SAN JOSE, Calif.—A chemist who worked at Theranos Inc. testified Friday about the scientific failures that plagued the defunct blood-testing startup, as the high-stakes fraud case wrapped up a second week of witness testimony in a federal courthouse.

Surekha Gangakhedkar, a senior scientist at the company for eight years who quit in 2013 because of her concerns about the blood-testing technology, described a workplace where employees were kept in silos and Elizabeth Holmes, company founder and defendant in the case, pressured employees to validate lab tests before they were ready to be used on patients.