Step-by-Step Lesson • Step 1 of 6
ARCH-B-07: Recombinant DNA: Restriction Endonucleases, Sticky Ends, Ligase & Plasmid Marker Screening
Step 1 of 6 • The Core Idea
ARCH-B-07: Why the 130s Classroom Method Fails Under the 90s Clock
Standard Classroom Method (Too Slow)~130s (Blows 90s Clock)
Evaluate each statement against recombinant DNA biochemistry:
1. Prokaryotes cannot splice eukaryotic introns, so mature -cell mRNA is reverse-transcribed into intron-free cDNA (1 is correct).
2. Digesting both vector and insert with the same restriction endonuclease produces complementary single-stranded overhangs ('sticky ends') (2 is correct).
3. Hydrogen bonds form spontaneously by base-pair complementarity; DNA ligase seals nicks in the sugar-phosphate backbone by forming covalent phosphodiester bonds (3 is incorrect).
1. Prokaryotes cannot splice eukaryotic introns, so mature -cell mRNA is reverse-transcribed into intron-free cDNA (1 is correct).
2. Digesting both vector and insert with the same restriction endonuclease produces complementary single-stranded overhangs ('sticky ends') (2 is correct).
3. Hydrogen bonds form spontaneously by base-pair complementarity; DNA ligase seals nicks in the sugar-phosphate backbone by forming covalent phosphodiester bonds (3 is incorrect).
The Fast ShortcutTarget: 35s (Saves 95s)
Classic ESAT trap: Ligase = Phosphodiester backbone, NOT hydrogen bonds eliminate 3 immediately 1 and 2 only.
Where Students Lose Time: In 5 past UAT-UK questions (2.15% of Biology), candidates lose 95s by confusing the cutting enzyme with the joining enzyme, or claiming ligase forms hydrogen bonds between bases (hydrogen bonds form spontaneously!). Instead, apply B6 with Maths 1 skill M3.5.
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