Negative Charge: DNA molecules carry a natural negative charge due to their phosphate backbone, causing them to move toward a positive electrode in an electric field.
Size Filtering: The agarose gel acts as a molecular sieve; its porous structure allows smaller fragments to pass through quickly while larger fragments are hindered and move more slowly.
Separation by Mass: Because all DNA has the same charge-to-mass ratio, the primary factor determining the distance traveled is the length (number of base pairs) of the fragment.
Inversely Proportional Movement: The distance a DNA fragment travels is roughly inversely proportional to the log of its molecular weight (or size).
Paternity Testing: A child inherits exactly half of their DNA from each parent. Therefore, every band in a child's DNA profile must have a corresponding match in either the mother's or the father's profile.
Forensic Identification: In criminal investigations, DNA profiles from crime scene evidence (e.g., blood or skin) are compared against profiles of suspects or databases to identify a match.
Eliminating Innocents: Profiling is just as important for exonerating innocent individuals as it is for identifying suspects; if a profile does not match, the individual is excluded from the investigation.
Species Classification: By comparing profiles between different organisms, scientists can determine evolutionary relatedness; the more similar the patterns, the more closely related the species are.
| Feature | Paternity Case | Forensic Case |
|---|---|---|
| Goal | Determine biological parentage | Identify an individual suspect |
| Matching Rule | Bands must come from mother OR father | Bands must match the evidence 100% |
| Number of Profiles | Usually three (Child, Mother, Alleged Father) | Usually two (Evidence and Suspect) |
| Logic Applied | Shared inheritance patterns | Exact sequence identity |
Check the Charge: Always remember that DNA is negatively charged; in exams, if asked for the direction of movement, it is always toward the positive anode.
Band Matching Strategy: When solving paternity problems, first identify all bands in the child that match the mother. Any remaining bands in the child must be found in the biological father.
Size and Distance: Be careful not to confuse fragment size with distance; the bands furthest from the starting wells are the smallest fragments, not the largest.
Vocabulary Precision: Use the term 'fragments' when discussing the cut DNA and 'bands' when discussing the visual results on the gel.
Privacy and Consent: Genetic information is highly personal; there are significant concerns regarding who has access to an individual's profile and how that data is stored.
Genetic Discrimination: There is a risk that insurance companies or employers could use profiling results to deny coverage or jobs based on a predisposition to certain health conditions.
Psychological Impact: Learning about a genetic risk for a serious disease can cause significant stress and anxiety for the individual and their family members.
Reproductive Choices: Screening of embryos or fetuses raises complex ethical questions regarding the termination of pregnancies or the selection of specific traits.
