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Understanding chromosome number...

    You learned in the last lesson (and in the mitosis lab) about how DNA molecules exist in a cell.  You also learned about the idea of homologous chromosomes.  These are important ideas for this week.

    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.   You understand the importance of having pairs from when we learned genetics...   If we only had one chromosome with a single allele for a trait, if anything ever happened that allele, we could end up somehow defective the trait it coded for.  So if we had only one allele for skin color and something happened to it, we would end up an albino.  But our cells all have two of each type of chromosome, so in the above example, we would have two chromosomes that each have an allele for skin color. This way, if something were to go wrong with one chromosome, we would still have one, fully functional chromosome left.

    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.   Most organisms that you are familiar with, though, are mainly composed of diploid cells.  Believe it or not:  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).   A haploid organism containing 28 chromosomes would have N = 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.

Chromosome appearance during meiosis

    Chromosomes at the beginning of mitosis have 2 sister chromatids... right?  Well at the beginning of meiosis, they look a little different.  You see, if meiosis is going to split up homologous chromosomes (to make diploid cells into haploid ones), the homologous chromosomes are going to have to associate with one another.

    Homologous chromosomes normally never touch each other or interact with each other.  But, in the one case of meiosis, they do.  So, when homologous chromosomes get together for meiosis, after DNA replication has occurred, each chromosome has sister chromatids, so you end up with 4 chromatids all stuck together.  This is called a tetrad.

    In the drawings in the table below, I have made one homologous chromosome green and the other orange.  The sister chromatids within a chromosome are the same color because they are identical.  The homologous chromosomes are not identical... they just code for the same traits (but may have completely different alleles for those traits).

Homologous chromosomes (each with sister chromatids) A tetrad
Keep in mind that homologous chromosomes never actually associate with each other in a cell unless that cell is going through meiosis.  Since most of the cells in your body NEVER go through meiosis, most of the homologous chromosomes never get together.  Only in those special cells in your gonads do the homologous chromosomes ever interact-- and they do so by forming the tetrads.


2006 STCC Foundation Press, content by Dawn A. Tamarkin, Ph.D.

Last changed: January 21, 2007