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Chromosome Classification Molecular Biology 5 (v1 as of 1/8/06) Go to Mol Bio 5 SIZE Magnified in the light microscope, the rod-like chromosomes within one cell vary considerably in size.  This size difference can be seen in the collection of drawn metaphase chromosomes (See Figure 1).  This is NOT  a karyotype because it only shows a chromatid pair joined at the centromere for each chromosome number.  A karyotype shows each homologous pair for each chromosome number.  To view a karyotype, click here. CENTROMERE POSITION Many chromosomes have clear constrictions in their rods. The primary constrictions are called centromeres. The ends are called telomeres. The positions of the centromeres are used to broadly classify chromosomes into three morphological groups:  (See Figures 1 &  2) acrocentric (constriction near the top),  or telocentric (no obvious constriction and thus thought to be at the end), and  metacentric (constriction in the middle), or  submetacentric (constiction off-center) Metacentric chromosomes have their centromere near the center of the chromosome. Submetacentric chromosomes have slightly off-center centromeres, such that one chromosome arm is longer than the other. Acrocentric chromosomes have the centromere located very near to one end  BANDING Another way to distinguish chromosomes from each other besides length and centromere position is to stain them with chemical dyes. Staining results in crosswise patterns of bands that are characteristic of that chromosome. (See Figures 1 & 2) For more information about banding patterns read: Chromosome Banding. The two homologues of a chromosome pair (e.g., chromosome 7 from father, and chromosome 7 from mother) exhibit identical banding patterns.  Certain organisms, like Drosophila, have chromosomes with such complex banding patterns that the position of genes can be pinpointed at a particular band position. Unfortunately, the banding patterns on human chromosomes are not this complex. Figure 1 Types of chromosomes based  on size from a human male. Figure 2 Types of chromosomes based  on centromere location Interesting Question:  Why are mules sterile? A mule is the product of two different species (a horse and a donkey) mating with each other. The fact that these two different types of animals can mate and produce viable offspring tells scientists that horses and donkeys are closely related.  However, mules are always sterile. Why? Horses and donkeys have different chromosome numbers (see below). The fact that horses and donkeys have different chromosome numbers tells scientists that these two are different species. For the mule, for the most part, having parents with different chromosome numbers isn't a problem. During mitotic cell division in the mule, each of the chromosomes copies itself and then distributes these two copies to the two daughter cells. In contrast, when the mule is producing sperm or egg cells during meiosis, each pair of chromosomes (one from Mom and one from Dad) need to pair up with each other. Since the mule doesn't have an even number of homologous pairs (his parents had different chromosome numbers), meiosis is disrupted and viable sperm and eggs are not formed.

References:
Background info:	http://opbs.okstate.edu/~melcher/MG/MGW1/MG11322.html Genes V, by Benjamin Lewin, Oxford University Press, Oxford, England,1995, p782 Centromere classification:  http://raven.umnh.utah.edu/units/karyotyping/criteria.html Idiogram Karyotype source: http://raven.umnh.utah.edu/units/karyotyping/normal.html Mule photo: http://members.aol.com/riolongear/tradinplace.html

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