In the sarcomere the protein that forms two twisted strands around a central rod-like protein is called

In the sarcomere, the protein that forms two twisted strands around a central rod-like protein is called actin.

Explanation:

The muscle consists of many proteins like actin, myosin, tropomyosin, nebulin, and titlin in the sarcomeres which help in muscular movement.

Actin is a globular protein with bead-like in appearance. To form the thin filament of the sarcomere, actin molecules attached themselves in a strand or string and get twisted to a helix, which finally look like two twisted pearl strings together. These thin filaments alternate with thick filaments or the myosin in the sarcomere.  

The myosin twists like two golf sticks.

step 1: nitrogen fixation- conversion of atmospheric no₂ into forms useful to plants by lightning, bacteria, and cyanobacteria.

step 2: nitrification-process of converting ammonia to nitrite or nitrate.

step 3: assimilation-plant roots absorb new form of nitrogen.

step 4: ammonification-conversion of organic nitrogen to ammonia by bacteria involved in decomposition.

step 5: denitrification-bacteria turns the decomposed matter back into n₂o, which is released into atmosphere; repeat cycle.