You started as one cell. You now have trillions, and many of them are still dividing to grow and to replace worn-out cells. Each of those cells follows the same repeating program, the cell cycle, built around the chromosomes it must copy and share out.
Key idea
A cell spends most of its life in interphase: G1 (growth), S (DNA synthesis: copying the chromosomes) and G2 (more growth and preparation). Then it divides in the mitotic (M) phase: mitosis and cytokinesis. Checkpoints decide whether it goes on.
- G1: the cell grows, makes proteins and organelles, and does its normal job.
- S: every chromosome is copied, so each becomes two sister chromatids.
- G2: the cell grows more, makes the proteins needed for division, and checks the copied DNA.
- M: mitosis divides the nucleus; cytokinesis divides the cytoplasm.
- G0: many cells, such as most nerve cells, leave the cycle and stop dividing, sometimes for good.
Why it works
Interphase is when a cell does its real work, and it is also the preparation for division: a cell must double its DNA and roughly double its contents before it can split into two complete cells. Because each phase takes a fixed share of the cycle, you can estimate how long each lasts by counting. In a slide of dividing cells, the fraction of cells in a phase is about the fraction of time the cycle spends in it.
Worked example: Time from a count
In an onion root tip, 180 of 200 cells are in interphase and 20 are in mitosis. If the whole cycle takes 24 hours, about how long is mitosis?
- Fraction in mitosis: $20 \div 200 = 0.10$.
- Time: $0.10 \times 24 = 2.4$ hours.
Answer: About $2.4$ hours
Worked example: Before and after S
A cell with $2n = 6$ goes from G1 through S into G2. What happens to its chromosomes and chromatids?
- In G1: 6 chromosomes, each a single chromatid: 6 chromatids.
- S phase copies each one.
- In G2: still 6 chromosomes (6 centromeres), now 12 chromatids.
Answer: 6 chromosomes throughout; chromatids go from 6 to 12.
Common mistake
Thinking interphase is a resting phase. The cell is busy growing, working and copying its DNA. It is only "resting" from division, and the S phase in the middle of it is one of the most important steps of the whole cycle.