Many organisms
pass their genes to their offspring
through sexual reproduction.
This begins
when two gametes unite to form an embryo
that is genetically unique
from the parent organisms.
The embryo then grows into an adult
who in turn passes their genetic information on to their own offspring.
Gametes are formed through a process called meiosis.
The cells that undergo meiosis to produce the gametes are called germ-line cells.
In diploid organisms,
germ-line cells have two copies
of each chromosome.
Germ-line cells undergo meiosis to produce
haploid gametes which only have one copy
of each chromosome.
These haploid gametes fuse to form a diploid
embryo that grows into the adult.
Meiosis is just one step
in the life cycle of a germ-line cell.
Similar to mitosis,
the cells also pass through the interphase,
G1, S, and G2 stages
before they enter meiosis.
The DNA inside a germ-line cell
is duplicated before meiosis begins
during the S phase.
The duplicated germ-line chromosomes
are called sister chromatids.
These chromatids remain attached to each other
until the second cell division event in meiosis.
There are two cell division events during meiosis.
The first division, meiosis I,
results in two unique daughter cells
that have half the amount of DNA
as the parent germ-line cell.
The second division, meiosis II,
results in four unique haploid cells
that only have one copy of each chromosome.
These haploid cells are the gametes
that could go on to produce an offspring through sexual reproduction.
Let’s look more closely at each of the division events.
Meiosis begins with prophase I.
In this stage,
the DNA condenses to form chromosomes.
Here we see the duplicated sister chromatids joined together at the centromere.
They stay fused at the centromere throughout Meiosis I.
Next,
each pair of homologous chromosomes
undergoes synapsis
to form a complex involving
two pairs of sister chromatids.
Chromosomal material is exchanged between the two pairs of sister chromatids.
This event is called recombination
or more commonly, crossing over.
After crossing over,
the sister chromatids for each chromosome
are no longer identical to one another.
This is one of the reasons why no two siblings (aside from twins) are genetically identical.
There are several more key steps in prophase I.
The nuclear membrane begins to break down.
Then the two centrosomes migrate to
opposite ends of the cell
and microtubules appear.
The microtubules then attach to the chromosomes.
The next phase of meiosis I is called metaphase I.
Here the synapsed chromosomes align at the equator of the cell.
The chromosomes align randomly which results in different combinations each time meiosis occurs.
The next phase is anaphase I.
During this phase, homologous chromosomes separate and migrate to the two poles of the cell.
Importantly, the sister chromatids remain attached at their centromeres.
The final steps of meiosis I
are telophase I and cytokinesis.
Here the cell divides into two daughter cells.
Each of these two cells now undergo meiosis II.
Meiosis II is similar to mitosis.
The first stage of meiosis II is prophase II.
Again, chromosomes condense,
the nuclear envelop breaks down,
and the spindle apparatus forms.
The major difference between prophase II and prophase I
is the fact that the daughter cells have only one copy of each homologous chromosome.
So, in prophase II, there is no synapsis of homologous chromosomes or crossing over.
In metaphase II, the chromosomes align
at the equator of the cell.
Again, the alignment is random.
Since the sister chromatids are no longer identical,
there will be many different possible ways for these chromosomes to align.
In anaphase II, the sister chromatids are pulled apart as the microtubules shorten.
Also, the ends of the cell are pushed further apart as microtubules elongate.
In telophase II, the nuclear membrane reforms,
and the cytoplasm is divided into the two haploid daughter cells.
This division is called cytokinesis.
Since meiosis II began with two cells,
and each of those cells were split into two cells,
we now have 4 unique haploid cells at the end of meiosis.
These cells are gametes.
Two gametes, one from the father and one from the mother,
may fuse to produce a diploid embryo.
The resulting embryo then grows through many cycles of mitosis.