barcode scanning in c#.net Figure 12.36 RecA-dependent postreplicative DNA repair. DNA in Software

Creating QR Code JIS X 0510 in Software Figure 12.36 RecA-dependent postreplicative DNA repair. DNA

Figure 12.36 RecA-dependent postreplicative DNA repair. DNA
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polymerase III skips past a thymine dimer during DNA replication (a). With the help of RecA, the single strand with the thymine dimer invades the normal sister duplex (b). An endonuclease nicks the new duplex at either side of the thymine dimer site, freeing the new duplex with the thymine dimer and leaving the sister duplex single-stranded (c). Repair enzymes then create two intact daughter duplexes (d).
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processes then complete both strands. In figure 12.36a, we see a replication fork with a gap in the progeny strand in the region of a thymine dimer. The RecA protein is responsible for the damaged single strand invading the sister duplex ( g. 12.36b). Endonuclease activity then frees the double helix containing the thymine dimer ( g. 12.36c). DNA polymerase I and DNA ligase return both daughter helices to the intact state ( g. 12.36d ). The thymine dimer still exists, but now its duplex is intact, and another cell cycle is avail-
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Tamarin: Principles of Genetics, Seventh Edition
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III. Molecular Genetics
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12. DNA: Its Mutation, Repair, and Recombination
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The McGraw Hill Companies, 2001
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Recombination
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Figure 12.37 The LexA protein represses its own gene, recA,
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and several other loci (uvrA, uvrB, uvrD, sulA, and sulB) by binding at the SOS box in each of the loci. Activated RecA protein causes autocatalysis of LexA, eliminating the repression of all these loci, which are then transcribed and translated.
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No repression LexA protein Repression No LexA autocatalysis Yes Catalyzed LexA proteins
RecA activated
LexA proteins
Translation
Transcription
SOS box
uvrA uvrB uvrD sulA sulB + others
SOS box
recA
SOS box
lexA
R E C O M B I N AT I O N
Although recombination, the nonparental arrangement of alleles in progeny, can come about both by independent assortment and crossing over, we are concerned here with recombination due to crossing over between homologous pieces of DNA (homologous recombination). We brie y discuss transpositional recombination in chapter 14 and site-speci c recombination (e.g., integration) in chapters 7, 14, and 16. Recombination is a breakage-and-reunion process. Homologous parts of chromosomes come into apposition and are then reconnected in a crosswise fashion (see g. 6.4). This general model ts what we know about the concordance of recombination and repair: Both involve breakage of the DNA and a small amount of repair synthesis, and both involve some of the same enzymes.
ously because a double-strand break was thought too dangerous a DNA lesion for cellular enzymes to create. However, we now know that the double-strand break model is generally correct, and we refer to the Holliday junction for an intermediate stage in the process. The model depends on DNA complementarity between the recombining molecules and is thus a model of great precision. We begin with two double helices lined up as they would be, for example, in a meiotic tetrad, ready to undergo recombination ( g. 12.38a). The rst step of the process is a double-stranded break in one of the double helices. In eukaryotes, the protein Spo11 accomplishes
R. Holliday (1932 ).
(Courtesy
of James L. German, III, M.D.)
Double-Strand Break Model of Recombination
In 1964, R. Holliday suggested a model of homologous recombination that involved simultaneous breaks in one strand each of the two double helices that were to cross over. In 1983, J. Szostak and colleagues put forth a different model, initiated by a double-strand break in one of the double helices. At rst, this model was not considered seri-
Tamarin: Principles of Genetics, Seventh Edition
III. Molecular Genetics
12. DNA: Its Mutation, Repair, and Recombination
The McGraw Hill Companies, 2001
Twelve
DNA: Its Mutation, Repair, and Recombination
Table 12.5 Some of the Enzymes and Proteins Involved in DNA Repair in E. coli, Not Including DNA
Polymerase I and III, DNA Ligase, and Single-Strand Binding Proteins
Enzyme Damage Reversal DNA photolyase DNA methyltransferase Base Excision Repair Uracil-DNA glycosylase Endonuclease IV Exonuclease, lyase, or phosphodiesterase Nucleotide Excision Repair UvrA UvrB UvrC UvrD (helicase II) Mismatch Repair MutH MutL MutS MutU (UvrD) Exonucleases DNA methylase Double-Strand Break Repair Ku PKCS DNA ligase IV XRCC4 Postreplicative Repair Polymerase IV Polymerase V Polymerase Polymerase RecA LexA SulA, SulB DinB UmuC, UmuD RAD30 REV3, REV7 recA lexA sulA, sulB DNA polymerase DNA polymerase DNA polymerase DNA polymerase Single-stranded DNA invades double-stranded DNA; causes LexA to autocatalyze; protease Represses SOS proteins Inhibit cell division Ku70, Ku80 PKCS LIG4 XRCC4 Binds to broken chromosomal ends Protein kinase Ligates broken ends of DNA Stabilization protein mutH mutL mutS mutU recJ, xseA, sbcB dam Nicks DNA at recognition sequence Recognizes mismatch Binds at mismatch Unwinds oligonucleotide Degrades unwound oligonucleotide Methylates 5 -GATC-3 DNA sequences uvrA uvrB uvrC uvrD With UvrB, locates thymine dimers and other distortions Nicks DNA on the 3 side of the lesion Nicks DNA on the 5 side of the lesion Unwinds oligonucleotide ung nfo several Removes uracils from DNA Nicks AP sites on the 5 side Removes base-free nucleotide phr ada Undimerizes thymine dimers Demethylates guanines in DNA Gene Action
this. The break is followed by 5 3 exonuclease activity to widen the gaps formed in the double helix and create 3 single-stranded tails ( g. 12.38b, c). These tails are coated with RecA protein that then catalyzes the invasion of one of the single strands into the intact double helix in direct apposition ( g. 12.38d ). Repair of singlestranded DNA by DNA polymerase I and DNA ligase then replaces sections of previously digested DNA (fig.
12.38e). At this point, there is no lost genetic material; however the two double helices are interlocked and need to be freed of each other. Before that happens, however, branch migration can take place, a process in which the crossover point can slide down the duplexes ( g. 12.38f ). In E. coli, the RuvAB complex, the product of the ruvA and ruvB genes that together form an ATP-dependent motor, moves the junction point
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