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Dictionary of important concepts
Kinesin is a motor protein. It is a dimmer with two identical motor heads. Each head consists of a catalytic core and a neck linker. In the cell, kinisen pull organelles along microtubule tracks. The organelle attaches to the other end of the long coiled-coil that holds the two motor heads together.
In solution, both kinesin heads contain tidily bound ADP and move randomly driven by Brownian motion. When one of the two kinesin heads encounters a microtubule, it binds tidily. Microtubule binding causes ADP to be released from the attached head. Then, ATP rapidly enters the empty nucleotide binding side. This nucleotide exchange triggers the neck linker to zip it onto the catalytic core. This action throws the second head forward and brings it near the next binding site on the microtubule. The attached trailing head hydrolyzes the ATP and releases phosphate. As the neck linker unzips from the trailing head, the leading head exchanges its nucleotide and it zippers its neck link into the catalytic core and the cycle repeats.
This way, kinesin dimmers move processively, step by step, along the microtubule.
Microtubules are straight hollow cylinders that built by the assembly of dimmers of alpha-tubulin and beta-tubulin. They have a diameter of about 25 nm, and a length varying from 200 nm to 25 um.
Microtubules grow at each end by the polymerization of tubulin dimmers, powered by the hydrolysis of GTP. They also shrink at each end by the release of tubulin dimmers in a process called depolymerization. Both processes always occur more rapidly at one end, called the plus end. The other, less active, end is the minus end.
Muscle myosin is a dimmer with two identical motor heads that act independently. Each myosin head has a catalytic core and an attached lever arm. A coiled-coil rod ties the two head together and attaches them to a thick filament.
At the beginning, the myosin heads contain bound ADP and phosphate and have weak affinity for actin. Once one of the heads connects properly into and actin subunit in the helical actin filament, phosphate is released. Phosphate release strengthens the binding of the myosin head to actin and it also triggers the force generating power stroke that moves the actin filament. ADP then dissociates, and ATP binds to the anti-nucleotide binding site, causing the myosin head to detach from the actin filament. On the detached head, ATP is hydrolyzed, which throws the lever arm back to its pre-stroke state. Thus, the lever arm stores the energy released by ATP hydrolysis, and the cycle can repeat.
The actin filament does not slide back after being released by the motor head because there are many other myosin molecules also attached to it holding it under tension.
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets in two nuclei. Definition1
Mitosis occurs exclusively in eukaryotic cells.
Meiosis is the cell division that produces reproductive cells in sexually reproducing organisms; the nucleus divides into four nuclei each containing half the chromosome number. Definition 2