A length of 15–25 bp is recommended, with a Tm of at least 48°C, estimated using 2°C per A/T and 4°C per G/C. Arms that are too short reduce recombination efficiency, while overly long arms may affect PCR specificity. Avoid three or more identical consecutive bases at the 3′ end of a primer, keep the Tm difference between forward and reverse primers within 2°C, and use tools such as SnapGene, Benchling, or NEBuilder for design support.

① Confirm that the vector was fully linearized by increasing the enzyme amount, extending digestion time, and gel-purifying the product. ② Dephosphorylate vectors cut with a single enzyme. ③ Confirm that the antibiotic is active and use freshly prepared selective plates. ④ Keep the insert-to-vector molar ratio at 2:1–3:1, or at least 5:1 for inserts under 200 bp. Recommended DNA purity is A260/A280 = 1.6–1.9 and A260/A230 ≈2.

The colony PCR may have amplified a nonspecific product, or homologous repetitive sequences in the vector or insert may have caused rearrangement. Gel-purify the PCR product, optimize amplification conditions, pick and prepare plasmid DNA from several additional colonies, and include a negative control to rule out false positives.

Confirm that absolute ethanol was added to the wash buffer as indicated on the bottle. If the gel-dissolving buffer turns orange-red or pink, indicating pH >7.5, add 10 μL of 3 M sodium acetate (pH 5.0) to restore a light-orange color. Use freshly prepared TAE or TBE. For long fragments of 10–15 kb, increase the input amount as appropriate, increase the elution volume to 50 μL, and prewarm the elution buffer to 65°C.

Residual ethanol is the most common cause. After washing, centrifuge the empty column at 12,000 rpm for 1 minute, then leave the cap open at room temperature for 5–10 minutes to allow the ethanol to evaporate completely. A diffuse electrophoresis band may indicate single-stranded DNA; heat at 95°C for 1 minute and cool slowly to reanneal. For elution, use a low-salt buffer at pH 8.0–8.5 and apply it to the center of the membrane for thorough wetting.

For low yield, use a fresh culture, confirm that the antibiotic is active, increase the culture volume for low-copy plasmids, up to 4–6 mL for a miniprep or 10 mL for a midiprep, and use an elution volume of at least 50 μL for a miniprep or 100 μL for a midiprep. Prewarming the elution buffer to 65°C can improve yield. For RNA contamination, confirm that all RNase A was added to Solution I. If the solution has been stored for more than three months, add fresh RNase A, or reduce the starting culture volume.

The most common cause is incomplete lysis due to excessive bacterial biomass. Reduce the culture volume or divide the sample between two tubes. Other possible causes include inadequate resuspension in Solution I, which should leave no visible cell clumps, or contamination by an unwanted microorganism.

1 kb-IV, with 13 bands from 250 bp to 12 kb, is suitable for verifying large fragments and long-insert vectors. 100 bp-II plus, with 11 bands from 100 bp to 1,500 bp, is suitable for precise sizing of small fragments and indel detection. 250 bp-I, with 6 bands from 100 bp to 2,000 bp, is suitable for rapid analysis of PCR and restriction-digestion products. 250 bp-II, with 9 bands from 100 bp to 5,000 bp, is suitable for cloning screens involving medium-sized inserts.