(BQ) Part 2 book “Radiobiology for the radiologist” has contents: Molecular techniques in radiobiology, cancer biology, clinical response of normal tissues, model tumor systems, alternative radiation modalities, the biology and exploitation of tumor hypoxia,… and other contents.
For Students of Radiation Oncology 475 chapter 17 Molecular Techniques Radiobiology Historical Perspectives The Structure of DNA RNA and DNA Transcription and Translation The Genetic Code Amino Acids and Proteins Restriction Endonucleases Vectors Plasmids Bacteriophage λ Bacterial Artificial Chromosomes Viruses Libraries Genomic Library cDNA Library Hosts Escherichia Coli Yeast Mammalian Cells DNA-mediated Gene Transfer Agarose Gel Electrophoresis Polymerase Chain Reaction Polymerase Chain Reaction–mediated Site-directed Mutagenesis Gene-Cloning Strategies Genomic Analyses Mapping 476 in DNA Sequence Analyses Polymorphisms or Mutations Comparative Genome Hybridization Gene Knockout Strategies Clustered Regularly Interspaced Short Palindromic Repeats and CRISPR Associated Protein Homologous Recombination to Knockout Genes Knockout Mice Gene Expression Analysis Northern Blotting RNA Interference Reverse Transcription Polymerase Chain Reaction Quantitative Real-Time Polymerase Chain Reaction Genetic Reporters Promoter Bashing Chromatin Immunoprecipitation Protein–DNA Interaction Arrays (Chromatin Immunoprecipitation-Chips) Microarrays to Assay Gene Expression RNA-Seq to Assay Gene Expression Chromatin Immunoprecipitation-Seq Protein Analysis Western Blotting Antibody Production Immunoprecipitation Far Western Blotting Fluorescent Proteins Two-Hybrid Screening Split Luciferase Complementation Assay Proteomics Two-Dimensional Electrophoresis Databases and Sequence Analysis Summary of Pertinent Conclusions Glossary of Terms Bibliography 477 HISTORICAL PERSPECTIVES Recombinant DNA technology has revolutionized research in biology It allows questions to be asked that were unthinkable just a few years ago It is also a technology that is moving so fast that anything written in a book is likely to be out of date before it appears in print This technology is invading every field of biologic research, and radiobiology is no exception To keep abreast of developments in the field, it is essential to know what recombinant DNA technology is and how it works A detailed description is beyond the scope of this book; for a more extensive account, the interested reader is referred to several excellent volumes that have appeared in recent years and are listed in the “Bibliography.” The goal here is to provide an overview of core recombinant techniques and core technologies that are commonly used today in radiobiologic research The birth of molecular biology could be ascribed to the one-page publication in Nature in 1953 by James Watson and Francis Crick describing the structure of DNA In short order, this work led the way to breaking the genetic code and understanding the process of transcription of DNA to messenger RNA (mRNA) and the translation of mRNA into proteins At about the same time, in the late 1940s and early 1950s, Linus Pauling realized that three-dimensional structures were built by amino acids and folded into proteins The whole concept emerged that the sequence of bases, which coded for a protein, ultimately determined function These remarkable discoveries were followed by a period of limited progress focusing mainly on simple systems such as viruses, bacteriophages, and bacteria, until new tools and techniques to work with DNA were perfected Recombinant DNA technology got its start with the first successful cloning experiment by Stanley Cohen, in which he joined two DNA fragments together (a plasmid containing a tetracycline resistance gene with a kanamycin resistance gene), introduced this recombinant molecule into Escherichia coli and demonstrated that the E coli with the plasmid now had dual antibiotic resistance This simple experiment was only possible because of the simultaneous development of several techniques for cutting DNA with restriction enzymes, joining the fragments together with ligases, and using E coli as a host to take up foreign DNA packaged as plasmid vectors This critical demonstration was quickly followed by the development of methods to sort pieces of DNA and 478 RNA by size using gel electrophoresis The stage was set for an explosion of new techniques What follows is a brief and simplified description of these techniques and technologies that followed THE STRUCTURE OF DNA The structure of DNA arrived at by Crick and Watson is elegant in its simplicity The molecule is composed of two antiparallel helices, looking rather like a gently twisted ladder The rails of the ladder, which run in opposite directions, contain units of deoxyribose sugar alternating with a phosphate Each rung is composed of a pair of nucleotides, a base pair, held together by hydrogen bonds (Fig 17.1) There is a complementary relationship between the bases: Adenine always pairs with thymine, and cytosine always pairs with guanine (complementary nucleotides) Thus, the nucleotide sequence of one strand of the DNA helix determines the sequence of the other FIGURE 17.1 The DNA double helix is held together by hydrogen bonds between base pairs These are shown as dotted lines in the figure This structure explains how a DNA molecule replicates during cell division so that each progeny cell receives an identical set of instructions The hydrogen bonds between the base pairs break, allowing the DNA ladder to unzip (Fig 17.2) Each half then constitutes a template for the reconstruction of the other half Two identical DNA molecules result, one for each progeny cell 479 FIGURE 17.2 The complementary nature of DNA is at the heart of its capacity for self-replication The two strands of the parental DNA unwind, and the hydrogen bonds break Each strand then becomes a template to specify a new progeny strand obeying the base-pairing rules RNA AND DNA Unlike DNA, which is located primarily in the nucleus, RNA is found throughout the cell Within the nucleus, RNA is concentrated in the nucleoli, dense granules attached to chromosomes The sugar molecule in RNA is a ribose (hence its name, ribonucleic acid), whereas in DNA, the sugar molecule is a deoxyribose (hence its name, deoxyribonucleic acid) In both DNA and RNA, the bases are made up of two purines and two pyrimidines The two purines, adenine and guanine, as well as the pyrimidine, cytosine, are common to both DNA and RNA However, although thymine is found only in DNA, the structurally similar pyrimidine uracil appears in RNA (Fig 17.3) FIGURE 17.3 Illustrating the pairing of complementary bases in DNA and 480 RNA Left: DNA contains the purines adenine (A) and guanine (G) as well as the pyrimidine’s thymine (T) and cytosine (C) A purine always pairs with a pyrimidine; specifically, A pairs with T, and G pairs with C Right: RNA contains uracil (U) instead of thymine In this case, A pairs with U, and G pairs with C TRANSCRIPTION AND TRANSLATION The flow of genetic information from DNA to protein requires a series of steps In the first step, the DNA code is transcribed in the nucleus into mRNA by RNA polymerase (Fig 17.4) It is at once obvious from a comparison of a mature cytoplasmic mRNA transcript with its parental DNA that the mRNA sequence is not contiguous with the DNA sequence Some blocks of DNA sequence are represented in the mRNA; others are not DNA is transcribed into pre-mRNA During the process of splicing, large regions called introns are removed and the remaining exons are joined together into what is termed an open reading frame Only the exons of the DNA are translated Almost all genes from higher eukaryotes contain introns; genes may have only a few or as many as 100 introns Typically, introns make up the bulk of the gene For example, in the gene involved with muscular dystrophy, the mRNA consists of 14,000 bases, whereas the gene spans more than million base pairs The mRNA transcript associates with a ribosome, at which, with the help of ribosomal RNA and transfer RNA (tRNA), the mRNA message is translated into a protein FIGURE 17.4 Transcription and translation The “information” in DNA is linear, consisting of combinations of the four nucleotides: adenine, guanine, cytosine, and thymine The information is transcribed into messenger RNA (mRNA), which in turn is a complementary version of the DNA code The 481 mRNA message is spliced in the nucleus to remove introns and is then transported to the cytoplasm for translation into protein Triplet RNA codons specify each of the 20 amino acids The sequence of amino acids determines the protein, which ultimately has three-dimensional form THE GENETIC CODE The genetic code was cracked by 1966 Triplet mRNA sequences specify each of the amino acids Because there are four bases, the number of possibilities for a three-letter code is × × 4, or 64 There are only 20 amino acids, however; consequently, more than one triplet can code for the same amino acid—that is, there is redundancy in the code Because nearly all proteins begin with the amino acid methionine, the initiation codon (AUG) represents the “start” signal for protein synthesis Three codons for which there are no naturally occurring tRNAs—UAA, UAG, and UGA—are “stop” signals that terminate translation (termination codons) Only methionine and tryptophan are specified by a unique codon; all other amino acids are specified by two or more different codons As a consequence of this redundancy, a single-base change in RNA does not necessarily change the amino acid coded for Given the position of the bases in a codon, it is possible from Table 17.1 to find the corresponding amino acid For example, 5′ CAU 3′ specifies histidine, whereas AUG specifies methionine Glycine is specified by any of four codons: GGU, GGC, GGA, or GGG Table 17.1 Codes for the Amino Acids SECOND POSITION FIRST POSITION (5′ END) U C A G THIRD POSITION (3′ END) U Phe Ser Tyr Cys U Phe Ser Tyr Cys C 482 A G C Leu Ser Stop Stop A Leu Ser Stop Trp G Ile Thr Asn Ser U Ile Thr Asn Ser C Ile Thr Lys Arg A Met Thr Lys Arg G Val Ala Asp Gly U Val Ala Asp Gly C Val Ala Glu Gly A Val Ala Glu Gly G Leu Pro His Arg U Leu Pro His Arg C 483 Leu Pro Gln Arg A Leu Pro Gln Arg G AMINO ACIDS AND PROTEINS Most of proteins are composed of a mixture of the same 20 amino acids Each polypeptide chain is characterized by a unique sequence of its amino acids Chain lengths vary from to more than 4,000 amino acids Most proteins contain only one polypeptide chain, but others are formed through the aggregation of separately synthesized chains that have different sequences Although most proteins are enzymes (i.e., they act as catalysts, inducing chemical changes in other substances but themselves remaining apparently unchanged), many have structural roles as well The essential fabric of both nuclear and plasma membranes is formed of proteins Once a polypeptide chain is synthesized from a string of amino acids, it tends to fold up into a three-dimensional form, the shape of which is governed by the weak chemical interactions between the side groups of the amino acids Each three-dimensional shape is unique to the amino acid sequence The shape of a protein is the key to its function RESTRICTION ENDONUCLEASES Restriction enzymes are endonucleases found in bacteria that have the property of recognizing a specific DNA sequence and cleaving at or near that site These enzymes can be grouped into three categories: types I, II, and III The restriction enzymes commonly used are of type II, meaning that they have endonuclease activity only (i.e., they cut the DNA without modification) at a predictable site within or adjacent to the recognition sequence Types I and III have properties that make them impractical for use in molecular biology More than a thousand type II enzymes have been isolated, and more than 70 are commercially available A few examples are shown in Table 17.2 They are named according to the following system: The first letter comes from the genus of the organism from which the enzyme was isolated The second and third letters follow the organism’s species name 484 Stone, R., 445, 445f stop codon, 260, 292 Strandquist, M A., 418f Strandquist plot, 418, 418f streptonigrin, 502t stress response, in hypoxia, 461–462 structurally defined FSUs, 353 structurally undefined FSUs, 353 sublethal damage (SLD), 67 sublethal damage (SLD) repair, 69–77 chemotherapy and, 498–500, 506 dose-rate effect and, 72–77 in in vivo systems in mice, 70–71, 70f mechanisms of, 72 processes in, 70 radiation quality and, 72, 72f split-dose experiments with, 69–72, 69f, 71f, 72f sucrose gradient sedimentation, 13 Sugimachi, K., 526 Suit, H D., 384, 384f, 449f, 454f “suitcase bombs,” 197–198, 199 sulfhydryl (SH) compounds covering with phosphate group, 127–129, 128t mechanism of action, 126–127, 127f as radioprotectors, 126–130 Sulman, E P., 440 sunitinib, 489t superficial malignancies, radiation and hyperthermia for, 525–526, 525f 1147 supportive care, as countermeasure, 131 Surveillance, Epidemiology, and End Results (SEER) program, 149 Sutherland, R M., 391f, 392f Swift, M., 316, 316t Symington, T., 412t symmetric translocations, 26–29, 31f frequency, FISH scoring of, 33 in Hiroshima/Nagasaki survivors, 33 as stable aberrations, 33 synchrocyclotron (Sweden), 448 synchronously dividing cell cultures, 58–62 definition of, 58 hydroxyurea-induced, 58–59, 58f, 59f techniques for producing, 58–59 x-ray effects on, 59–62, 60f, 61f, 62f synergistic interaction, chemotherapy and radiation, 509 synthetic lethality, 496–497, 497f T cell checkpoint therapy, 321–322, 322f, 497–498 T lymphocytes (T cells), radiosensitivity of, 363 tamoxifen, 486t, 493 tandem repeats, 275 Tannock, I F., 410t Taq polymerase, 271–272, 292 target organ, in nuclear medicine, 225 targeted radiotherapy, hypoxia monitoring for, 93 targeted therapies (chemotherapy), 487t–490t, 494–496 taxanes, 484t–485t, 493 adjunct use with radiation, 508, 509f 1148 cell-cycle specificity of, 476, 477f sensitivity to, 500f Taxol See paclitaxel Taxotere See docetaxel Taylor, L., 237 Tc See cell cycle time TCD50 See tumor cure (TCD50) assay TC-NER See transcription-coupled repair TCP See tumor control probability TCR See transcription-coupled repair technetium-99m, 225 Teicher, B A., 501f, 502t teletherapy, 77 telomerase, 25, 312f, 357 telomeres, 24–25, 312–313, 312f, 357 telophase, 24 temozolomide, 480t temperatures See also hyperthermia cytotoxic, 517–520 noncytotoxic, 521–522, 521f, 522f template, DNA, 259, 292 TER See thermal enhancement ratio Terasima, T., 58, 60–61, 61f termination codons, 260, 292 terrorism, radiologic, 197–205 availability of radioactive material for, 199–201, 200f, 201f “dirty bomb,” 198–199, 198f, 199f external exposure in, 201–202 1149 health effects in, 201 improvised nuclear device, 197–198, 197t, 199 internal contamination in, 202 medical countermeasures in, 126, 128–129, 132, 202–203, 203t nuclear power station attack, 198 patient management priorities in, 203, 203t possible scenarios for, 197–199 radiation exposure device, 199 resources on, 203 Terz, J J., 412t testes, 359t, 367 testes stem cells, 329, 332–336, 335f, 336f TGF-β See transforming growth factor β TH-302, 469 Thames, H D., 334f, 336f therapeutic gain factor, in hyperthermia, 523 therapeutic ratio (index), 327–328, 327f definition of, 327 drug or radiosensitizer and, 327–328, 328f thermal ablation, 520–521 thermal dose, 523–524 thermal enhancement ratio (TER), 522–523 thermal isoeffective dose formulation, 524 thermal neutrons, 447 thermometry clinical, 530 invasive methods of, 530 noninvasive, progress toward, 530 1150 thermosensitive liposomes, 527, 528f thermotolerance, 519–520 thiotepa, 477 Thomlinson, R H., 86–88, 87f, 88f, 96f thorium, 137, 208 Thorotrast, 137 Thrall, D E., 524, 525–526 threshold dose, 135, 351 threshold-sigmoid, 135, 136f, 243, 243f thrombopenia, 361 thrombospondin (TSP-1), 313–314 thymidine, for cell labeling, 54–55, 55f thymine, 11, 12f, 16, 16f, 259–260, 259f–261f thyroid cancer, 140–141, 141f in A-bomb survivors, 140 age at exposure and, 148 in children receiving radiotherapy, 138, 140 radioactive iodine-131 for, 227 thyroid cells, 329, 339–341, 340f thyroid gland, 357, 359t, 374t thyroid scan, 224t tiling arrays, 283 Till, J E., 329, 336–339, 338f time, treatment See treatment time time since exposure, 139 time-dependent relative risk model, 139 tin-113, 225 tinea capitis, carcinogenesis in children treated for, 138, 141, 143, 144 1151 TIPS See transjugular intrahepatic portosystemic shunt tirapazamine, 467–469, 467f, 468f, 502t Tishler, R B., 509f tissue, normal See normal tissues tissue reactions (deterministic effects), 135–136, 136f, 242–243 cataractogenesis as, 194 definition of, 135, 201, 210, 254 dose–response relationship in, 243–244, 243f interventional radiology and, 219–220 occupational limits for, 245–246, 245t prevention of, 243 terrorism and, 201 tissue rescue unit (TRU), 327, 353 tissue weighting factor (WT), 240, 241t, 254 TNF See tumor necrosis factor Todaro, G J., 297 Tokaimura accident (Japan), 111, 118 tolerance dose, 244, 354 Tolmach, L J., 58, 60–61, 61f, 89–90, 90f tongue, 363 tonsils, 363 topoisomerase inhibitors, 487t, 494 Tpot See potential doubling time track average, 102, 102f Tradescantia paludosa, anaphase bridge in, 26, 30f training, exposure limits for, 245t, 247 transcription, 258, 260, 261f ChIP studies of, 280–281, 281f 1152 definition of, 292 transcription-coupled repair (TCR or TC-NER), 16–18, 17f transfection, 270 in cancer biology, 299, 299f in chromatin immunoprecipitation, 280 definition of, 293 in gene transfer, 268, 270 in knockout strategies, 277, 278f, 279f in promoter bashing, 280, 281f transfer RNA (tRNA), 260, 293 transformation in cancer biology, 299–300, 299f, 306–307 definition of, 293 in mammalian cells, 268–270, 268f transforming growth factor β (TGF-β), 358 transgenic tumor models, 389–390 transjugular intrahepatic portosystemic shunt (TIPS), 218, 219 translation, 258, 260, 261f, 293 translocations, 167 in Hiroshima/Nagasaki survivors, 33 oncogene activation in, 300–301, 301f, 301t Robertsonian, 165t, 167 symmetric, 26–29, 31f, 33 trastuzumab, 489t Travis, E L., 343, 345f Travis, L B., 151 treatment time influence on early- vs late-responding tissue, 423 1153 overall, importance of, 424–426, 426f Strandquist plot of, 418, 418f triage, in radiation exposure, 121–123, 203, 203t triazine derivatives, 477 Tribondeau, L., 352, 414 tricyclic triazine-di-N-oxides (TTOs), 469 tRNA See transfer RNA Trofimov, A V., 451f TRU See tissue rescue unit TSP-1 See thrombospondin TTOs See tricyclic triazine-di-N-oxides tuberculosis, fluoroscopy and breast cancer in, 138, 141, 142f Tubiana, M., 412–413, 412t, 415 tumor control probability (TCP), 327, 328, 428, 428f tumor cure (TCD50) assay, 380, 382–384, 384f tumor growth measurements, 380, 382, 382f, 383f, 393 tumor kinetics cell loss in, 381, 409–411, 410t Gompertz function in, 411, 411f growth fraction in, 407–409, 409t, 413, 475, 476 growth kinetics in, 411–414 histology and, 412 in human tumors, 412–414, 412t, 413t potential doubling time in, 409 tumor metabolism hypoxia-inducible factor in, 460 targeting to enhance radiotherapy, 470 1154 to kill hypoxic cells, 469–470, 469f, 470f tumor model systems See model tumor systems tumor necrosis factor (TNF), 358 tumor suppressor genes definition of, 295 familial breast cancer and, 303–304 function of, 306–315 gatekeepers and caretakers, 315–316 Li–Fraumeni paradigm of, 301, 302–303, 303f mutation or inactivation of, 295, 296f, 301–305 retinoblastoma paradigm of, 301–303, 302f somatic homozygosity and, 304–305, 305f two-dimensional electrophoresis, 288, 293 two-hit hypothesis, 302 two-hybrid screening, 284–285, 285f ultraviolet radiation, 4f, 511, 511t unfolded protein response (UPR), 461–462 United Kingdom CT scanners in, 213, 215f Hammersmith neutron experience in, 445–446 sources of radiation exposure in, 210, 211f United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), 139, 238 on computed tomography, 213, 215f on DDREF, 146 on doubling dose, 170 on effective dose, 222–223 on heritable effects, 165, 166t, 170–173, 171, 171t 1155 on linear no–threshold, 155–156 on nuclear medicine, 222–223 United States collective effective dose in, 217 cosmic radiation in, 207, 208f CT scanners in, 213, 215f sources of radiation exposure in, 210, 211f unity, in oxygen enhancement ratio, 83, 84f UNSCEAR See United Nations Scientific Committee on the Effects of Atomic Radiation Uppsala cyclotron (Sweden), 448, 450 UPR See unfolded protein response uracil, 260, 261f uranium decay of, in earth’s crust, 208 uranium miners, 137, 142–143 ureters, radiosensitivity and response of, 359t urinary bladder doses from diagnostic radiology, 212t QUANTEC analysis of, 373t radiosensitivity and response of, 359t, 366 retreatment and, 438 U.S Food and Drug Administration (FDA), 129, 202, 212 uterus, 360t vagina, 360t, 368 vaginal metastases, recurrent, 441 Valdagni, R., 526 1156 van der Kogel, A J., 343–347, 345f, 346f, 347f Van Putten, L M., 91f, 94, 96, 96f Vó, E., 220f vascular endothelial growth factor (VEGF), 313, 460–461, 494–496 vascular system, 368 vectors, 264–266 bacterial artificial chromosomes as, 264, 266 bacteriophages as, 264, 265, 265f definition of, 264, 293 plasmids as, 264–265, 264f viruses as, 264, 266 vegetative intermitotic cells, 356–357, 356t VEGF See vascular endothelial growth factor Velban See vinblastine velocity, 3–4, 4f vemurafenib, 490t Vent polymerase, 272 Vernon, C C., 441 Vicia faba oxygen effect in, 82 synchrony induced in, 59, 59f vinblastine, 483t, 492–493 vinca alkaloids, 483t, 492–493 vincristine, 483t, 492–493 hyperthermia combined with, 526t oxygen effect and, 501, 502t vinorelbine, 483t viruses, as vectors, 264, 266 1157 visible light, 4, 4f vitamin E succinate, 132 Vogelstein, B., 305f volume effect, 354–355 kidney, 354–355 lung, 354–355 skin and mucosa, 355 spinal cord, 346, 347f, 354 vomiting, 351, 364 von Hevesy, G., 222, 222f von Hippel-Lindau gene/protein (VHL/VHL), 286f, 313, 459, 460 von Kölliker, R A., Vriesendorp, H M., 117t vulva, 368 Wagner, L K., 219t Wakeford, R., 157, 186–187 Walter Reed Institute of Research, 127–128 Wang, Z., 285f Warburg effect, 460, 461f Ward, J., 13, 511t Watson, J., 258, 259 wavelengths, 3–4, 4f Weber, D C., 451f Weber, U., 455t Weichselbaum, R., 46f Weinert, T A., 398 Weinstein, G D., 412t Werner syndrome, 320 1158 Westermark, F., 516 Western blotting, 283–284, 288, 293 Weyrather, W K., 453f WHO See World Health Organization whole genome shotgun sequencing, 273 Willers, H., 15 Wilson, C W., 91f, 384 Wilson, R R., 448 Withers, H R on age–response function, 63–64, 64f on dose–response in normal tissue, 354f on dose-response assays, 329–336 on early- vs late-responding tissue, 421f, 422f on fractionation, 421f, 422f, 423, 425, 425f on jejunal crypt cells, 331–332, 333f, 334f on kidney tubules, 336, 337f, 338f on skin colonies, 329–331, 330f, 331f on testes stem cells, 332–336, 335f, 336f women medical exposure of, 187 occupational exposure of, 187 pregnant (See also embryonic or fetal effects) care and medical exposure during, 187 exposure limits in, 245t, 246 nuclear medicine in, 230–231 Worgul, B V., 192, 193f working level, 254 working level month, 254 1159 World Health Organization (WHO), 119–120, 119t, 120t World Trade Center attack (9/11), 132, 197 WR See radiation weighting factor WR-638 (cystaphos), 128, 128t WR-1065 (amifostine metabolite), 129 WR-2721 (amifostine), 128–130, 128t See also amifostine WT See tissue weighting factor X chromosome, 164 xenografts chemotherapy assays in, 510 definition of, 388 of human tumors, 388–389, 389f, 393–394 patient-derived, 389, 394 xerostomia amifostine for prevention of, 129 salivary gland regeneration for, 375–376 X-linked diseases, 165t, 166–167 X-linked traits, 164 x-rays, 3–5, 4f absorption of, 6–8 Compton process and, 6–8, 7f discovery of, first medical use of, first publicly taken radiograph, 2, 2f first therapeutic use of, ionization density of, 101, 102f LET values of, 103, 103t oxygen enhancement ratio for, 82–83, 84f 1160 photoelectric process and, 7–8, 7f radiosensitivity effects of, 59–62, 60f, 61f, 62f Y chromosome, 164 yeast cell cycle in, 267 cell survival curve for, 50, 51f molecular techniques using, 267, 284–285 yeast artificial chromosome, 266, 293 yellow fluorescent protein (YFP), 284 Young, R C., 412t ytterbium-169, 78t yttrium-90, 228 Yugoslav scientists, radiation exposure in, 121, 122f Yun, Z., 281f Z (atomic number), zinc DTPA (ZnDTPA), 131 1161 ... the DNA ladder to unzip (Fig 17 .2) Each half then constitutes a template for the reconstruction of the other half Two identical DNA molecules result, one for each progeny cell 479 FIGURE 17 .2. .. Table 17 .2 They are named according to the following system: The first letter comes from the genus of the organism from which the enzyme was isolated The second and third letters follow the organism’s... that the sequence on one strand is the same as on the other For example, EcoRI recognizes the sequence 5′ GAATTC 3′; the complementary strand is also 5′ GAATTC 3′ EcoRI cuts the DNA between the