Complex sentence: In 1973, Arthur Pardee demonstrated that a single restriction point exists in G 1 1 was defined simply as the time between mitosis and S-phase, and no place-markers for a cell's position in G 1 were known.
Split into simpler sentences: In 1973, Arthur Pardee demonstrated that a single restriction point exists in G 1. Previously, G 1 was defined simply as the time between mitosis and S-phase, and no place-markers for a cell's position in G 1 were known.