November 17, 2013

What are Chromosome? Their Role in Cell Division? Chromosome number in different species

Humans, as you know, have 23 pairs of chromosomes, such that each pair is made up of homologous chromosomes.  Cells that have this type of arrangement of their chromosomes, meaning, cells that have pairs of chromosomes, are called diploid.
Some organisms are composed of cells with only one of each type of chromosome.  These cells do not have homologous chromosomes and are called haploid.
Some other organisms contain cells that are made up of many of each type of chromosome, so they have 3 or 4 or 6 or whatever homologous chromosomes!  These are called polyploid.

Different organisms contain different numbers of chromosomes.
The number of different types of chromosomes is represented by "N." In a haploid cell, where no pairs of chromosomes are found, the number of chromosomes that it has is simply N. In a diploid cell, the number of chromosomes is 2 times the number of different types of chromosomes, because it has pairs of chromosomes. So the total number of chromosomes in a diploid cell is 2N. Humans have 46 chromosomes in total. But we also are diploid. So we have 23 pairs of chromosomes (only 23 different types of chromosomes).   A diploid organism containing 28 chromosomes would have N = 14 (because 2N = 28).
What does chromosome number have to do with cell division?
 Meiosis, as you read in the introduction for this lesson, is the way we make our gametes.  A man and a woman each have 46 chromosomes.  To have a child through sexual reproduction, they have to jumble up their genes and combine them together using their gametes.  If the man put all of his 46 chromosomes in his sperm and the woman put all of her 46 chromosomes in her egg, when the sperm and the egg combined, the zygote would have 92 chromosomes!  That is way too many!  The zygote could not develop into a person.  So, instead, the man and woman have to only put half the number of chromosomes into each gamete.  That means that the sperm would get 23 chromosomes, the egg would get 23 chromosomes, and the zygote would end up with the correct number of chromosomes, 46, after fertilization.
   To reduce the number of chromosomes from 46 to 23 in the gamete, the gamete has to be made through meiosis.  And, now that you know a lot about homologous chromosomes, it should make sense to you that the only way to cut down the number of chromosomes while still ensuring that each parent gets to contribute an allele to each gene, is to only give one of each pair of chromosomes.  When we do that, we take our diploid cells and make haploid gametes out of them.
    Therefore, humans do have some haploid cells-- but they are all located in our gonads (testes and ovaries).  There are no haploid skin or liver cells.

    And, also, that means that if we are going to make new cells (the gametes), but make them different from the parent cell (haploid, not diploid, gametes), we can't use mitosis to make them.  And Pasteur showed us that we can't wait for spontaneous generation to make them.  So we need to use another method of cell division... and that method is meiosis.  So, meiosis serves to make haploid cells out of a diploid one.

Common Name
Genus and Species
Diploid Chromosome
Bison bison
Felis catus
Bos taurus, B. indicus
Canis familiaris
E. asinus
Capra hircus
Equus caballus
Homo sapiens
Sus scrofa
Ovis aries


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