--- title: "S Phase — AP Bio Definition & Exam Guide" description: "S phase is the part of interphase where DNA replicates so each new cell gets a full chromosome set. Learn how it ties to checkpoints, cancer, and meiosis on the AP Bio exam." canonical: "https://fiveable.me/ap-bio/key-terms/s-phase" type: "key-term" subject: "AP Biology" unit: "Unit 4" --- # S Phase — AP Bio Definition & Exam Guide ## Definition In AP Bio, S phase (synthesis phase) is the part of interphase where DNA is replicated, copying every chromosome so that the resulting daughter cells each receive a complete, identical set of genetic material. ## What It Is S phase is the "copy" step of the [cell cycle](/ap-bio/unit-4/homeostasis-feedback-loops/study-guide/OgMSpwCrEez0qyWtlCAC "fv-autolink"). The S stands for **synthesis**, meaning DNA synthesis. During this window, the cell replicates all of its DNA so that every [chromosome](/ap-bio/key-terms/chromosome "fv-autolink") gets duplicated into two identical sister chromatids held together at the centromere. S phase sits in the middle of **interphase**, sandwiched between G1 (the cell grows and gets ready) and G2 (final prep before division). The whole point is to make sure that when the cell divides later, both [daughter cells](/ap-bio/unit-5/meiosis/study-guide/FC0aTuODYikjJuhlBO1Z "fv-autolink") walk away with a full, accurate copy of the genome. No S phase, no second copy, and division would shortchange one of the cells. ## Why It Matters S phase lives in **[Unit 4](/ap-bio/unit-4 "fv-autolink"): Cell Communication and Cell Cycle**, specifically topic **4.6 Regulation of the Cell Cycle**. It connects directly to **[AP Bio](/ap-bio "fv-autolink") 4.6.A**, which asks you to describe how checkpoints regulate the cycle. There's a G1 checkpoint right before S phase that decides whether the cell is healthy enough to commit to copying its DNA, and a G2 checkpoint right after that verifies the DNA was copied correctly. Cyclins and cyclin-dependent kinases (CdKs) drive the cell into and through S phase. Because S phase is where the genome gets duplicated, errors here feed straight into **AP Bio 4.6.B**, where uncontrolled cycling leads to cancer and damaged cells get sent to apoptosis. ## Connections ### DNA Replication (Unit 6) S phase is the WHEN; DNA replication is the WHAT. Everything you learn about semiconservative replication, helicase, and DNA polymerase in [Unit 6](/ap-bio/unit-6 "fv-autolink") is the molecular machinery actually running during S phase. ### [Cyclin-Dependent Kinases (Unit 4)](/ap-bio/key-terms/cyclin-dependent-kinases) CdKs paired with [cyclins](/ap-bio/unit-4/cell-cycle/study-guide/4ztGMFvp0v4KAzL65pOP "fv-autolink") are the switches that push a cell past the checkpoint and into S phase. If cyclin levels don't rise, the cell stalls in G1 and never starts copying DNA. ### Cancer and Apoptosis (Unit 4) If a cell enters S phase with damaged DNA or skips its checkpoint, it copies mistakes into both daughter cells. Functioning checkpoints either pause to fix the problem or trigger [apoptosis](/ap-bio/key-terms/apoptosis "fv-autolink"); broken ones can lead to cancer. ### Meiosis (Unit 5) S phase happens once before meiosis too, then is followed by two divisions. That single round of DNA copying before two splits is exactly why meiosis halves the chromosome number to make gametes. ## On the AP Exam S phase shows up most often in checkpoint and cell-cycle-arrest questions. A classic MCQ stem describes a cell treated with something that blocks progression and asks where it arrests. For example, overexpressing the CdK inhibitor p21 stalls cells before S phase, and a growth factor that pushes G0 cells to "initiate DNA synthesis" is describing entry into S phase. On the 2024 long FRQ, you were asked to describe the function of S phase of interphase in the context of meiosis. The expected answer is that S phase replicates the DNA so each chromosome becomes two sister chromatids before division. So you need to be able to state plainly that S phase copies the genome and explain why that copying must happen before any chromosomes separate. ## S Phase vs G2 phase S phase is where DNA actually gets copied. G2 comes right after and is a growth and quality-check phase where the cell makes proteins for division and confirms the DNA was replicated correctly. If a question says "DNA synthesis," that's S phase; if it says "preparing to divide after replication," that's G2. ## Key Takeaways - S phase is the synthesis phase of interphase, the only time DNA is replicated during the cell cycle. - After S phase, each chromosome consists of two identical sister chromatids joined at the centromere. - The order is G1, then S, then G2, then mitosis (or meiosis); S sits in the middle of interphase. - Checkpoints before and after S phase make sure DNA is intact and accurately copied (AP Bio 4.6.A). - Skipping checkpoints or copying damaged DNA in S phase can lead to cancer or trigger apoptosis (AP Bio 4.6.B). - Before meiosis, S phase runs once, then two divisions follow, which is why gametes end up with half the chromosomes. ## FAQs ### What does the S phase do in the cell cycle? S phase replicates the cell's entire DNA, duplicating every chromosome into two identical sister chromatids so that both daughter cells will receive a complete genome after division. ### Does the cell divide during S phase? No. S phase only copies the DNA. Actual division happens later, during mitosis (or meiosis) and cytokinesis. S phase is purely the synthesis step within interphase. ### What is the difference between S phase and G2 phase? S phase is when DNA is actually copied. G2 phase comes after, when the cell grows and double-checks that replication finished correctly before entering division. "DNA synthesis" always means S phase. ### What happens if a cell enters S phase with damaged DNA? It can copy those errors into both daughter cells. Checkpoints (AP Bio 4.6.A) normally catch this and pause the cycle or trigger apoptosis; if checkpoints fail, the unchecked division can contribute to cancer. ### Why does S phase matter for meiosis? S phase runs once before meiosis, then two divisions follow. That single round of DNA copying before two splits is exactly why meiosis produces gametes with half the original chromosome number. ## Related Study Guides - [4.6 Regulation of the Cell Cycle](/ap-bio/unit-4/cell-cycle/study-guide/4ztGMFvp0v4KAzL65pOP) ## Structured Data ```json {"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/ap-bio/key-terms/s-phase#resource","name":"S Phase — AP Bio Definition & Exam Guide","url":"https://fiveable.me/ap-bio/key-terms/s-phase","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/ap-bio/key-terms/s-phase#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-06-12T23:22:13.911Z","isPartOf":{"@type":"Collection","name":"AP Biology Key Terms","url":"https://fiveable.me/ap-bio/key-terms"},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/ap-bio/key-terms/s-phase#term","name":"S Phase","description":"In AP Bio, S phase (synthesis phase) is the part of interphase where DNA is replicated, copying every chromosome so that the resulting daughter cells each receive a complete, identical set of genetic material.","url":"https://fiveable.me/ap-bio/key-terms/s-phase","inDefinedTermSet":{"@type":"DefinedTermSet","name":"AP Biology Key Terms","url":"https://fiveable.me/ap-bio/key-terms"}},{"@type":"FAQPage","mainEntity":[{"@type":"Question","name":"What does the S phase do in the cell cycle?","acceptedAnswer":{"@type":"Answer","text":"S phase replicates the cell's entire DNA, duplicating every chromosome into two identical sister chromatids so that both daughter cells will receive a complete genome after division."}},{"@type":"Question","name":"Does the cell divide during S phase?","acceptedAnswer":{"@type":"Answer","text":"No. S phase only copies the DNA. 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