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Home >> Dictionary of Bioinformatics, Biochemistry, Biotechnology >> Hollow Fibre - Homologous Recombination
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Hodgkin’s disease
A type of cancer of the lymphatic system that includes the lymph nodes, tonsils, spleen and thymus. The disease is characterised by weight loss, fever and enlargement of the lymph node.
Hogness box
See TATA box
Holiday intermediate
An intermediate formed during homologous recombination in which two double stranded DNA molecules are joined by a reciprocal crossover that involves one strand of each molecules.
Holiday junction
An X-shaped structure seen in DNA undergoing recombination. The two strands of DNA at the site of crossing over are from two different parental molecules. See Holiday intermediate.
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Hollow fibre
A tube of a porous material, with an internal diameter of a fraction of a millimetre. As a result, the ratio of surface area to internal volume is very large. Hollow fibres can be use as filters and because they have a huge surface area, they take longer to clog up than normal filters. They are also used in the hollow fibre bioreactor, in which cells are placed inside the hollow, porous fibres, and the culture medium is circulated outside the reactor. The fibres allow nutrients in and remove the products but do not allow the passage of cells.
Hollow fibre bioreactors are very effective for maintaining mammalian cells in culture for the following reasons: they have a very large surface area for the growth of cells and there is no need for a large reactor, the nutrient reaching the cells can be kept fresh. The reactor also provides an easy method of removing the product made by the cells such as monoclonal antibodies.
Holocentric
A chromosome with a diffuse centromere and spindle attachment over the entire chromosome.
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Holoenzyme
Catalytically active enzyme including prosthetic group and cofactors
Homeobox
A conserved sequence of 18bp that encodes a protein domain (homeodomain) found in many eukaryotic regulatory proteins. It is involved in the specification of organs and segmentation in animals. It is part of the coding region of Drosophila melanogaster.
Homeodomain
A DNA binding domain that defines a class of regulatory proteins that plays an important role in animal development. See homeobox
Homeodomain gene
A gene that controls the activity of other genes involved in the development of a body part. These genes are found in many organisms from plant to humans.
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Homing inteins
polypeptides spliced out of nascent proteins after translation. They may encode endonucleases that cleave within an ORF of an inteinless allele of the same gene and initiate transfer of the intein segment by gene conversion
Homoalleles
Mutations that are both functionally and structurally allelic; mutations at the same site in the same gene.
Homocysteine
A sulphur containing amino acid, and an intermediate in the metabolism of methionine. Elevated homocysteine levels in the blood have been associated with increases risk of cardiovascular disease.
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Homocystinurias
Refers to several biochemical and clinically distinct disorders in the metabolism of sulphur containing amino acids resulting in elevated levels of homocysteine in circulation and excessive excretion in urine. Homocystinuria type I is the most common form of Homocystinuria due to the deficiency of cystathionine b synthase, a pyridoxal phosphate dependent enzyme that catalyses the condensation of homocysteine and methionine in cells and body fluids. The formation of cysteine and cystine is impaired leading to low levels of these amino acids. Homocystinuria Type II is characterised by a deficiency of N5, N10 methylene tetrahydrofolate reductase. This enzyme is required for the formation of methylenetetrahydrofolate, a substrate for methylenetransferase, which catalyses the conversion of homocysteine to methionine. The reductase activity is also crucial for DNA and RNA synthesis.
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A decrease in enzyme activity results in low levels of methionine and impairment in nucleic acid synthesis leading to central nervous system dysfunction. Several forms of Homocystinuria characterised by defects in the synthesis of methylcobalamin required for the methyltransferase reaction that catalyses the formation of methionine from homocysteine have also been recognised.
Homeotic genes
Genes that regulate the development pattern of segments in Drosophila melanogaster. Such genes are seen in most vertebrates. These are also present in amphibians and mammalians where they are expressed during early embryonic development.
Homeosis
Transformation of one body part to another as a result of mutation or misexpression of certain genes critical for development.
Homeostasis
The state of equilibrium in the body with reference to chemical composition and functions of the tissues and fluids.
Homeotic mutation
Genes that are defined by mutation that lead to conversion of one body part into another. For example, replacement of insect leg by antenna.
Holiday model
A model for homologous recombination proposed by Robin Holliday in 1964. The general steps include synapsis, strand transfer, and resolution.
Homodimer
A protein containing two identical subunits.
Homogametic sex
Refers to similar gametes with regard to the sex chromosomes.
In mammals, the male (XY) is heterogametic whereas the female XX is homogametic.
Homogenotisation
A genetic technique that is used to replace one copy of a gene, or DNA sequence within a genome with an altered copy of that sequence. The DNA is first cloned and then altered, for example, by insertion of a transposon into gene. The mutated gene copy is used to replace the original gene by recombination in vivo. The mutated gene, which is incorporated is then selected because it contains transposon encoded antibiotic resistance.
Homogenous system
A system which has the same chemical composition throughout such as a mixture of gases.
Homokaryon
A cell that contains two or more identical nucleic as a result of fusion.
Homolog
See homologue.
Homologous
1. From the same source or sharing similar evolutionary structure or function. 2. Paired chromosomes in diploid organisms 3. similar DNA sequences.
Homologous chromosome
Chromosome that have the same pattern of genes but not the same alleles.
Homologous proteins
Proteins that have similar sequences and functions in different species (for eg. Haemoglobin)
Homologous scanning mutagenesis
A technique that involving replacing each segment of a protein with a homologous region from a related protein to identify functionally specific residues.
Homologue
1. A biomolecule related to another by evolution from a common ancestor. 2. In phylogenetics, particular features in different individuals that are genetically descended from the same feature in a common ancestor.
Homology
1. The extent of identity (similarity) between individuals, or characters. 2. Two or more biological species, systems or molecules that share a common evolutionary ancestor 3. The extent of identity (similarity) between the nucleotide sequences of two nucleic acid molecules or the amino acid sequences of two protein molecules. The degree of homology can be tested by sequencing or by either DNA DNA or DNA RNA hybridisation. Sequence “homology” search era are performed with a query DNA or protein sequence to identify known genes or gene products that share significant similarity. This gives information on the ancestry, heritage and possible of the query gene.
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