Final answer:
The DNA concentration in the solution with an A260 reading of 1.69 is calculated as 84.5 µg/mL using the 260 absorbance rule, multiplying the reading by 50 µg/mL per A260 unit.
Step-by-step explanation:
To determine the concentration of their DNA solution from an A260 reading of 1.69, we use the fact that an absorbance of 1 at 260 nm corresponds to a concentration of approximately 50 µg/mL for double-stranded DNA. This relationship is a rule of thumb in molecular biology known as the 260 absorbance rule. Therefore, by multiplying the A260 reading by 50 µg/mL, the student's DNA concentration is calculated to be:
1.69 A260 reading × 50 µg/mL/A260 unit = 84.5 µg/mL
This is the concentration of the DNA in the undiluted solution since the reading of 1.69 A260 was for the undiluted sample. If dilution factors are provided, as in the Exercise 4.12, these must be taken into account to determine the original concentration.