The Cell Cycle and Mitosis

Core Concepts

Chromosome organization

In eukaryotic cells, DNA is organized into chromosomes — structures of DNA and associated proteins (histones). The complex of DNA and proteins is called chromatin.

• Before DNA replication, each chromosome is a single, long chromatin fiber.

• After DNA replication (S phase), each chromosome consists of two identical copies called sister chromatids, held together at a region called the centromere.

• Sister chromatids are identical — they are the product of DNA replication. One will be distributed to each daughter cell during mitosis.

• The number of chromosomes in a cell varies by species. Humans have 46 chromosomes (23 pairs). The number of chromosomes is NOT a measure of organism complexity.

The cell cycle: Interphase (~90%): G1 (growth, normal function; may exit to G0), S (DNA replication → sister chromatids), G2 (preparation for division). M phase: Mitosis (nuclear division: prophase → prometaphase → metaphase → anaphase → telophase) + cytokinesis (cytoplasmic division).

Mitosis stages

Prophase

• Chromatin fibers condense into visible chromosomes (each consisting of two sister chromatids joined at the centromere).

• The mitotic spindle begins to form — microtubules grow from the centrosomes, which move to opposite poles of the cell.

• In the cytoplasm, the nuclear envelope is still intact.

Prometaphase

• The nuclear envelope breaks down.

• Microtubules from the spindle attach to the kinetochores — protein complexes at the centromere of each sister chromatid.

• Chromosomes begin to move.

Metaphase

• Chromosomes align at the metaphase plate — an imaginary plane equidistant from the two spindle poles.

• Each chromosome is attached to microtubules from both poles (sister chromatids are attached to opposite poles).

• The alignment ensures that each daughter cell will receive one copy of each chromosome.

Anaphase

• The centromeres split, and sister chromatids separate, becoming individual chromosomes.

• Motor proteins pull the chromosomes toward opposite poles along the microtubules.

• The cell elongates as non-kinetochore microtubules push against each other.

Telophase

• Chromosomes arrive at the poles and begin to decondense (return to chromatin).

• Nuclear envelopes reform around each set of chromosomes.

• The mitotic spindle disassembles.

• Mitosis — division of the nucleus — is complete.

Cytokinesis

Cytokinesis divides the cytoplasm:

• Animal cells: A cleavage furrow forms — a ring of actin microfilaments contracts, pinching the cell in two.

• Plant cells: A cell plate forms in the middle of the cell. Vesicles from the Golgi apparatus deliver materials to build a new cell wall between the daughter cells.

Mitosis produces two genetically identical diploid daughter cells. Used for growth, repair, and asexual reproduction.

Cell-cycle checkpoints

Checkpoints are control points in the cell cycle where the cell assesses whether conditions are favorable for proceeding to the next phase. The major checkpoints:

• G1 checkpoint (restriction point): Is the cell large enough? Is the environment favorable? Is the DNA undamaged? If conditions are not met, the cell may exit into G0.

• G2 checkpoint: Has DNA replication been completed? Is the DNA undamaged? Is the cell large enough?

• M checkpoint (spindle checkpoint): Are all chromosomes properly attached to the spindle? This occurs during metaphase and ensures that sister chromatids will separate correctly.

The cell cycle is regulated by proteins called cyclins and cyclin-dependent kinases (CDKs). Cyclin concentrations fluctuate throughout the cycle, activating CDKs at the appropriate times. Different cyclin-CDK complexes trigger different cell-cycle events.

Cancer: Uncontrolled cell division from mutations in proto-oncogenes (→ oncogenes, stuck accelerator) and tumor-suppressor genes (→ disabled brakes; p53 mutated in >50% of cancers). Cancer cells ignore density-dependent inhibition and anchorage dependence, escape cell-cycle controls, and may divide indefinitely. Tumors: benign (local) or malignant (invasive, may metastasize).

Binary fission: Prokaryotic division — chromosome replicates, copies separate, cell pinches in two. Faster than mitosis (some bacteria divide every 20 minutes under optimal conditions). Result: two genetically identical daughter cells.

ELI Example

Mitosis: photocopy every card (S phase), paper-clip copies to originals (sister chromatids), remove clips (anaphase), deal one complete set to each side, cut the table in half (cytokinesis). Two identical decks. Cancer: the copy machine runs out of control, ignoring stop signals.

Do Not Confuse

Lab Link

Mitosis is most commonly observed in onion root tips or whitefish blastulae. The root tip is a region of rapid cell division, so many cells will be in various stages of mitosis. Students identify and count cells in each stage, and the relative frequency of each stage reflects the duration of that stage relative to the total cell cycle (interphase is always the most common). Common errors include misidentifying late prophase as metaphase and confusing telophase with early prophase of two new daughter cells.

High-Yield Memory Anchors

• Interphase: G1 (growth), S (DNA replication), G2 (prep). Mitosis: PMAT (prophase, metaphase, anaphase, telophase).

• Sister chromatids separate in anaphase of mitosis.

• Mitosis = 2 genetically identical diploid daughter cells.

• Cancer = failure of cell-cycle checkpoints = uncontrolled division.

• Binary fission (prokaryotes) = simpler but functionally analogous to mitosis.

Quick Check

Q1 (Foundational): List the stages of mitosis in order. At which stage do sister chromatids separate?

Q2 (Application): A drug inhibits the formation of the mitotic spindle. At which stage of mitosis would cells treated with this drug become arrested (stopped)? Explain your reasoning.

Q3 (Comparison/Reasoning): How many chromosomes are present in a human cell during G1 phase? How many chromatids are present in that same cell during G2 phase? Explain the relationship between these numbers.

Quick Check Answers

A1: Prophase, prometaphase, metaphase, anaphase, telophase. Sister chromatids separate during anaphase.

A2: Cells would be arrested at metaphase (specifically, at the M checkpoint / spindle assembly checkpoint). Without a functional mitotic spindle, the kinetochores of sister chromatids would not attach to spindle microtubules. The spindle checkpoint prevents the cell from entering anaphase until all chromosomes are properly attached. Without spindle attachment, this checkpoint is never satisfied, and the cell remains arrested in metaphase. This is the mechanism of action of some chemotherapy drugs (e.g., taxanes like paclitaxel).

A3: In G1 phase, a human cell has 46 chromosomes, each consisting of a single chromatid (46 chromatids total). During S phase, DNA is replicated. In G2 phase, the cell still has 46 chromosomes, but each now consists of two sister chromatids — so there are 92 chromatids total (46 chromosomes × 2 chromatids each). The chromosome number has not changed; the number of DNA molecules has doubled.

Chapter Summary

The cell cycle: interphase (G1, S, G2) → M phase (mitosis + cytokinesis). Mitosis: prophase → prometaphase → metaphase → anaphase → telophase. Checkpoints ensure proper progression. Cancer = checkpoint failure. Prokaryotes divide by binary fission.

Common Mistakes

Mistake: "Chromosomes are always X-shaped."

Reality: Chromosomes are X-shaped only after DNA replication (when they consist of two sister chromatids joined at the centromere) and only when condensed. Before S phase, each chromosome is a single, long chromatin fiber. The X-shape is a snapshot of a replicated, condensed chromosome — not the permanent form.

Mistake: "Mitosis produces four daughter cells."

Reality: Mitosis produces TWO daughter cells. Meiosis produces four. This is a critically important distinction.

Mistake: "Interphase is a resting phase."

Reality: Interphase is a period of intense metabolic activity — growth, protein synthesis, organelle production, and DNA replication. It is anything but rest. The cell may spend 90% of its life in interphase.

Mistake: "Cancer is caused by a single gene mutation."

Reality: Cancer typically requires multiple mutations that accumulate over time — affecting both proto-oncogenes and tumor-suppressor genes. It is a multi-step process, which is why cancer incidence increases with age.