Cell Division
Cell Division
Slide 1 – Introduction
We will now begin the fourth and final unit of this course. In unit four we will explore the
process by which cells divide and make gametes, as well as the basics of genetics. We will end
this unit discussing some of the implications our knowledge of on genetics has on our lives.
Let’s start by focusing on cellular reproduction.
Slide 2 – Reproduction
Reproduction is defined as the creation of new individuals from existing ones. This is one of the
characteristics of life. In biology, there are two different types of reproduction, asexual
reproduction and sexual reproduction. We will discuss each of these in more detail.
Slide 3 – Asexual Reproduction
Asexual reproduction is the production of offspring from one parent. The offspring produced
through asexual reproduction are genetically identical to their parent. This type of reproduction
occurs through mitosis. One advantage of asexual reproduction is that an organism does not
have to find another organism of its own species to produce offspring. Some examples of
organism that reproduce asexually are sea stars, sea anemones and sponges. Because all of the
individuals produced through asexual reproduction are genetically identical, this can leave a
population of organisms vulnerable to any sudden changes in their environment. A population of
identical organisms lacks the genetic diversity that may be needed to adapt to a changing
environment.
Slide 4 – Sexual Reproduction
Sexual reproduction is the production of offspring from two parents. This occurs through the
fusion of gametes. The offspring of sexual reproduction are not genetically identical to either
parent. Sexual reproduction leads to genetic variability within a species which allows for
adaptability to changing environmental conditions. The majority of organisms reproduce
sexually.
Slide 5 – Cell Division
There are two types of cell division we will be studying, mitosis and meiosis. Mitosis is the cell
division that occurs when a cell makes an exact duplicate of itself. Mitosis occurs when we are
growing or when we need to replaced damaged or worn out cells of the body.
Meiosis is the cell division that occurs when we make gametes. In meiosis, we do not make
duplicate copies of the cells. We only do meiosis when making eggs and sperm.
Slide 6 – Haploid versus diploid
Before beginning our study of cell division, we need to learn two terms, haploid and diploid.
The diploid number of chromosomes is the number of chromosomes we find in normal, everyday
body cells. For example, humans have a normal diploid number of 46 chromosomes. If a cell
sample was taken from your skin, liver, or brain, we would find 46 chromosomes in the nucleus
of each of these cells.
The haploid number is half the diploid number. In the case of humans the haploid number is 23.
The haploid number of chromosomes is only found in eggs and sperm. Think about why this is
so. Eventually the egg and sperm come together, producing a new individual that has 46
chromosomes, since the egg had 23 chromosomes and the sperm had 23 chromosomes.
Slide 1 – Introduction
We will now begin the fourth and final unit of this course. In unit four we will explore the
process by which cells divide and make gametes, as well as the basics of genetics. We will end
this unit discussing some of the implications our knowledge of on genetics has on our lives.
Let’s start by focusing on cellular reproduction.
Slide 2 – Reproduction
Reproduction is defined as the creation of new individuals from existing ones. This is one of the
characteristics of life. In biology, there are two different types of reproduction, asexual
reproduction and sexual reproduction. We will discuss each of these in more detail.
Slide 3 – Asexual Reproduction
Asexual reproduction is the production of offspring from one parent. The offspring produced
through asexual reproduction are genetically identical to their parent. This type of reproduction
occurs through mitosis. One advantage of asexual reproduction is that an organism does not
have to find another organism of its own species to produce offspring. Some examples of
organism that reproduce asexually are sea stars, sea anemones and sponges. Because all of the
individuals produced through asexual reproduction are genetically identical, this can leave a
population of organisms vulnerable to any sudden changes in their environment. A population of
identical organisms lacks the genetic diversity that may be needed to adapt to a changing
environment.
Slide 4 – Sexual Reproduction
Sexual reproduction is the production of offspring from two parents. This occurs through the
fusion of gametes. The offspring of sexual reproduction are not genetically identical to either
parent. Sexual reproduction leads to genetic variability within a species which allows for
adaptability to changing environmental conditions. The majority of organisms reproduce
sexually.
Slide 5 – Cell Division
There are two types of cell division we will be studying, mitosis and meiosis. Mitosis is the cell
division that occurs when a cell makes an exact duplicate of itself. Mitosis occurs when we are
growing or when we need to replaced damaged or worn out cells of the body.
Meiosis is the cell division that occurs when we make gametes. In meiosis, we do not make
duplicate copies of the cells. We only do meiosis when making eggs and sperm.
Slide 6 – Haploid versus diploid
Before beginning our study of cell division, we need to learn two terms, haploid and diploid.
The diploid number of chromosomes is the number of chromosomes we find in normal, everyday
body cells. For example, humans have a normal diploid number of 46 chromosomes. If a cell
sample was taken from your skin, liver, or brain, we would find 46 chromosomes in the nucleus
of each of these cells.
The haploid number is half the diploid number. In the case of humans the haploid number is 23.
The haploid number of chromosomes is only found in eggs and sperm. Think about why this is
so. Eventually the egg and sperm come together, producing a new individual that has 46
chromosomes, since the egg had 23 chromosomes and the sperm had 23 chromosomes.
Cell Division
Scientists have devised a quick abbreviation for the diploid and haploid numbers. The diploid
number is designated as 2n, while the haploid number is designated as n.
Slide 7 – Mitosis
Now let’s turn our attention to mitosis. Mitosis is a very small part of a cell’s existence. Most of
the time, the cell is in a state called interphase. During interphase, the cell is doing its necessary
jobs to help the body. For example it is during interphase that pancreatic cells make insulin and
glucagon. At some point the cell may get a signal that it is time to divide. At this point the cell
begins to prepare for mitosis.
Slide 8 – DNA duplication
When a cell gets a signal to divide, the first thing it does is duplicate its genetic material. DNA
is our genetic material. DNA acts like a blueprint for a cell, giving it all the necessary
information it needs. DNA is found in the cell in units called chromosomes. A chromosome is a
segment of DNA wound very tightly around various proteins. In preparation for division, a cell
must duplicate all of its chromosomes. This ensures that the new cell will have all the
instructions it needs to perform its job properly.
Slide 9 – Chromosomes
When a cell makes an exact copy of a chromosome, it connects them together at a point called a
centromere. The fact that the duplicated chromosomes are attached together ensures that the new
cells will receive copies of the chromosomes. Two chromosomes attached at the centromere are
called sister chromatids.
Slide 10 – End result of mitosis
Mitosis results in two daughter cells that are identical to the original cell that divided. Mitosis
occurs to replace damaged tissues, to create B and T cells for the immune system, during growth,
and for many other reasons. It is important to understand that after mitosis, there are two diploid
daughter cells that are identical to each other, as well as identical to the parent cell they derived.
Mitosis is a form of asexual reproduction.
Slide 11 – Mitosis
Animation of mitosis from YouTube
https://www.youtube.com/watch?v=O8rgwTYyuOc
Slide 12 – Check Your Understanding
Now that we have learned about mitosis, let’s check your knowledge of the subject. The
following slides will have a series of questions on the topic. Be sure to click “Submit” after
answering each question.
Slides 13 through 15 – Interactive Quiz
A non–graded assessment on your knowledge on mitosis.
Slide 16 – Meiosis
Meiosis is more complicated than mitosis. It is considered a reduction division because the
number of chromosomes in each cell will be reduced by half. Meiosis begins just like mitosis,
with the duplication of chromosomes. Unlike mitosis, meiosis goes through two divisions. At the
Scientists have devised a quick abbreviation for the diploid and haploid numbers. The diploid
number is designated as 2n, while the haploid number is designated as n.
Slide 7 – Mitosis
Now let’s turn our attention to mitosis. Mitosis is a very small part of a cell’s existence. Most of
the time, the cell is in a state called interphase. During interphase, the cell is doing its necessary
jobs to help the body. For example it is during interphase that pancreatic cells make insulin and
glucagon. At some point the cell may get a signal that it is time to divide. At this point the cell
begins to prepare for mitosis.
Slide 8 – DNA duplication
When a cell gets a signal to divide, the first thing it does is duplicate its genetic material. DNA
is our genetic material. DNA acts like a blueprint for a cell, giving it all the necessary
information it needs. DNA is found in the cell in units called chromosomes. A chromosome is a
segment of DNA wound very tightly around various proteins. In preparation for division, a cell
must duplicate all of its chromosomes. This ensures that the new cell will have all the
instructions it needs to perform its job properly.
Slide 9 – Chromosomes
When a cell makes an exact copy of a chromosome, it connects them together at a point called a
centromere. The fact that the duplicated chromosomes are attached together ensures that the new
cells will receive copies of the chromosomes. Two chromosomes attached at the centromere are
called sister chromatids.
Slide 10 – End result of mitosis
Mitosis results in two daughter cells that are identical to the original cell that divided. Mitosis
occurs to replace damaged tissues, to create B and T cells for the immune system, during growth,
and for many other reasons. It is important to understand that after mitosis, there are two diploid
daughter cells that are identical to each other, as well as identical to the parent cell they derived.
Mitosis is a form of asexual reproduction.
Slide 11 – Mitosis
Animation of mitosis from YouTube
https://www.youtube.com/watch?v=O8rgwTYyuOc
Slide 12 – Check Your Understanding
Now that we have learned about mitosis, let’s check your knowledge of the subject. The
following slides will have a series of questions on the topic. Be sure to click “Submit” after
answering each question.
Slides 13 through 15 – Interactive Quiz
A non–graded assessment on your knowledge on mitosis.
Slide 16 – Meiosis
Meiosis is more complicated than mitosis. It is considered a reduction division because the
number of chromosomes in each cell will be reduced by half. Meiosis begins just like mitosis,
with the duplication of chromosomes. Unlike mitosis, meiosis goes through two divisions. At the
Cell Division
end of meiosis we will end up with four daughter cells that are not identical to each other, nor the
original cell that divided. The end product of meiosis is 4 haploid cells. These cells can then go
on to become gametes. Meiosis is a process essential for sexual reproduction.
Slide 17 – Homologous Chromosomes
In order to study meiosis, we will have to learn another term related to chromosomes. Recall
that you have 46 chromosomes. You got 23 chromosomes from your mother and the other 23
from your father. These chromosomes were not identical; however they did carry information
about the same characteristics. For example, one of the chromosomes that you got from your
mother had a gene on it for eye color and one of the chromosomes that you got from your father
also had a gene for eye color. Chromosomes that carry information for the same trait are called
homologous chromosomes.
Slide 18– Meiosis
Animation of mitosis from YouTube
https://www.youtube.com/watch?v=VzDMG7ke69g
Slide 19 – Mitosis versus meiosis
Take a moment to review the similarities and differences between mitosis and meiosis. In both
processes, you will duplicate the chromosomes prior to beginning. Mitosis will produce two
diploid cells that are identical to each other, as well as the original parent cell. There is only one
cell division during mitosis.
Meiosis requires two division cycles to produce four non–identical (remember crossing over)
haploid daughter cells.
Slide 20 – Mitosis versus Meiosis
Animation from YouTube
https://www.youtube.com/watch?v=zrKdz93WlVk
Slide 21 – Check Your Understanding
Now that we have learned about meiosis, let’s check your knowledge of the subject. The
following slides will have a series of questions on the topic. Be sure to click “Submit” after
answering each question.
Slides 22 through 35 – Interactive Quiz
A non–graded assessment on your knowledge on the differences between mitosis and meiosis.
Slide 36 – Nondisjunction
Sometimes meiosis can go wrong and you can end up with an abnormal number of chromosomes
in either an egg or a sperm. Non–disjunction is when the homologous chromosomes fail to
separate during meiosis I, or the sister chromatids fail to separate during meiosis II. When this
occurs and the egg or sperm participate in fertilization, the child will have three copies of a
chromosome as is the case in Down syndrome, or only one copy of a chromosome as in the case
of Turner syndrome. There are only a few cases in which the offspring can survive with too
many or too few chromosomes.
end of meiosis we will end up with four daughter cells that are not identical to each other, nor the
original cell that divided. The end product of meiosis is 4 haploid cells. These cells can then go
on to become gametes. Meiosis is a process essential for sexual reproduction.
Slide 17 – Homologous Chromosomes
In order to study meiosis, we will have to learn another term related to chromosomes. Recall
that you have 46 chromosomes. You got 23 chromosomes from your mother and the other 23
from your father. These chromosomes were not identical; however they did carry information
about the same characteristics. For example, one of the chromosomes that you got from your
mother had a gene on it for eye color and one of the chromosomes that you got from your father
also had a gene for eye color. Chromosomes that carry information for the same trait are called
homologous chromosomes.
Slide 18– Meiosis
Animation of mitosis from YouTube
https://www.youtube.com/watch?v=VzDMG7ke69g
Slide 19 – Mitosis versus meiosis
Take a moment to review the similarities and differences between mitosis and meiosis. In both
processes, you will duplicate the chromosomes prior to beginning. Mitosis will produce two
diploid cells that are identical to each other, as well as the original parent cell. There is only one
cell division during mitosis.
Meiosis requires two division cycles to produce four non–identical (remember crossing over)
haploid daughter cells.
Slide 20 – Mitosis versus Meiosis
Animation from YouTube
https://www.youtube.com/watch?v=zrKdz93WlVk
Slide 21 – Check Your Understanding
Now that we have learned about meiosis, let’s check your knowledge of the subject. The
following slides will have a series of questions on the topic. Be sure to click “Submit” after
answering each question.
Slides 22 through 35 – Interactive Quiz
A non–graded assessment on your knowledge on the differences between mitosis and meiosis.
Slide 36 – Nondisjunction
Sometimes meiosis can go wrong and you can end up with an abnormal number of chromosomes
in either an egg or a sperm. Non–disjunction is when the homologous chromosomes fail to
separate during meiosis I, or the sister chromatids fail to separate during meiosis II. When this
occurs and the egg or sperm participate in fertilization, the child will have three copies of a
chromosome as is the case in Down syndrome, or only one copy of a chromosome as in the case
of Turner syndrome. There are only a few cases in which the offspring can survive with too
many or too few chromosomes.
Cell Division
Slide 37 – Visualizing Nondisjunction
Animation from YouTube
https://www.youtube.com/watch?v=nQ3Rjh661X8
Slide 38 – Chromosome screening
How do doctors know if a child will be born with an abnormal number of chromosomes?
Doctors can perform some procedures in which they obtain some of the cells of the fetus. The
least invasive is a blood test that looks for markers in the mother’s blood that suggest an
increased likelihood of a chromosomal abnormality. Another method is through amniocentesis.
In amniocentesis, a large needle is used to pass through the pregnant women’s abdomen and into
the uterus. Some of the amniotic fluid that surrounds the fetus is drawn off and analyzed. A third
method of chromosomal screening is through chorionic villi sampling. In chorionic villi
sampling, doctors insert a tube through the vagina to remove some of the cells that are part of the
placenta.
Slide 39 – Blood Test
Animation from YouTube
https://www.youtube.com/watch?v=TOdEbBlhIgU
Slide 40 – Amniocentesis
Video clip about amniocentesis from YouTube
https://youtu.be/w60pBBXBnDM
Slide 41 – Chorionic Villus Sampling
Video clip about chorionic villus sampling from YouTube
https://youtu.be/PKG9yhkd8zQ
Slide 42 – Karyotypes
When doctors obtain cells from the growing fetus, they make a karyotype, which is a
photographic representation of the chromosomes arranged in order from largest to smallest. All
of the human chromosomes are numbered. You should have two of each of the numbered
chromosomes; one from your mother and one from your father. These are the homolougus
chromosomes we mentioned earlier. There are a total of 23 pairs of chromosomes. Twenty–two
of these pairs are called the autosomes, while the chromosomes in the 23rd pair are the sex
chromosomes. Females have two X chromosomes; males have an X chromosome and a Y
chromosome.
Slide 43 – X chromosome inactivation
In females, one X chromosome is inactive in each cell. The same X chromosome is not
necessarily inactivated each cell. The inactive chromosome is known as the barr body. The X
chromosome, unlike the Y chromosome, carries a lot of information that is not related to sexual
characteristics. This can be seen in the tortoise shell coat color of cats. In these cats the X
chromosome carry genes for color, either orange or black. In areas where the black X
chromosome is inactivated, the cat has an orange color. In places where the orange X
chromosome is inactivated, the cat has a black color.
Slide 44 – Incidence of Down syndrome
Slide 37 – Visualizing Nondisjunction
Animation from YouTube
https://www.youtube.com/watch?v=nQ3Rjh661X8
Slide 38 – Chromosome screening
How do doctors know if a child will be born with an abnormal number of chromosomes?
Doctors can perform some procedures in which they obtain some of the cells of the fetus. The
least invasive is a blood test that looks for markers in the mother’s blood that suggest an
increased likelihood of a chromosomal abnormality. Another method is through amniocentesis.
In amniocentesis, a large needle is used to pass through the pregnant women’s abdomen and into
the uterus. Some of the amniotic fluid that surrounds the fetus is drawn off and analyzed. A third
method of chromosomal screening is through chorionic villi sampling. In chorionic villi
sampling, doctors insert a tube through the vagina to remove some of the cells that are part of the
placenta.
Slide 39 – Blood Test
Animation from YouTube
https://www.youtube.com/watch?v=TOdEbBlhIgU
Slide 40 – Amniocentesis
Video clip about amniocentesis from YouTube
https://youtu.be/w60pBBXBnDM
Slide 41 – Chorionic Villus Sampling
Video clip about chorionic villus sampling from YouTube
https://youtu.be/PKG9yhkd8zQ
Slide 42 – Karyotypes
When doctors obtain cells from the growing fetus, they make a karyotype, which is a
photographic representation of the chromosomes arranged in order from largest to smallest. All
of the human chromosomes are numbered. You should have two of each of the numbered
chromosomes; one from your mother and one from your father. These are the homolougus
chromosomes we mentioned earlier. There are a total of 23 pairs of chromosomes. Twenty–two
of these pairs are called the autosomes, while the chromosomes in the 23rd pair are the sex
chromosomes. Females have two X chromosomes; males have an X chromosome and a Y
chromosome.
Slide 43 – X chromosome inactivation
In females, one X chromosome is inactive in each cell. The same X chromosome is not
necessarily inactivated each cell. The inactive chromosome is known as the barr body. The X
chromosome, unlike the Y chromosome, carries a lot of information that is not related to sexual
characteristics. This can be seen in the tortoise shell coat color of cats. In these cats the X
chromosome carry genes for color, either orange or black. In areas where the black X
chromosome is inactivated, the cat has an orange color. In places where the orange X
chromosome is inactivated, the cat has a black color.
Slide 44 – Incidence of Down syndrome
Cell Division
Down syndrome is the most common trisomy, or case of three chromosomes of the same
number. Down syndrome results in certain facial characteristics and individuals that are shorter
in stature. The incidence of having a child with Down syndrome increases with the age of the
mother. Women who are over 40 are much more likely to have a child with Down syndrome.
Slide 45 – Non–disjunction of the sex chromosomes
There are several syndromes that occur as a result of non–disjunction of the sex chromosomes.
Kleinfelter syndrome results when an individual has two X chromosomes and a Y chromosome.
This situation produces males that are sterile and that have some female characteristics, such as
breast development.
A super male is a male that has one X and two Y chromosomes. These individuals have no
unusual characteristics and are not sterile.
A metafemale has three copies of the X chromosome. Metafemales have no unusual
characteristics and are not sterile.
Turner Syndrome results when an individual has just one X chromosome and no other sex
chromosome. These individuals are sterile females that are short in stature and who have neck
webbing.
Slide 46 – Summary
This slide is a summary of all of the “Check Your Understanding” questions from this lecture.
Be sure to review the questions you answered incorrectly.
Down syndrome is the most common trisomy, or case of three chromosomes of the same
number. Down syndrome results in certain facial characteristics and individuals that are shorter
in stature. The incidence of having a child with Down syndrome increases with the age of the
mother. Women who are over 40 are much more likely to have a child with Down syndrome.
Slide 45 – Non–disjunction of the sex chromosomes
There are several syndromes that occur as a result of non–disjunction of the sex chromosomes.
Kleinfelter syndrome results when an individual has two X chromosomes and a Y chromosome.
This situation produces males that are sterile and that have some female characteristics, such as
breast development.
A super male is a male that has one X and two Y chromosomes. These individuals have no
unusual characteristics and are not sterile.
A metafemale has three copies of the X chromosome. Metafemales have no unusual
characteristics and are not sterile.
Turner Syndrome results when an individual has just one X chromosome and no other sex
chromosome. These individuals are sterile females that are short in stature and who have neck
webbing.
Slide 46 – Summary
This slide is a summary of all of the “Check Your Understanding” questions from this lecture.
Be sure to review the questions you answered incorrectly.
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