Connect nucleotide structure, complementary strands and semi-conservative copying.
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Method and worked examples
A DNA nucleotide contains a sugar, phosphate and base. Two antiparallel strands form a double helix: A pairs with T and C with G. Hydrogen bonds join complementary bases; covalent bonds form the backbones. Eukaryotic nuclear chromosomes contain linear DNA associated with histones. Bacterial chromosomes are typically circular, and mitochondria and chloroplasts also contain DNA.
Semi-conservative replication gives each daughter molecule one original and one new strand. Each original strand templates its complement; polymerases extend new DNA 5′ to 3′. Write strand directions before finding a complement. After one ideal generation from fully heavy DNA in light medium, every molecule has intermediate density. After the next, half are intermediate and half fully light.
Example 1
A strand is 5′−AGTC−3′. Give its aligned complement.
The aligned complementary strand is 3′−TCAG−5′. Writing that complementary strand in the usual 5′ to 3′ direction gives 5′−GACT−3′.
Example 2
Heavy-labelled DNA replicates once in light medium. What does semi-conservative copying predict?
Every molecule has one heavy original strand and one light new strand, giving one intermediate-density band.
Watch out:That is conservative copying. Semi-conservative replication distributes an original strand into each daughter molecule.
DNA and replication practice | Year 12 Biology | Ascent Arcade