Chapter 15 Summary

Chapter 15 goes over The Eukaryotic Cell cycle, Mitosis, Meosis, and Variations in chromosomes.  All topics which are essential for a proper education in Biology.  

Cytogenetic is a field in science that covers microscopic study of the cell cycle and chromosomes.  Studying chromosomes is very important because you can learn two important things.  Before i go into that, I’ll tell you how scientists even study the chromosomes.  During cell division, chromosomes condense, allowing a human eye to see them via a light microscope.  Pictures are taken and the chromosomes can be organized from largest to smallest.  This is called a karyotype.  When you have a karyotype you can see the number, size, and form of all the chromosomes in that cell.   Now that you have a karyotype, you can see that eukaryotic chromosomes occur in sets.  Each set has different types of chromosomes.  This means that in a set, none of the chromosomes are the same.  The set contains different sized chromosomes that have many different purposes.  A set contains 22 autosomes, and 1 sex chromosomes.  This makes 23 different chromosomes.  But 46 total chromosomes because a human cell contains 2 sets of chromosomes.  1 set from their father, and 1 set from their mother.  This makes them diploid because they have 2 sets of chromosomes. (2n)  n representing the number of sets.  When a cell has 1 set of chromosomes it is called haploid.  Human sex cells or gametes are haploid because the sperm and the egg have to combine to form a zygote.  So the sperm contains 1 set of 23 chromosomes, and the egg contains 1 set of 23 chromosomes.  When they combine it makes a diploid cell with 2 sets of 23 chromosomes.  The two chromosomes in the pair are called homologous chromosomes.  This means that they are similar in size, and form.  This is why they are paired up.  The only difference is the gene they express.  For example if a homologous pair represented eye color.  One homologue would express blue pigment and the other homologue would express brown pigment.   The cell cycle is what makes new cells.  It is composed of multiple steps.  The G1 phase, the S phase, the G2 phase, and the M phase.  The G1 phase is when the cell is making sure it is ready to go proceed into the cell cycle, and divide.  During this time the cell grows.  In the S phase DNA replication occurs, and the chromosomes get replicated, so now there are double the chromosomes in the cell.  Sister chromatids are made in the S phase.  During the G2 phase, cell growth occurs, and proteins are made for cell division.  During the M phase, mitosis occurs.  Mitosis has many steps in which the end result is an identical daughter cell.  Throughout the cell cycle there are checkpoint proteins.  These proteins check, the progress and make sure the next step can occur without any problems occurring.  There are checkpoint proteins in the G1, G2, and the M phase.   

Mitosis is when the steps interphase, prophase, prometaphase, metaphase, anaphase, telophase, and cytokinesis occur.  In order to make an identical daughter cell.  Mitosis is essential for asexual reproduction.  So reproduction for bacteria and fungi.  It is also essential for the growth, and the continual growth and maintenance of eukaryotic species.  During Interphase, the steps of G1, S, and G2 occur.  These steps were already talked about.  Prophase is when the chromosomes condense, and the nuclear envelope is diminishing.  Prometaphase is when the nuclear envelope completely diminishes.  And the centrioles start to migrate to opposite poles of the cell.  Spindle fibers come out of the centrioles and start to attach to the sister chromatids.  During mitosis, the spindle fibers have attached to the kinetochores of the sister chromatids and aligned them not he equator of the cell.  The orientation of the sister chromosomes isn’t always one way.  They can be lined up in many different ways.  During anaphase, the spindle fibers shorter pulling apart the sister chromatids to opposite sides of the cell.  In telophase and cytokinesis, the nuclear envelope forms around the new chromosomes and the chromosomes begin to de- compact, and a nuclear envelope starts to form around the chromosomes.  The cell splits in two creating an identical daughter cell form the original mother cell. 

Meiosis is a little more complex that mitosis.  After G1, S, and G2 a tetrad is made, and crossing over occurs.  This is when parts of the sister chromatids switch onto the other chromosomes creating diversity.  Meiosis has Meiosis I and Meiosis II.  So, prophase I, prometaphase I, metaphase, anaphase I, telophase I and cytokinesis.  Than Prophase II, prometaphase II, Metaphase II, Anaphase II, and Telophase II with cytokinesis.  The object of Meiosis is to create four haploid cells from a diploid cell.  Meiosis is used for the production of gametes for eukaryotic cells.  The new cells need to be haploid because they need to form with other haploid cells to dorm a diploid zygote.   During Meiosis I a diploid cell is made into two haploid cells containing 1 sister chromatid in each cell.  During Meiosis II.  Those two new cells containing one sister chromatid are broken down into 4 cells with 1 chromosome in each.  During prophase I the chromosomes condense, and the homologous chromosomes form bivalents, and crossing over occurs.  Lastly, the nuclear envelope starts to diminish.  In prometaphase I, the nuclear envelope am gone, and spindle fibers start to attach to the sister chromatids.  During Metaphase I, the bivalents are lined up along the center of the cell.  During Anaphase I, the homologues seperate to different sides of the cell.  In telophase I, the cell starts to split apart containing the two different homologues.  The chromosomes starts to de- condense, and the nuclear envolope forms.  During cytokinesis, the cell is split apart.  During Meiosis II, the same thing happens during prophase II, Prometaphase II, Metaphase II, but in Anaphase II, the sister chromatids will separate, not the bivalents.  Telophase II occurs, with cytokinesis, and it produces 4 haploid cells. 

Variation in chromosomes happens all the time.  Sometimes it is normal and it won’t have any effect, while other times the variation is not normal, and there will be a great effect.  The chapter describes different -ploids that can be a characteristic of chromosomes.  It ends talking about the different birth defects, like Down syndrome when you inherent 3 sex chromosomes instead of 2.

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So this images is all about cytokinesis.  image A shows cytokinesis of an animal cell. The animal cell cytokinesis involves a cleavage furrow.  That constricts like a drawstring to sepearte the cell.  

For image B, it is showing cytokinesis of a plant cell.  In plants vesicles fromt he Golgi move along microtubules to the center of the cell and form a cell plate.  The cell plate forms a cell wall between teh two duaghter cells.  

This is a video of Mitosis.  It is a great video for anyone who doesnt fully understand the phases of mitosis.  Mitosis is very important because it is how bacteria reproduce, and its how eukaryotics gain new cells.

http://www.ncbi.nlm.nih.gov/pubmed/21301226 - This is a very nice article about the importance of checkpoint proteins.  In this articke they give an example of a check point protein.  They explain what happends to the chromosome when it is inactive or missing.  The article is basically a study of this.