1.5% agarose should work.I would run the gel slowly to keep it cool and minimise band broadening so slower and for a longer time ( half voltage and double time at least). You might consider a double digest if one of the fragments has a cut site that the other does not so you separate 291 from 150+123 for instance. Acrylamide will work ,of course, but you will only get one try per run so it is very good for many samples run at different times because it is very reproducible but agarose gives you flexibility for each run to have a look and then run further if that will improve the visualisation of the result

I agree with Paul, I can also suggest a longer gel because your bands will reach more than 3/4 of the total gel, I think the important thing is that you do the run slowly to separate, after digestion, the two bands,

I really thank you, I will try it with conditions that you recommended me, I have an other question, I was using TAE buffer for electrophoresis, it will be recommendable change to TBE?

And Paul the restriction enzyme that I used cut one bp after the primer I mean the primer Forward was change in one nucleotide to identify the SNP of my interest with Taq l, for this I need to identify the fragments of heterozygous genotype, 293 and 271 (it was a mistake in description), some days ago I tried with 2.5% agarose gel but I did not see the separation; for the amplification I was using 1% of agarose and I see the fragment without any problem. In literature that I found recommend increase to 3% or more because the camera that I have is not to long to make a long gel (length 15 cm aprox), to a better recomendation I put the conditions that I have been using in electrophoresis of the digestion reaction:

Thank you for your help, I appreciate all your contribution.

ok 2 things possible.

1 cut with a common enzyme then the separation is for example 150 from 170 and a common fragment, The relative size makes the separation better.

2 design a new forward (acrs? ) primer with maybe a 15 base poly A ( or anything really) tail on the 5'end. Then when you cut it will be an easier separation of about 308 and 271 +37 which is easier to visualise.

Don't worry about the size of the picture of the gel you take as only the part of the gel with the cut fragments is important for comparison between samples

Hi

The two bands should easily separate, as suggested by Paul. Even in a very commonly used DNA marker (Phi X174 cut with HaeIII) there is a 281-271 band pair (in the size range your are using) and it is seen as two bands. See also that you don't load too much DNA in the well.