Re: Can the length and Mr be determined for DNA molecule if no of bp is known?

If the size of a DNA molecule is known in Kbp eg by 
electrophoresis would it possible to work out it's length in eg 
cm and it's Mr in daltons. If so how would you go about it? 

Both length and mass are very easy to determine if the number of base pairs (bp) is known, since the molecular mass and height of each base pair is easy to find.

First, the molecular mass of each nucleotide is dependent on the base it contains, so the molecular mass of a base pair would be the sum of the two complementary nucleotides. Since there are two types of base pairs, A:T and G:C, this yields two numbers: 653 g/Mol for A:T, and 652 g/Mol for G:C. These are close enough for a rough estimate of 652 g/Mol per base pair. Actually, with very long pieces of DNA, the difference between A:T and G:C becomes noticeable, especially in cases where there is a sizeable difference between A:T pairs and G:C pairs - this difference is refered to as the "GC content" of the DNA and is expressed as a percentage. (Beside MW, knowing the length and GC content of a strand of DNA can also tell you the "melting point" at which the two strands can be separated.)

Next, the height of a base pair is almost entirely dependent on the nature of the DNA helix: A-DNA (dehydrated DNA, DNA:RNA, and RNA helices), B-DNA (normal DNA), or Z-DNA (left-handed DNA helix, very rare). Assuming you're working with B-DNA, the height of a single base pair is 3.4 Å (Ångstroms: 1 Å equals 10^-10 meters).

So, putting the two numbers together, a 10 Kb (kilobases: 1Kb equals 1000 bp) DNA fragment would have a molecular mass around 6,520,000 g/Mol and be 34,000 Å (3.4 µm, 3.4 x 10^-6 meters) long. Similarly, the human genome is about 3 x 10⁹ bp long, so placed end to end the chromosomes from a single cell would stretch about 1 meter and have a mass around 40 pg (picograms: 10^-12 grams).