Question 108·Hard·Command of Evidence
DNA Lesion Counts in Zebrafish Embryos 24 Hours after UV Exposure
| Experimental group | Visible light during recovery? | Mean DNA lesions (per bases) |
|---|---|---|
| Control (no injection) | no | 120 |
| Control (no injection) | yes | 70 |
| Photolyase injected | no | 60 |
| Photolyase injected | yes | 30 |
Molecular biologists investigating DNA repair mechanisms injected half of a group of UV-exposed zebrafish embryos with photolyase, an enzyme that uses light energy to repair damaged DNA. All embryos were then allowed to recover either in complete darkness or under normal visible light. The researchers concluded that photolyase can enhance DNA repair even in the absence of visible light.
Which choice best describes data from the table that support the researchers’ conclusion?
For SAT Reading & Writing questions that ask which data best support a researcher’s conclusion, first restate the conclusion in your own words and underline any limiting phrases (like “even in the absence of visible light”). Then, scan the table for the rows that match those exact conditions and compare only the groups that differ in the key variable (here, injected vs. not injected). Finally, choose the option that most directly summarizes that specific comparison without adding extra claims.
Hints
Focus on the key phrase in the conclusion
Underline the words “even in the absence of visible light” in the conclusion. Which rows in the table correspond to embryos recovering without visible light?
Narrow down to comparable groups
Among the rows with no visible light, find the two groups that differ only in whether they were injected with photolyase. What are their mean DNA lesion counts?
Connect the data back to the claim
Ask yourself: Do the data for those two dark-recovery groups show that the photolyase-injected embryos had better DNA repair (fewer lesions) than the uninjected ones? Which option summarizes that relationship?
Step-by-step Explanation
Identify what the conclusion is really claiming
The conclusion says that photolyase can enhance DNA repair even in the absence of visible light. This means we need evidence that the enzyme helps repair DNA when there is no visible light, not just when light is present. So we should focus on data from embryos recovering in darkness and see whether photolyase makes a difference there.
Find and compare the relevant groups in the table
Look only at the rows where Visible light during recovery? is "no" (darkness):
- Control (no injection), no light: 120 DNA lesions
- Photolyase injected, no light: 60 DNA lesions
These two groups are treated the same way except for the photolyase injection, so any difference in DNA lesions between them is evidence of the effect of photolyase without visible light.
Interpret the comparison and match it to an answer choice
In darkness, the control embryos have 120 lesions, while the photolyase-injected embryos have 60 lesions. Since is half of , the injected embryos in darkness have half as many DNA lesions as the uninjected embryos in darkness. This directly supports the idea that photolyase enhances DNA repair even when no visible light is present. The answer choice that states this is: “The photolyase-injected embryos kept in darkness had half as many DNA lesions as the uninjected embryos kept in darkness.”