A drug that stops the spindle from forming is added to a culture of dividing human cells (2n = 46)…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A drug that stops the spindle from forming is added to a culture of dividing human cells (). The cells can still copy their DNA and condense their chromosomes.
- At which stage of mitosis do the cells collect?
- How many chromosomes, chromatids and units of C of DNA does such a cell have?
- After some time, a cell leaves mitosis without dividing. How many chromosomes and how much DNA does its single new nucleus hold?
- Why is such a drug used to count the chromosomes of an animal?
Need a reminder? The key ideaShow
Mitosis runs in the order prophase, prometaphase, metaphase, anaphase and telophase. Each chromatid has its own kinetochore. At anaphase the centromeres split, and every chromatid is counted as a chromosome. The whole cell then holds twice the chromosome number of a G1 cell, but the same 4C of DNA.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. (a) At metaphase. The chromosomes condense and gather in the middle, but without a spindle the chromatids cannot be pulled apart.
Step 2. (b) chromosomes, chromatids and C of DNA.
Step 3. (c) The chromatids have separated and are single chromosomes. The single nucleus holds chromosomes and C of DNA. It is tetraploid.
Step 4. (d) The cells pile up at metaphase, where the chromosomes are fully condensed and easy to count. A spread of such a cell shows the number and the shapes of the chromosomes.
Answer: (a) metaphase; (b) , , C; (c) chromosomes, C; (d) condensed chromosomes of many cells at one stage can be counted.
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