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Intro to Anatomy 4: Cell Structure and Function
Intro to Anatomy 4: Cell Structure and Function
The Lone Ranger
Published by The Lone Ranger
01-07-2007
Default Mitosis and Meiosis


During most of a cellís life cycle, the DNA in the nucleus is not readily visible. Instead, it is in an unraveled state and is called chromatin. Just before the cell is ready to reproduce, it duplicates all of its DNA. The DNA then condenses to form the chromosomes. In this condensed, highly compacted form, DNA cannot function, so cells donít spend much time in this state.



The normal process of cellular reproduction is called mitosis. In mitosis, a diploid parent cell duplicates its DNA, then divides into two daughter cells, each of which is genetically identical to the original parent cell.



Sex cells (gametes) are produced by a variation of mitosis known as meiosis. Meiosis starts out like mitosis in that a diploid cell first duplicates its DNA. But before the cell divides, the homologous chromosomes come together and exchange bits of DNA between themselves. As a result, a gene that was inherited from the organismís mother might wind up on the chromosome inherited from its father, and vice versa. This process, in which genetic material is swapped between the homologous chromosomes, is known as crossing over.

After crossing over occurs, the cell divides into two daughter cells. Because of crossing-over and other mechanisms that rearrange the cellís DNA, the two daughter cells that result are diploid, but they are not genetically identical.

The daughter cells then divide again, but without duplicating their DNA first. The end result of meiosis is therefore four haploid cells (each has only one set of DNA) Ė none of which are genetically identical.


Mitosis involves a single cell division and produces 2 diploid daughter cells
Meiosis involves two cell divisions and produces 4 haploid daughter cells

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