This untangles the DNA releasing the DNA spine again. Snce the chemical composition of the DNA remains the same, te untangled DNAs are chemical isomers. Terefore, tpoisomerases are isomerase enzymes which work on the DNA topology. Te N-terminal domain is then preceded by a highly conserved, 421 amino acid core domain that contains all of the catalytic residues except the active site tyrosine. Aprotease-sensitive and poorly conserved linker domain comprising 77 amino acidsconnects the core domain to the 53 amino acid C-terminal domain. A active form of the be reconstituted by mixing together fragments corresponding approximately to the core domain (residues 175 to 659) and the C-terminal domain (residues 713 to 765), ad thus the linker is dispensable for relaxation activity in vitro.
Te active site Tyr723 can be observed within the C-terminal domain. Te structure of the enzyme (pdb entry 1A36) is viewed from the side with the DNA axis horizontally oriented (a) and looking down the axis of the DNA (b). Cre subdomains I (residues 215–232 and320–433), I (residues 233–319) and III (residues 434–633) are bue, ad red, rspectively.
Te linker (residues 641–712) and C-terminal domain (residues 713–765) are shown in orange and green, rspectively. I panel a, te amino terminus (N) and carboxyl terminus (C) of the protein are indicated, ad the active site tyrosine is shown in black ball and stick. Te long-helix that connects the cap to the base of the core is labeled “Connector” in panel a. I panel b, te “Lips” region where the protein opens during DNAbinding and unbinding is shown with opposing arrows, ad the hinge at top of the connector helix is labeled “Putative hinge”.
Gnomic integrity must be maintained in a cell, hnce after replication, te two new DNA structures should be totally and completely separated. Hnce when there is resumption of transcription bubble, tere is an over wound of DNA which is behind transcription fork, wile the DNA that is behind the bubble becomes under wound. I replication the DNA that is ahead becomes over wound while the DNA that is behind the fork becomes under wound. Te climax of replication a situation requires disentangling before an occurrence of mitosis.
I the figure 1. 3above, i shows how the enzyme binds to DNA (in. ..
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