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Unformatted text preview: X X X X X X V V V . . . D D D N N N A A A R R R e e e p p p l l l i i i c c c a a a t t t i i i o o o n n n Watson and Crick in 1953 proposed a model of DNA as a double helix (in a one-page letter to the journal Nature) according to which the DNA strands are complementary. By complementary, they meant that each strand contains the information to make the other strand, e.g. if one strand reads 3'-G-T-C-5', the other strand must be 5'-C-A-G-3'. S S S e e e m m m i i i--- c c c o o o n n n s s s e e e r r r v v v a a a t t t i i i v v v e e e R R R e e e p p p l l l i i i c c c a a a t t t i i i o o o n n n Because Watson and Crick had almost no experimental data, Kornberg (1957) incubated DNA with a polymerase (a polymerizing enzyme) from E. coli and a mixture of bases. His mixture synthesized DNA daughter molecules identical to those he began with. Replication of the strand could have been: (i) conservative (mother double helix makes totally new daughter double helix, (ii) semi-conservative (mother splits into two separate strands, and each of them makes the other missing strand), or (iii) random (the mother DNA is randomly chopped up, and the bases from the mother end up in both strands of offspring DNA)....
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