Final answer:
Thermal stress on a polytene chromosome can lead to increased depurination and possibly induce protective structural changes such as further condensation to prevent DNA damage.
Step-by-step explanation:
If a polytene chromosome is exposed to a thermal stress, several events can occur on a molecular level. One such event is an increase in the rate of depurination, which is the loss of purine bases from the DNA backbone. This type of damage is seen in thermophilic bacteria that grow at high temperatures of 85-250°C. Given that the rate of depurination in these organisms can be high, it implies that these cells might have evolved adaptive mechanisms to deal with such stress. Another potential event could involve changes in the chromosome structure; for example, chromosomes may undergo further condensation to protect the DNA during stress, which is similar to what happens during cell division to prevent damage as the chromosomes are moved to opposite poles of the cell.