CHAPTER 16 DEVELOPMENT, STEM CELLS, AND CANCER
Scientific Skills Exercise
Teaching objective: Students will compare quantitative data with spatial data related to gene
expression during animal development.
Teaching tips: A version of this Scientific Skills Exercise can be assigned in MasteringBiology.
The two approaches to studying gene expression shown in this exercise complement each other.
The mRNA measurements (done by quantitative reverse-transcription PCR in this case) are
quantitative but have low spatial resolution because the entire digit-forming area is dissected out
and used for a single analysis. The in situ hybridizations are advantageous for showing spatial
impress upon students the value of carefully observing subtle aspects of the patterns of mRNA
distribution in the paws. Note that the control embryo shows fairly even dark staining across the
whole digit-forming area of the paw and has the highest amount of mRNA, while deleting all
three possible regulatory segments (A, B, and C) eliminates most of the mRNA and staining,
again in a fairly even pattern. The other two deletions, however, show intermediate levels of
mRNA but an uneven pattern of expression. When C is eliminated there is one patch of lighter
staining near the thumb region that accounts for most of the reduction in overall mRNA level.
If you want to challenge your students, you can ask them what type of control element they
conclude might be present in region C. You can help them think through the scientific logic to
arrive at the idea that there must be one or more control elements that specifically up-regulate
expression in the pre-thumb area. (This is, in fact, what the researchers have concluded.)
Answers:
1. (a) The top sample, with all three DNA regulatory segments and a 100% level of mRNA, was
Suggested Answers for End-of-Chapter Essay Questions
See the general information on grading short-answer essays and a suggested rubric at the
beginning of this document.
6. Scientific Inquiry
Hypothesis: Estrogen is activating genes normally controlled by testosterone and other
7. Focus on Evolution
Because of a series of genetic changes such as mutations in the ras or p53 genes, cancer cells
evade the normal cell cycle controls and are able to divide in an unregulated fashion. During the
8. Focus on Organization
Sample key points:
Apoptosis is essential to embryonic development and the destruction of damaged cells in the
adult.
Sample top-scoring answer:
Apoptosis plays a crucial role in embryonic development, including formation of the nervous
system and the separation of digits. It is also part of the removal of damaged cells in the adult.
-in cell suicide program. Timely and
9. Synthesize Your Knowledge
Plant stem cells would give rise to plant tissues (such as grape vines and grapes, in this case)
under the appropriate conditions. The advertisement is claiming somehow that rubbing grape