--- title: "Griffith's Experiment | General Biology I" description: "Griffith's experiment showed that heat-killed S bacteria could transform live R bacteria, revealing transformation in General Biology I and the roots of DNA heredity." canonical: "https://fiveable.me/college-bio/key-terms/griffiths-experiment" type: "key-term" subject: "General Biology I" unit: "Unit 14" --- # Griffith's Experiment | General Biology I ## Definition Griffith's experiment is the 1928 bacterial study that showed genetic information could move from dead S cells to live R cells, changing the R cells' traits. In General Biology I, it is the classic first clue that heredity can be transferred by transformation. ## What It Is Griffith's experiment is the classic General Biology I demonstration of transformation, the process where one bacterium takes in genetic material from another and changes its traits. Frederick Griffith worked with two strains of Streptococcus pneumoniae: the smooth (S) strain, which had a capsule and caused disease, and the rough (R) strain, which lacked the capsule and was harmless. He tested them in mice. Live S bacteria killed the mice, live R bacteria did not, and heat-killed S bacteria did not. The surprising result came when he mixed heat-killed S bacteria with live R bacteria. The mice died, and living S-type bacteria were recovered from the animals. That meant something from the dead S cells had changed the live R cells into virulent cells. That “something” was later called the transforming principle. Griffith did not know the molecule itself, but his experiment showed that hereditary information could survive cell death and pass into another cell. In other words, traits were not fixed just because a bacterium was born one way. They could be altered by acquired genetic material. In a biology course, the point is not just the mouse result. The experiment is a mechanism story: dead donor cells supplied information, recipient cells took it up, and the recipient phenotype changed. That is transformation. The capsule mattered because it made the S strain slippery and disease-causing, while the R strain's lack of capsule made it nonvirulent. This is also why Griffith's experiment sits at the start of the DNA heredity story. It raised the question Avery, MacLeod, and McCarty later answered by showing that DNA, not protein, was the transforming material. So when you see Griffith's experiment, think “first evidence that genetic material can move between bacteria and change phenotype,” not “proof that DNA is the heredity molecule.” ## Why It Matters Griffith's experiment matters because it introduces one of the biggest shifts in genetics: traits can come from transferred genetic information, not just direct inheritance from parent to offspring. That idea shows up again in bacterial genetics, molecular biology, and any unit on how DNA moves through cells. It also gives you a clean way to connect genotype and phenotype. The R strain had one phenotype, harmless colonies without a capsule, and the S strain had another, virulent smooth colonies with a capsule. When R cells became transformed, their phenotype changed because they acquired new genetic information. That cause-and-effect link is a theme you will keep seeing in biology. The experiment is a setup for later evidence. Once Griffith showed transformation, later scientists could ask what the transforming principle was. That makes it a starting point for the history of DNA research, not the final answer. It is one of those experiments that matters because it changes the question scientists ask next. It also helps with bacterial gene transfer topics. Even though Griffith was not studying plasmids, conjugation, or modern lab methods, the basic idea that bacteria can acquire new traits from external DNA is the foundation for those ideas. If you understand Griffith's setup, the rest of bacterial genetics makes a lot more sense. ## Connections ### [Transformation](/college-bio/key-terms/transformation) This is the process Griffith's experiment revealed. A cell takes in DNA from outside itself and uses that information to change its traits. In bacteria, transformation can create a new phenotype without sexual reproduction. Griffith's result is the famous proof-of-concept that made the word transformation worth learning in genetics. ### Smooth and Rough Strains These are the two Streptococcus pneumoniae forms Griffith compared. The smooth strain had a capsule, which made it appear smooth and helped it evade a host immune response. The rough strain lacked that capsule, so it looked rough and was nonvirulent. The contrast between them made the transformation result easy to see. ### Avery-MacLeod-McCarty Experiment Griffith showed that a transforming factor existed, but he did not identify it. Avery, MacLeod, and McCarty later tested extracts from the S strain and showed that DNA was the molecule responsible for transformation. If Griffith is the observation, this later experiment is the chemical identification step. ### [competent cells](/college-bio/key-terms/competent-cells) Competent cells are cells that can take up foreign DNA more easily. That idea is closely related to Griffith's work because transformation only happens when the recipient can actually absorb genetic material. In lab settings, scientists often make bacteria competent so they can transform them on purpose. ## On the AP Exam A quiz or short-answer question may give you the S and R strains and ask you to explain why the mixed sample killed the mouse. The move is to identify transformation, not just memorize the outcome. You should connect the heat-killed S cells to the transfer of hereditary material into live R cells and explain that the R cells became virulent because they gained new genetic information. You may also be asked to sequence the logic of the experiment from the controls to the surprise result. Look for what each tube or injection proves: live S kills, live R does not, heat-killed S does not, and heat-killed S plus live R does kill. That pattern is what supports the transformation claim. If there is a lab or discussion question, use the experiment to distinguish phenotype from genotype, and to explain why Griffith's work mattered before DNA was identified as the genetic material. ## Griffith's experiment vs Avery-MacLeod-McCarty Experiment Griffith's experiment showed that a transforming factor existed, but it did not identify the molecule. Avery-MacLeod-McCarty tested that factor more directly and found that DNA was the substance causing transformation. If a question asks who proved DNA was the genetic material, the answer is not Griffith. ## Key Takeaways - Griffith's experiment showed that something from dead S bacteria could transform harmless R bacteria into virulent cells. - The S strain was smooth and disease-causing because it had a capsule, while the R strain was rough and nonvirulent because it lacked one. - The experiment gave the first strong evidence that genetic information can move between bacterial cells. - Griffith did not identify DNA, but his results set up the later experiments that did. - If you remember one thing, remember this: transformation changes a bacterium's traits by letting it acquire new genetic material. ## FAQs ### What is Griffith's experiment in General Biology I? It is the 1928 experiment showing that heat-killed disease-causing S bacteria could transform live harmless R bacteria. The live R cells picked up information from the dead S cells and became virulent. In biology, that result is the classic introduction to transformation and bacterial heredity. ### What happened when Griffith mixed heat-killed S strain and live R strain? The mice died, and scientists later recovered live S-type bacteria from them. That meant the live R bacteria had changed after contact with material from the dead S strain. The experiment showed that the change was not random, it was heritable transformation. ### Is Griffith's experiment the same as the Avery-MacLeod-McCarty experiment? No. Griffith showed that a transforming principle existed, but he did not know what it was made of. Avery, MacLeod, and McCarty later showed that the transforming principle was DNA. If you are asked about the molecule itself, that is the later experiment. ### Why are the smooth and rough strains important in Griffith's experiment? They gave Griffith a clear before-and-after comparison. The smooth strain had a capsule and caused disease, while the rough strain lacked the capsule and was harmless. After transformation, the rough cells took on the smooth strain's harmful trait, which made the result easy to detect. ## Related Study Guides - [14.1 Historical Basis of Modern Understanding](/college-bio/unit-14/1-historical-basis-modern-understanding/study-guide/kNxjuF5TccvxOUzU) ## About This Document Canonical Fiveable pages are available as Markdown at the same path plus `.md`. - [llms.txt](https://fiveable.me/llms.txt): index of Fiveable's sections and URL patterns - [llms-full.txt](https://fiveable.me/llms-full.txt): complete subject and unit listing - [MCP server](https://fiveable.me/mcp): call Fiveable as tools instead of fetching pages (`https://fiveable.me/api/mcp`) - [MCP server for AP teachers](https://fiveable.me/mcp/teachers): a teacher's classes, assignments and AP-rubric grading (`https://fiveable.me/api/mcp/teacher`) ## Structured Data ```json {"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/college-bio/key-terms/griffiths-experiment#resource","name":"Griffith's Experiment | General Biology I","url":"https://fiveable.me/college-bio/key-terms/griffiths-experiment","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/college-bio/key-terms/griffiths-experiment#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-07-03T02:21:09.281Z","isPartOf":{"@type":"Collection","name":"General Biology I Key Terms","url":"https://fiveable.me/college-bio/key-terms"},"author":{"@type":"Organization","@id":"https://fiveable.me/editorial#content-team","name":"Fiveable Content Team","url":"https://fiveable.me/editorial","parentOrganization":{"@id":"https://fiveable.me/#organization"},"member":[{"@type":"Person","name":"Amanda DoAmaral","sameAs":["https://www.linkedin.com/in/adoamaral"]},{"@type":"Person","name":"Becky Bahr","sameAs":["https://www.linkedin.com/in/becky-bahr-b52743132"]}]},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/college-bio/key-terms/griffiths-experiment#term","name":"Griffith's experiment","description":"Griffith's experiment is the 1928 bacterial study that showed genetic information could move from dead S cells to live R cells, changing the R cells' traits. 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