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12.21
The process of cell replication
  
  That cells carry within them the memory, instructions and capacity to build other cells is the key to the existence of the cellular universe and the existence of complex multi-cellular life forms such as whales and human beings.  
  CELL REPLICATION is when a cell contributes in some way- either by delivering part or all of their existence into the creation of one or more new cells.  
  The importance of cell replication cannot be underestimated as a subject of importance to human beings. Rapid cell replication- new growth in a good way, is a key to staying healthy and young as new tissue grows to replace old. On the other hand- understanding cell replication in terms of bad rapid cell growth in the case of a tumor, cancer or parasite is vital to understand how we get sick and what is required to get healthy again.  
  In this section, we seek to define the different types of cell replication and the key processes involved in cell replication- the creation of new cells.  
12.13.1 The differing types of cell replication and the different levels of life  
  Mitosis, division of a living cell nucleus (control centre), leading to the production of two offspring or daughter cells, normally with the same genetic information. Mitosis is the standard way that cells multiply. It occurs all the time in the human body and other multi cellular living things, especially during growth to make more cells, and during maintenance to replace damaged and worn-out cells. In single-celled organisms, it represents asexual reproduction. In plants, it is the basis of asexual or vegetative reproduction (making cells for sexual reproduction involves another type of cell division. Genes exist as chemical codes on lengths of the chemical deoxyribonucleic acid (DNA) inside the nucleus. During a cell's "resting" period, or interphase, the DNA copies or replicates itself to form two complete sets. Mitosis then occurs in four main stages.  
  Meiosis, specialized division of the nucleus (control centre) in a living cell, leading to the production of sex cells, called gametes, for sexual reproduction. The female sex cells are usually known as eggs and the male sex cells as sperm. Meiosis ensures that a parent passes its genes, via its sex cells, to its offspring. It also ensures that the offspring vary, since each receives a unique selection or combination of genes from its two parents. In wild animals and plants, this variation among offspring is essential for evolution by natural selection. We will discuss Meiosis in Chapter 13.  
12.13.1 PROPHASE, METAPHASE, ANAPHASE, AND TELOPHASE  
  The lengths of DNA begin to twist and coil, becoming more compacted and distinct under a light microscope as thread-like bodies, chromosomes. Because of the DNA replication, each chromosome consists of a pair of identical chromatids. They are attached at a region towards the middle, the centromere, giving the chromosome an X-like appearance. At one end or pole of the cell are two pairs of rod-like centrioles. One pair of centrioles moves to the opposite pole of the cell. At the same time, the nuclear membrane around the nucleus begins to break down.  
  In the metaphase, mitotic spindle fibres, made of groups of microtubules, now stretch between the two opposing centriole pairs. The chromosomes align across the cell's middle, or equator. As the nuclear membrane disintegrates, the centromeres of the chromosomes attach to the spindle. In the next anaphase the two halves of the chromosome are pulled apart, separating at the centromere.  
  They move in opposite directions along the spindle fibres, towards the centrioles at the poles. In the following telophase the spindle fibres break up. A new nuclear membrane forms around each new set of chromosomes, which themselves uncoil and unravel, gradually appearing less distinct.  
  Meanwhile, the rest of the cell is also dividing, a process known as cytokinesis. In flexible-walled animal cells, the outer membrane pinches inward, like a tightening belt around the equator, and eventually splits the cell into two. In rigid-walled plant cells, a new cell wall is built across the middle of the parent cell.  
     
 
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