Daily Newsletter March 20, 2012
Today's Topic: Nuclear Division
Mitosis describes nuclear division. We commonly use the term loosely to describe cell division, but that is an incorrect usage of the word Mitosis. Cell division is better described by the term cytokinesis. It is important to use these terms correctly, as it will help you as you go further through biology.
Mitosis describes a nuclear division event. Most of the time we think of nuclear division occuring just prior to cytokinesis (cell division), but there are examples of cells that can undergo nuclear division without undergoing cytokinesis (many fungi do this). For most eukaryotic cells though, mitosis will be followed by cytokinesis. Remember though that these are two different events.
Mitosis is also described as somatic nuclear division. The word somatic refers to general body cells. This is to contrast difference between a general cell and a reproductive cell. A reproductive cell, or gametic cell, will undergo a special type of nuclear division known as meiosis, which will be discussed in Thursday's newsletter.
The goal of mitosis is to produce two daughter nuclei that are genetically identical. So when we couple mitosis and cytokinesis, the goal is to create two genetically identical daughter cells.
Going into mitosis, DNA replication will have had to occur. Every molecule of DNA will have undergone replication (DNA synthesis). We will discuss the process of DNA replication in class on Thursday.
The process of mitosis occurs in 4 Main Phases (there are additional phases that have been added in recent years). Below is a phrase to remember:
"Prophase sets up the process, metaphase aligns the chromosomes, anaphase seperates the chromosomes, and telophase returns the cell to normal."
This phrase describes the basic action of the four main steps: Prophase, Metaphase, Anaphase and Telophase.
Review the University of Arazonia's Mitosis Tutorial. This will take you through the main phases, and includes the Prometaphase addition.
In Prophase, we see the condensation of the chromosomes, which were discussed in yesterday's newsletter. We also see the dissolution of the nuclear envelope and the formation of the mitotic spindle.
In Prometaphase, microtubules of the mitotic spindle reach toward the centromere of each chromosome, forming the kinetochore.
During Metaphase, the chromosomes are aligned down the imaginary equitorial line of the cell. This line is equidistant between the centrioles. The alignment is important. If you look at most drawings, the chromosomes are shown as aligning so that their centromeres are on the equitorial line, and individual chromatids are on either side of the line. This is to represent an equitorial division. As each sister chromatid of a chromosome represents a complete DNA molecule, the division of these will result in an equal number of chromosomes on either side of the equitorial line. So, a cell with 46 molecuels of DNA (chromosomes) will produce two dauther nuclei (and then cells) with 46 moledules of DNA (chromosomes).
Beyond showing the arrangment of chromosomes, the image above also shows the different types of microtubules: Kinetochore, Polar and Aster. Aster, meaning star (Greek), comes from the starlike appearance of these microtubules. This starlike arrangment is also seen in the plant genus Aster, which is noted for the radial startlike flower petals. Note: they are sometimes referred to as Astral microtubules (Astral is an English adjetive derived from Aster).
Once the chromsomes have been arranged, they can be seperated. Anaphase is when there is visible seperation of the chromosomes into daughter chromatids. This link to an Anaphase Image is an excellent reference for what occurs in Anaphase (remember that you have motors that can move down microtubules).
During Telophase, the cell returns to normal interphase operation. The mitotic spindle will be released, the nuclear envelope will reform, and the DNA will be released from the supercoiled packing that produced the chromatids and chromosomes.
Study Note: You will notice that above I gave you a number of links. Do you think that they are important for your overall understanding of this topic?
Remember This: The following phrase will help you as we move through genetics. Remember it!
"Base complementarity is the foundation of all genetic processes."
Daily Challenge: Mitosis
Describe the process of mitosis in your own words. Feel free to use images, just reference where you got the image. Remember that this leads to your milestone paper and exam, spend some time to build a personal description of mitosis.
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