ELEMENTS OF ERROR-CORRECTION IN MITOSIS - MICROTUBULE CAPTURE, RELEASE, AND TENSION

The correction of certain errors in mitosis requires capture and relea se: new kinetochore microtubules must be captured and old, misdirected ones must be released. We studied capture and release in living grass hopper spermatocytes. Capture is remarkably efficient over a broad ran ge in the angle at which a microtubule encounters a kinetochore. Howev er, capture is inefficient when kinetochores point directly away from the source of properly directed microtubules. Capture in that situatio n is required for correction of the most common error; microtubule-kin etochore encounters are improbable and capture occurs only once every 8 min, on average. Release from the improper attachment caused by misd irected microtubules allows kinetochore movement and the completion of error correction. We tugged on kinetochores with a micromanipulation needle and found they are free to move less than one time in two. Thus error correction depends on two improbable events, capture and releas e, and they must happen by chance to coincide. In spermatocytes this w ill occur only once every 18 min, on average, but a leisurely cell cyc le provides ample time. Capture and release generate only change, not perfection. Tension from mitotic forces brings change to a halt by sta bilizing the one correct attachment of chromosomes to the spindle. We show that tension directly affects stability, rather than merely const raining kinetochore position. This implies that chromosomes are attach ed to the spindle by tension-sensitive linkers whose stability is nece ssary for proper chromosome distribution but whose loss is necessary f or the correction of errors.