Cell Cycle, Mitosis, and Meiosis… and Giant Chocolate Cake.

Hello, and welcome to the Cell-fie Life. Thank you so much for listening! My name is Nikaela and today we are going to be reviewing the cell cycle, meiosis, and mitosis.

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Okay, let’s do
this—mitosis and meiosis.

There are two main types
of cell division that we humans take part in: mitosis and meiosis.

Mitosis is the process
of making new body cells, so it is how two identical daughter cells are created
from a single cell.

Meiosis is the type of
cell division that creates gametes, eggs, and sperm. Meiosis results in up to
four non-identical daughter cells.

When I first learned about mitosis and meiosis, I could not keep their names straight. Which one happens in somatic cells and which one creates germ cells? And, because in science we like to name things very similar to one another to make things extra fun—just wait till we get to centromeres, centrosomes, centrioles, and kinetochores, and we will get to those—I had to think of a clever (or dumb) way to remember which one was mitosis and which one was meiosis.

And, let’s be honest, I
went with the dumbest and most ridiculous way to remember: Have you guys ever
seen the movie Singing in the Rain? There is a song that has the line,
“Moses supposes his toeses are roses, but Moses supposes erroneously…”

And for some reason, I combined “Moses” and “toeses” when I was singing, and it came out “Mos-toesis, and from there it is a really was a short leap to: “Mitosis happens in the toeses”.

If mitosis is happening
in your “toeses”, well, your toes do not need to create sex cells—your toes are
somatic cells. In somatic cells, mitosis is what occurs to create genetically
identical diploid daughter cells.

So if mitosis is
creating somatic cells, then meiosis is happening in germ cells . If my
“Singing in the Rain” song wasn’t enough to help you keep mitosis and meiosis
straight meiOOOsis happens in your ovaries—or, you know, testes, if you
are male.

If you guys want to check out the song I’m talking about, “Singing in the Rain”, I’ll post the YouTube clip on the website cellfielife.com. There is also some excellent tap dancing, you know if that’s your thing. To see the whole thing, here’s the YouTube link:

So now that you will
never switch up the location of mitosis and meiosis, let’s get into the
nitty-gritty:

A lot of people approach the cell cycle with a pie chart, which is great—I love pie. And I’m willing to bet that a lot of you have seen this breakdown. I’ll post it on my Insta and on the website, but I’m going to approach the cell cycle, not with a pie chart, but with a cake meme.

Remember that cake meme?
It’s been on the Internet for years. It’s a cartoon in which a slice of cake is
cut from a whole cake, and the person—instead of taking that single slice for
themselves—removes the rest of the cake and leaves the single slice sitting on
the plate.

That
almost-whole-cake-slice is what we call “interphase”.

Interphase is where a
cell spends approximately 90% of its life. The single slice left on the serving
dish? That would be mitosis. Mitosis is the process where the one nucleus
splits into two nuclei.

But first, interphase!
Or, how I like to think of it, the correct size of a slice when it comes to
chocolate cake.

Interphase is where the
cell is really just living as a cell: growing, making proteins, and all the
other functions that a cell might have.

Interphase can be further
divided into G₁, S, and G₂, phases, as well as G₀.
G “now” is the G with the little subscript zero after it. The G₀
phase is sometimes thought of as outside the cell cycle, or as an extended G₁
phase. G₀ is a phase where the cell is either not dividing or is
preparing to divide. It’s simply living its life, carrying out its daily
functions.

Can you think of any cells that would enter this type of inactive phase and hang out there for their life? I’ll give you a hint: think super-specialized cells. My favorite example is neurons.

Okay, G₁: G₁
is when you have a new cell and it’s gonna start growing. It’s getting bigger.
It’s creating more organelles. It’s called the G₁ phase because it
is the first phase of growing in the cells.

The S phase: This is the
phase where the cell replicates its DNA so that it has two identical copies.
This happens in the synthesis phase. DNA replication happens before mitosis.
And this is an important point to remember! DNA replication happens before
mitosis—a cell that enters in the G₂ phase has twice as much DNA as
it did the G₁ phase.

Remember—DNA replication
happens in the S phase; “S” as in “synthesis” phase.

In the S phase, one copy of DNA will become two copies. Once a chromosome replicates, it is still considered one chromosome. For example, let’s say we have a cell that has three chromosomes. This cell with three chromosomes enters S phase and all its DNA is replicated. How many chromosomes does this cell have now?

The cell still has three chromosomes, despite the fact that it has double the amount of DNA. The DNA strands are still attached, so they are still considered one chromosome. Recall in your mind the traditional X chromosome. Right now, it’s a stringy, loose spaghetti-like X, free-floating in the S phase; it won’t actually get the super tight X shape until mitosis. The middle, where the X’s cross, is called the “centromere”. So, there are two copies of the DNA that are attached in a specialized region called the “centromere”. Because they are still attached, they are still considered one chromosome. Each individual copy can be called a “chromatid”. These are the sister chromatids (I mean, really, they are twins, but they are called sister chromatids). During mitosis, the two sister chromatids will get split apart, and then they will become two separate chromosomes.

Centromere: This is the
middle point connecting the two sister chromatids. But we’re going to think of
it like centroMere. “M”, as in “middle”—this is legit how I remember
this. Centromeres are the middle of the two sister chromatids, making them one
chromosome. M, centroMere, for middle.`

With all of this DNA
replication happening, there is also some other additional duplicating
happening—the centrosomes duplicate.

Side note: I think of
centrosomes as centro-SUMS—like they are just “sum” (some) organelle
hanging out in the cytoplasm. I put the emphasis on pronouncing it incorrectly
so that I can remember what it is and what it does.

The centrosomes
(centro-SUMS): They are the little organelles close to the nucleus of the cell
which will help in the physical splitting of the genetic material. When the
chromosomes get pulled apart in mitosis, it is the spindle fibers that are made
from microtubules that will pull the chromosomes to opposite sides (we will be
going over this is more detail, I just wanted to get the word “centrosome” in
early so that it’s in our vocab and I can keep repeating it).

After the synthesis
phase, there is one more growth phase. This second growth phase is called G₂.

Also, the cell checks
itself before it wrecks itself.

The cell has checkpoints
where it makes sure that it is healthy enough to continue forward. One really
important checkpoint is the one between G₁ and S. This checkpoint
makes sure that the DNA is looking good, not damaged, and can be replicated. If
the DNA is damaged, the cell cycle is arrested until the DNA can be repaired.
The protein that is in charge of this checkpoint is called p53.

Another checkpoint is
the one right before the G₂/M phase. This checkpoint makes sure that
there are enough organelles, and that it is large enough to replicate.

So, at the end of the G₂
phase, the cell is now ready for the M phase, mitosis. Yay!!!

There are four phases of
mitosis:

• Prophase
• Metaphase
• Anaphase
• Telophase

I will talk about each
phase individually, but I find it helpful to know what phase is where in the
process. For me personally, I don’t have a super clever way of remembering
this. I just think PMAT—which sounds kinda like a test, similar to the MCAT,
and MCAT is always on the brain, so PMAT kinda just sticks. But I did
google some mnemonic devices and my favorite was: Pass Me A Taco. If you have a
really great way to remember hit me up on my Instagram and I’ll share
them—@thiscellfielife.

Okay, so, PMAT…

Prophase: the DNA goes
from its chromatin form—which is the loosey-goosey,
floating-around-in-the-nucleus state—to its condensed form. Remember that we
have two sister chromatids connected by a centromere, and now it is in its X
shape, which can be seen by a light microscope. So, in prophase, the chromosomes
condense and the nuclear membrane starts to dissolve. The centrosomes that were
those little organelles that were hanging out in the cytoplasm (close to the
nucleus) start migrating to opposite sides of the cell.

Metaphase
(“M” as in “middle” again): the chromosomes start lining up in the middle of
the cell. The
centrosomes, the organelles,
are now on opposite sides of the cell. Centrioles exist
inside the centrosomes; each centrosome has two centrioles. I know that naming
is cruel and unusual.

Is
there a tongue twister for all of this? ‘Cause there should be… Maybe I’ll
make one. It would go with the “Singing in the Rain” clip…

Anaphase: the
centrosomes’ microtubules pull the sister chromatids apart, so now there are
two chromosomes. Remember how I told you that the area where the X’s cross is
called the centromere? The actual protein structure that the microtubules
attach to on the chromosome is called a kinetochore. Think of it as though the
centromere is the city you live in, but the kinetochore is the street. Also, in
anaphase, the cytoplasm begins to pinch in a little bit.

Telophase: Telophase is
basically the opposite of prophase. This means that the nuclear envelope starts
reforming, and the chromosome starts unwinding into its chromatin, spaghetti
form.

Basically, you have a mitosis
sandwich, where the prophase and telophase make up the bread and do the
opposite of each other.

The last part is
cytokinesis, where the cytoplasm finishes pinching together and creates two
new, separate cells. Which puts us back at the beginning of the cell cycle! G₁
baby—time for our newly minted baby cells to grow.

Clarification and
reminder: centromeres and centrosomes both have M’s in them. In my head, to
keep them straight, I say centroMeres, putting the accent on the M in the
middle of the word. And the centrosomes I say in my head like centro-SUMS,
because they are just “sum” (some) organelle in the cytosol.

Before we move on to
meiosis, I want to review a couple of general concepts.

Chromosomes: You
(humans) have 46 chromosomes grouped into 23 pairs; 22 pairs plus the sex
chromosomes.

Cells with all 46
chromosomes are called “diploid” cells, which just means they contain two
copies of each chromosome. Cells with 23 are called “haploid”, and these are
your sex cells—sperm and egg—and contain only one set of chromosomes.

Diploid cells are
referred to as 2n, as they have twice the number of chromosomes; that
is, they have two copies, one set from mom and one set from dad. Haploid cells
are n cells, as they have one set of chromosomes and will be combined
with another set of chromosomes if fertilization happens to create babies with 2n
diploid cells. So, in mitosis, 2n cells create two daughter 2n
cells.

Meiosis creates gametes,
so a 2n cell will create up to four cells, all haploid.

It’s important to note
that the more specialized a cell is the less likely it’s going to replicate
itself – neurons, blood cells (← Not sure what you’re trying to say
here)

By now you are a pro at
mitosis, so on we go to meiosis. Meiosis has two rounds, that is, two each of
prophase, metaphase, anaphase, and telophase. So, we literally call them
meiosis I and meiosis II.

Round one of meiosis
takes a 2n cell and creates two haploid daughter cells. The daughter
cells then undergo meiosis II (which is actually super similar to mitosis, but,
don’t worry, we will get into that), and we end up with up four cells, all
haploid. I should say “up to four” because, in egg development, we get polar
bodies that are just discarded. But we will go into the specifics of that in
the next podcast.

So here we are in
meiosis and, just like in mitosis, these cells spend most of their time hanging
out in interphase. They are growing and hanging out and synthesizing and
growing, just like in mitosis, but with a few differences.

During the S phase of
the interphase, you have your DNA, your homologous pairs of chromosomes. This
just means you have a copy from your mom and a copy from your dad that code for
the same stuff. They aren’t identical, they just code for the same genes.

Pop quiz for
understanding: How many chromatids are there at the end of G₂ phase?

96 chromatids, organized
into 46 chromosomes, which are in 23 homologous pairs. Got it? Great!

Now here is where
differences start as we enter meiosis. Do you remember the phases?

Pass me a taco! PMAT:

• Prophase I (because in meiosis we undergo two rounds)
• Metaphase I
• Anaphase I
• Telophase I

Then,

• Prophase II
• Metaphase II
• Anaphase II
• Telophase II

So back to prophase I of
meiosis I. The nuclear envelope starts disappearing. The centrosomes start
migrating to the opposite sides of the cell and start forming their spindles.
The DNA condenses from its spaghetti form into the recognizable X form.

Now, in meiosis, the
homologous pairs line up and can undergo genetic recombination. So the two
pairs of homologous chromosomes join together in what is called a tetrad. Get
it? “Tetra” means “four”, and there are four chromatids connected. Homologous
points on the chromosomes can cross over; they switch little parts of each
other that code for the same thing. Imagine you are back in elementary school,
at lunch with your friends, and you’re unpacking your lunches. Your friend has
a banana pudding and you have a chocolate pudding and you decide to trade. You
both still have the same lunch and each have a desert, but you’ve traded a
portion. Now, this metaphor isn’t perfect, but I think you get the picture. The
homologous pairs can trade puddings. They can trade similar genes in a process
called “crossing over”.

This crossing over is
the main difference between prophase I in meiosis and prophase in mitosis. If
you think about it, it makes sense. You can have traits from both sides of the
family; the crossing over results in a mixture of parental characteristics in
the offspring.

Now, this is
important—MCAT ALERT!

Remember—crossing over
happens in prophase I of meiosis. I’ve heard that this is often tested on the
MCAT.

If you have some time
later and want to see a good visual explanation, check out the Khan Academy
video on crossing over. I’ll link it on my podcast notes.

Khan
Academy video

Metaphase I: The nuclear
envelope is gone, the centrosomes (centro-SUMS) are on opposite sides of the
cell, and the fibers are growing out and attaching to the kinetochores on
centromeres, which hold the sister chromatids together. The chromosomes are
lined up in the middle and getting ready to divide.

Anaphase I: In the
anaphase of mitosis, the sister chromatids are pulled apart. That is not what
happens here. In meiosis, the entire chromosome (made of two sister chromatids)
is pulled to opposite sides of the cell. Again, there will be genetic variation
because not all of your mom’s go to one side, and not all your dad’s go to
another. It is random and adds to the genetic variation!

MCAT ALERT! Remember—in
anaphase I of meiosis, the homologous pair is being pulled apart, NOT the
sister chromatids.

Telophase I: Again,
telophase I is basically the opposite of prophase I. The nuclear envelope
starts reforming. The microtubules start to disassemble. Cytokinesis is
starting, so the cytoplasm is splitting, and we are going to end up with two
haploid cells.

The cell can now go into
interphase II, where it can either take a rest or continue on to meiosis II,
which is super similar to mitosis. The first phase of meiosis, or meiosis I, is
also called reductional division, which makes total sense, because the number
of chromosomes is being reduced. The daughter cells are haploids.

Now, meiosis II…

Prophase II: The
centrosomes have to duplicate, the nuclear envelope needs to dissolve.

Metaphase II: The
chromosomes line up in the middle of the cell. The centrosomes spindle fibers
grow out and attach to the kinetochores of the sister chromatids.

Anaphase II: The
spindles pull the sister chromatids apart, just like it mitosis.

Remember that in
anaphase I the homologous pairs are separated and in anaphase II the sister
chromosomes are pulled apart. The sister chromosomes are separated.

Telophase II: The
opposite of prophase II. The nuclear envelope reforms, the extra centrosomes
disintegrate, and then cytokinesis happens.

So now we have four
cells that all have one copy of all the chromosomes. They are haploid, also
notated as n. These are called gametes, and we will talk extensively
about these in the next podcast.

And, just like that, you
have made it through the cell cycle, mitosis, and meiosis! So, to recap:

Interphase is the big
piece of cake. It has the growth phases and the synthesis phase, where the DNA
is replicated. Mitosis is the single slice of cake left on the plate. It might
be a single slice but it is very important.

Ya, I’m also going to
put that clip up on the notes…

The following phases are
part of interphase, and they proceed in this order: G₁ (for growth
phase I), S (for synthesis, like DNA synthesis), and G₂ (growth II).

WRAP UP

One thing I should
mention is nondisjunction.

If you were reading
about the cell cycle and you came across the term “nondisjunction”, what would
you think it meant, just from looking at the word alone? Assuming you don’t
already know what this means.

Nondisjunction: This
means that something is not working. Some sort of junction didn’t come apart
correctly. What is pulled apart in mitosis or meiosis? Pairs of homologous
chromosomes or sister chromatids. So nondisjunction would be the failure of homologous chromosomes
or sister chromatids to separate normally during nuclear division, usually
resulting in an abnormal distribution of chromosomes in the daughter nuclei.

A final review:

Mitosis is the process by which the one nucleus becomes two nuclei, each of which has the original genetic information

After
we exit mitosis we get into cytokinesis, which splits the cytoplasm in two, creating
two, whole individual cells.

Let’s see if we have a
firm grasp on the number of chromosomes in the following phases:

Before replication? 46

After replication? Still
46! Remember, they are still connected by the centromere. So basically as long
as the DNA is connected, they are still considered 46 chromosomes. They can
also be called sister chromatids.

Vocab:

• chromatin
• diploid
• haploid
• homologous
• sister chromatids
• centromere
• kinetochore

When you get, like, 30
seconds today, sketch out what each phase of mitosis would look like, and then
check your answers tonight. The test is known for giving pictures, and then
making us identify the phase.

Ooh, tag me on your
drawings and I’ll repost some of them on Instagram! @thecellfielife

Remember PMAT—pass me a
taco!

One last thing that I
really, really want you to remember: When does crossing over occur? Crossing
over occurs in meiosis, prophase I.

• P (prophase): chromosomes condense and the nuclear envelope dissolves;
• M (metaphase): the chromosomes line up in the middle;
• A (anaphase): the sister chromosomes are pulled apart to the opposite sides of the poles;
• T (telophase): the nuclear envelope reforms, the chromosomes decondense, cytokinesis happens, the cytoplasm splits, and two cells are created.

Study Hard Friends