Nase See deoxyribonuclease

DNA sequencing Determination of the sequence of nucleotides in DNA or in RNA which has been reverse-transcribed into DNA. The DNA is cloned, treated by base-specific chemical reactions and the products separated by size on sequencing gels then read as a sequence of bases: A, C, G and T. The Maxam-Gilbert method involves chemical cleavages at specific bases. The DNA is end-labeled, for example using polynucleotide kinase, and after completion of the four specific cleavage reactions the products are separated on a polyacrylamide gel. This method is labor-intensive and more difficult to perform than the Sanger method. Here, the DNA to be sequenced is cloned in phage M13 in single-stranded form and a specific primer is annealed to the sequence immediately 5' to the cloned insert. A DNA polymerase reaction is then carried out using the Klenow fragment of bacterial DNA polymerase 1 and four reactions, each containing four deoxyribonucleotides (one of which is labeled by a fluorescent or radioactive marker) and one of the four dideoxyri-bonucleotides (A, C, G or T). During DNA synthesis, incorporation of a dideoxyribonucleotide prevents further chain elongation, and reaction conditions are chosen such that the DNA is terminated at all possible positions. The four reaction mixtures each yield a nested set of fragments of increasing length, and after separation on adjacent lanes of a polyacrylamide gel the DNA sequence (up to 350 nucleotides) can be read directly.

Maxam AM and Gilbert W (1977) Proc Natl Acad Sci 74, 560

Sanger F et al (1977) Proc Natl Acad Sci 74, 5463

DNA topoisomerase An enzyme that catalyzes the breaking and rejoining of DNA strands, allowing the strands to pass through one another and altering the topology of the DNA. Type 1 activity catalyzes the removal of superhelical turns and the linking of single-stranded rings of complementary DNA sequences. Type 2 activity catalyzes the removal or introduction of superhelical turns and the linking of double-stranded DNA rings. A type 1 DNA topoisomerase activity is found in poxviruses, and is associated in Vaccinia virus with a 314 amino acid polypeptide that is essential for virus replication and probably plays a role in transcription and replication of the virus genome DNA. See also DNA gyrase and DNA relaxing enzyme.

DNA transcriptase Synonym for DNA-dependent RNA polymerase.

DNA tumor viruses Many of the known DNA-containing viruses have been associated with oncogenesis.

Dobrava-Belgrade virus (DOBV) A species in the genus Hantavirus. Two viruses that are closely similar and can be considered a single virus species. Dobrava virus was isolated from the yellow-necked field-mouse, Apodemus flavicollis, in Slovenia, and Belgrade virus came from a fatal case of hemorrhagic fever with renal syndrome in the former Yugoslavia. Dobrava virus has also been found in the striped mouse, Apodemus agrarius, which appears to be a reservoir of the virus in Central Europe.

Avsic-Zupane T et al (1995) J Gen Virol 76, 2801 Nemirov K et al (1999) J Gen Virol 80, 371 Sibold C et al (2001) J Med Virol 63, 158

DoCl1 (S+L2) cells (CCL 34.1) The DoCl1 cell line is one of four canine lines established from focus-derived clones. A clonal substrain of MDCK (ATCC CCL 34) was infected by murine sarcoma virus (MSV) in the absence of the associated RD-114 helper virus to yield the parent foci. Provides a non-murine cell host system for studies on the interaction of RNA tumor viruses with mammalian cells.

doctor fish virus (DFV) A tentative species in the genus Ranavirus.

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