Final answer:
The first transition point in yeast cells for entering the S phase is just before the end of the G1 phase, known as the restriction point, determined by the G1 checkpoint.
Step-by-step explanation:
The first transition point in yeast cells that commits to entering the S phase occurs just before the end of the G1 phase and is called the restriction point. The G1 checkpoint is crucial as it determines whether all conditions are favorable for cell division to proceed. Growth factors, cell size, energy reserves, and checking for DNA damage are all part of this checkpoint. A cell that does not meet the requirements at the G1 checkpoint will not proceed to the S phase, where DNA replication occurs. Instead, it may either try to remedy any issues or enter a quiescent state (G0) until conditions improve.