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 onto their own offspring.
Gametes are formed through a process called meiosis.
The cells that undergo Meiosis to produce the gametes are called germline cells.
In diploid organisms, germline cells have two copies of each chromosome.
Germline cells undergo meiosis to produce haploid gametes, which only have 1 copy of each chromosome.
These haploid gametes fuse to form a diploid embryo that grows into the adult.
Meiosis is just 1 step in the life cycle of a germline cell.
Similar to Mitosis, the cells also pass through the interphase, G1, S, and G2 stages, before they enter meiosis.
The DNA inside a germline cell is duplicated before Meiosis begins during the S phase.
The duplicated germline chromosomes are called sister chromatids.
These chromatids remain attached to each other until the second cell division event in Meiosis.
There are 2 cell division events during Meiosis.
The first division: Meiosis I, results in 2 unique daughter cells that have half the amount of DNA as the parent germline cells.
The second division: Meiosis II, results in 4 unique haploid cells that only have 1 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 1.
In this stage, the DNA condenses to form chromosomes.
Here we see the duplicated sister chromatids join 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 2 daughter cells, each of these 2 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 envelope breaks down, and the spindle apparatus forms.
The major difference between Prophase II & Prophase I,
Is the fact that the daughter cells only have 1 copy of each homologous chromosome.
So in Prophase II, there is so 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 Cytokenesis.
Since Meiosis II began with 2 cells and each of those cells were split into 2 cells,
We now have 4 unique haploid cells at the end of meiosis.
These cells are gametes.
Two gametes; one from a father, and one from a mother may fuse to produce a diploid embryo.
The resulting embryo then grows through many cycles of Mitosis.