Final answer:
In the sec12 yeast mutant, TEM would show accumulation of vesicles near the RER and possibly an enlarged RER due to defective vesicle budding impeding protein transport to the Golgi.
Step-by-step explanation:
In the sec12 yeast mutant where vesicle budding is affected, and vesicles are unable to travel from the RER to the Golgi, the appearance of the cells as observed by transmission electron microscopy (TEM) would likely show an accumulation of vesicles near the RER. This build-up occurs because the proteins synthesized and modified in the RER, which are typically transported via vesicles to the Golgi for further processing and sorting, get stalled due to the budding process being defective
Consequently, the RER might appear swollen or enlarged compared to wild-type cells.