In standard constant-field agarose gel electrophoresis, DNA molecules larger than $30 - 50 \text{ kb}$ migrate together regardless of size (reptation limit). Pulsed-Field Gel Electrophoresis (PFGE), developed by Schwartz and Cantor, periodically alternates the direction of the electrical field across the gel. Larger DNA molecules require more time to reorient each time the electric field changes direction, allowing the resolution of very large DNA molecules ranging from $50 \text{ kb}$ up to $10 \text{ Mb}$ (intact fungal chromosomes, YACs, and bacterial genomes).

A standard PCR thermal cycle consists of three fundamental steps:

• Denaturation: Carried out at the highest temperature ($94^\circ\text{C} - 98^\circ\text{C}$) to disrupt inter-strand hydrogen bonds and melt double-stranded DNA into single strands.
• Annealing: Carried out at the lowest temperature ($50^\circ\text{C} - 65^\circ\text{C}$, typically $3 - 5^\circ\text{C}$ below the primer Tm) to permit oligonucleotide primers to bind complementary sequences.
• Extension/Elongation: Operated at $72^\circ\text{C}$, the enzymatic optimum for thermostable Taq DNA polymerase.

Bacterial genetic exchange occurs via three primary mechanisms:

• Transformation: Uptake of naked foreign DNA from the surrounding extracellular medium by competent cells. Because the DNA is exposed in solution, the process is completely inhibited by addition of deoxyribonuclease (DNase).
• Transduction: Bacteriophage-mediated transfer where viral capsids protect the donor DNA from environmental DNase.
• Conjugation (including Sexduction): Direct physical cell-to-cell transfer through a conjugation bridge (sex pilus), preventing exposure of DNA to external DNase.

The cross $AaBb \times aabb$ is a standard dihybrid testcross involving two independently assorting loci. The heterozygous parent ($AaBb$) produces four distinct gametic genotypes ($AB$, $Ab$, $aB$, and $ab$) in equal frequencies of 25% each ($1:1:1:1$). The homozygous recessive parent ($aabb$) produces only $ab$ gametes. Consequently, the resulting progeny exhibit four genotypes: $AaBb$, $Aabb$, $aaBb$, and $aabb$, in a strict 1:1:1:1 ratio. Note: Dominance relationships (complete, incomplete, or codominance) alter only the phenotypic ratios, never the underlying Mendelian genotypic segregation ratios.