Cell Cycle — Free AP Biology Lesson

The idea, then worked examples solved step by step, then practice questions whose numbers change every time you try them.

The idea

The cell cycle: how a cell copies itself

Growth and repair run on one route: a cell grows, copies its contents, and splits in two. The cell cycle is an ordered series of events of cell growth and cell division that makes two new daughter cells.

Interphase: the long working and copying stretch

Interphase is the part of the cycle in which the cell grows and its DNA is copied. It has three stages, G1, S and G2. First misconception to drop: cells are not mostly busy dividing. In fast-growing human cells in culture the mitotic phase lasts about one-half hour of a cycle of about 24 hours; interphase takes the rest.

In the G1 phase the cell is metabolically active and busy: the G1 phase involves the duplication of organelles and cytosolic components.

The S phase is the copying stage. S stands for synthesis: in S phase, DNA is in the form of chromatin and the cell replicates its DNA, forming two sister chromatids connected at a centromere. Picture one photocopy of a book, still stapled to the original: the sister chromatids are identical copies, held together until the moment they must part.

The G2 phase is the final preparation. In G2 the cell makes proteins, makes ATP in large quantities, and its centrosomes replicate. The G2 phase involves protein synthesis, and the G2 phase involves the making of ATP in large quantities. Together, interphase involves cell growth and interphase involves DNA replication.

The cell cycle contains the sequential stages G1, S, G2, mitosis and cytokinesis. A checkpoint is a point in the eukaryotic cell cycle at which the cell's progress to the next stage can be halted until conditions are favorable; how that control works is the next topic.

G0: the exit ramp

Not every cell keeps cycling. The G0 phase is a quiescent stage that cells enter when they exit the cycle and are not actively preparing to divide. Mature cardiac muscle cells and nerve cells stay in G0. A cell in G0 can re-enter the cycle in response to appropriate cues, and an external signal can trigger the onset of G1 in cells that have entered G0. Nondividing cells may also be held at a particular stage in the cell cycle without proceeding to division.

Mitosis: one nucleus becomes two, exactly

The mitotic phase is the part of the cycle in which the copied DNA and the cytoplasm are separated and the cell divides. Its first part is mitosis itself (karyokinesis, division of the nucleus); its second is cytokinesis, the physical separation of the cytoplasm. Mitosis ensures the transfer of a complete genome from a parent cell to two genetically identical daughter cells. That exactness is the whole point: mitosis plays a role in growth, in tissue repair, and in asexual reproduction, and all three need faithful copies rather than new combinations.

Mitosis is divided into prophase, prometaphase, metaphase, anaphase and telophase. In prophase, the sister chromatids condense — coiling more tightly so that they become visible — the mitotic spindle begins to form, the centrosomes move to opposite poles of the cell, and the nuclear envelope breaks into small vesicles. The spindle is the apparatus that orchestrates the movement of chromosomes: its microtubules extend between the centrosomes like ropes strung between two posts.

In metaphase, spindle fibers align the chromosomes along the equator of the cell, the metaphase plate — the plane midway between the two poles. Before metaphase, in prometaphase, a kinetochore develops on each sister chromatid and spindle microtubules attach to the kinetochores. A kinetochore is a protein structure that each sister chromatid develops in the centromere region, to which spindle microtubules bind.

The picture shows a cell with 2n = 4 at metaphase: the duplicated chromosomes, each still two sister chromatids joined together, are lined up on the equator. In anaphase, the sister chromatids separate and are pulled toward opposite poles. In telophase, the chromosomes decondense, the mitotic spindle breaks down, and new nuclear envelopes form around the chromosomes.

Here the same cell is in anaphase: the sisters have parted. Each separated chromatid is now a chromosome, and each pole gets four — the same number the parent had.

Cytokinesis: pinching in animals, building in plants

Cytokinesis finishes the job, and it finishes differently in animals and in plants. Cytokinesis in animal cells involves a contractile ring composed of actin filaments, which pinches the cell in two, forming a cleavage furrow. In plant cells a cell plate forms, and cytokinesis in plant cells produces a new cell wall between the daughter cells. Cytokinesis in animal cells produces two new daughter cells, and cytokinesis in plant cells produces two new daughter cells.

Reading the clock: how long each stage takes

In fast-growing human cells in culture, the whole cycle lasts about 24 hours: G1 about nine hours, S about 10 hours, G2 about four and a half hours, and the mitotic phase about half an hour. If you count cells on a slide, the percent of cells seen in a stage, as a decimal, times the length of the cycle estimates the time spent in that stage. Because the mitotic phase lasts about one-half hour of about 24 hours in those cells, almost every cell you see is in interphase.

Warning.

Three traps to leave behind. First, cytokinesis is not a stage of mitosis; it is the second part of the mitotic phase, after the nucleus has divided. Second, two sister chromatids joined at one centromere count as one chromosome. Sister chromatids are the identical pair of DNA molecules made by replication in S phase, firmly attached at the centromere. Third, G0 is not a graveyard — mature nerve cells stay in G0, just not preparing to divide.

Adapted from Biology by OpenStax (Rice University), licensed under CC BY 4.0. Access for free at https://openstax.org/details/books/biology

Worked examples

Each example is the kind of question the AP exam asks on this topic, worked the way a teacher would talk it through.

Example.

1. Reading a slide. A student counts 400 cells in an onion root tip. She finds 368 in interphase, 18 in prophase, 6 in metaphase, 4 in anaphase and 4 in telophase. The cycle in these cells lasts about 20 hours. About how long does division take, and why are so few cells caught in it?

Read the script above: the dividing cells add to 32 of 400, a share of 0.08 of the cells.
The time in a stage is the percent of cells seen in it, as a decimal, times the length of the cycle: the script's 0.08 of 20 hours is about 1.6 hours for division.
So few cells are caught dividing because the mitotic phase is a small share of the clock. Interphase takes the rest — the script's last figure, about 18.4 hours of the 20.

Result.

About 1.6 hours in mitosis and cytokinesis on this count; the snapshot matches the clock.

Example.

2. Counting chromosomes, stage by stage. A species has 2n = 6. How many chromosomes, and how many DNA molecules, does one cell hold at metaphase of mitosis? What about at anaphase?

Read the counts above. At metaphase the cell holds 6 chromosomes and 12 DNA molecules: every chromosome has been copied, and each sister chromatid is one DNA molecule. Sister chromatids are the identical pair of DNA molecules made by replication in S phase, firmly attached at the centromere, so the chromosome count has not yet risen.
At anaphase there are 12 chromosomes and 12 DNA molecules in the one cell: the sisters have parted, and each chromatid, now called a chromosome, is counted on its own.
After cytokinesis each daughter is back to 6 chromosomes, one complete set each.

Result.

Metaphase: 6 chromosomes, 12 DNA molecules. Anaphase: 12 and 12, on the way to two cells of 6.

Example.

3. The skin cut. A shallow cut in your skin heals in days, covered by new skin cells identical to the ones lost. Which process made them, and what would go wrong if the copying stage were skipped?

Mitosis made them: it is used in tissue repair, and it passes a complete genome to two genetically identical daughter cells — exactly what replacement skin needs.
The copying stage is S phase, when DNA replicates to form two sister chromatids. Without the S phase, mitosis could not ensure the transfer of a complete genome from the parent cell to two genetically identical daughter cells.
Growth uses the same process for the same reason: more cells, each with the full set.

Result.

Mitosis, fed by S phase; without copying there is nothing complete to hand on.

Practice

Answer AP-style questions on cell cycle. Each answer is checked, with the full solution, and every try brings new numbers. Practice needs a free account: it keeps your progress.