Carrier RNA or indeed inert carriers like glycogen increase recovery and thus yield of DNA in this case but also RNA following extraction

Extraction is perfectly feasible in the absence of carrier and is thus not strictly mandatory but when dealing with small ug amounts of total DNA or RNA recovery can be proportionately and significantly lower in the absence of a carrier 

Carrier RNA or indeed inert carriers like glycogen increase recovery and thus yield of DNA in this case but also RNA following extraction

Extraction is perfectly feasible in the absence of carrier and is thus not strictly mandatory but when dealing with small ug amounts of total DNA or RNA recovery can be proportionately and significantly lower in the absence of a carrier 

I think it work because you need precipitation and with tiny amounts of dna it may stay in solution at the precipitation step but adding inert rna or glycogen saturates the regent mix and starts the precipitation process and give small particles that the dna can stick to making a larger particle which spins out easier so it co-precipitates. Also a very small precipitate may not stick to the tube so is lost on  aspiration or decanting but a larger volume precipitate  will stick to the tube better . Some co-precipitants are coloured to make it easier to see the precipitate to help with supernatant aspiration

Hello Paul.

Agreed. The solubility product is reduced and consequently things more readily precipitate. In addition as I understand things the conformation is altered to reduce the so called Stokes radius which increases sedimentation. I have always likened it to the difference between a sky diver acted on by gravity stretching out their body and falling gradually and curling into a ball wherein they fall to earth more quickly 

hi Lawrence,

I think that is particularly true for large dna which is heavily solvated so in conditions which do not favour hydrogen bonding the molecule becomes very small in diameter and dense so spins down well as you say. I think there may be a further effect where small degraded dna might form a colloid which is difficult to centrifuge butif rna or glycogen is also forming non solvated particles these may aggregate and give a much larger , also dense, mass which spins down better so my feeling is that co-precipitants are mainly useful for small amounts of small size dna

Another possible reason may be that it helps DNA leave the surface. Some amount of DNA will never leave, so add carrier so that the analyte DNA becomes a small fraction of amount bound, and therefore a small fraction of the amount that stays bound.