| helium | <element> A gas. Certain types of radiation therapy used charged, radioactive particles of helium. (16 Dec 1997) |
|---|---|
| helium speech | The peculiar high-pitched, often unintelligible speech sounds produced when one breathes a mixture of up to 80 per cent helium and 20 per cent oxygen. (05 Mar 2000) |
| helium-3 | The rare stable isotope of helium (1.37 parts per million of ordinary helium); produced by the beta decay of tritium. (05 Mar 2000) |
| helium-4 | The common helium isotope, making up 99.999% of natural helium; it is emitted in the form of alpha rays (which are helium nuclei), from a variety of radionuclides. (05 Mar 2000) |
| helix | <chemistry, molecular biology> A spiral structure in a macromolecule that contains a repeating pattern. (09 Oct 1997) |
| helix (snails) | A genus of chiefly eurasian and african land snails including the principal edible snails as well as several pests of cultivated plants. (12 Dec 1998) |
| helix destabilising protein | <molecular biology, protein> Proteins involved in DNA replication. They bind cooperatively to single stranded areas of a DNA molecule in a double helix, causing the helix to unwind and preventing the reformation of the duplex and extending the DNA backbone, thus making the exposed bases more accessible for base pairing. (10 Oct 1997) |
| helix loop helix | <molecular biology> A motif associated with transcription factors, allowing them to recognise and bind to specific DNA sequences. Two _ helices are separated by a loop. Examples: myoblast MyoD1, c myc, Drosophila genes daughterless, hairy, twist, scute, achaete, asense. Not the same as helix turn helix. (18 Nov 1997) |
| helix turn helix | <molecular biology> A motif associated with transcription factors, allowing them to bind to and recognise specific DNA sequences. Two amphipathic _ helices are separated by a short sequence with a _ sheet. One helix lies across the major groove of the DNA, while the recognition helix enters the major groove and interacts with specific bases. An example in Drosophila is the homeotic gene fushi tarazu, that binds to the sequence TCAATTAAATGA. Not the same as helix loop helix. (18 Nov 1997) |
| helix-coil transition | <molecular biology> A change in the structure of a nucleic acid or protein molecule from a highly ordered, complex structure to a random, chaotic structure. Also means that the protein or nucleic acid becomes denatured. (09 Oct 1997) |
| helix-loop-helix motifs | A group of 20-residue peptides characterised by two alpha helices separated by a non-helical segment. These recurring supersecondary structural patterns are found in many sequence-specific DNA-binding proteins. (12 Dec 1998) |
| helix-turn-helix motifs | The first DNA-binding protein motif to be recognised. Helix-turn-helix motifs were originally identified in bacterial proteins but have since been found in hundreds of DNA-binding proteins from both eukaryotes and prokaryotes. They are constructed from two alpha helices connected by a short extended chain of amino acids, which constitute the "turn." the two helices are held at a fixed angle, primarily through interactions between the two helices. (12 Dec 1998) |