biophysics / single-molecule measurement · BEGINNER LESSON
TPM
Tethered Particle Motion (TPM) is an in vitro optical method that measures how much a small bead moves when it is attached to a fixed surface by a single molecule such as DNA. The motion is used to infer changes in the tether’s effective length and conformation.
What TPM is and what it measures
Tethered Particle Motion (TPM) is a laboratory technique used on molecules outside cells, especially DNA. A DNA molecule or other macromolecule attaches a mobile reporter particle, usually a small bead, to a fixed point on a surface. The main measurement is the bead’s random motion, often summarized by the root mean square of its excursions around the anchor point. That motion reflects the tether’s effective length and changes in molecular conformation, such as looping or unlooping. Because it is a relative measurement, TPM is useful for observing whether the tether behaves as a longer or shorter connection over time.
[1] Tethered Particle Motion: an easy technique for probing DNA topology … [3] Multiplexed, tethered particle microscopy for studies ...
How the measurement works
The bead undergoes Brownian motion, which means random movement caused by thermal energy in the surrounding liquid. The tether restricts this movement, so the particle explores only the space allowed by the attached molecule. When the tether changes shape, the allowed motion also changes. For example, a looped DNA tether is shorter than an unlooped one, so the bead tends to show a smaller excursion amplitude. In some experiments, protein binding, DNA looping, transcription, or other DNA-processing events change the tether length and can therefore be tracked from the bead position over time.
[1] Tethered Particle Motion: an easy technique for probing DNA topology … [3] Multiplexed, tethered particle microscopy for studies ...
Why time scale matters
TPM does not detect every change instantly. If a tether change is too brief compared with the diffusion time of the bead, the bead may not sample enough positions for the change to be recognized reliably. The sources explain that for short observation intervals, the tether can have little apparent effect and the motion may look nearly unrestricted. As a result, changes in tether properties must last longer than the characteristic diffusion time to be resolved with confidence. This also means fast, small changes can be missed, especially when the bead is large or the tether is long.
[1] Tethered Particle Motion: an easy technique for probing DNA topology … [3] Multiplexed, tethered particle microscopy for studies ...
Limits and cautions
TPM is simple, but its readout has important limits. The bead’s motion contains noise from diffusion, so the start and end of a tether-length change may not be located precisely. Accuracy can worsen as the tether gets longer, and short or transient changes may be obscured. The method is therefore best for observing relatively large changes that persist long enough for averaging. TPM also requires matching the experimental time resolution to the process being studied.
[1] Tethered Particle Motion: an easy technique for probing DNA topology … [3] Multiplexed, tethered particle microscopy for studies ...
Important note on a different meaning of TPM
In genomics, TPM can also mean transcripts per million, a unit for relative RNA abundance. That meaning is unrelated to tethered particle motion. In this lesson, TPM means tethered particle motion because the supplied sources describe that experimental method.
SEE THE IDEA
How TPM infers DNA behavior from bead motion
Connects a bead to a fixed point on a surface
Random thermal motion makes the bead wander
Looping, unlooping, or binding changes effective tether length
Bead position over time is summarized as excursion amplitude
The tether limits random bead motion, and changes in tether structure change the measured motion.
TPM reads molecular structure indirectly by tracking how a tethered bead moves.
TRY IT YOURSELF
Test your understanding
Choose an answer, then check the explanation.
PUT IT TOGETHER
Match each concept to its meaning
Drag a label onto its description, or select a label and then select a description. This also works with a keyboard.
4 pairs to match
Sources and further reading
This general lesson was generated from public source material. It explains a concept; it does not interpret a private paper or provide medical advice.