| DNA restriction-modification enzymes | Systems consisting of two enzymes, a modification methylase and a restriction endonuclease. They are closely related in their specificity and protect the DNA of a given bacterial species. The methylase adds methyl groups to adenine or cytosine residues in the same target sequence that constitutes the restriction enzyme binding site. The methylation renders the target site resistant to restriction, thereby protecting DNA against cleavage. (12 Dec 1998) |
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| DNA reverse gyrase | <enzyme> A topoisomerase II subclass which introduces positive superhelical turns into DNA Registry number: EC 3.1.- Synonym: reverse gyrase (26 Jun 1999) |
| DNA, ribosomal | DNA sequences encoding ribosomal RNA. These sequences regulate amplification and transcription initiation and contain transcribed and nontranscribed spacer segments. (12 Dec 1998) |
| DNA-RNA hybrid | Double-stranded polynucleic acids in which one strand is DNA and the other strand is the complementary RNA; formed during transcription and during multiplication of oncogenic RNA viruses. (05 Mar 2000) |
| DNA-RNA hybridisation | <molecular biology> A type of hybridisation. In this case, a strand of DNA is joined with a complementary strand of RNA to form a double-stranded molecule (or one which is partly double-stranded, if one of the original single strands is shorter than the other). (09 Oct 1997) |
| DNA, satellite | DNA that contains many tandem (not inverted) repeats of a short basic repeating unit. Satellite DNA is located at very specific spots in the genome (on chromosomes 1, 9, 16 and the Y chromosome, the tiny short arms of chromosomes 13-15 and 21 and 22, and near the centromeres of chromosomes). (12 Dec 1998) |
| DNA sequence | <molecular biology> The relative order of base pairs, whether in a fragment of DNA, a gene, a chromosome, or an entire genome. See: base sequence analysis. (09 Oct 1997) |
| DNA sequence analysis | <molecular biology> Determination of the nucleotide sequence of a length of DNA. Typically, this is performed by cloning the DNA of interest, so that enough can be prepared to allow the sequence to be determined, usually by the Sanger dideoxy chain temination or Maxam Gilbert chain degradation techniques. The resulting reactions are then run on a large sequencing gel, capable of resolving single nucleotide differences in chain length. Recently, PCR based methods have obviated the need to clone the DNA under some conditions and automated DNA sequencing has become widely available. (18 Nov 1997) |
| DNA sequence, unstable | DNA region comprised of a variable number of repetitive, contiguous trinucleotide sequences. Presence of these regions is associated with diseases such as fragile x syndrome and myotonia atrophica. (12 Dec 1998) |
| DNA sequencing | <molecular biology> Any lab technique used to find out the sequence of nucleotide bases in a DNA molecule or fragment. Examples are dideoxy sequencing and Maxam-Gilbert sequencing. (09 Oct 1997) |
| DNA shearing | <molecular biology> The tearing apart of unusually long DNA molecules by mechanical rather than chemical means, sometimes by accident when the molecule is being prepared in the lab for testing. (09 Oct 1997) |
| DNA, single-stranded | A single chain of deoxyribonucleotides that occurs in some bacteria and viruses. It usually exists as a covalently closed circle. (12 Dec 1998) |
| DNA supercoiling | <molecular biology> The condition of DNA coiling up onitself because its helix has been bent, overwound or underwound. (09 Oct 1997) |
| DNA, superhelical | Circular duplex DNA isolated from viruses, bacteria and mitochondria in supercoiled or supertwisted form. This superhelical DNA is endowed with free energy. During transcription, the magnitude of RNA initiation is proportional to the DNA superhelicity. (12 Dec 1998) |
| DNA synaptase | <enzyme> Fuses double stranded DNA molecules at a region of homology Registry number: EC 6.- (26 Jun 1999) |