To test the lever-arm model of myosin movement, an investigator utilizes recombinant DNA technology to attach myosin heads to various lengths of neck domains. In the schematics shown below (1-4), the y-axis represents the velocity of myosin in μm/sec on the actin filament, and the x-axis shows the recombinant myosins (a-d; shown on the left) that were utilized to calculate the velocity. Considering that all the appropriate conditions were applied to estimate the velocity of recombinant myosin, choose the graph that correctly represents the velocity of recombinant myosin.

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Correct option is B

Lever-Arm Hypothesis Overview:

• The lever-arm hypothesis suggests that the length of the neck domain of myosin influences the size of each step it takes along actin filaments. This, in turn, affects the velocity of movement.
• A longer neck domain allows myosin to take larger steps, which should lead to increased velocity.

Myosin Constructs:

• a: Shortest neck domain.
• b: Slightly longer neck domain.
• c: Even longer neck domain.
• d: Longest neck domain.

Prediction According to the Lever-Arm Model:

• The longer the neck domain, the higher the velocity.
  • d (longest neck domain) should have the highest velocity, as a longer neck allows for larger steps.
  • a (shortest neck domain) should have the lowest velocity, as it takes smaller steps.

Thus, the correct velocity order would be:

• d > c > b > a

This prediction fits the relationship between neck domain length and velocity.

Analysis of Graphs:

• Graph 2 shows the velocity order of d > c > b > a, which aligns with the expected relationship between neck length and velocity.