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I like to give my students different ways to practice chromosome counting. Below is a micrograph taken by a former undergraduate student in my lab, Maya Benavides. To take this picture, Zea mays (corn) seeds were germinated and the root tips were removed. The root tip is where most of the mitosis is occurring in the root and, since we wanted to capture condensed chromosomes, using a tissue with many mitotic cells was important. After digesting the cell walls and squashing the tissue on a slide it was possible to find nice chromosome spreads like the one shown below.
|Maya Benavides, Cal Poly, 2008|
The circled shape is a pair of sister chromatids. I give my students this picture along with the following questions as a way to reinforce their understanding of chromosome numbers during mitosis and meiosis. (Get this as a worksheet from the download page.)
1) How many pairs of sister chromatids are shown in the picture?
Answer...20. Question 1 is straightforward...you just need to count how many "blobs" there are in the picture. Sometimes students overthink this step and say 40. The number of chromatids is 40 but the question is asking for the number of sister chromatid pairs. (I usually interrupt the students after they have been working for a minute or two and make sure that they have the correct answer for question 1...if they get off track here they will miss most of the remaining questions.)
2) A diploid corn cell has _____ total chromosomes.
Answer...20. Each of the pairs of sisters is a single chromosome despite the fact that each pair of sisters is actually composed of two double-stranded DNA molecules (see my previous post on chromosomes and chromatids). There were 20 chromosomes in this cell before DNA replication and there are 20 chromosomes in the cell now (at the start of mitosis). DNA replication doesn't change chromosome number.
3) A diploid corn cell has _____ homologous pairs of chromosomes.
Answer...10 Corn is diploid meaning that it has homologous chromosomes. For each chromosome in the picture there is another chromosome with similar length, centromere position, and gene content. (An expert could look at this picture and match all the homologs up.) These homologs are not be identical...they have the same genes in the same order, but, they can have different alleles of those genes.
Answer...10. In meiosis I homologous chromosomes pair up to form a bivalent (also known as a 'tetrad', see picture below). Since there are 20 total chromosomes, 10 bivalents form when the homologs come together.
"Bivalent" by internet - http://188.8.131.52/bahan-ajar/modul_online/biologi/meiosis/glossary/glossary_frameset.htm. Licensed under CC BY-SA 2.5 via Wikimedia Commons.
5) A Z. mays gamete will have _____ total chromosomes.
Answer...10. A gamete is a haploid cell produced by meiosis. If one corn cell was to undergo meiosis the result would be four haploid cells. During meiosis I the homologous chromosomes separate into different cells...each of those cells has 10 total chromosomes and no homologous pairs. (Even though there are 10 chromosomes in the cell at this point each chromosome is still represented by a pair of sister chromatids.) During meiosis II the sister chromatids separate and go into different cells. Meiosis II doesn't reduce the number of chromosomes (again, this is just the weird way geneticists count chromosomes.)
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