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Joined 2026-09-21 Message
Incoming links: Antibiotic resistance
1973 plasmid construction experiment 2026-10-18
In his account of the 1973 work, Stanley Norman Cohen describes purified R6-5 plasmid DNA cut with EcoRI, reassociated and joined by DNA ligase. The mixture entered calcium chloride-treated Escherichia coli; plates containing antibiotics selected biological colonies showing antibiotic resistance. The recovered pSC102 carried three R6-5 fragments and kanamycin antibiotic resistance. The team then joined material from pSC102 to the carrier pSC101 providing tetracycline antibiotic resistance, recovering pSC105 with both forms of antibiotic resistance.[ref]
DNA cloning: A personal view after 40 years by Stanley Norman Cohen. Cohen (2013), The Hawaii Meeting and the Initial DNA Cloning Experiments, Figure 2 and Figure 3 caption; pSC102, pSC101 and pSC105 identities
Stained agarose gel electrophoresis, gradient centrifugation and electron microscope comparisons characterized recovered plasmids. Parental and mixture lanes in Figure 3 supplied fragment comparisons; pSC101 alone gave one EcoRI fragment, while pSC105 combined its carrier fragment with the resistance-bearing fragment. This supported joined, propagating genetic material beyond selection alone. The comparisons did not determine every nucleotide sequence or establish a universal construction efficiency.[ref]
DNA cloning: A personal view after 40 years by Stanley Norman Cohen. Cohen (2013), paragraphs describing the two laboratories' analyses and Figure 3, top and bottom: parental, mixed and recovered DNA comparisons
The interesting inference combines two kinds of evidence: a recoverable phenotype identifies descendants worth examining, and the extracted DNA tests a structural explanation. The fragment comparisons are molecular scientific controls; they answer a different objection from a control for background growth. The hypothetical diagrams here illustrate that logic without reproducing the historical fragment lengths or biological colony counts.
Antibiotic resistance 2026-10-18
Antibiotic resistance is the ability of a microorganism to grow or survive under specified antibiotic exposure that inhibits a susceptible comparison.
Plasmid Created 2026-10-13 Updated 2026-10-18
A plasmid is an extrachromosomal DNA replicon: genetic material maintained by DNA replication separately from the host's main chromosomes. Separate copying does not mean independence of the host's enzymes or resources. Many plasmids are circular, but naturally occurring linear plasmids also exist.[ref]
Conservation of plasmid maintenance functions between linear and circular plasmids in Borrelia burgdorferi by Philip E. Stewart and others. Stewart, Chaconas and Rosa (2003), abstract opening sentence: natural linear and circular plasmids
Engineered plasmids can carry genes for genetic transformation and functional complementation. Typical laboratory plasmids combine an origin of replication, a selectable marker and a plasmid insert. These are engineering roles, not requirements that every natural plasmid carry an experimentally chosen selectable marker or plasmid insert.[ref]
Molecular Biology Reference by Addgene. Plasmid Elements, Figure 2 and table: origin, antibiotic-resistance gene, insert and promoter; description applies to engineered vectors
The useful separation is between keeping a DNA fragment, finding its carriers and testing its function. Molecular cloning uses propagation to recover more copies for study. Plasmid restriction mapping tests a proposed arrangement, while DNA sequencing examines its letters. A successful genetic transformation or a biological colony showing antibiotic resistance answers neither question completely. The yeast cross-species complementation experiment goes a step further by separating plasmid selection from a test of functional complementation.