csomes&ctids copy 2

csomes&ctids copy 2 - The number of chromosomes is...

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Of chromosomes and chromatids The number of pairs of chromosomes in a diploid organism is represented as N. N is also referred to as the haploid chromosome number. In humans, N = 23. A chromosome may be thought of as the basic “unit” of DNA in a cell. The book defines a chromosome as “an end-to-end arrangement of genes and other DNA, sometimes with associated proteins and RNA”. Each chromatid is a single, linear piece of DNA. After mitosis (and after meiosis II), each chromosome consists of one chromatid. The sister chromatid, which is effectively identical to the original chromatid, is synthesized during the S phase in somatic cells. The two chromatids are attached by the centromere. In gametes, a sister chromatid is not synthesized until after fertilization.
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Unformatted text preview: The number of chromosomes is the same as the number of centromeres. Before a cell enters mitosis or meiosis, the sister chromatid is synthesized. Left: one chromosome, with one chromatid. Right: same chromosome, after S phase, now with two chromatids (new sister chromatid shown in grey). Centromeres are shown as white circles. Human karyotypes are made from cells in late prophase or occasionally metaphase, so that the chromosomes are condensed and easy to see; also, both chromatids are present for each chromosome. Hence our typical representation of chromosomes as X-shaped. Image borrowed from: http://homepages.uel.ac.uk/V.K.Sieber/s olidktp.jpg To try making a karyotype yourself, go to this link: http://learn.genetics.utah.edu/units/disord ers/karyotype/karyotype.cfm...
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This note was uploaded on 12/02/2011 for the course PCB 3063 taught by Professor Marta during the Fall '08 term at University of Florida.

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