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Filaments that treadmill in place

Science · Organisms & Life Processes · ages 20-21

Name ______________________ Date ____________

In the basic ATP-actin model, free actin is between the two end critical concentrations. What are the net changes?
- Plus grows; minus shrinks
- Both ends grow; neither shrinks
- Both ends shrink; neither grows
Under comparable conditions, which actin-filament end adds subunits faster?
- The minus end of the filament
- The plus end of the filament
- Both ends always add equally
Actin assembly near a crawling cell's leading edge can push its membrane outward.
Circle one: True False
Which sequence of nucleotide changes helps produce older, less stable ADP-rich regions after ATP-actin joins a filament?
- Phosphate release before ATP hydrolysis
- ATP hydrolysis, then phosphate release
- ADP conversion to ATP without energy input
A drug blocks all actin-subunit addition in a crawling cell. What follows for its actin network?
- Addition continues; protrusion remains unchanged
- Both filament ends immediately stop all subunit loss
- Addition stops; subunit loss can continue
A cell protrudes only at its right edge. Adhesion and contraction work normally. In this model, which way does it advance?
- Toward its upper edge
- Toward its left edge
- Toward its right edge
In the basic ATP-actin model, free actin falls below both end critical concentrations. What is the net result?
- Net subunit gain at both ends
- Net subunit loss at both ends
- Net subunit gain at the plus end
Three subunits join a filament's plus end while three leave its minus end. Its length stays constant. What does this show?
- Constant length proves every subunit stays in place
- The two ends must always have identical reactions
- Balanced turnover can keep length unchanged

Answer key

For grown-ups. Fold this page away before handing over the rest.

Filaments that treadmill in place W1-mt_sJlYoA-BnA-s1

Plus grows; minus shrinks · The concentration is above the lower plus-end threshold and below the higher minus-end threshold.
The plus end of the filament · Addition is faster at the plus end under comparable conditions. This does not mean that end grows at every concentration.
True · Assembly can produce local protrusion. Adhesion and contraction help the rest of the cell follow.
ATP hydrolysis, then phosphate release · ATP hydrolysis produces ADP and phosphate. Later phosphate release leaves ADP-rich regions that are generally less stable.
Addition stops; subunit loss can continue · Blocking addition prevents assembly-driven protrusion, but existing filaments may continue losing subunits.
Toward its right edge · The right edge is the protruding front. Working adhesion and contraction allow the rest of the cell to follow that front.
Net subunit loss at both ends · Below an end's critical concentration, loss exceeds addition. Both ends therefore shrink overall.
Balanced turnover can keep length unchanged · Three additions balance three losses. Constant length does not prove that the filament contains an unchanged set of subunits.