In many ways this unit is the opening to the next several units on genetics and molecular biology. This unit deals with the processes by which eukaryote cell divide both in the non reproductive (or somatic) cells and more importantly in the reproductive (or germinal) cells. An understanding of what is happening to the chromosomes and the genes they carry during these processes is essential for a complete understanding of the genetics that will follow.
Learning Objectives: the successful student will be able to ...
- describe the basic structure of the chromosome including the chromatids, nucleosomes, and centromeres.
- explain the difference between diploid and haploid karyotypes.
- list the stages of the cell cycle and describe the important events that occur in each stage.
- compare and contrast the processes of mitosis and meiosis in terms of chromosome numbers, the number of daughter cells produced, the types of tissues involved, and the general functions of these two processes.
- list the stages of mitosis and describe the important events that occur in each stage as well as recognize diagrams/images of each stage.
- list the stages of meiosis and describe the important events that occur in each stage as well as recognize diagrams/images of each stage.
Lesson One: Chromosome Structure and Karyotypes.
Read the brief section in Kimball for an introduction to the structure of chromosomes. Go to The Cell Online to see an animation of nucleosome organization. You should understand the differences and relationships among the terms: chromosome, chromatid, and centromere.
A common method for studying chromosomes is to produce a "karyotype," an image of the homologous pairs of chromosomes ordered in standard arrangement usually stained to indicate some of the internal structure of the chromatin. Go to this brief description from WebMD. Understand the general technique of preparing a karyotype and some of the important uses this information is used for.
Homework: Due 26 January. Go to the Genomics page of the U.S. Department of Energy and view the images of the 23 pairs of human chromosomes. This diagram list a number of the genes and genetic disorders located on these chromosomes. Pick any chromosome (try to pick one that your classmates are not using for this assignment) and list at least 4 different genetic traits or disorders located on that chromosome. Be prepared to share this information in class. No emails are necessary.
Now go to the karyotyping exercise from the Biology Project of the University of Arizona. Complete the three patient karyotype problems and email the answers to the questions using your FBA account.
Lesson Two: The Cell Cycle and Mitosis.
Most cells go through a predictable series of events as they grow, prepare to divide, and finally undergo cell division (mitosis). The period of time between cell divisions has been called "interphase," but is in fact more complex than this term suggests. Go to the animation from the Human Anatomy online Learning Center for an introduction to the cell cycle. For more information, go to this YouTube video.
Now go to the Medical Biochemistry page and read the sections through the first three paragraphs of the "Check points and cell cycle regulation" section. Appreciate the complex sequence of events that are necessary to correctly choreograph the steps of the cell cycle and mitosis.
The stages of the cell cycle that are directly involved with the division of the chromosomes is Mitosis. During these four stages (prophase, metaphase, anaphase, and telophase) the nucleus falls apart, the chromosomes condense, actin fibers connect to the chromosomes and pull apart the chromatids of each chromosome. Finally a new nuclear membrane is formed around two new daughter nuclei. Notice that at the end of mitosis there is still only one cell. Mitosis is often, but not always, followed by cyctokinesis, the actual division of the original cell into two daughter cells. Understand that these daughter cells, absent any somatic mutations, are genetically identical to the original cells. Go to the Kimball pages for a summary of the steps of mitosis and cytokinesis. You should understand the sequence of important events that take place during mitosis, especially as they relate to the number and location of the chromosomes. For an animation of mitosis go to John Kyrk's pages.
Homework, Due 31 January. Go to this web site from Indiana University that discusses the connection between the cell cycle and cancer. Email your answers to the following questions using your FBA account.
- What cell cycle controlling genes are implicated in the development of cancer?
- Is the action of these genes the same? In other words, do they "cause" cancer in the same way and do they interact with one another? Explain.
- Are all tumors "cancerous" and does mutation in a cell cycle controlling gene mean that a person carrying these mutations will necessarily get cancer? Explain.
Lesson Three: Meiosis.
You will find that meiosis is in many ways similar to mitosis. The basic events of nuclear disintegration, chromosome condensation, spindle fiber development, and cyctokinesis are virtually identical. However, there are very important differences, especially in terms of the movement of the chromosomes that must be recognized. Most of these differences can be traced to the events of the Prophase I stage. Here homologous chromosomes pair to form structures called tetrads. During this stage, the homologous chromosomes often break and exchange genetic material in a process called crossing over. In addition, during the first division the chromosomes of each tetrad separate and move to the poles of the cell; not the chromosomes as in mitosis. This movement of chromosomes rather than chromatids leads to the reduction of the chromosome number from diploid to haploid. You will find a summary of meiosis at Kimball's pages. A somewhat clearer description and a good comparison of mitosis vs meiosis can be found at the Online Biology pages. A good animation can be found here. The animation at John Kyrk's pages gives a clear demonstration of crossing over.
Homework, Due 2 February. Read the short article from Discover Magazine and the bit longer article from the same source. Summarize these two articles using the format provided to you at the beginning of the year.
