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- In the 1850's, Rudolf Virchow put forth the idea that every cell derives from another cell.
- In the late 1870's and early 1880's, cytologists examined fertilization, initially observed by N. Pringsheim in1856. Cytologists noticed that despite the large inequality of the size of the cells, the nuclei of egg and sperm were of equivalent size.
- Before or during their union, each the sperm and egg nucleus is transformed into a definite number of rodlike structures which appear to be of the same shapes, sizes, and number in the two cells.
- These structures were soon recognized also in the division processes of mitosis and meiosis. In 1888, Wilhelm Waldeyer coined the term chromosome to describe these structures obviously important in nuclear division (rev).
- Chromosomes also condense in preparation for cell division. The chromosomes at metaphase are particularly distinctive.
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- Magnified, the rod-like chromosomes within one cell vary considerably in size.
- 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: acrocentric (constriction near the top), telocentric (no obvious constriction and thus thought to be at the end), and metacentric (constriction in the middle).
- Secondary constrictions are present on some chromosomes. In maize, there is 1 on chromosome #6. In humans there are five, on #s 13, 14, 15, 21, 22 and in Drosophila there is only 1 on each the X and Y. The number of secondary constrictions agrees closely with the number of nucleoli.
- Size and shape are used to number and identify chromosomes.
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