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In biology, there are often vocabulary terms that sound pretty similar.
Chromosome.
Chromatid.
Chromatin.
Transcription.
Translation.
Mitosis.
Meiosis.
You probably have encountered this.
When I was first learning about mitosis and meiosis, I learned them both separately first.
And then I tried to figure out what was the same about them, what was different, why did
they both matter?
I would try to compare the stages by flipping through images.
You know what would have helped me?
A side by side comparison.
And that’s what this video is.
We assume you already have a background of mitosis and meiosis---if not take a look at
our videos on them---but this video is a side by side comparison.
Presented in a split screen.
Mitosis on the left.
Meiosis on the right.
Both of these processes, along with the cytokinesis that follows them to split the cytoplasm,
are involved in making new cells.
Mitosis results in body cells.
Meiosis results in sperm and egg cells, otherwise knows as the fancy term, gametes.
Before we start mitosis and meiosis, let’s look at what you start with.
Your starting cell in both mitosis and meiosis is diploid, written here as 2n.
That means it has 2 sets of chromosomes---in humans, that’s including one set of 23 chromosomes
from mom and one set of 23 chromosomes from dad.
46 chromosomes total in humans.
During interphase, the cell duplicates the chromosomes.
When you duplicate 46 chromosomes, you still say there are 46 chromosomes as the newly
duplicated portion is still attached at the centromere region---but there are actually
92 chromatids.
Interphase isn’t part of mitosis or meiosis, but it’s a really important phase because
it duplicates chromosomes before we get started.
Just to point out, it’s really hard to draw 46 chromosomes which is how many humans have.
We’re going to use 6 chromosomes in our diagrams when we illustrate what’s happening
because it’s much easier to draw and visualize.
Oh and just a fun fact: some insects have 6 chromosomes.
Like mosquitoes.
Unfortunately, I am not a fan of mosquitoes.
But mosquitoes do mitosis and meiosis too.
When learning the stages, we give the acronym PMAT which is helpful for understanding the
stages.
Both mitosis and meiosis go through these stages, but meiosis goes through them twice
and therefore has a number next to each PMAT stage.
We’re going to show some basic events for each PMAT stage, but please know there is
way more detail to explore than what we can include in this quick video.
Prophase in mitosis.
Remember that “pro” can mean “before” and this stage comes before the others.
The chromosomes are visible; in fact, we say they’re condensing which means they are
thickening.
Prophase I in meiosis.
Happening here too, but the chromosomes are actually going to match up with their homologous
pairs.
The word homologous means that the chromosomes are approximately the same size and that they
contain the same types of genes in the same locations.
With each pair, one came from mom and one came from dad.
In this formation, chromosomes can transfer their genetic information and exchange it
between each other.
It’s called crossing over!
It can make for what we call recombinant chromosomes.
Metaphase in mitosis.
The nuclear envelope which had surrounded the nucleus was already disassembled before
metaphase started.
For metaphase, I like to remember the M for middle because in this stage the chromosomes
line up in the middle of the cell in a single file line.
Metaphase I in meiosis.
The chromosomes are in the middle as well, but they’re still going to be in pairs in
the middle of the cell so it’s not a single file line.
Anaphase in mitosis.
I like to think as the A is for “away.”
The chromatids are pulled away by the work of the spindles.
They are moving to the opposite sides of the cell.
Anaphase I in meiosis.
Same thing but in this case, it’s the chromosomes- not chromatids- being pulled away to opposite
sides of the cell.
Telophase in mitosis and telophase I in meiosis.
The chromosomes are at the complete opposite ends and new nuclei are forming on each side
to make these two new cells.
And they are starting to surround the chromosomes on both sides as this will eventually form
2 cells.
Cytokinesis follows to split the cytoplasm to complete the actual dividing
of the cell.
So at the end of mitosis and cytokinesis, we end with two identical, diploid cells.
In humans, they would both have 46 chromosomes.
This is great for organism growth---growing requires making more cells after all---or
replacing damaged cells.
On to meiosis II!
Prophase II.
Chromosomes condensing in both cells.
It’s not going to be as eventful as it was in prophase I because they are not going to
have homologous pairs and crossing over.
Metaphase II.
M for middle, but this time, the chromosomes are in a single file line.
Similar to how metaphase looked in mitosis.
Anaphase II.
Think A for away.
This time, though, it’s actually the chromatids that are getting pulled away.
Telophase II.
Chromosomes are at the complete opposite ends and new nuclei are forming on each side to
make new cells.
Cytokinesis will follow meiosis II to completely split the cytoplasm.
We are now finished with meiosis: and we end with four non-identical cells.
Gametes.
Males makes sperm cells in meiosis and females make egg cells in meiosis.
These gametes are haploid, meaning they have half the number of chromosomes as the original
starting cell.
In the case of humans, the resulting cells would each have 23 chromosomes.
By the way, when a sperm and egg cell combine, it results into a diploid cell, a fertilized
egg otherwise known as a zygote, which will then start a series of divisions using mitosis
to give rise to a brand new organism.
Well, that’s it for the Amoeba Sisters, and we remind you to stay curious!