안녕하세요~

 제가 그렇게 말했다면 
The first cell is in mitosis, which means it has already passed the G1 checkpoint. 이라는 의미였을거고

학생말대로 더 정확히 말하자면, 

The first cell is in mitosis, which means it has already passed the G1 checkpoint, S phase, and G2 checkpoint

입니다 :)
G1 check point을 이제 막 지나자마자 들어오는 phase은 s phase가 맞습니다 :)