| Term | Description | SeeAlso |
| Haploid | A cell is haploid if it has only a single set of unpaired chromosomes. Gamete cells are haploid. | Diploid, Chromosome, Gamete, Somatic |
| Haplotype | A group of alleles on a chromome that tend to be inherited together. More specifically, a haplotype is a set of Single Nucleotide Polymorphisms (SNPs) on a chromosome that tend to be inherited together. | Single Nucleotide Polymorphism, SNP, Polymorphism, Allele, Chromosomal Theory of Inheritance |
| Heavy Chain | Antibodies (Immunoglobulins) are made up of a heavy chain and a light chain. Note that both the heavy and light chains have a stochastic (random) fusion of V,D, and J Segments. | Antibody, V Segment, D Segment, J Segment |
| Heterologous | A biologicial experiment is heterologous if it combines biological materials which have origins in different species. For example, if an experiment used microsomes from a dog, and extract from wheat germ, then the experiment is heterologous. | Microsome |
| Heterozygous | A genotype consisting of two different alleles for the same gene on a pair of chromosomes. The individual has received a different allele for the given gene from each parent. | Homozygous, Allele, Genotype, Gene |
| Histidine | One of the 20 amino acids used for building proteins. C6H9N3O2. Histidine is essential for the growth and repair of tissues | Amino Acid, Protein |
| Homeostatis | The maintenance of equilibrium by a cell or organism, usually using feedback loops. Examples include the regulation of body temperature, and the regulation of red blood cell counts. | Red Blood Cells |
| Homologous | Two chromosomes are homologous if they both have the same genes in the same order and locations, and each chromosome came from a different parent. Homologous chromosomes pair up with each other. | Chromosome, Gene |
| Homozygous | A genotype consisting of two identical alleles for the same gene on a pair of chromosomes. The individual has received the same allele for the given gene from each parent. | Heterozygous, Allele, Genotype, Gene |
| Humoral Immunity | Soluble Immunity. The production of soluble Antibody molecules. | Cellular Immunity |
| Husbandry | The cultivation, production, and breeding of plants or animals. | |
| Hydrogen Bond | A weak chemical bond in which a hydrogen atom in one covalently bonded molecule becomes slightly positive, and attracts an electronegative atom in another molecule. Because the Hydrogen atom has a low electron affinity, the Hydrogen atom's single electron is pulled toward the other atom in the covalent bond (e.g. Oxygen, Nitrogen, or Flourine), giving the Hydrogen a slight polarity, being more negative near its covalent bond, and more positive elsewhere, allowing it to form a weak bond with yet another molecule. Hydrogen bonds are stronger than Van der Waal's forces, but are weaker than Covalent and Ionic bonds. Hydrogen bonds hold the two strands of DNA together, making it easy to separate the two strands during transcription and replication. Hydrogen and covalent bonds are used as intramolecular and intermolecular bonds in proteins. | Covalen't Bonds, Ionic Bonds, Van Der Waal's Forces, Transcription, Replication, Intramolecular, Intermolecular |
| Hydrolysis | a chemical reaction which breaks down a compound into two molecules by the addition of a water molecule. The opposite of esterification. For example, hydrolysis is used to break down polymers (e.g. starch into glucose) | Esterification |
| Hydrophilic | Water loving; molecules which are hydrophilic tend to dissolve in water, and are usually polar (have a net charge) | Hydrophobic |
| Hydrophobic | Water hating; molecules which are hydrophobic do not dissolve in water, are usually not polar (charged), and tend to clump together when placed in water | Hydrophilic, Lipid, Membrane |
| Hydroxyurea | A compound biochemists use to block synthesis of DNA (DNA Synthesis Inhibitor). For example, a biochemist trying to determine the time the cells spends in the G2 phase of the cell cycle will add Hydroxyurea to the cells, and then wait for all of the cells to enter the G1->Synthesis phase boundary (where they get stuck because they can't synthesis DNA -- note that some cells will be stuck in the middle of S phase at the moment Hydroxyurea was added.) Then, the biochemist will remove the hydroxyurea, allowing the cells to again proceed through the cell cycle. The biochemist will add Colcemid (which stops the cell cycle at Mitosis Phase) and also Tritiated Thymidine. By timing how long it takes to see evidence of radioactive reactions (on silver) directly above chromosomes (viewable under the microscope in M phase), the biochemist can measure the amount of time the cells spent in G2 phase. | Cell Cycle Phases, G2 phase, chromosome, chromatid, Microtubule Antagonist |
| Hypermutation | The immune system stochastically modifies genes in different cells, so that each cell can produce a different antibody. Besides fusing V, D, and J Segments, the immune system also uses enzymes like Cytidine Deaminase to create more mutations, resulting in a more diverse antibody population. Note that hypermutation also occurs during monclonal expansion (after a B Cell has encountered its antigen). In this way, mutations can occur which create antibodies which bind more "avidly" (better) to the antigen. | V Segment, D Segment, J Segment, Cytidine Deaminase |