Professor Dave again, let’s learn about meiosis.
Every living creature on earth begins as a single cell.
That cell undergoes mitosis, dividing over and over again, with certain cells becoming
specialized, until eventually, you have a frog, or a penguin, or Tony Danza.
But where does that first cell come from?
Not mitosis of course, because cell division begins with an existing cell.
Instead, that first cell came about because of meiosis and fertilization.
Let’s take a look at the details of meiosis, and how it differs from mitosis.
First, let’s understand that meiosis is another type of cell division, but it doesn’t
produce two identical cells like mitosis does.
Human cells have 46 chromosomes, which are two of each of 23 types, shown here in this
karyotype, which is a display of all the chromosomes in an organism.
Each pair of chromosomes of a particular type are called homologous, meaning they carry
genes controlling the same characteristics.
More on this later.
After mitosis occurs in somatic cells, which are most of the cells in your body, you end
up with two new cells with identical copies of those 46 chromosomes.
These are called diploid cells, represented by 2n, meaning they contain two sets of chromosomes.
With meiosis, when one cell divides, the product is four haploid cells.
These have 23 chromosomes each, or just a single set, represented by n, and these haploid
cells are reproductive cells called gametes.
These are the cells that transmit genetic information from one generation of an organism
to the next.
In humans, these gametes take the form of sperm and egg cells.
As we said, these are haploid cells with 23 chromosomes, and when fertilization takes
place, the sperm and egg merge, and a new cell is formed, complete with the full 46
chromosomes, which are a combination of genetic material from the mother and father.
So while mitosis, a kind of asexual reproduction, just produces identical copies of an original
cell, meiosis and subsequent fertilization comprise sexual reproduction, which produces
new cells with a novel set of genes.
This is how we get variation from generation to generation.
All humans have a combination of the genetic material from their parents, so no child will
be 100% identical to either one of them.
We will talk all about variance, heredity, and genetics later.
For now, let’s focus on meiosis as a cellular process.
Meiosis is similar to mitosis in a lot of ways, and we will assume that mitosis is already
well understood while discussing meiosis.
So if you haven’t yet watched that tutorial, please go back and check it out now.
If we are up to speed, we will first note that the main difference between these processes
is that meiosis consists of two cell divisions rather than one.
These are called meiosis one and meiosis two.
In meiosis one, homologous chromosomes, one maternal and one paternal, duplicate and are
separated during cell division to produce haploid cells with duplicated chromosomes.
Then in meiosis two, these haploid cells divide again to produce four haploid daughter cells,
each with just a single set of chromosomes.
Let’s zoom in on meiosis one first.
This consists of prophase one, metaphase one, anaphase one, and telophase one followed by
cytokinesis.
In prophase one, each chromosome, already duplicated, exchanges information with the
homolog, which is a process called crossing over.
This begins when certain proteins break apart the DNA of two nonsister chromatids at exactly
the same location.
Then during synapsis, a complex of proteins called the synaptonemal complex holds the
homologs together and each broken end of DNA is joined to the corresponding section of
DNA from the other chromatid.
So you can see that the two chromosomes have essentially swapped a tiny section of DNA
with one another.
The areas on the chromosome where crossing over occurs are called chiasmata, and there
will be at least one instance of this per chromosome, resulting in what we call recombinant
chromosomes.
This is just a fancy way of saying that they carry genes from what were originally two
completely different chromosomes.
In this case, they carry genes from both the maternal and paternal chromosome.
Then the nucleus comes apart just like in mitosis, and the mitotic spindle forms, attaching
to kinetochores at each centromere.
In metaphase one, chromosomes line up at the metaphase plate, and in random fashion.
The maternal and paternal chromosomes for each homologous pair can be in either order.
Then in anaphase one, the homologs separate and are pulled towards the poles by the spindle.
Notice that both chromatids of each chromosome are pulled to one side or the other together,
rather than being pulled apart at the centromere, like in mitosis.
Lastly, in telophase one, the nuclear membrane reforms, cytokinesis occurs, and we get two
haploid daughter cells.
So meiosis one differs from mitosis in that the sister chromatids of an individual chromosome
are not getting pulled apart, it is a pair of homologous chromosomes that are getting
pulled away from each other after having exchanged some genetic material.
This is why the daughter cells are haploid, because each of them has only half the genome,
with only one chromosome from each pair of homologs.
Then in meiosis two, again we have prophase two, metaphase two, anaphase two, telophase
two and cytokinesis.
This part looks just like mitosis.
In prophase two, the spindle apparatus forms.
In metaphase two, the chromosomes align at the metaphase plate, but unlike mitosis, the
sister chromatids are not all genetically identical, because of the crossing over that
occurred in prophase one.
Again, the spindle attaches to kinetochores so that in anaphase two, the sister chromatids
are pulled apart towards the poles.
Then telophase two and cytokinesis occur, where nuclei form, and we are left with four
haploid cells, each with 23 unduplicated chromosomes.
Each of these four daughter cells is different from the parent cell, and they are all different
from each other.
In fact, due to all the different possibilities present for the assortment and distribution
of the chromosomes, each haploid daughter cell, or gamete, represents one unique outcome
out of millions of possible outcomes.
This is the secret to biological variation that sexual reproduction offers, which gives
rise to the wide variety of phenotypes in living organisms that we will discuss later.
So the human life cycle begins with haploid cells, in this case a sperm and an egg.
These specialized cells are products of meiosis, and contain just one set of 23 chromosomes
each.
When these fuse during fertilization, the product is a single diploid cell with both
sets of chromosomes, one from each of the parents.
From here, it is mitosis that leads to the development of a human being, which will then
exhibit characteristics from both parents.
But what factors determine which characteristics from the mother and father will show up in
a child and which will not?
To discuss this is to discuss genetics, so let’s move on to this fascinating topic now.