occur if fertilization occurs and at meiosis II the secondary oocytes will
divide into an ovum which will be 1N and 1C and a second
polar body, which will also be 1N 1C. The first polar body may divide
usually in humans disintegrates. Meiosis II is occurring as fertilization occurs
the sperm actually has to penetrate the egg for the second meiotic division in
humans to be complete. Once that second meiotic division is
complete then the pro nuclei will fuse and you'll begin to get the zygotic
divisions. We'll talk about this in much more detail when we talk about the
female reproductive system, but meiosis is clearly under endocrine control, and
this is just to put that in perspective for you. I'm not gonna go over it in
detail here because we'll go over it in detail in the female reproductive system,
and you clearly go over it in much more detail in your physiology course, as well.
Just some points to wrap up about meiosis versus mitosis. In mitosis,
chromosomes align as separate entities on a metaphase plate, that's what's shown
here. In meiosis, homologous chromosomes pair and the line of synapse, bivalents,
or tetrads on the metaphase plate of meiosis I. So, for example, let's compare
these two figures. This would be paternal chromosome I, maternal chromosome I,
they aligned separately. In mitosis, maternal chromosome II, paternal
chromosome II, they align separately in mitosis. So, during metaphase each sister
chromatid goes to opposite poles. Here in meiosis, the homologous chromosomes
synapse. So, maternal chromosome I, paternal chromosome I aligned as a
homologous pair. Maternal chromosome II, paternal chromosome II align as a
homologous pair. So here in humans, there are 23 homologous pairs on the metaphase
I plate, whereas in mitosis in humans there are 46 individual chromosomes
aligned on the metaphase plate. Metaphase I of meiosis the homologous pairs
separate. So, an entire chromosome consisting of both sister chromatids
goes to one pole or the other. This figure shows the tetrad or bivalent
consists of four chromatids. So, these would be the maternal chromatids, the
paternal chromatids, the chiasmata, the point of crossing over between the
homologous chromosomes. Chiasmata are essentially a manifestation of
recombination that actually occurs in zygotene or pachytene. You don't see
the chiasmata until much later in prophase I, sort of a diplotene or
diakinesis. This just shows the 46 human chromosomes, 22 pairs of autosomes,
and the pair of sex chromosomes, so 2N = 46 and they're just displayed
nicely on the right side of the figure. This figure finally compares and
contrasts mitosis and meiosis again. The first meiotic division, we call it the
reduction division. DNA content goes from 4C to 2C, chromosome number goes from 46
to 23 at the first meiotic division. The second mitotic division we call it an equational
'division, it's similar to mitosis, DNA content goes from 2C to 1C, chromosome
number remains at 23. And I will leave it to you to look at this figure and try to
consolidate in your mind the differences between mitosis and meiosis. With respect
to the stages of meiosis, we talk about a pre-meiotic S phase where DNA synthesis
occurs and then meiosis I, the reduction division sometimes called the heterotypic
division, prophase I is very protracted, it's very elaborate and
inhuman females that can last for decades. But, there's a leptotene stage
where there's a condensation of chromatin and assembly of lateral
elements of the synaptonemal complex complex the zygotene stage is the assembly of the
central elements of the synaptonemal complex. The synaptonemal complex, of course,
is the structural basis to allow homologous recombination
to occur. That recombination occurs in pachytene, then you have diplotene or
diakinesis and in females there's a D condensation of chromatin here and you
have an intense amount of transcriptional activity, especially for
ribosomal RNA. You're going to have translation of
proteins that are specific proteins needed for the egg to survive. Then at
metaphase I, there's going to be recondensation of chromatin, transcription
and translation is going to cease and in human females this is only going to
occur if ovulation occurs. Then there will be anaphase I, telophase I, there's a very
brief interface after telophase, there's no DNA synthesis, in humans you can't
hardly recognize that interphase, meiosis II is the equational division of the homotypic
division. In human females, this only occurs if the egg is fertilized. So,
they'll be a very quick prophase II, metaphase II, anaphase II, then telophase II
and in sperm after telophase II, there's a whole series of maturation events that
are going to occur as the sperm is going to mature. In the egg, after telophase II
the pro nuclei will fuse and then you'll get into the first embryonic divisions.
This final slide is to remind you that meiosis uses two mechanisms to mix up or
shuffle genes and that's going to lead to genetic or biologic variability.
The diagram on the left shows independent assortment in meiosis to produce 2 to
the N different combinations of haploid gametes. So, this would show a cell that
potentially has 6 chromosomes, 3 homologous chromosomes. You can, I think,
in your mind's eye do the pairing and come up with 1, 2, 3, 4, 5, 6, 7, 8 possible
gametes. 2 to the 3rd, 3 homologous pairs, 8. It'd be a little bit hard to do that
in humans but true to the 23rd combinations, as we said, well over 8
million. And then, the diagram on the right showing
homologous recombination. In meiosis I is exchanging segments on homologous
chromosomes, it's sorting the genes. This diagram shows two crossover events on
one pair of homologous chromosomes. The two crossover events there would lead to
four possible gametes. So, here you can imagine, combine these numbers by
multiplication: 8,388,608
possible combination of gametes times ever how many crossover
events occur, you're going to multiply that number times that number. It's an
infinite number of possible gametes that can occur as a result of meiosis. Thus,
assuring that no gamete is the same. Thus, assuring genetic variability. Thus, an
absolute necessity for the evolution of species. Finally, the last slide of this
internet film from McgGraw-Hill. It's got a nice simple animation to show how
meiosis works, but you'll see as you click on that video the animation will come up.