The cell cycle - The cell cycle, mitosis and meiosis...

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The cell cycle, mitosis and meiosis Introduction Although there are minor differences in how authors may define what is living, one thing common to any definition; living things come from other living things. To put it another way, living things are able to replicate themselves. The cell theory included this with its statement that the cell was the smallest unit of life and all cells arose from pre-existing cells. Everything in a cell, its organelles, cytoplasmic components, and the molecules that are dissolved in the cytoplasmic mix are ultimately coded for in the DNA of the organism. So, if a cell is to replicate and make an exact copy of itself one of the first things it must do is replicate its DNA, and this was the subject of our first homework assignment. Once a precise duplicate copy of the DNA is made then one copy has to be placed into each of the two new daughter cells that result from the division of the parent cell. Mitosis is only the duplication of the DNA. Subsequent partitioning of the DNA along with the cell contents into two new cells from a single cell is called cytokinesis. The combination of the two is the cell cycle. Be sure you don’t mix up these two very different steps of the cell cycle; mitosis and cytokinesis. It’s only as a cell is about to divide that its DNA is easily seen under the microscope. The long delicate strands are packed and wound around each other in folds on more folds of DNA held in place by special packaging proteins that will protect the genetic material as it is pulled and pushed into the new cells during division. Packaged this way, unreplicated DNA appears under the microscope as a chromosome, and when replication is complete as two identical sister chromatids. Most organisms have chromosomes arranged in pairs, with one coming from the maternal parent (mother) and the other from the paternal parent (father). The matching pairs are homologous chromosomes, or homologs, and organisms whose cells have homologous chromosomes are diploid. Mitosis works fine if we want to produce two new cells that are duplicates of the parent cell - asexual reproduction is how somatic cells reproduce. But if we want to combine the genetic material of two parents, it won’t work by combining two diploid parental cells. Under these circumstances a new cell resulting from the fusion of the parental and maternal cell would have twice as many chromosomes as each parent, and in most cases would not be a viable organism. To prevent this, germ cells are set aside early in the development of the organism and these cells undergo a special sequence of two cell divisions that split the duplicated homologous chromosomes then split the duplicated DNA. The process is meiosis, and the cell products have only half the genetic information as the original parent cells and are referred to as haploid. These specialized cells are gametes, with the male gamete becoming sperm and the female gamete
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becoming the egg. Sexual reproduction combines the two haploid gametes and the result
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This note was uploaded on 12/03/2011 for the course SCIENCE BIO1130 taught by Professor Fenwick during the Spring '11 term at University of Ottawa.

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The cell cycle - The cell cycle, mitosis and meiosis...

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