BIO130H1 Lecture Notes - Lecture 6: Ribose, Deoxyribose, Dna Repair
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BIO130H1 Full Course Notes
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Dna topoismerases prevent dna tangling during replication (cid:862)(cid:449)i(cid:374)di(cid:374)g p(cid:396)o(cid:271)le(cid:373)(cid:863) every 10 bp replicated at fork corresponds to one turn about axis of double helix. Swivel is formed in dna helix by proteins known as dna topoisomerases (energetically more favourable than having the entire chromosome ahead to rotate rapidly: topoisomerase a reversible nuclease that adds itself to dna backbone phosphate. Breaks phosphodiester bond; is reversible, so when topoisomerase leaves, phosphodiester bond reforms. Topoisomerase i produces nick, allowing two sections of dna helix to rotate freely relative to each other (tension in dna helix drives this rotation, hence relieving the tension: prevents severe dna tangling problems. Okazaki fragments encounter special problem when replication fork reaches the end of a linear chromosome: no place to produce rna primer needed to start last okazaki fragment at the tip of linear dna molecule. Bacteria solve this by having circular dna.
