The following are required steps in the sliding filament theory of muscle contraction:
calcium ions bind to troponin.
tropomyosin moves.
calcium pumps of the sarcoplasmic reticulum open.
myosin heads bend to attach to actin filaments.
what would be the correct order of the steps above?
1, 4, 3, 2
1, 2, 3, 4
3, 1, 2, 4
3, 2, 1, 4
i picked c (3, 1, 2, 4)

The correct order of the sliding filament theory of muscle contraction is third ( that is 3,1,2,4) -

1) Calcium pumps of the sarcoplasmic reticulum opens.

2) Calcium binds to troponin.

3) Troponin moves.

4)  Myosin heads bend to attach to actin filaments.

During muscle contraction, the thin filaments ( actin filaments) slide pass over the thick filaments ( myosin) and the sarcomere ( unit of muscle contraction ) becomes short. This is explained through sliding filament theory.

When action potential arrives at sarcoplasmic reticulum, calcium ions are released and arrive at sarcomere. They bind to troponin that changes its shape, removing tropomyosin from active sites on actin. Myosin binds to actin and pulls it towards the center of sarcomere ( crossbridge formation that is the contraction phase). Finally, when action potential ends, calcium ions are sequestered back into the sarcoplasmic reticulum and tropomyosin restores the blocking of active site on actin.

humans, along with other animals and plants, have linear chromosomes that are arranged in pairs within the nucleus of the cell. the only human cells that do not contain pairs of chromosomes are reproductive cells, or gametes, which carry just one copy of each chromosome.