Hairpin loop - An area where single-stranded DNA or RNA has folded back on itself and nucleotides from the two separate segments have base paired, so that the resulting structure appears as a hairpin..

Haploid - A Single set of chromosomes present in the egg and sperm cells of animals and in the egg and pollen cells of plants

Hayflick Limit -  Cells die (nonaccidentally) either when they have completed a fixed number of division cycles (around 60). This called the Hayflick limit.

Heterozygous - A genotype in which the 2 copies of the gene that determine a particular trait are different e.g.(Aa)

Histones - Small proteins with a high frequency of the basic and positively charged amino acids lysiine and arginine. The histone's strong positive charge enables them to bind and to neutralise the negatively charged DNA throughout the chromatin.

Homlogue - One member of a related pair of human chromosomes.

Homology - Similarity in DNA or protein sequences between individuals of the same species or among different species.

Homologous chromosomes - Chromosome pairs of the same length, centromere position, and staining pattern that posses genes for the same traits at corresponding loci..One homologous chromosome is inherited from the organism's father , the other from the mother.

Homozygous - A genotype in which the 2 copies of the gene that determine a particular trait are the same. e.g.(AA)

Human gene therapy - Insertion of normal DNA directly into cells to correct a genetic defect.
Human Genome Initiative: Collective name for several projects begun in 1986 by DOE to (1) create an ordered set of DNA segments from known chromosomal locations, (2) develop new computational methods for analyzing genetic map and DNA sequence data, and (3) develop new techniques and instruments for detecting and analyzing DNA. This DOE initiative is now known as the Human Genome Program. The national effort, led by DOE and NIH, is known as the Human Genome Project.

Hybridization - The process of joining two complementary strands of DNA or one each of DNA and RNA to form a double-stranded molecule.

Hydrogen bond - (H-bonding). An electrostatic attraction between a hydrogen atom in one polar molecule (as of water) and a small electronegative atom (as of oxygen, nitrogen, or fluorine) in usually another molecule of the same or a different polar substance.

Hydroxyl Group - The chemical group or ion OH that consists of one atom of hydrogen and one of oxygen and is neutral or negatively charged.