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How to Determine Alpha Helical Length

A helical object is one that makes a three-dimensional curve in space. This curve is continuous and therefore becomes akin to a spiral, such as a spiral staircase or Christmas decoration. To determine the length of an Alpha helical spiral, the right hand side of a double helix and usually used in DNA measurement, you need to know the number of spirals and the length on one spiral.

Things You'll Need

Calculator

Instructions

- 1
  Measure the change of height in one turn of the spiral. In a DNA helix, this is 34 angstroms.
- 2
  Measure the radius of the cylinder that the spiral exists in. For DNA, this is 10 angstroms.
- 3
  Place these values in the equation to compute helical length to get L = (34^2+2(10) pi^2)(0.5). L then equals (1156 + 3943.84)^(0.5), or 71.41 angstroms.
- 4
  Multiply the length of one turn, 71.41 angstrom,s by the total number of turns in a DNA helix, approximately 2.9 x 10^8. Because an angstrom is 10^-8 cm and the helix is multiplied by 10^8, these values cancel, leaving 71.41 x 2.9, or approximately 207 cm.

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