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Chapter 15 Blog:  Cell Division

Page history last edited by Peter Falk 13 years, 1 month ago

 

A.  Chapter Summary 

     Chapter fifteen is all about cell growth and cell division.  The chapter starts off by describing chromosomes. Cell division is the reproduction of of cells, and it is highly regulated at various points. In eukaryotes, the two types of cell division are meiosis and mitosis.  In eukaryotes, genetic information is stored in the form of chromosomes. Humans have 23 pairs of chromosomes, giving us 46 total chromosomes. Ploidy is important to consider when discussing organisms, humans are diploid and have two copies of every chromosomes, thus making 2n=23 for humans. In eukaryotic species, memebers of a pair of chromosomes are called homolouges (homologous chromosomes).  Each homolouge is nearly identical in size and genetical composition. This is true for all chromosomes except sex chromosomes, which are extremely different in size and composition. A karyotype is a mapping of all the chromosomes in an organism. 

     The Cell Cycle is composed of four different phases. These are G1 (first gap), S (synthesis of DNA), G2 (Second gap), and M (mitosis and cytokinesis). G0 is a replacement for G1, this is the phase that the cell assumes when it has no intention of dividing. G1 through G2 are all parts of inter phase. During the G1 phase, the cell increases the rate of its biosyntehtic activities and begins producing enzymes necessary for DNA synthesis. During the S phase DNA is synthesized. By the end of the S phase, each chromosome has two sister chromatids. During the G2 phase, micro-tubules are produced.  The M phase is where the magic happens. This where the cell actually divides and goes through the process of mitosis. Mitosis is a process which can be divided into six different phases, Prophase, prometaphase, metaphase, anaphase, telophase and cytokinesis. The process of mitosis is fairly straight forward and I feel that regurgitating the information is unproductive. However, it is important to note that at the end of mitosis a two new daughter cells are formed. The decision that a cell makes to divided is influenced by several factors. These factors include external factors, such as environmental conditions and and signaling molecules, as well as internal factors, which includes cell cycle control molecules and checkpoints. In order to pass through the different phases of the cell cycle, they must first be approved of cyclin or cdk checkpoint proteins. These are turned on and off through phosphorylation. Loss of checkpoint proteins is very bad for the cell, and often results in cancer. 

     Meiosis is the process by which haploid cells are produced from a diploid cell. Similar to mitosis, meiosis begins after a cell has progressed through the G1, S, and G2 phases of the cell cycle. The difference is that the homologous pairs form a bivalent (tetrad) and crossing over of genetic information occurs. Crossing over is the physical exchange between chromosome pieces of the crossing biavalent. Crossing over increases the genetic variation of a species, however crossing over is tightly regulated by the cell.  While Mitosis produces two diploid daughter cells that are genetically identical, meiosis produces four haploid daughter cells each with a random mix of chromosomes. 

 

B.  Useful Materials

 

 

An Important Role for CDK2 in G1 to S Checkpoint Activation and DNA Damage Response in Human Embryonic Stem Cells

 
This article from PubMed discusses the newly discovered function of Cyclin Dependent Kinase 2 (cdk2) in G1 in Human Embryonic Stem cells. This article is dated Feb 11 2011, so this discovery is fairly recent. This article says that downregulation of CDK2 is responsible for triggering the G1 checkpoint through the activation of the ATM- CHK2-p53-p21 pathway. Now I am not familiar with that pathway so it is difficult for me to follow it exactly, however I can see the protein CDK2 is necessary and how so many different proteins are needed to control so many different pathways that are responsible for maintaining genomic integrity and keeping the cell intact. 

Animation: The Cell Cycle

This website has a very helpful animation that details the cell cycle and is then followed by a quiz that test basic comprehension of the cell cycle. This isn't helpful with higher learning comprehensions, but is extremely useful for establishing the fundamentals.  
Animated Outline of meiosis  This is an extremely useful animation that goes through the whole process of meiosis. It simplifys it a bit,  but it covers the most important aspects of meiosis. 

 

 

 

 

 

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