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Biology Definitions

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HaploidA cell is haploid if it has only a single set of unpaired chromosomes. Gamete cells are haploid.Diploid, Chromosome, Gamete, Somatic
HaplotypeA 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 ChainAntibodies (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
Helical SymmetryOne of the simplest structures a virus can have. Helical Symmetry describes a single strand of RNA or DNA wrapped in a protein. The virus may or may not have an envelope, but helical symmetry without an envelope has only been seen in plant viruses. Examples include Tobacco Mosaic Virus, which has a single RNA strand wrapped in a protein (no envelope), Sendai Virus and Rabies Virus both have genomes wrapped in protein, surrounded by an envelope. Note that a virus without an envelope is often referred to as "naked".Virus, Tobacco Mosaic Virus, Viral Envelope
Hemagglutination assayA laboratory test which enables a scientist to quickly estimate the amount of virus or bacteria in a sample. Some viruses (e.g. influenza) and bacteria have surface proteins (called hemagglutinin) which are able to bind to (agglutinate) sialic acid receptors on cells. These microbes can also bind to erythrocytes (red blood cells). As the microbe (virus or bacteria) attaches to more than one blood cell, a lattice forms. To perform the assay, the microbes and red blood cells are placed in the wells of a plastic tray, and the red blood cells that don't bind to the microbe form a "button" at the bottom of the tray. By observing (or not observing) the formation of a lattice, the scientist can estimate the amount of microbe in a sample of unknown molarity.Assay
Hepatitis VirusA broad term for any virus that can infect the liver. Note that hepatis viruses vary widely in their forms, structures, and mechanisms/receptors for entering cells of the liver.Baltimore Scheme, Virus Classification, Viral Genomes
Hershey-Chase experimentA 1952 experiment which showed that DNA (and not protein) is the moleclue that contains the genetic material of viruses (note there are also viruses that use only RNA has their genetic material). A follow up to the Avery-MacLeod-McCarty Experiment which showed that DNA was the molecule that caused Bacterial Transformation. In their first experiements, they labeled DNA with radioactive Phosphorus-32 (Phosphorous is present in DNA, but not in any of the 20 amino acids used to build proteins). In subsequent experiments, they labeled the phage using radioactive Sulfur-35 (sulfur is present in some Amino Acids, but not in DNA). Hershey and Chase then observed that infected bacteria had the radioactive Phosphorus, but not (much of) the radioactive Sulfur. This showed that the bacteria phage was using DNA as its genetic material. Because a blender was used to separate the radioactive protein shells from the cell surface, this is also called the "blender experiment".Avery-MacLeod-McCarty Experiment
HeterologousA 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
HeterozygousA 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
HistidineOne of the 20 amino acids used for building proteins. C6H9N3O2. Histidine is essential for the growth and repair of tissuesAmino Acid, Protein
HomeostatisThe 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
HomologousTwo 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
HomozygousA 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 ImmunitySoluble Immunity. The production of soluble Antibody molecules.Cellular Immunity
HusbandryThe cultivation, production, and breeding of plants or animals. 
HybridizeTo form hydrogen bonds between two complementary strands of molecules. For example, complementary strands of DNA will form Hydrogen bonds when brought together. Microarrays take advantage of this hybridization to help biologists determine if an unknown genome contains any of the sequences which were robotically added to the Microarray probes.Microarray, DNA, RNA, Protein
Hydrogen BondA 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
Hydrolysisa 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
HydrophilicWater loving; molecules which are hydrophilic tend to dissolve in water, and are usually polar (have a net charge)Hydrophobic
HydrophobicWater hating; molecules which are hydrophobic do not dissolve in water, are usually not polar (charged), and tend to clump together when placed in waterHydrophilic, Lipid, Membrane
HydroxyureaA 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
HypermutationThe 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

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