When does a scientist become a scientist? It is not when a person knows many facts and even understands in depth some aspect of the natural world. I would suggest the transition occurs when curiosity about a phenomenon leads to an inquiry for new knowledge. This can occur in a person with little or lots of knowledge about a subject. It is an attitude of inquiry.
A substantive undergraduate research experience should be part of every chemistry or biochemistry student's degree program. This is just as true for a student who plans a career in pharmaceutical sales, high school teaching, or medicine as for a student determined to be a PhD research scientist.
Laboratory research is qualitatively different from typical course-associated laboratories where lab manuals define procedures, TAs supply the reagents, and the results are no surprise. Research requires planning, time-management skills, attention to detail, persistence, and an attitude of inquiry. These aspects must be experienced to be appreciated.
Ideally undergraduate research should start before the senior year. Winter or Summer Sessions are good times to begin because they provide blocks of time for learning techniques and becoming familiar with the laboratory. Supervision often is more available at these times.
Because I have cut back my biochemistry research program and increased my activities in other areas, I am unable to supervise more than than a few dedicated and self-motivated students at a time.