| DNA gyrase | <enzyme, molecular biology> A type II topoisomerase of Escherichia coli, that is essential for DNA replication. This enzyme can induce or relax supercoiling. (18 Nov 1997) |
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| DNA helicase | <enzyme, molecular biology> A prokaryote enzyme that uses the hydrolysis of ATP to unwind the DNA helix at the replication fork, to allow the resulting single strands to be copied. Two molecules of ATP are required for each nucleotide pair of the duplex. (18 Nov 1997) |
| DNA helicases | <enzyme> Proteins that promote unwinding of duplex DNA during replication by binding cooperatively to single-stranded regions of DNA or to short regions of duplex DNA that are undergoing transient opening. Registry number: EC 5.99.- (12 Dec 1998) |
| DNA helix | The helical structure assumed by two strands of deoxyribonucleic acid, held together throughout their length by hydrogen bonds between bases on opposite strands, referred to as Watson-Crick base pairing. See: base pair. Synonym: DNA helix, double helix, twin helix. (05 Mar 2000) |
| DNA, helminth | Deoxyribonucleic acid that makes up the genetic material of helminths. (12 Dec 1998) |
| DNA homology | <molecular biology> How closely related two or more separate strands of DNA are to each other, based on their base sequences. (09 Oct 1997) |
| DNA hybridisation | <molecular biology> The process of joining two complementary strands of DNA or one each of DNA and RNA to form a double-stranded molecule. Technique in which single stranded nucleic acids are allowed to interact so that complexes or hybrids, are formed by molecules with sufficiently similar, complementary sequences. By this means the degree of sequence identity can be assessed and specific sequences detected. The hybridisation can be carried out in solution or with one component immobilised on a gel or, most commonly, nitrocellulose paper. Hybrids are detected by various means: visualisation in the electron microscope, by radioactively labelling one component and removing noncomplexed DNA or by washing or digestion with an enzyme that attacks single stranded nucleic acids and finally estimating the radioactivity bound. Hybridisations are done in all combinations: DNA DNA (DNA can be rendered single stranded by heat denaturation), DNA RNA or RNA RNA. In situ hybridisations involve hybridising a labelled nucleic acid (often labelled with a fluorescent dye) to suitably prepared cells or histological sections. This is used particularly to look for specific transcription or localisation of genes to specific chromosomes (FISH analysis). <zoology> The mating of individuals from different species or sub-species. (13 Oct 1997) |
| DNA hybridization | A technique used to determine the relatedness of microorganisms by the speed and efficiency of the reassociation of single-stranded DNA to form double-stranded DNA when one of the strands originates from one organism and the other strand from another organism; occurs when the base sequences are complementary or nearly so. (05 Mar 2000) |
| DNA insertion elements | Discrete transposable segments of DNA which can insert into chromosomal, phage, and plasmid DNA. Some insert at random while others are site-specific; most have not been found to exist except in the inserted state. Their insertion into a genome always produces a mutation ("insertion mutation"), and their excision frequently results in a loss of host genetic information. Types of transposable elements include is elements (insertion sequence elements), which are composed of between 700 and 1400 bases and contain no genes unrelated to insertion function and tn elements (transposon elements), which are generally larger than 1400 bases and contain genes unrelated to insertion function. The concept also includes the delta element of saccharomyces cerevisiae and the integration site. (12 Dec 1998) |
| DNA iteron | <molecular biology> Repeated DNA sequence found near the origin of replication of some plasmids. (18 Nov 1997) |
| DNA, kinetoplast | Mitochondrial DNA of trypanosomes and related parasitic protozoa within the order kinetoplastida. Kinetoplast DNA is used as a DNA probe and consists of a complex network of numerous catenated rings of two classes; the first being a large number of small DNA duplex rings, called minicircles, approximately 2000 base pairs in length, and the second being several dozen much larger rings, called maxicircles, approximately 37 kb in length. (12 Dec 1998) |
| DNA library | <molecular biology> A collection of DNA molecules, derived from restriction fragments that have been cloned in vectors, that includes all or part of the genetic material of an organism. (18 Nov 1997) |
| DNA ligase | <enzyme, molecular biology> Enzyme involved in DNA replication. The DNA ligase of E. Coli seals nicks in one strand of double stranded DNA, a reaction required for linking precursor fragments during discontinuous synthesis on the lagging strand. Nicks are breaks in the phosphodiester linkage that leave a free 3_ OH and 5_ phosphate. The ligase from phage T4 has the additional property of joining two DNA molecules having completely base paired ends. DNA ligases are crucial in joining DNA molecules and preparing radioactive probes (by nick translation) in recombinant DNA technology. (18 Nov 1997) |
| DNA ligases | <enzyme> Poly(deoxyribonucleotide):poly(deoxyribonucleotide)ligases. Enzymes that catalyze the joining of preformed deoxyribonucleotides in phosphodiester linkage during genetic processes during repair of a single-stranded break in duplex DNA. The class includes both ATP and NAD. Registry number: EC 6.5.1.- (12 Dec 1998) |
| DNA ligation | <molecular biology> The joining of two DNA strands by their ends with a phosphodiester bond. (09 Oct 1997) |