Emily Stewart
Career Tech Description- Audience: Patient
Meiosis (My-OH-siss)- A Guide to Sexual Reproduction
Have you ever wondered what happens internally before a man and woman are able
to create a baby? Have you ever wondered how the sperm in males and the eggs in
females are created inside your body? Meiosis is a specialized type of cell division,
and through two phases, Meiosis I and Meiosis 2, the chromosome numbers are cut
in half. A chromosome is the DNA molecule located in the nucleus of each cell,
packaged in a thread like structure. The products of Meiosis are four haploid cells,
or a cell that has one complete set of chromosomes. These haploid cells become
unfertilized eggs in women and sperm in men.
©beatricebiologist
Phase 1: Meiosis I
Overview: The first cell division of meiosis is known as meiosis I. During this phase,
members of homologous chromosome pairs are separated. Homologous chromosomes
are a set of one maternal chromosome (from the mother), and one paternal chromosome
(from the father). These chromosomes pair up with each other inside the cell during
meiosis. These copies have the same genes, or the basic physical and functional unit of
heredity. The result of this first phase is these genes segregated into two gametes, or a
mature sexual reproductive cell that unites with another cell from another organism.
Examples of gametes include sperm and egg. Meiosis I is broken down into four different
steps. Let’s take a look!
Step 1: During this first phase, condensation of chromosomes, pairing of homologous
chromosomes, and crossing over all occur. The condensation of chromosomes allows
them (chromosomes) to be viewed under a microscope. The pairing of the homologous
chromosomes was mentioned in the overview it is where these chromosomes, one set
from the mother and one set from the father pair side by side. Once they are beside one
another, crossing over occurs, which is when genetic recombination, or the exchange of
genetic material occurs. During this time the spindle fibers are formed. Spindle fibers
form a protein structure that divides the genetic material in the cell. These fibers are
pointed out in the image below. This can be compared to the shaking up of a snow globe.
Once it is shook, the snow is falling and landing where it wants at the bottom of the
globe. The same goes for crossing over. Imagine if you could just pick up a cell and
shake it around. The snow that is in the globe is the genetic material of the cell. Once it is
shook up, the genetic material lands on either chromosome at random.
Photo by Robert Bear and David Rintoul
(CC BY 4.0)
Step 2: During this second phase, the chromosomes are lining up at the equator of
the cell.
The “equator” of the cell, is known as the metaphase plate. The chromosomes
position at this metaphase plate at random, whether it be a maternal or paternal
cell. After the chromosomes are lined up at the “equator,” or metaphase plate, at
random, there are four different ways that chromosomes can be aligned.
Photo by Robert Bear and David Rintoul
(CC BY 4.0)
Step 3: During this step, the homologous chromosomes separate. Each homologous
chromosome contains two chromatids, or one of two identical strands. These
chromatids each move to separate poles. Yes, the cell has its own poles just like the
Earth does. The Earth has the North and South Pole, so we can say the same for the
cell.
Photo by Robert Bear and David Rintoul
(CC BY 4.0)
Step 4: Part 1- the homologs, or strands of DNA arrive at opposite poles of the cell.
During this time a nuclear membrane surrounding the genetic material forms. The
nuclear membrane is a form of protection, and acts as a bumper to the genetic
material. Compare this to a bumper car. The bumper on the car is the nuclear
envelope, and the car itself is the genetic material.
Part 2- During Part 2 in step 4, the cell is divided into two daughter cells, or one of
the two identical cells when the form divides. Each daughter cell is now haploid,
with half of the number of chromosomes as it started with. These two haploid cells
are the final product of meiosis I.
Photo by Robert Bear and David Rintoul
(CC BY 4.0)
Final Product of meiosis I: two haploid cells
Photo by Robert Bear and David Rintoul
(CC BY 4.0)
Phase 2: Meiosis II
Overview: The second cell division of meiosis is meiosis II. During this phase, the
chromosomes line up and the sister chromatids, or either copy of two identical
copies formed by replication of a single chromosome. As mentioned before, the
daughter cells are genetically identical to one another. At the start of meiosis II, the
cells are haploid because that was the final product of the first phase, meiosis I; two
haploid cells. Meiosis II occurs in four steps. Let’s take a look!
Step 1: During this step, the spindle fibers that were mentioned before, reform.
They attach to the centromeres, or regions of linear chromosomes where two
chromatids are joined. As noted in the figure of Step 1 in meiosis I, the green lines
that are in the image below are the spindle fibers.
Photo by Robert Bear and David Rintoul
(CC BY 4.0)
Step 2: During this phase, the chromosomes align on the “equator,” or metaphase
plate again. This time, instead of being linear or parallel, they are perpendicular.
Photo by Robert Bear and David Rintoul
(CC BY 4.0)
Step 3: During this phase, the chromosomes divide at the centromeres, or the
regions of linear chromosomes where the two chromatids joined. The resulting
chromosomes, once again, move toward each pole.
Photo by Robert Bear and David Rintoul
(CC BY 4.0)
Step 4: Part 1- During Part 1, four haploid nuclei, the plural form of nucleus (center
point where genetic material is stored), are formed.
Part 2- During Part 2, the cytoplasm, or the thick liquid that fills up the cell divides.
This division of the cytoplasm results in four haploid cells. Recall from meiosis I that
the genetic material was shook up and exchanged at random through crossing over.
Because of this, the four haploid cells are not identical. They each have their own
genetic composition.
Photo by Robert Bear and David Rintoul
(CC BY 4.0)
Final Products:
The product of meiosis is four genetically different haploid daughter cells. Each one of
these daughter cells has 23 chromosomes each that consist of one chromatid. These
haploid cells become unfertilized eggs in females and the sperm in males.
Photo by Robert Bear and David Rintoul
(CC BY 4.0)
Examples of the products of meiosis, four haploid cells (shown in the figure above),
are egg in females and sperm in males (gametes). The gametes are shown in the
figure below. These haploid cells, or gametes, formed in meiosis (a special form of
sexual reproduction), allow males and females to reproduce organisms. The
reproduction of an organism results in a little bundle of joy, a baby. You have now
successfully learned what happens inside the body in order to create a family!
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Photo by Mitchinson
(CC BY 3.0)
GAMETES
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Photo by Artemelza
(CC BY 2.5)
Photo by Reema
(CC BY 2.5)