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Chapter 9 Answers
Mitosis
Visual Understanding
1. Figure 9.4
This shows a set of human chromosomes. At what stage of the cell cycle are such photos
taken, and why?
They are all coiled up, and appear to have duplicated, so they are in some stage of
mitosis, usually near metaphase. At other times in the life cycle many areas of the DNA
molecule are stretched out so that they can be transcribed by mRNA. It would be very
difficult to try to divide with the chromosomes all uncoiled.
2. Figure 9.6
During interphase the DNA is not visible through a microscope; it only becomes visible
as chromosomes during mitosis. Why isn’t the DNA visible during interphase and why
would this be the case?
The DNA is not visible during interphase because during this phase, portions are being
used in transcription of mRNA for protein synthesis. DNA is in an “unraveled”
configuration that will allow segments to be “unzipped” and transcribed. During
interphase, the cell is carrying out many life functions, and proteins such as enzymes are
necessary to carry out the chemical reaction required for life.
Challenge Questions
1. Why does the DNA need to change periodically from a long, double-helix chromatin
molecule into a tightly wound-up chromosome? What does it do at each stage that it
cannot do at the other?
When the DNA is an open chromatin molecule, portions of it are actively being
transcribed by mRNA. The cell will then be using those instructions to manufacture
amino acid chains and proteins. When the DNA winds up into a short chromosome it is
preparing for mitosis, which would be very difficult if the DNA were all stretched out.
2. Despite all we know about cancer today, some types of cancers are still increasing in
frequency. Lung cancer in women is one of those. What reason(s) might there be for this
increasing problem?
Lung cancer is typically caused by a mutation in the DNA of a cell. Mutations can be
caused by chemicals or environmental factors. The two main causes of chemicals in the
lungs are air pollution and smoking. Both are serious problems in some places, and the
resulting lung cancers are terribly expensive and painful.