Advancing Epigenetics Towards Systems Biology

Glossary

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Term Definition
Haemoglobin
Oxygen-carrying protein found in the red blood cells of vertebrates and some invertebrates, made up from four subunits.
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Haemopoietic
(haematopoietic) The formation of blood cellular components. This process occurs in myeloid tissue, which is found in the bone marrow and lymphatic tissue, such as lymph nodes or the spleen. All of the cellular components of the blood are derived from haematopoietic stem cells.
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Heterochromatin
A type of chromatin that is darkly staining and tightly packaged or coiled throughout the cell cycle and that is, for the most part, genetically inactive.
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Histone
The chief proteins of chromatin that act as spools around which DNA winds; they play a role in gene regulation.
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Histone acetylation
See histone acetytransferase.
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Histone acetyltransferase
An enzyme that acetylates conserved lysine amino acids on histone proteins.
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Histone code
Theory that proposes that specific histone-tail modifications effected by enzymes, control the activity of genes in the nucleus, by providing binding sites for regulatory proteins.
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Histone deacetylase
HDACs are enzymes that remove acetyl groups from histone proteins. This has the general effect of increasing the positive charge of histones and increasing their attraction for the negatively charged phosphate groups in DNA (see heterochromatin).
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Histone methylation
Modification of the primary protein sequence of a histone protein by the addition of one, two, or three methyl groups. This modification alters the properties of the nucleosome and affects its interactions with other proteins. Specific kinds of methylation, e.g. methylation of lysine 9 within the histone H3 molecule are associated with inactive chromatin (see heterochromatin) where genes are relatively inactive.
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Histone methyltransferase
(HMT) Enzymes that catalyze the transfer of methyl groups from certain types of histone proteins.
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Histone modification
Acetylation, methylation, poly(ADP-ribose)ylation, phosphorylation, ubiquitylation of specific-amino-acid residues on histones, which are believed to control chromatin states.
Homeostasis
Encompasses physiological processes that allow an organism to maintain internal equilibrium.
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Homeotic (homeobox, hox) genes
Homeotic genes in general transform parts of the body into structures appropriate to other positions. Genes with a homeobox or Hox domain involved in embryonic development in many organisms from fruit flies to humans.
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Homologue
Describes a gene, protein or morphological structure that is similar in both DNA sequence and amino-acid sequence and function between organisms of different species. Real homologies are based on a common evolutionary descent in contrast to convergent evolution or analogies. Genes discovered initially in fruit flies often have homologues in mammals.
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Housekeeping gene
Genes required throughout development for the smooth running of the cell's metabolism. Such genes are expressed continuously.
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