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Biochemistry, 4th Edition P94

Biochemistry, 4th Edition P94. Continuing Garrett and Grisham's innovative conceptual and organizing framework, "Essential Questions," BIOCHEMISTRY guides students through course concepts in a way that reveals the beauty and usefulness of biochemistry in the everyday world. Streamlined for increased clarity and readability, this edition also includes new photos and illustrations that show the subject matter consistently throughout the text. New end-of-chapter problems, MCAT practice questions, and the unparalleled text/media integration with the power of CengageNOW round out this exceptional package, giving you the tools you need to both master course concepts and develop critical problem-solving skills you can draw upon. | 28.10 Do Proteins Ever Behave as Genetic Agents 893 Figure 28.30 . Less often 5-BU is inserted into DNA at cytosine sites not T sites. Then if it base-pairs in its keto form mimicking T a C-G to T-A transition ensues. The adenine analog 2-aminopurine recall that adenine is 6-aminopurine normally behaves like A and base-pairs with T. However 2-AP can form a single H bond of sufficient stability with cytosine Figure 28.31 that occasionally C replaces T in DNA replicating in the presence of 2-AP. Hypoxanthine Figure 28.32 is an adenine analog that arises in situ in DNA through oxidative deamination of A. Hypoxanthine base-pairs with cytosine creating an A-T to G-C transition. Chemical Mutagens React with the Bases in DNA Chemical mutagens are agents that chemically modify bases so that their basepairing characteristics are altered. For instance nitrous acid HNO2 causes the oxidative deamination of primary amine groups in adenine and cytosine. Oxidative deamination of cytosine yields uracil which base-pairs the way T does and gives a C-G to T-A transition Figure 28.33a . Hydroxylamine specifically causes C-G to T-A transitions because it reacts specifically with cytosine converting it to a derivative that base-pairs with adenine instead of guanine Figure 28.33c . Alkylating agents Figure 28.33e are also chemical mutagens. Alkylation of reactive sites on the bases to add methyl or ethyl groups alters their H bonding and hence base pairing. For example methylation of O6 on guanine giving O6-methylguanine causes this G to mispair with thymine resulting in a G-C to A-T transition Figure 28.33d . Alkylating agents can also induce point mutations of the transversion type. Alkylation of N7 of guanine labilizes its N-glycosidic bond which leads to elimination of the purine ring creating a gap in the base sequence. An enzyme AP endonuclease then cleaves the sugar-phosphate backbone of the DNA on the 5 -side and the gap can be repaired by enzymatic removal of the 5 .