The Cell Cycle and Mitosis: Phases, Checkpoints, and Regulation
The cell cycle is the series of stages a eukaryotic cell goes through to grow, copy its DNA, and divide into two identical daughter cells.
The cell cycle is the series of stages a eukaryotic cell goes through to grow, copy its DNA, and divide into two identical daughter cells. Most cells in the body are somatic cells — diploid (“2N”), meaning they carry two copies of each chromosome, one from each parent. Every time one of these cells divides, it runs through the same regulated sequence, ending in mitosis, the process that splits one diploid cell into two genetically identical diploid cells.
Key Takeaways
The cell cycle has four phases: G1 (growth), S (DNA replication), G2 (final preparation), and M (division). G1, S, and G2 together make up interphase.
Two checkpoints control progression: the G1/S checkpoint (restriction point), enforced primarily by p53, and the G2/M checkpoint, which also draws on p53 for DNA-damage-triggered arrest.
Cyclins and CDKs drive the cycle forward — a CDK is inactive until it binds a cyclin, and the resulting complex switches on the genes needed for the next phase.
Mitosis produces two identical diploid daughter cells through five stages: prophase, metaphase, anaphase, telophase, and cytokinesis.
Checkpoint failure, such as loss-of-function p53 mutations, allows damaged cells to divide unchecked — a hallmark of cancer.
The Four Phases of the Cell Cycle
For actively dividing cells, the cell cycle has four main phases: G1, S, G2, and M.
G1 (Gap 1). The cell grows and carries out its normal functions, producing more organelles like mitochondria and ribosomes. A checkpoint here confirms the cell is ready to move forward.
S (Synthesis). DNA replication occurs. Every chromosome is copied, producing two identical sister chromatids joined at a central region called the centromere.
G2 (Gap 2). The cell continues growing and finishes replicating organelles and other components needed for division. A second checkpoint confirms replication was successful and the cell is ready to divide.
M (Mitosis and Cytokinesis). The nucleus divides first, followed by the rest of the cell.
G1, S, and G2 together are called interphase. During interphase the cell isn’t dividing, but it’s far from idle — it’s growing, functioning, and preparing for mitosis. Throughout this time, DNA stays in its relaxed, uncondensed form called chromatin, rather than being packed into visible chromosomes.
Some cells exit the cycle altogether into G0, a resting state where the cell is alive and functional but not preparing to divide. Neurons, for example, spend most of their lives in G0.
How the Cell Cycle Is Regulated
The cell doesn’t move blindly from one phase to the next — built-in checkpoints confirm that everything is happening correctly before allowing progression.
The G1/S Checkpoint (Restriction Point)
Also called the restriction point, this checkpoint assesses whether the cell’s DNA is intact and undamaged before committing to replication. The tumor suppressor protein p53 — often called the “guardian of the genome” — is the key enforcer here: if DNA damage is detected, p53 halts the cycle so the cell can attempt repairs before entering S phase. This is p53’s primary, best-characterized role in cell cycle control.
The G2/M Checkpoint
Between G2 and M, the cell checks that it has grown large enough and that all organelles and proteins have been properly duplicated to support division into two healthy daughter cells. This checkpoint is enforced mainly by a separate signaling pathway, but p53 also contributes here by suppressing cyclin B production when DNA damage is detected, helping maintain the arrest until repairs are complete.
Cyclins and CDKs: The Molecular Engine
Progression through both checkpoints is driven by cyclins and cyclin-dependent kinases (CDKs). CDKs are enzymes that are always present in the cell but inactive on their own — a CDK needs to bind a cyclin to switch on. Cyclins, by contrast, are regulatory proteins whose concentrations rise and fall throughout the cycle.
Once a cyclin binds its CDK, the activated complex phosphorylates specific transcription factors, which then switch on the genes needed for the next stage. G1/S cyclins push the cell from G1 into S phase; M cyclins promote entry into mitosis. Think of the cyclin as a timer building up toward a checkpoint, and the CDK as the engine that timer switches on.
MCAT Callout — Checkpoint Failure and Cancer: If a checkpoint fails — for example, if p53 is mutated and can’t stop a cell with damaged DNA from dividing — that cell can continue through the cycle unchecked. Damaged cells dividing uncontrollably is a hallmark of cancer, and loss-of-function mutations in checkpoint proteins like p53 are found in many cancers.
The Stages of Mitosis
Mitosis occurs in somatic cells and is used for growth, repair, and tissue maintenance. The goal is simple: take one diploid cell and produce two identical diploid daughter cells. By the time mitosis begins, the cell has already gone through G1, S, and G2 — DNA is copied, so every chromosome exists as two sister chromatids joined at the centromere, and the cell’s centrosomes have also been duplicated.
Prophase. The two centrosomes — each built from a pair of centrioles — move to opposite poles of the cell. From each, microtubule filaments grow outward, forming the mitotic spindle. On each chromosome’s centromere, a protein complex called the kinetochore assembles, giving spindle fibers a point to attach to. Meanwhile, chromatin condenses into visible chromosomes (each now X-shaped, made of two sister chromatids), and the nuclear membrane breaks down.
Metaphase. Chromosomes align along the metaphase plate, an imaginary line through the center of the cell. Each chromosome’s kinetochore is attached to spindle fibers coming from both poles, ensuring each daughter cell will get one complete copy of every chromosome.
Anaphase. The sister chromatids are pulled apart at the centromere by shortening spindle fibers and dragged toward opposite poles. Once separated, each chromatid counts as its own individual chromosome — this is why the chromosome count appears to double during anaphase, even though no new DNA has been made. It’s simply sister chromatids being recounted as separate chromosomes once they split.
Telophase. A new nuclear envelope forms around each set of chromosomes, which decondense back into chromatin, and the spindle apparatus breaks down.
Cytokinesis. Technically distinct from mitosis but occurring alongside telophase, cytokinesis is the physical division of the cytoplasm and organelles, producing two separate diploid daughter cells, each genetically identical to the original.
MCAT Callout — Chromatin vs. Chromosome vs. Sister Chromatid: These terms describe the same DNA at different points in the cycle. Chromatin is DNA in its loose, uncondensed form during interphase. Once condensed and visible, it’s called a chromosome. If that chromosome has already been copied (post-S phase) and still has both identical copies attached at the centromere, each copy is a sister chromatid — and the pair together is still counted as one chromosome, until anaphase splits them into two.
Why the Cell Cycle and Mitosis Matter for the MCAT
The cell cycle and mitosis connect directly to several other heavily tested MCAT topics:
Cancer biology passages frequently center on checkpoint failure — knowing that p53 is the primary G1/S enforcer but only a contributing factor at G2/M lets you reason through experiments involving p53 mutants rather than just recalling the name.
Chromosome-counting questions are a classic MCAT trap — passages often ask how many chromosomes or chromatids a cell contains at a specific phase, which depends on correctly tracking when DNA content doubles (S phase) versus when chromosome count doubles (anaphase).
Structure-function reasoning about the mitotic spindle and kinetochore shows up in passages about cell division drugs (e.g., microtubule-targeting chemotherapy agents), since these drugs work by disrupting the exact spindle machinery described here.
Common MCAT Mistakes
Assuming chromosome number doubles during S phase. DNA content doubles, but chromosome count doesn’t — each chromosome now has two sister chromatids joined at the centromere, and it’s still counted as one chromosome until anaphase splits the chromatids apart.
Treating p53 as the sole enforcer of both checkpoints. p53 is the primary enforcer at the G1/S checkpoint, but at the G2/M checkpoint it only contributes (via cyclin B suppression) alongside a separate signaling pathway that mainly governs that transition.
Mixing up cyclins and CDKs. CDKs are constantly present in the cell but inactive on their own; cyclins are the regulatory proteins whose rising and falling levels determine when a CDK gets switched on.
Confusing mitosis with meiosis. Mitosis produces two genetically identical diploid daughter cells for growth, repair, and tissue maintenance — it does not reduce chromosome number or produce haploid gametes.
MCAT-Style Concept Check
Question: A human somatic cell contains 46 chromosomes as it enters S phase. After DNA replication is complete but before anaphase begins, how many chromosomes and how many chromatids does the cell contain?
A) 46 chromosomes, 46 chromatids
B) 46 chromosomes, 92 chromatids
C) 92 chromosomes, 92 chromatids
D) 92 chromosomes, 46 chromatids
Answer: B
Explanation: DNA replication in S phase doubles the amount of DNA by producing a sister chromatid for every chromosome, but it does not change the chromosome count — each chromosome now simply consists of two sister chromatids joined at the centromere. So the cell still has 46 chromosomes, but 92 chromatids total. The chromosome count only reaches 92 once anaphase pulls the sister chromatids apart and each is recounted as an individual chromosome, ruling out C and D. Choice A ignores that replication has already occurred, so it's incorrect.
FAQ
What are the four phases of the cell cycle?
G1 (growth), S (DNA replication), G2 (final preparation before division), and M (mitosis and cytokinesis). G1, S, and G2 together make up interphase.
What’s the difference between the G1/S and G2/M checkpoints?
The G1/S checkpoint (restriction point) checks that DNA is undamaged before committing to replication and is primarily enforced by p53. The G2/M checkpoint confirms the cell is ready to divide and is enforced mainly by a separate signaling pathway, though p53 also contributes by suppressing cyclin B when DNA damage is present.
What’s the difference between chromatin, a chromosome, and a sister chromatid?
Chromatin is DNA in its loose, uncondensed interphase form. Once condensed and visible, it’s called a chromosome. If that chromosome has already been replicated and still has both identical copies joined at the centromere, each copy is a sister chromatid — the joined pair still counts as one chromosome until anaphase separates them.
Why does the chromosome count appear to double during anaphase?
No new DNA is made during anaphase — sister chromatids, which were already present after S phase, are simply pulled apart and each is recounted as its own individual chromosome once separated.
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