H

Hairpin:

A helical (duplex) region formed by base pairing between adjacent (inverted) complementary sequences within a single strand of RNA or DNA.

Haploid:

A single set of chromosomes (half the full set of genetic material), present in the egg and sperm cells of animals and in the egg and pollen cells of plants.

Haplotype:

A particular combination of alleles or sequence variations that are closely linked Ñ that is, are likely to be inherited together Ñ on the same chromosome.

Helix-Loop-Helix:

A protein structural motif characteristic of certain DNA-binding proteins.

Helix-Turn-Helix:

A protein structural motif characteristic of certain DNA-binding proteins.

Heteroduplex DNA:

Generated by base pairing between complementary single strands derived from different parental duplex molecules; heteroduplex DNA molecules occur during genetic recombination in vivo and during hydridization of different but related DNA strands in vitro. Since the sequences of the two strands in a heteroduplex differ, the molecule is not perfectly base-paired; the melting temperature of a heteroduplex DNA is dependent upon the number of mismatched base pairs.

Heterozygous:

An individual containing dissimilar alleles for a given gene or locus.

Heterochromatin:

Compact, gene-poor regions of a genome, which are enriched in simple sequence repeats.

Heterozygosity:

The presence of different alleles at one or more loci on homologous chromosomes.

Histones:

Highly basic proteins which associate with the chromosomal DNA to package it into a compact, higher order structure.

hnRNA:

Heterogeneous nuclear RNA; refers collectively to the variety of RNAs found in the nucleus, including primary transcripts, partially processed RNAs and snRNA. The term hnRNA is often used just for the unprocessed primary transcripts.

Homeobox:

A short stretch of nucleotides whose base sequence is virtually identical in all the genes that contain it. It has been found in many organisms from fruit flies to human beings. In the fruit fly, a homeobox appears to determine when particular groups of genes are expressed during development.

Homeotic Mutation:

A mutation in a homeotic gene that is responsible for controlling the activities of numerous other genes, usually during embryologic development in higher organisms.

Homologous Recombination:

The exchange of sequence between two related but different DNA (or RNA) molecules, with the result that a new "chimeric" molecule is created. In DNA recombination, breakage of single strands of DNA in the two recombination partners is followed by joining of strands present in opposing molecules, and may involve specific enzymes. Recombination of RNA molecules may occur by other mechanisms.

Homologous Chromosomes:

A pair of chromosomes containing the same linear gene sequences, each derived from one parent.

Homology:

Indicates similarity between two different nucleotide or amino acid sequences, often with potential evolutionary significance. It is probably better to use more quantitative and descriptive terms such as nucleotide "identity" or, in the case of proteins, amino acid "identity" or "relatedness" (the latter refers to the presence of amino acids residues with similar polarity/charge characteristics at the same position within a protein).

Homozygous:

An individual containing identical alleles for a given gene or locus.

Host Strain (Bacterial):

The bacterium used to harbor a plasmid. Typical host strains include HB101 (general purpose E. coli strain), DH5a (ditto), JM101 and JM109. It is available in a form with no plasmids.

Hot Spot:

It is a site at which the frequency of mutation is very much increased.

Housekeeping Gene:

Genes that are widely expressed in abundance and are usually used as reference genes for normalization in real-time PCR with the assumption of 'constant expression'.

Human Gene Therapy:

Insertion of normal DNA directly into cells to correct a genetic defect.

Hybridoma:

A clone of plasmacytoma cells which secrete a monoclonal antibody; usually produced by fusion of peripheral or splenic plasma cells taken from an immunized mouse with an immortalized murine plasmacytoma cell line (fusion partner), followed by cloning and selection of appropriate antibody-producing cells.

Hybridization:

The reaction by which the pairing of complementary strands of nucleic acid occurs. DNA is usually double-stranded, and when the strands are separated they will re-hybridize under the appropriate conditions. Hybrids can form between DNA-DNA, DNA-RNA or RNA-RNA. They can form between a short strand and a long strand containing a region complementary to the short one.

Hybridization Probe:

One of the main fluorescence-monitoring systems for DNA amplification. LightCycler probes are hybridization probes and are not hydrolyzed by Taq Polymerase.

Hydrophilicity Plot:

A computer plot which examines the relative summed hyrophobicity /hydrophilicity of adjacent amino acid sidechains along the primary sequence of a polypeptide chain.

Hydrolysis Probe:

One of the main fluorescence-monitoring systems for DNA amplification. TaqMan® probes are an example. These kinds of probes are hydrolyzed by the 5' endonuclease activity of Taq Polymerase during PCR.

Hyperchromocity:

It is the increase in the optical density that occurs when DNA is denatured.