Educerie · AP · Biology
AP Biology 4.5 Cell Cycle
In one breath
- The eukaryotic cell cycle is interphase (G1, S, G2), then mitosis, then cytokinesis.
- In G1 the cell grows and duplicates organelles and cytosol; in S each chromosome's DNA, as chromatin, is replicated to give two sister chromatids joined at a centromere, doubling the DNA but not the chromosome number; in G2 the cell makes proteins and plenty of ATP and duplicates its centrosome.
- A cell can leave the cycle into G0 and sometimes return.
- Mitosis gives two identical nuclei: in prophase chromosomes condense, the spindle forms and centrosomes move to the poles; in metaphase spindle fibres line the chromosomes up at the equator; in anaphase sister chromatids separate and are pulled to opposite poles; in telophase the spindle breaks down and new nuclear envelopes form.
- Cytokinesis follows: a cleavage furrow in animal cells, a cell plate in plant cells.
- Mitosis serves growth, tissue repair and asexual reproduction.
- From counts: mitotic index is the percentage of cells in mitosis, and the fraction of cells in a phase times the cycle length gives the time spent in it.
What you must be able to do
- Describe the stages of the cell cycle in order: G1, S, G2, mitosis, cytokinesisOrder the stages; name the stage where DNA is copied
- Describe what happens in each stage of interphase, and the G0 state"In which phase does this cell spend most of its time?" A cell that has stopped dividing
- Explain how mitosis gives two daughter cells with identical, complete genomesDescribe each phase; explain why daughter cells are identical
- Describe cytokinesis in animal and plant cellsCleavage furrow against cell plate
- State the roles of mitosis: growth, tissue repair, asexual reproduction"Explain why a wound heals with cells identical to those around it"
- Describe and calculate from cell-cycle dataMitotic index, time in a phase, percent change, DNA-content graphs
Once you have read the note, tick off each one you can do in Check your understanding.
In the exam. Section I multiple choice: order the phases; identify a phase from a description, a micrograph-style drawing, or a graph of DNA content per cell; and calculate from cell counts (the percentage of cells dividing, the time spent in a phase). In Section II it supplies Analyze Data and Interpreting and Evaluating Experimental Results questions with counts from root tips or cell cultures (means, percentages, percent change), Scientific Investigation (design a test of a chemical's effect on mitosis), and the comparison of mitosis with meiosis that Unit 5 completes.
1Interphase: growing and copying
Figure 1 shows the phases to scale for a cell that divides once every 24 hours. Mitosis and cytokinesis together are about an hour. Everything else is interphase. Interphase is not a resting stage: the cell is working hard, preparing to divide.
G1 (first gap). The cell is metabolically active: it grows, makes proteins, and duplicates its organelles and the contents of its cytosol, so that there will be enough for two cells. G1 is the most variable phase: in fast-dividing cells it is short; in slow ones it is long.
S (synthesis). The DNA, loosely packed as chromatin, is replicated. Each chromosome, one long DNA molecule, becomes two identical DNA molecules, the two sister chromatids, held together at the centromere. The number of chromosomes does not change (two joined sister chromatids still count as one chromosome), but the amount of DNA in the nucleus doubles.
G2 (second gap). The cell makes the proteins needed for division, such as those of the spindle, produces ATP in large quantities to power division, and its centrosome, the structure that organises microtubules in animal cells, is duplicated so that there are two, one for each pole of the spindle.
G0: leaving the cycle. Not every cell keeps dividing. A cell can leave the cycle from G1 and enter G0, a non-dividing state in which it carries out its function. Most mature nerve cells and heart muscle cells stay in G0 for life. Liver cells sit in G0 but can re-enter the cycle when the right signals arrive, for example after part of the liver is damaged. Cells can also be held at a particular stage until conditions are right (topic 4.6).
2Mitosis: sharing out the copies
Mitosis is the division of the nucleus that gives each daughter nucleus one complete copy of the genome. It is continuous, but it is described in four steps. Step through Figure 2, drawn for a cell with four chromosomes ().
Prophase. The replicated chromosomes condense, coiling tightly so that each becomes short, thick and visible as two sister chromatids joined at a centromere. The mitotic spindle, made of microtubules, begins to form, and the two centrosomes move to opposite poles of the cell. The nuclear envelope breaks down, so the spindle can reach the chromosomes.
Metaphase. Spindle fibres attach at each centromere, one from each pole, and line the chromosomes up along the equator of the cell, the metaphase plate. Each chromosome is now pulled equally toward both poles.
Anaphase. The sister chromatids of every chromosome separate at the same moment, and the spindle fibres shorten, pulling one chromatid of each pair toward each pole, centromere first. Once separated, each chromatid counts as a chromosome in its own right.
Telophase. An identical set of chromosomes has arrived at each pole. The spindle breaks down, a new nuclear envelope forms around each set, and the chromosomes begin to uncoil. Cytokinesis usually begins as telophase ends.
Why are the two daughter nuclei identical? Because S phase made two exact copies of every DNA molecule, and anaphase sends exactly one copy of each chromosome to each pole. Nothing is shuffled and nothing is left out.
Mitosis keeps the chromosome number the same: S phase doubles the DNA, anaphase halves it, and each daughter cell gets one copy of every chromosome.
3Cytokinesis: two cells from one
Cytokinesis divides the cytoplasm. It is different in animals and plants because plant cells have a wall.
- Animal cells. A ring of protein filaments just inside the membrane at the equator contracts, pinching the membrane inwards as a cleavage furrow until the cell is split in two.
- Plant cells. A rigid wall cannot be pinched. Vesicles carrying wall material gather at the equator and fuse, forming a cell plate that grows outward until it joins the existing wall, and becomes the new wall between the two cells.
4What mitosis is for
Because it produces genetically identical cells, mitosis is used for:
- growth: a fertilised egg becomes a body of trillions of cells by repeated mitosis;
- tissue repair: cells around a wound divide to replace lost cells with identical ones;
- asexual reproduction: many single-celled eukaryotes, and plants growing from cuttings or runners, produce offspring identical to the parent.
5Data from the cell cycle
The CED pairs this topic with describing data and calculations. Three kinds come up.
DNA content per cell. If you plot the amount of DNA in a cell against time, it stays level in G1, doubles gradually during S, stays at the doubled level through G2 and mitosis, and halves at cytokinesis. A rising section of such a graph is always S phase.
Mitotic index. In a squashed, stained root tip, count the cells in each phase in a field of view. The mitotic index is the percentage of cells in mitosis.
An invented count of 200 cells from an onion root tip:
| Phase | Interphase | Prophase | Metaphase | Anaphase | Telophase |
|---|---|---|---|---|---|
| Number of cells | 172 | 14 | 6 | 3 | 5 |
Time in a phase. If the cells are dividing at random, not in step, the fraction of cells in a phase equals the fraction of the cycle spent in it. With a cycle of 20 hours, time in mitosis is
and time in metaphase is .
To compare a treatment with a control, use percent change: new − original ÷ original × 100. Counts from several fields of view should be averaged as a mean, and more fields make the estimate more reliable.
Practice questions
8 questions on this note, from easy to hard. Each one you get right lights a star of this topic's constellation.