Thymine Nucleotides: Phosphate esters of THYMIDINE in N-glycosidic linkage with ribose or deoxyribose, as occurs in nucleic acids. (From Dorland, 28th ed, p1154)Megaloblasts: Red blood cell precursors, corresponding to ERYTHROBLASTS, that are larger than normal, usually resulting from a FOLIC ACID DEFICIENCY or VITAMIN B 12 DEFICIENCY.ThymineAnemia, Megaloblastic: A disorder characterized by the presence of ANEMIA, abnormally large red blood cells (megalocytes or macrocytes), and MEGALOBLASTS.Leukemia, Lymphoid: Leukemia associated with HYPERPLASIA of the lymphoid tissues and increased numbers of circulating malignant LYMPHOCYTES and lymphoblasts.Methotrexate: An antineoplastic antimetabolite with immunosuppressant properties. It is an inhibitor of TETRAHYDROFOLATE DEHYDROGENASE and prevents the formation of tetrahydrofolate, necessary for synthesis of thymidylate, an essential component of DNA.Nucleotides: The monomeric units from which DNA or RNA polymers are constructed. They consist of a purine or pyrimidine base, a pentose sugar, and a phosphate group. (From King & Stansfield, A Dictionary of Genetics, 4th ed)Thymine DNA Glycosylase: An enzyme that removes THYMINE and URACIL bases mispaired with GUANINE through hydrolysis of their N-glycosidic bond. These mispaired nucleotides generally occur through the hydrolytic DEAMINATION of 5-METHYLCYTOSINE to thymine.Molecular Sequence Data: Descriptions of specific amino acid, carbohydrate, or nucleotide sequences which have appeared in the published literature and/or are deposited in and maintained by databanks such as GENBANK, European Molecular Biology Laboratory (EMBL), National Biomedical Research Foundation (NBRF), or other sequence repositories.UracilPolymorphism, Single Nucleotide: A single nucleotide variation in a genetic sequence that occurs at appreciable frequency in the population.DNA: A deoxyribonucleotide polymer that is the primary genetic material of all cells. Eukaryotic and prokaryotic organisms normally contain DNA in a double-stranded state, yet several important biological processes transiently involve single-stranded regions. DNA, which consists of a polysugar-phosphate backbone possessing projections of purines (adenine and guanine) and pyrimidines (thymine and cytosine), forms a double helix that is held together by hydrogen bonds between these purines and pyrimidines (adenine to thymine and guanine to cytosine).Base Sequence: The sequence of PURINES and PYRIMIDINES in nucleic acids and polynucleotides. It is also called nucleotide sequence.Adenine NucleotidesPyrimidine Dimers: Dimers found in DNA chains damaged by ULTRAVIOLET RAYS. They consist of two adjacent PYRIMIDINE NUCLEOTIDES, usually THYMINE nucleotides, in which the pyrimidine residues are covalently joined by a cyclobutane ring. These dimers block DNA REPLICATION.Deoxyribonuclease (Pyrimidine Dimer): An enzyme which catalyzes an endonucleolytic cleavage near PYRIMIDINE DIMERS to produce a 5'-phosphate product. The enzyme acts on the damaged DNA strand, from the 5' side of the damaged site.Amino Acid Sequence: The order of amino acids as they occur in a polypeptide chain. This is referred to as the primary structure of proteins. It is of fundamental importance in determining PROTEIN CONFORMATION.Guanine NucleotidesBromouracil: 5-Bromo-2,4(1H,3H)-pyrimidinedione. Brominated derivative of uracil that acts as an antimetabolite, substituting for thymine in DNA. It is used mainly as an experimental mutagen, but its deoxyriboside (BROMODEOXYURIDINE) is used to treat neoplasms.Nucleic Acid Conformation: The spatial arrangement of the atoms of a nucleic acid or polynucleotide that results in its characteristic 3-dimensional shape.Escherichia coli: A species of gram-negative, facultatively anaerobic, rod-shaped bacteria (GRAM-NEGATIVE FACULTATIVELY ANAEROBIC RODS) commonly found in the lower part of the intestine of warm-blooded animals. It is usually nonpathogenic, but some strains are known to produce DIARRHEA and pyogenic infections. Pathogenic strains (virotypes) are classified by their specific pathogenic mechanisms such as toxins (ENTEROTOXIGENIC ESCHERICHIA COLI), etc.Pentoxyl: 5-Hydroxymethyl-6-methyl- 2,4-(1H,3H)-pyrimidinedione. Uracil derivative used in combination with toxic antibiotics to lessen their toxicity; also to stimulate leukopoiesis and immunity. Synonyms: pentoksil; hydroxymethylmethyluracil.Purine Nucleotides: Purines attached to a RIBOSE and a phosphate that can polymerize to form DNA and RNA.Cytosine: A pyrimidine base that is a fundamental unit of nucleic acids.Osmium Tetroxide: (T-4)-Osmium oxide (OsO4). A highly toxic and volatile oxide of osmium used in industry as an oxidizing agent. It is also used as a histological fixative and stain and as a synovectomy agent in arthritic joints. Its vapor can cause eye, skin, and lung damage.Cloning, Molecular: The insertion of recombinant DNA molecules from prokaryotic and/or eukaryotic sources into a replicating vehicle, such as a plasmid or virus vector, and the introduction of the resultant hybrid molecules into recipient cells without altering the viability of those cells.DNA Repair: The reconstruction of a continuous two-stranded DNA molecule without mismatch from a molecule which contained damaged regions. The major repair mechanisms are excision repair, in which defective regions in one strand are excised and resynthesized using the complementary base pairing information in the intact strand; photoreactivation repair, in which the lethal and mutagenic effects of ultraviolet light are eliminated; and post-replication repair, in which the primary lesions are not repaired, but the gaps in one daughter duplex are filled in by incorporation of portions of the other (undamaged) daughter duplex. Excision repair and post-replication repair are sometimes referred to as "dark repair" because they do not require light.Mutation: Any detectable and heritable change in the genetic material that causes a change in the GENOTYPE and which is transmitted to daughter cells and to succeeding generations.Pyrimidine Nucleotides: Pyrimidines with a RIBOSE and phosphate attached that can polymerize to form DNA and RNA.Endodeoxyribonucleases: A group of enzymes catalyzing the endonucleolytic cleavage of DNA. They include members of EC 3.1.21.-, EC 3.1.22.-, EC 3.1.23.- (DNA RESTRICTION ENZYMES), EC 3.1.24.- (DNA RESTRICTION ENZYMES), and EC 3.1.25.-.DNA, Bacterial: Deoxyribonucleic acid that makes up the genetic material of bacteria.ThymidineUltraviolet Rays: That portion of the electromagnetic spectrum immediately below the visible range and extending into the x-ray frequencies. The longer wavelengths (near-UV or biotic or vital rays) are necessary for the endogenous synthesis of vitamin D and are also called antirachitic rays; the shorter, ionizing wavelengths (far-UV or abiotic or extravital rays) are viricidal, bactericidal, mutagenic, and carcinogenic and are used as disinfectants.Oligodeoxyribonucleotides: A group of deoxyribonucleotides (up to 12) in which the phosphate residues of each deoxyribonucleotide act as bridges in forming diester linkages between the deoxyribose moieties.Adenine: A purine base and a fundamental unit of ADENINE NUCLEOTIDES.Kinetics: The rate dynamics in chemical or physical systems.GuanineSequence Analysis, DNA: A multistage process that includes cloning, physical mapping, subcloning, determination of the DNA SEQUENCE, and information analysis.Nucleotides, CyclicGuanine Nucleotide Exchange Factors: Protein factors that promote the exchange of GTP for GDP bound to GTP-BINDING PROTEINS.Potassium Permanganate: Permanganic acid (HMnO4), potassium salt. A highly oxidative, water-soluble compound with purple crystals, and a sweet taste. (From McGraw-Hill Dictionary of Scientific and Technical Information, 4th ed)Sequence Homology, Nucleic Acid: The sequential correspondence of nucleotides in one nucleic acid molecule with those of another nucleic acid molecule. Sequence homology is an indication of the genetic relatedness of different organisms and gene function.Oligonucleotides: Polymers made up of a few (2-20) nucleotides. In molecular genetics, they refer to a short sequence synthesized to match a region where a mutation is known to occur, and then used as a probe (OLIGONUCLEOTIDE PROBES). (Dorland, 28th ed)