Jill Daniels
Biology 1010
Assignment #4 research paper
Sexual Reproduction:
How Humans Reproduce
Personal Reflection
I decided to write my report on sexual reproduction with a focus on
meiosis because; I find it fascinating how the human species can become so
diversified through this process of meiosis. There are millions of people living on
the planet and not one of us is an exact replica of another. “Two people can
create offspring that are genetically different from themselves and from each
other” (Mader 129). In my family you can see we are from the same parents yet
we are so different at the same time. I have pinkish skin with lighter hair and
green eyes, while my older sister has olive skin, dark brown hair and brown eyes.
We are the same height and build but some of our features are so different. Also
our personalities are different, she can study for hours on end and do really well
in school while my attention span is much shorter and I struggle studying
sometimes and try to rush through tests. Not only are the differences fascinating
but also, what we can learn from studying our DNA and sex chromosomes.
Scientists have discovered how humans reproduce through meiosis and
the crossing over of traits on chromosomes since this discovery there have been
leaps and bounds of new information. For example stem cell research has
allowed us to find cures for disease and malfunction, they can generate large
quantities of cells with altered genes and therefore have new opportunities for
more research on genetic disorders (Alteration. ScienceDaily). The study of how
we reproduce has allowed us to cure a lot of the problems that happen during
that process. Science has come so far and I find their achievements on sexual
reproduction alluring, this is why I have chosen to write my paper on this topic.
Since the beginning of our species, humans have been able to sexually
reproduce; this biological cell division is called meiosis. Meiosis is a type of cell
division for eukaryotes that results in two daughter cells each with half the
chromosome number of the parent cells. Every human male and female has 23
pairs of chromosomes, during meiosis the chromosome number is reduced and
also produces genetically different gametes, for men this means sperm and for
women this means eggs. The new mixture of DNA gives results in offspring that
are genetically different from their parents allowing all human species to have
distinct DNA and diverse traits.
Meiosis has been occurring for who knows how long, maybe since the
beginning of man, but was only noted first in 1876 by a German biologist named
Oscar Hertwig. Hertwig was observing sea urchin eggs and was able to see the
chromosome reduction during meiosis in the eggs but didn’t fully develop the
theory of meiosis. After Hertwig, a man named Edouard Van Beneden
discovered this same idea of meiosis at the chromosome level in worms’ eggs,
but it wasn’t until 1890 when August Weisman a German biologist noted it was
necessary to have two cell divisions to transfer one diploid cell into four haploid
cells if the number of chromosomes had to be maintained. Most recently in 1911
an American geneticist named Thomas Hunt Morgan observed crossover in
meiosis which is an exchange of genetic material between chromosomes. This
was important because it proved genes are transmitted on chromosomes.
There are two divisions of meiosis each division has four different phases;
prophase, metaphase, anaphase, and telophase, these phases are named the
same for both divisions. During prophase 1, the chromosomes coil up
sometimes exchanging fragments by a process called crossing over. Metaphase
1, is where the pairs align independently at the metaphase plate. Anaphase 1,
begins when homologous chromosomes separate and move toward the poles.
Telophase 1, daughter cells have one chromosome from each homologous pair.
During the second division of meiosis Prophase 2, cells have one chromosome
from each homologous pair. Metaphase 2, chromosomes align at the metaphase
plate. Anaphase 2, sister chromatids separate and become daughter
chromosomes. Telophase 2, spindle disappears, nuclei form, and Cytokinesis
takes place. Cytokinesis is the cytoplasm division of the cell or bringing about
separation into two daughter cells. Mitosis is also very important to the process
of meiosis because both are necessary for a human life to evolve. After meiosis
and mitosis take place to form the chromosomes and DNA and cells that make
us who we are mitosis is continued through growth of humans especially during
childhood and for the repair of tissues at any time (Mader.104).
After mitosis and meiosis take place humans receive 23 pairs of
chromosomes one of each pair is inherited from one male parent and one female
parent. Due to cell division the body contains copies of these chromosomes in
the nucleus. In studying the chromosomes after meiosis scientists have been
able to discover many chromosomal abnormalities, the most common being
down syndrome. Down syndrome is the when a human has inherited three
number 21 chromosomes. A person with Down syndrome has many side effects
just from this simple chromosomal malfunction that occurred during meiosis.
They can be short, have an eyelid fold, a flat face, stubby fingers; poor muscle
tone, heart defects, a large, fissured tongue; and, even mental retardation, which
can vary in severity (Mader. 111). Turner Syndrome is when a female has only
one sex chromosome, this affects the females ovaries never allowing them to be
fully functional and these females never undergo puberty, menstruation, and their
breasts don’t fully develop. Physical side affects of Turner syndrome are usually
a shorter build, folds of skin on the back of the neck, difficulty recognizing various
spatial patters, and luckily normal intelligence. Another common disorder is
Klinefelter syndrome, these individuals are male in appearance, but the testes
are underdeveloped, and the breasts may be enlarged. Physical side affects
may be sparse body hair, longer limbs with poor muscular development. Jacob
syndrome can be due to non-disjunction during meiosis 2 of spermatogenesis.
These are males and are usually taller than average, suffering from persistent
acne, and tend to have speech and reading problems (Mader.111).
These are just a few of a hundred various types of Chromosome
abnormalities in humans. Since the discovery of meiosis and knowing how it
works scientists have been working on finding out how to manipulate the process
of meiosis and altering the chromosomes in sexual reproduction. If this becomes
successful we could cure not just chromosomal abnormalities but thousands of
diseases. Stem cell research is one of the ways scientists have been able to
cure hundreds of diseases thus far. In an article about stem cell research UC
Irvine researchers found a new technique that blends cell-handling methods to
improve cell survival rates which allows them to research the cells. Scientists
can use the technique to populations of cells with abnormalities that lead to
disease, allowing them to learn about the disorder and cure the disease. Think if
scientists could find a way to manipulate meiosis and get it controlled to the point
where they could find what causes the chromosomal defects, then treat them.
Thousands of people could potentially benefit from these possibilities and the
human race would be that more advanced.
Sexual reproduction through meiosis is common in human beings and
how unfortunate it is some suffer from biological malfunctions during this
process. Today we see researchers working hard at finding how to cure the
causes for these various types of disorders. I predict that in the future
researchers will have the technology they need to find the problem and solve it,
curing chromosome abnormalities.
Works cited:
Mader, Sylvia S. Essentials of Biology. New York, 2010. Print
“Alteration.” Science Daily, Web. April 2012.
<http://www.sciencedaily.com/releases/2008/03/080307150657.htm>
Meiosis. Wikipedia. Web. April 2012.
<http://en.wikipedia.org/wiki/Meiosis#History>
Mader, Sylvia S. Lab Manual Concepts of Biology, New York. 2010. Print