Download Chapter 24 – end of chapter critical thinking questions

Answers to Thinking Critically Questions
Mader: Inquiry into Life, Twelfth Edition
Chapter 24
1. Use what you have learned in this chapter about the scientific experiments
conducted to determine that DNA is the genetic material and how scientists
deduced the structure of DNA.
Explanation/Answer: The Hershey-Chase experiment used radioactivity to label
both the proteins and DNA of viruses that infect bacteria (bacteriophages). The
DNA was the material that resulted in radioactivity inside the bacterial cells,
while the protein remained outside the cell. This experiment confirmed that DNA
is the genetic material. To deduce the structure of DNA, Watson and Crick relied
on three key discoveries: 1) DNA is constructed of a phosphate group, a sugar
(deoxyribose) and a nitrogen containing base (either A, C, T or G); 2) the number
of purines (A and G) always equals the number of pyrimidines (T and C) and
there is always equal proportions of A and T, and C and G. These are called
“Chargraff’s rules;” 3) Rosalind Franklin and Maurice Wilkins used X-ray
diffraction to take a photograph of DNA, showing it is a double helix of constant
diameter with bases stacked on top of each other.
2. In this chapter, you learned about gene cloning. Now, as you have likely heard in
the news, we can clone whole organisms – most recently dogs, cats, and mules.
Soon, we may even be able to clone human beings! Should cloning of pets be
allowed (for a cost of about $30,000 to the consumer)? What about humans?
What are the scientific arguments for and against cloning?
Explanation/Answer: Students should discuss the pros and cons of cloning.
They may bring in religious arguments against cloning. In the news, some of the
scientific arguments include the fact that because adult cells are used in cloning,
the clones may age faster due to DNA damage that is inherent in adult cells.
Also, clones are not necessarily the same as the “originals” because there are
effects of the environment, as well as genetics. Arguments for cloning include
satisfaction of pet owners, cloned plants that can improve agricultural output, and
cloned animals that may have high meat yield or low fat, etc.
3. In the same way that viruses were used to infect bacteria in the Hershey-Chase
experiments, we can use transgenic viruses to infect humans and help treat genetic
disorders. That is, the viruses are genetically modified to contain “normal”
human genes to try to replace functional human genes. Using this type of gene
therapy, a person is infected with a particular virus, which then delivers the
“normal” human gene to cells by infecting them. What are some pros and cons of
viral gene therapy?
Explanation/Answer: Pros include treatment of genetic disorders. Recently in
the news, viral gene therapy has been used to treat tumors that do not respond to
traditional chemotherapy or radiation treatments. Viruses can be used to deliver
“self destruct” genes to cancer cells. Cons of viral gene therapy include the fact
that you are infecting patients with a virus, which could have toxic side effects or
cause illness. Also, gene therapy is usually fairly short lived, mostly
experimental, and thus very expensive, making gene therapy only available to
wealthy people or those with the best health insurance.
4. Forensic genetics is now used in many court cases. While it is easy to prove
innocence using genetics (by identifying a mismatch at a single microsatellite locus), it is
more difficult to prove guilt. Suppose DNA found at a crime scene does not belong to
the victim and matches the DNA of a suspect. Is the suspect guilty? What information
would you need to prove guilt “beyond a reasonable doubt”?
Explanation/Answer: First, it would depend on how the DNA was collected.
For example, semen collected from a victim of a rape or skin under a victim’s
fingernails is much more compelling than DNA from fingerprints on a doorknob.
In addition, the prosecutor usually has to establish a motive for the murder. Also,
what if the suspect has a solid alibi that would not place them at the crime scene
at the time of the crime? DNA evidence alone is usually not sufficient to convict
someone of a crime, but, with other evidence, it can be quite compelling.