A typical mammalian cell completes its 24-hour cell cycle. Approximately how long does the cell spend in mitosis (M phase)?
A12 hours
B6 hours
C1–2 hours
Dless than 10 minutes
Most of the cell cycle is interphase (G1, S, G2), which accounts for roughly 22–23 hours. Mitosis itself — the dramatic condensation, segregation, and cytokinesis — takes only about 1–2 hours. Students often assume the 'important' part (division) must take the most of the time, but interphase is where the bulk of preparation occurs.
Question 2 True / False
A cell in G0 is permanently inactive and no longer carries out metabolic functions.
TTrue
FFalse
Answer: False
G0 is a quiescent state, not cellular death. Neurons and muscle cells, for example, exit the cycle into G0 but remain fully metabolically active — synthesizing proteins, responding to signals, and performing specialized functions. G0 simply means the cell is not currently progressing through the division cycle.
Question 3 Short Answer
What is the purpose of cell cycle checkpoints, and what happens if they fail?
Think about your answer, then reveal below.
Model answer: Checkpoints are quality-control gates that verify each phase has completed correctly before the cell proceeds to the next. The G1/S checkpoint ensures the cell is large enough and DNA is undamaged; the G2/M checkpoint confirms DNA replication is complete; the spindle assembly checkpoint verifies all chromosomes are attached to the spindle. Checkpoint failure allows cells with damaged or incompletely replicated DNA to divide, producing daughter cells with mutations — a mechanism that can drive cancer.
Understanding checkpoints as surveillance mechanisms — not just timers — is key. Each checkpoint monitors a specific condition (DNA integrity, replication completion, chromosome attachment) and halts the cycle until that condition is met. The link to cancer is direct: many cancer-driving mutations disable checkpoint proteins like p53.