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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.
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 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 nongraded 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 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 nonidentical (remember crossing over) haploid daughter cells. Slide 20 Mitosis versus Meiosis Animation from YouTube 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 nongraded 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. Nondisjunction 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 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 Slide 40 Amniocentesis Video clip about amniocentesis from YouTube Slide 41 Chorionic Villus Sampling Video clip about chorionic villus sampling from YouTube 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. Twentytwo 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 Nondisjunction of the sex chromosomes There are several syndromes that occur as a result of nondisjunction 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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