AP BiologyUnit 4: Cell Communication and Cell Cycle4.5
AP Biology · Unit 4 · Topic 4.5

Cell Cycle

Play it as a 3-minute lesson → short cards, one idea at a time

The lesson3 min read

S phase doubles DNA, not chromosome number. More cells caught in a phase means a longer phase.

The cell cycle is copy, then split. In interphase the cell grows and replicates its DNA. Mitosis sends one copy of every chromosome to each of two nuclei, and cytokinesis divides the cytoplasm. You get two genetically identical cells.

Key terms

  • Interphase — G1, S, and G2 together. The cell grows, does its job, and copies its DNA. It fills most of the cycle.
  • Sister chromatids — the two identical copies of a chromosome made in S phase. They stay joined until anaphase.
  • Mitosis — division of the nucleus. Each new nucleus gets the same chromosomes as the parent cell.
  • Cytokinesis — division of the cytoplasm, which finishes making two separate cells.
  • G0 — a non-dividing state outside the cycle. Mature nerve cells stay here.
  • Mitotic index — the share of cells in a sample caught in mitosis. A high index marks a zone of active division.

How it works

  1. G1: the cell grows and works. At the G1 checkpoint it commits to dividing only if growth signals and conditions say go. If not, it can exit to G0.
  2. S: DNA replication. Each chromosome becomes two identical sister chromatids. The DNA doubles; the chromosome count does not.
  3. G2: the cell keeps growing and gets ready to divide. Mitotic cyclin builds up and switches on the kinase that starts mitosis.
  4. Prophase: chromosomes condense, the spindle forms, and the nuclear envelope breaks down. Metaphase: chromosomes line up across the middle of the cell.
  5. Anaphase: sister chromatids separate and move to opposite poles. Telophase: a nuclear envelope re-forms around each group, and the chromosomes loosen.
  6. Cytokinesis, usually overlapping telophase, splits the cytoplasm. Animal cells pinch in with a ring of protein filaments. Plant cells build a cell plate that fuses with the wall.

Why it matters

Mitosis makes genetically identical cells for growth, repair, and asexual reproduction, and it keeps the chromosome number constant. Meiosis, in the next unit, is the division that halves it. Timing matters too. DNA damaged while it is being copied in S phase can be written into the new strand before repair reaches it. The error becomes a mutation the daughter cells inherit.

Exam traps

TrapActually
Interphase is a resting phase between divisions.It's working time. The cell grows, does its job, and copies its DNA. Most cells on a root-tip slide are in it.
After S phase, a human cell has 92 chromosomes.It still has 46, each made of two sister chromatids. Chromatids count as separate chromosomes only once anaphase splits them.
Mitosis splits the cell in two.Mitosis splits the nucleus. Cytokinesis splits the cytoplasm. In a plant cell, the split is complete only when the cell plate joins the wall.
A cyclin that never breaks down keeps a cell dividing faster.Cyclin has to rise and then be destroyed. A mitotic cyclin that can't be destroyed locks the cell in mitosis.
Worked example

In the growing zone of a root, cells complete one cycle about every 20 hours, and they are not in step with each other. A student scores 400 cells: 352 in interphase, 28 in prophase, 8 in metaphase, 4 in anaphase, and 8 in telophase. About how long does mitosis last, and which stage of mitosis takes longest?

  1. The cells are out of step, so a snapshot works like a clock. The share of cells in a stage equals the share of the cycle spent in it.
  2. Add the mitotic cells: 28 + 8 + 4 + 8 = 48. That's 48 of 400, or 12% of the cells.
  3. 12% of 20 hours is 2.4 hours. Prophase holds the most mitotic cells, 28 of 400 or 7%, so it lasts about 1.4 hours.

Mitosis lasts about 2.4 hours. Prophase is its longest stage, at about 1.4 hours.

A stage that lasts longer catches more cells in any snapshot. The near-miss is 48 ÷ 352, which leaves the interphase cells out of the total and gives about 2.7 hours. Every scored cell is part of the clock, so divide by all 400.

Now try it on real exam-style questions

Ten questions on Cell Cycle, each with a ten-second explanation. Free, no account.

Three questions to check yourself

Question 1 · 4.5 Cell Cycle

What is the main activity of a cell during G1 of the cell cycle?

  1. A It grows and performs its usual functions
  2. B It makes a copy of each of its chromosomes
  3. C It condenses its chromosomes for division
  4. D It pulls its sister chromatids apart
Show the answer

A — During G1 the cell grows, makes proteins and organelles, and carries out its normal job in the body. Copying the chromosomes is tempting, but DNA replication happens later, in S phase.

Question 2 · 4.5 Cell Cycle

A cell that has exited the cell cycle and stopped preparing to divide is in which phase?

  1. A G1 phase
  2. B S phase
  3. C G2 phase
  4. D G0 phase
Show the answer

D — Cells that leave the cycle, such as most mature nerve and muscle cells, enter a nondividing state called G0, where they keep working but make no preparations to divide. G1 is the tempting wrong answer because G0 branches off from it, but a G1 cell is still moving toward S phase.

Question 3 · 4.5 Cell Cycle

Which sequence lists the stages of mitosis in the order they occur?

  1. A Prophase, anaphase, metaphase, telophase
  2. B Metaphase, prophase, anaphase, telophase
  3. C Prophase, metaphase, anaphase, telophase
  4. D Prophase, metaphase, telophase, anaphase
Show the answer

C — Chromosomes condense in prophase, line up at the cell's middle in metaphase, separate in anaphase, and reach the poles as new nuclei form in telophase. Swapping metaphase and anaphase is the common slip, but chromatids must line up before they can be pulled apart.