= Gel band
A <gel band> is a localized population of stained or otherwise detected <molecules> with sufficiently similar migration in one sample lane. One visible <gel band> need not contain only one <nucleotide sequence>. Two different linear <DNA> fragments of 375 <base pairs> can occupy the same unresolved <gel band>; equal migration supplies size evidence under suitable conditions, not a sequence determination.
<Fluorescence> from a bound stain can support an estimate of <DNA> quantity, but suitable staining and detector response matter.<Five Main Nucleic Acid Gel Electrophoresis Steps by Thermo Fisher Scientific>[Step 4, Sample visualization, Fluorescent dyes: relation between fluorescence and bound nucleic acid]{ref}
For a quantitative model with equal staining response per unit <mass>, background-subtracted, integrated <fluorescence> $I$ satisfies $I=km$. This requires an unsaturated detector and a common proportionality constant $k$; visual peak brightness alone does not supply $I$.
The <amount of substance> is $n=m/M$, where $M$ is <molar mass>. For double-stranded <DNA> with approximately equal average <mass> per <base pair>, $M$ is proportional to length $L$, so $n$ is proportional to $I/L$. In a constructed example, equal integrated signals from 250-<base pair> and 1000-<base pair> <gel bands> imply approximately equal <masses> but four times as many short <DNA> <molecules>. Conversely, equal numbers of those <DNA> <molecules> would give about four times the signal for the longer fragments in this model. Neither conclusion follows from darkness in a schematic drawing.
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