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Chapter 24: Problem 78

You are working in a biotechnology lab and are analyzing DNA. You obtain a sample of a short dodecamer of DNA that contains 12 base pairs. (a) What must the ratio of adenine to thymine be in your sample? (b) What must the ratio of cytosine to guanine be in your sample? (c) Assume the counterions present in your DNA solution are sodium ions. How many sodium ions must there be per dodecamer? Assume the \(5^{\prime}\) end phosphates each bear a-1 charge.

Short Answer

Expert verified
(a) The ratio of adenine to thymine in the DNA dodecamer is 1 : 1. (b) The ratio of cytosine to guanine in the DNA dodecamer is 1 : 1. (c) There are 2 sodium ions per dodecamer in the DNA solution.

Step by step solution

01

Find the ratio of adenine to thymine

In a DNA molecule, adenine always pairs with thymine. Since there are 12 base pairs in the dodecamer, the amount of adenine must be equal to the amount of thymine. So, Ratio of A to T = A : T = 1 : 1
02

Find the ratio of cytosine to guanine

Similarly, cytosine always pairs with guanine in a DNA molecule. Since there are 12 base pairs in the dodecamer, the amount of cytosine must be equal to the amount of guanine. So, Ratio of C to G = C : G = 1 : 1
03

Determine the total negative charges of the dodecamer

The total negative charge of the dodecamer will come from the \(5^{\prime}\) end phosphates. As stated in the exercise, each \(5^{\prime}\) end phosphate bears a -1 charge. Since there are two strands in the DNA molecule, we have two \(5^{\prime}\) end phosphates: Total negative charges = -1 charge/phosphate × 2 phosphates = -2 charges
04

Determine the number of sodium ions in the dodecamer

To neutralize the negative charges on the dodecamer, there must be an equal number of positive charges coming from the sodium ions. Since each sodium ion has a +1 charge, we need the same number of sodium ions (+2 charges) to neutralize the -2 charges of the dodecamer. Therefore: Number of sodium ions = +2 charges / +1 charge/sodium ion = 2 sodium ions per dodecamer. The answers to the exercise are: (a) The ratio of adenine to thymine in the DNA dodecamer is 1 : 1. (b) The ratio of cytosine to guanine in the DNA dodecamer is 1 : 1. (c) There are 2 sodium ions per dodecamer in the DNA solution.

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