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How are homologous chromosomes defined?

If a chromosome consists of 2 chromatids, how do we have 23 pairs of chromosomes?

  • In my bio text book it reads, Humans have 23 pairs of chromosomes for a total of 46 chromosomes. But a Chromosome is defined as 2 chromotids. why doesn't it read as, Humans have ...show more

  • Answer:

    " But a Chromosome is defined as 2 chromotids." Each sister chromatid is an exact copy of the other. I have two Peter Benchley novels "Jaws" and "The Deep". If I have two copies of each novel, I still have two Peter Benchley novels. I disagree that a Chromosome is "defined" as two chromatids. A chromosome is but one chromatid before S phase, and it's two identical sister chromatids after S phase. MARK makes an excellent point. His answer reminded me that there are ways that the chomatids can actually become a new set of chromosomes and the chromosome number of the organism will actually double. Plant cells that are treated with the chemical colchicine will double their chromosome number. The resulting plants cultured from such cells are polyploid. http://www.carnivorousplants.org/howto/Propagation/Colchicine.php

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First of all your DNA is not always in chromosomes. Your DNA is attached to proteins called histone proteins. The DNA and his tones are diffused throughout the nucleus as chromatin, which cannot be seen under the light microscope. If a cell is going to divide let's say by mitosis the cell must copy its DNA so when it divides into 2 cells each one gets a full complement of DNA. Each DNA molecule copies itself. The copy is attached to the original. When the DNA condenses and becomes visible under the light microscope there are 46 chromosomes. Each chromosome consists of 2 chromatids. During mitosis the chromatids separate and goes to opposite poles of the cell. Each chromatid is now a chromosome. I suppose you could argue that at the end of mitosis just before the cell divides it temporarily has 92 chromosomes. 46 for each of the 2 new cells just about to be formed. Remember this only happens for cell division and some cells never divide such as nerve cells.

MARK

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