The Transforming Principle: Definition & Explanation

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  • 0:00 Transforming Principle Defined
  • 0:30 Griffith's Experiment
  • 1:30 Transformation
  • 2:09 How Bacteria Can Eat…
  • 2:44 Lesson Summary
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Lesson Transcript
Instructor: Beth Skwarecki
Bacteria aren't above cannibalism. Discover how one bacterium can 'eat' another's genetic material - and how this freakish behavior led to the discovery of DNA.

Transforming Principle Defined

The transforming principle was an early name for DNA. In 1928, scientists didn't know yet that DNA carried genetic information, but they knew that there was something that could cause bacteria to transform from one type to another.

Transformation occurs when one bacterium (you know, those microscopic, single-celled creatures) picks up free-floating DNA and incorporates it into its own genome. The idea of the transforming principle was discovered during an experiment by Frederick Griffith.

Griffith's Experiment

The year was 1928. The scientist, Frederick Griffith, was posing this question: why was a non-virulent germ killing his mice?

He was studying pneumococcus bacteria. There was a rough type (call it R) that didn't usually kill mice, and another type with a smooth protective shell. The smooth type was harder for the mouse's immune system to fight, so the smooth type (call it S) was the dangerous, or virulent, type.

If Griffith injected the S-type into mice, they would get sick and die. If he injected the R-type, the mice would live. So far so good. He could also kill the S-type with heat, and those dead bacteria couldn't make the mouse sick, so that mouse would still live.

The tricky part came when Griffith mixed the heat-killed S-type bacteria with living, normally harmless R-type bacteria, and injected this mixture into a mouse. The mouse should live, right? But it didn't. It got sick and died.


Griffith tested the blood of that fourth, mysterious mouse. He expected to only find living R bacteria, but the blood also contained living S bacteria. How could that be? He was sure he'd killed the S bacteria.

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