AP® Biology review sheet from Aim for Five (aimforfive.com/bio/units/4/4-5)
Unit 4 · Topic 4.5
4.5 Cell Cycle
The cell cycle is the ordered series of events in which a eukaryotic cell grows, copies its DNA and divides. Interphase (G₁, S, G₂) prepares the cell, mitosis separates the copied chromosomes into two identical sets, and cytokinesis splits the cell in two. Mitosis is how you grow, how you heal, and how some organisms reproduce asexually.
Key terms
- interphase
- S phase
- G₀
- mitosis
- sister chromatids
- cytokinesis
Interphase: getting ready
Most of a cell's life is spent in interphase, the part of the cycle between divisions. The cell is not resting; it's busy with three stages.
- G₁ (first gap): the cell grows, is metabolically active, makes proteins and copies its organelles and other cytoplasm components.
- S (synthesis): the DNA, which is in its loose, spread-out form called chromatin, is replicated. Each chromosome becomes two identical sister chromatids joined at a region called the centromere.
- G₂ (second gap): the cell keeps growing, makes the proteins it needs for division and produces lots of ATP. The centrosome, which organizes the spindle, finishes copying itself, so by the end of G₂ the cell has two centrosomes ready to move apart.
Leaving the cycle: G₀
Not every cell keeps dividing. A cell can exit the cycle into G₀, a non-dividing state. Many mature nerve and muscle cells stay in G₀ for a very long time. Other cells, such as liver cells, can leave G₀ and reenter the cycle when they get the right signals, for example to repair an injury. Cells can also be held at a particular stage until conditions are right (4.6).
Mitosis: dividing the chromosomes
Mitosis separates the sister chromatids so each new cell gets one complete, identical copy of the genome. It runs in four phases, and it alternates with interphase.
Some textbooks treat the end of prophase, when the nuclear envelope breaks down and spindle fibers attach to the chromosomes, as a separate phase called prometaphase. The AP course uses the four phases in the table.
| Phase | What happens | How it looks under a microscope |
|---|---|---|
| Prophase | Chromatin condenses into visible chromosomes, each with two sister chromatids. The spindle (a set of protein fibers called microtubules) starts to form, and the centrosomes move toward opposite poles. The nuclear envelope breaks down. | Thick, visible chromosomes; no clear nucleus |
| Metaphase | Spindle fibers attached to each chromosome line the chromosomes up along the middle of the cell. | Chromosomes in a single line across the center |
| Anaphase | Sister chromatids separate and are pulled toward opposite poles. Each chromatid is now counted as its own chromosome. | Two V-shaped groups moving apart |
| Telophase | Chromosomes reach the poles and begin to unwind, a new nuclear envelope forms around each set, and the spindle breaks down. | Two clusters, with nuclei re-forming at each end |
Cytokinesis and why mitosis matters
Cytokinesis divides the cytoplasm, usually starting during late anaphase or telophase. In animal cells, a ring of proteins pinches the cell in two, forming a cleavage furrow. In plant cells, which have rigid walls, vesicles gather in the middle and fuse into a cell plate that becomes a new wall between the two cells.
The result is two genetically identical daughter cells, each with the same number of chromosomes as the parent. Mitosis is used for growth (from one fertilized egg to a whole body), tissue repair (healing skin or bone), and asexual reproduction in single-celled eukaryotes like yeast and in many plants.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Counting chromosomes and DNA copies
A human body cell has 46 chromosomes. How many chromosomes and how many DNA molecules (each double-stranded copy of DNA counts as one) are in the cell (a) in G₁, (b) in G₂, (c) in metaphase, (d) in anaphase, and (e) in each daughter cell after cytokinesis? At which of these stages are there sister chromatids?
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- Step 1: Count chromosomes by counting centromeres. Count DNA molecules separately: before S phase, each chromosome is one DNA molecule; after S phase, each chromosome is two sister chromatids, and each chromatid is its own DNA molecule.
- Step 2: (a) G₁: 46 chromosomes and 46 DNA molecules. Nothing has been copied yet, so there are no sister chromatids.
- Step 3: (b) G₂: the DNA has been copied, but each pair of sister chromatids is still joined at one centromere. So there are still 46 chromosomes, now with 92 DNA molecules (92 sister chromatids).
- Step 4: (c) Metaphase: same as G₂: 46 chromosomes and 92 DNA molecules, lined up in the middle.
- Step 5: (d) Anaphase: the sister chromatids separate, and each one now counts as its own chromosome. The cell briefly has 92 chromosomes and 92 DNA molecules, and no sister chromatids remain.
- Step 6: (e) Each daughter cell: 46 chromosomes and 46 DNA molecules, the same as the G₁ cell.
Answer: (a) 46 chromosomes, 46 DNA molecules; (b) 46 and 92; (c) 46 and 92; (d) 92 and 92; (e) 46 and 46. Sister chromatids are present only in G₂ and metaphase here (from S phase until they separate in anaphase).
- Example 2Calculator allowed
Mitotic index and phase timing
A student counts 200 cells in an onion root tip: 160 in interphase, 20 in prophase, 8 in metaphase, 6 in anaphase and 6 in telophase. The cell cycle in these cells takes about 24 hours. Calculate the mitotic index, then estimate how long metaphase lasts.
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- Step 1: Mitotic index = cells in mitosis ÷ total cells. Cells in mitosis = 20 + 8 + 6 + 6 = 40. So 40 ÷ 200 = 0.20, or 20%.
- Step 2: Assume the cells are dividing at random times, so the fraction of cells in a phase matches the fraction of the cycle spent in that phase.
- Step 3: Metaphase fraction = 8 ÷ 200 = 0.04.
- Step 4: Time in metaphase ≈ 0.04 × 24 h = 0.96 h ≈ 58 minutes.
Answer: Mitotic index = 20%. Metaphase lasts about 0.96 hours (about 58 minutes).
Common mistakes
- Saying DNA is copied during mitosis. DNA replication happens in S phase of interphase; mitosis only separates the copies.
- Counting chromatids as chromosomes in G₂ or metaphase. Count centromeres: 46 chromosomes, each with two sister chromatids.
- Calling interphase a resting phase. The cell is growing, copying organelles and replicating DNA.
- Swapping cleavage furrow and cell plate. Animals form a cleavage furrow; plants build a cell plate.
On the exam
- Expect diagrams or micrographs where you identify a mitotic phase, or questions that ask you to count chromosomes, chromatids or DNA content at a given stage.
- Graphs of DNA amount per cell over time are common: DNA doubles during S phase and halves at cytokinesis.
Connected topics
Videos
Check yourself
4 questions on 4.5 Cell Cycle. Pick an answer to see if you got it, and why.
Which sequence correctly orders these events in a dividing animal cell? (1) Sister chromatids separate and move to opposite poles. (2) Chromosomes condense and become visible. (3) Nuclear envelopes form around two sets of chromosomes. (4) Chromosomes line up along the middle of the cell.
| Relative DNA content per cell | Percent of cells |
|---|---|
| 2 units | 58 |
| Between 2 and 4 units | 27 |
| 4 units | 15 |
Experimental data: the DNA content of 10,000 cells from a population of actively dividing cultured cells was measured.
Cells with between 2 and 4 units of DNA are most likely in which phase of the cell cycle?
A drug that prevents DNA from being copied is added to the culture. After 24 hours, which change in the data would be expected?
In an onion root tip, 90 percent of the cells are in interphase and 10 percent are dividing. If the whole cell cycle takes 24 hours, about how long do the cells spend in interphase?
0 of 4 answered