What is the molecular weight of a single-stranded DNA molecule of 17 nucleotides (e.g. a primer)? The average molecular weight of a nucleotide is 320 Da.
What is the molecular weight of a single-stranded DNA molecule of 17 nucleotides (e.g. a primer)? The average molecular weight of a nucleotide is 320 Da
The Correct Answer and Explanation is:
Correct Answer:
The molecular weight of a single-stranded DNA molecule of 17 nucleotides is approximately 5,120 Daltons (Da).
Explanation
To determine the molecular weight (MW) of a single-stranded DNA (ssDNA) molecule, we multiply the number of nucleotides by the average molecular weight of one nucleotide. In this case, we are told that the average molecular weight of one nucleotide is 320 Da, and the DNA strand contains 17 nucleotides. Therefore: Molecular Weight=17 nucleotides×320 Da/nucleotide=5,440 Da\text{Molecular Weight} = 17 \text{ nucleotides} \times 320 \text{ Da/nucleotide} = 5,440 \text{ Da}
However, this calculation slightly overestimates the molecular weight of a DNA strand. Here’s why:
When nucleotides form a polymer such as DNA, they are connected by phosphodiester bonds, which are created through condensation reactions. Each time a nucleotide is added to the growing DNA strand, a molecule of water (H₂O) (18 Da) is removed. Therefore, for a chain of 17 nucleotides, there are 16 phosphodiester bonds, and hence 16 water molecules are lost: Mass loss due to water=16×18=288 Da\text{Mass loss due to water} = 16 \times 18 = 288 \text{ Da}
So, we correct the initial estimate by subtracting this loss: 5,440 Da−288 Da=5,152 Da5,440 \text{ Da} – 288 \text{ Da} = 5,152 \text{ Da}
For practical purposes, especially in molecular biology, the average molecular weight per nucleotide in a polymerized DNA strand is often approximated as approximately 300 Da (instead of 320 Da) to account for the water loss: 17×300=5,100 Da17 \times 300 = 5,100 \text{ Da}
Thus, depending on the level of precision and the assumptions used, the molecular weight of a 17-mer ssDNA primer is typically approximated between 5,100 to 5,152 Da, with ~5,120 Da being a commonly accepted midpoint for quick estimates.
