Cell Cycle

Core Explanation

The cell cycle is the ordered sequence of events by which a cell duplicates its contents and divides into two daughter cells. It consists of two major phases:

• Interphase (~90% of the cycle): The cell grows, replicates its DNA, and prepares for division.
• M phase (mitotic phase): The nucleus divides (mitosis) and the cytoplasm divides (cytokinesis).

Interphase

G1 phase (Gap 1):

• The cell grows in size and synthesizes proteins and organelles.
• Metabolic activity is high.
• Duration varies enormously — from hours to years. Some cells exit the cycle from G1 into a non-dividing state called G0 (e.g., neurons, skeletal muscle cells).

S phase (Synthesis):

• DNA replication occurs. Each chromosome is duplicated, producing two identical sister chromatids held together at the centromere by cohesin proteins.
• The centrosome is also duplicated (in animal cells).

G2 phase (Gap 2):

• The cell continues to grow and synthesizes proteins needed for mitosis (e.g., tubulin for spindle fibers).
• Organelles may be replicated.
• The cell checks for DNA damage and ensures replication is complete before committing to mitosis.

M Phase: Mitosis

Mitosis is the division of the nucleus, distributing one complete copy of the genome to each daughter nucleus. It is a continuous process but is conventionally divided into five stages:

Prophase

• Chromatin condenses into visible chromosomes (each consisting of two sister chromatids).
• The mitotic spindle begins to form — microtubules grow from the duplicated centrosomes, which migrate to opposite poles.
• In the cytoplasm, the spindle apparatus assembles.

Prometaphase

• The nuclear envelope breaks down into fragments.
• Microtubules from the spindle invade the nuclear region.
• Each sister chromatid has a protein structure called a kinetochore at its centromere; spindle microtubules attach to kinetochores.
• Non-kinetochore microtubules interact with microtubules from the opposite pole.

Metaphase

• Chromosomes align at the metaphase plate (the equator of the spindle).
• Each chromosome's kinetochores are attached to microtubules from opposite poles.
• This alignment ensures that when sister chromatids separate, each daughter cell receives one copy of each chromosome.

Anaphase

• Cohesin proteins are cleaved, allowing sister chromatids to separate.
• Separated chromatids (now individual chromosomes) are pulled toward opposite poles as kinetochore microtubules shorten.
• Simultaneously, the poles are pushed apart by elongating non-kinetochore microtubules.
• This is the shortest stage of mitosis.

Telophase

• Two daughter nuclei begin to form.
• Nuclear envelopes reassemble around each set of chromosomes.
• Chromosomes decondense (return to chromatin).
• Nucleoli reappear.
• The mitotic spindle disassembles.

Cytokinesis

Cytokinesis is the division of the cytoplasm, which usually begins during late anaphase or telophase.

• Animal cells: A cleavage furrow forms. A contractile ring of actin and myosin microfilaments constricts the cell like a drawstring, pinching it into two daughter cells.
• Plant cells: A rigid cell wall prevents furrowing. Instead, vesicles from the Golgi apparatus coalesce at the metaphase plate, forming a cell plate that grows outward until it fuses with the plasma membrane. New cell wall material is deposited between the two membranes.

How It Works — Regulation and Checkpoints

The cell cycle is not a simple timer — it is tightly regulated by a molecular control system. Checkpoints at critical transitions ensure that each phase is completed accurately before the next begins:

The molecular drivers of the cell cycle are cyclins and cyclin-dependent kinases (Cdks). Cdk levels are relatively constant, but cyclin concentrations oscillate:

• Cyclin binds to Cdk, activating it.
• The active cyclin-Cdk complex phosphorylates target proteins that trigger cell-cycle events.
• Different cyclin-Cdk complexes act at different stages (e.g., G1/S cyclin-Cdk drives entry into S phase; M cyclin-Cdk drives mitosis).

MPF (Maturation-Promoting Factor) is the cyclin-Cdk complex that triggers the G2 → M transition. Its discovery and characterization were landmark achievements in cell biology.

Common Misconceptions and Exam Traps

• Exam trap: Confusing chromosome number with chromatid number. A duplicated chromosome (2 chromatids) is still counted as ONE chromosome. After anaphase, when sister chromatids separate, each chromatid becomes an independent chromosome — the chromosome number temporarily doubles.
• Misconception: "Interphase is a resting phase." Interphase is metabolically active — DNA replication, protein synthesis, and growth all occur during interphase. The cell is far from resting.
• Exam trap: Forgetting that plant cells do cytokinesis differently. "Cleavage furrow" = animal cells; "cell plate" = plant cells.
• Misconception: "Mitosis is the entire cell cycle." Mitosis is only the nuclear division portion of M phase; the cell cycle also includes interphase and cytokinesis.