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Biology Definitions

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T Cell Receptor (TCR)A receptor on a T Helper cell which has a transmembrane protein which is very similar to the antibody protein receptor on a B Cell (having constant and variable domains which are stochastically chosen by the immune system early in its development). The TCR can bind to an antigen on an MHC Class II receptor on a dendritic cell or macrophage. Once bound to its matching antigen, the T Cell becomes "activated" and also grows and divides.Dendritic Cell, Macrophage
T Helper CellsCells of the immune system that are created in a similar fashion to B Cells. Each T Cell displays on its surface a receptor which binds to a specific antigen (just as a B Cell produces antibodies that bind to a specific antigen). The binding of the MHC Class II molecule acts as a mitogenic signal, causing the cell to grow and divide.B Cell, V Segment, J Segment, D Segment
TelomeraseA protein which adds repeating sequences to the end of a chromosome. Because the lagging DNA strand cannot replicate its last few bases (see Okazaki Fragments), telomerase must add some repeating sequences at the end of the new DNA molecule so that DNA replication does not cause the strands to get shorter after each replication. Many cancers depend on telomerase for rapid replication of cells, so telomerase inhibitors are an active area of cancer research.Replication, Telomere, Okazaki Fragments
TelomereShort repetitive sections of DNA bases that appear at the ends of chromosomes in eukaryotes. Telomeres do not code for proteins, and are believed to protect the ends of chromosomes. In humans, telomeres contain repeats of the sequence TTAGGG.Telomerase, Eukaryotes, Chromosomes
TelophaseThe final sub-phase of Mitosis during which the mitotic spindle dissolves, the two new nuclei form, and the cell completes its division into two daughter cells.Cell Cycle Phases, Mitosis
Termination of TranscriptionThe mechanism by which RNA Polymerase is "told" to stop transcription of DNA to produce complementary RNA. Shouldn't be confused with a "stop codon" which is used to stop translation of RNA.DNA Polymerase, Transcription, Translation
Tertiary StructureWhile the secondary structure defines domains of the macromolecule that are alpha helices or beta pleated sheets, the tertiary structure defines the three dimensional orientation of alpha helices and beta pleated sheets with respect to each other. Primary Structure, Secondary Structure, Tertiary Structure, Quaternary Structure, Macromolecule, Protein
Test of ComplementationA test geneticists use to determine if two recessive mutations, affecting the same phenotype, are in two different genes or are on the same gene. For example, say you have two colonies of mutant, haploid yeast which are unable to produce a particular protein (they have the same phenotype). Cross the two colonies of yeast together to create a diploid (yeast can switch from haploid to diploid). If the new colony can't create the protein (has the same phenotype as the parents), the two mutant colonies may have a mutation on the same gene (or one gene may be inhibiting the expression of the other). If the resulting colony has some children who can create the protein ("wild type"), then there is "genetic complementation" and the two mutations appear on different genes. The two mutations are likely disruptions in separate steps of the biochemical pathway that produces the protein. By using mutants in this fashion, geneticists working with biochemists are able to map out the steps in biochemical pathways.Genetics, Yeast, Recessive, Haploid, Phenotype, Test of Recessivity, Test of Epistasis
Test of EpistasisImagine a biochemical pathway in which two genes create two proteins which are used to create a particular biomolecule. Also, assume that each of these mutants has a measurable phenotype. Imagine that a geneticst has isolated two mutants that cannot create this particular biomolecule, and that a Test of Complementation has shown that these two mutants have mutations in different genes. A Test of Epistasis will show which of the two proteins are used first in the biochemical pathway. If an organism which is Homozygous for both gene mutations shows the phenotype of the first mutant, then the first mutant's mutated gene is the one that is used first in that biochemical pathway.Test of Recessivity, Test of Complementation
Test of RecessivityBy crossing a mutant with wild-type, a geneticist can determine if the phenotype (observed trait) of the mutant is recessive or dominant. If the trait is recessive, the offspring will not show the phenotype because they will be heterozygotes. A geneticist will often do the Test Of Recessivity, followed by a Test of Complementation, followed by a Test of EpistasisRecessive, Dominant, Mutant, Test of Complementation, Test of Recessivity
Test-CrossIn genetics, a test-cross is used to determine if an individual exhibiting a dominant trait is homozygous or heterozygous for that trait. This is done by crossing (breeding) that individual (whose genotype is unknown) with an individual which is known to be homozygous recessive for that particular trait.Genetics, Mendelian Genetics, Dominant, Recessive, Homozygous, Heterozygous
Thermus AquaticusBacteria capable of living in hot water. It uses a polymerase called "Taq" to replicate its DNA. This polymerase is often used for PCR, so that the biochemist does not have to add new polymerase to the solution after each denaturing (heating) cycle.PCR, Polymerase, Bacteria
Tobacco Mosaic VirusThe first virus ever discovered, Tobacco Mosaic Virus infects plants, and in particular Tobacco plants, causing discoloration of the leaves. It consists of a single RNA genome wrapped in protein (one of the simplest virus structures, helical symmetry, note that there is no envelope). Note that there are no known animal viruses with a protein capsid and no envelope, although there are other plant viruses that have this structure.Virus
TopoisomerTwo chemicals are Topoisomers if they are the same except for their topology. For example, during DNA replication, the new DNA strands are wrapped around their old DNA strands until an enzyme called topoisomerase cuts them and rejoins them outside their parent strand, so that they are no longer tangled. In this way, topoisomerase changes the topology of the new DNA strand, but not its chemical structure.Topoisomerase
TopoisomeraseAn enzyme which cuts and rejoins new DNA strands during replication so that they are no longer tangled with their parent. In this way, the DNA strands can separate and be sent on to two different daughter cells. Because rapidly growing cancer cells require much Topoisomerase, there is ongoing research to inhibit some cancers by inhibiting Topoisomerase.Topoisomer, DNA, Replication
Trafficking StepSome viruses can reproduce in the Cytoplasm of the cell. Alternatively, some viruses need to inject their nucleic acid (and possibly some proteins) into the nucleus of the cell. The latter type of viruses partake in a "trafficking step" which moves the viral molecules into the nucleus of the infected cell.Virus
Transcribed StrandDuring RNA Transcription, the DNA strand that is being read is called the "Transcribed Strand". Note that the RNA molecule that is polymerized has complementary bases (A->U, T->A, C->G, G->C). The Transcribed Strand is also called the "Non-Coding Strand".Transcribe, RNA, Non-Transcribed Strand, Coding Strand, Non-Coding Strand
TranscriptionThe process by which a complementary RNA strand is made from a section of DNA. RNA Polymerize begins transcribing a gene from 5' to 3' right after a Promoter Site. When DNA Polymerase detects Adenine on the DNA, it adds Uracil (not Thymine) to the RNA. Similarly it polymerizes Guanine (G) for each Cytosine (C), Cytosine (C) for each Guanine (G), and Adenine (A) for each Thymine(T). It stops when it detects a stop sequence. In Eukaryotes, Transcription takes place in the Nucleus. In Prokaryotes, Transcription takes place in the Cytoplasm.Central Dogma of Molecular Biology, Replication, Translation, Transcribed Strand
Transcriptional RegulationA type (level) of Gene Regulation whereby a gene's expression is turned on or off by preventing or allowing the transcription of the gene into mRNA. This can be done by controlling the initiation of transcription (preventing or allowing RNA Polymerase to attach -- this is the most common form of gene regulation). This method of regulation also includes Alternative Splicing. mRNA stability is also used to regulate. The less stable the mRNA, the less likely the target protein will be produced.Gene Regulation, Alternative Splicing
TransfectionTo introduce and integrate foreign DNA into a cell; gene transfer.Integrase
Transfer RNATransfer RNA (tRNA) contain sequences of 3 nucleotides and also a bond to a particular amino acid. Each 3 letter nucleotide corresponds to a particular amino acid. For example, a tRNA with the nucleotide sequence GUU will attach to a Glutamine amino acid. As a Ribosome "reads" a messenger RNA (mRNA), it uses a tRNA whose 3 letter nucleotide sequence (anti-codon) complements the currently read mRNA codon (3 letter code) to receive and attach the correct amino acid to the protein it is building. Thus, tRNA acts as a kind of adapter, converting a 3-letter nucleotide sequence into a specific amino acid.Messenger RNA, Nucleotide, Amino Acid, Protein
TransformA cell is said to be "transformed" if it is genetically changed in some way. For example, a cell can be "transformed" by a mutation into a malignant cell which then grows and divides constitutively, forming a tumor (note that tumors form from a single, malignant parent cell). If a Molecular Biologist introduces new DNA into a bacterial cell, thus modifying it, the cell is said to be "transformed". For example, in DNA cloning, bacterial cells are "transformed" when recombinant DNA is added to the bacterial cell.DNA Cloning
Transforming PrincipleDuring the 1920s, Griffiths discovered that combining dead, virulent, smooth, glistening pneumococcus bacteria with live, non-virulent, rough pneumococcus bacteria would cause the future generations of bacteria to "transform" into virulent bacteria. He called the mysterious, unknown substance which caused the non-virulent bacteria to change into virulent bacteria "The Transforming Principle". This would give motivation for later experments which determined that DNA was the molecule which contained heredity.Molecular Biology, DNA
TranslationThe process by which a protein is created from an RNA template. During translation, the single-stranded RNA molecule is read from 5' to 3', and 3 bases (a triplet code, or codon) are read to determine which Amino Acid should be added next (polymerized) onto the protein. Translation begins at an initiator codon (AUG). Translation takes place in the Cytoplasm.Central Dogma of Molecular Biology, Replication, Transcription, Codon
Translational RegulationA type of Gene Regulation which enables or disables the expression of a gene at during Translation. For example, the cell might sequester the mRNA, keeping it from the Ribosome and preventing Translation. Also, micro-RNAs can interfere with an mRNA, preventing its translation.Gene Regulation
TransloconA complex of proteins on the Endoplasmic Reticulum (ER), located next to a Signal Recognition Protein Receptor, which regulate the movement (translocation) of proteins in the ER.Endoplasmic Reticulum, Signal Recognition Protein
Transmembrane DomainThe portion (sequence of amino acids) of a trans-membrane protein (a protein that straddles the membrane) that reside inside the membrane. The transmembrane domain usually consists of 20-25 amino acids, and is hydrophobic ("water hating"). It anchors the protein in the membrane. Protein, Amino Acid, Hydrophobic
TransposonA segment of DNA that is capable duplicating itself and then inserting the copy into a different part of the genome. Some transposons make an RNA copy, and then insert their genome into DNA using a process similar to that of retroviruses. Other transposons use a DNA intermediate. It is currently believed that half of the human genome is made up of transposons. Although it is currently believed that most transposons just "take up space", there are a few that are known to have been co-opted into working genes. They are sometimes described as a "genomic symbiot"DNA, Genomics, Reverse Transcriptase, Retrovirus
Tritiated ThymidineRadio labelled Thymidine that a biochemist can use to study biochemical reactions. For example, a biochemist studying cell cycle phases may do the following: 1.) Add Colcemid to the solution of cells, causing all the cells to get stuck in Metaphase (a sub-phase of Mitosis) after a period of time. 2.) Remove the Colcemid, freeing the cells from Metaphase, and then add Tritiated Thymidine to the solution. 3.) After some time (hours), measure how much Tritiated Thymidine is still in solution. The missing Tritiated Thymidine will be found in DNA of any cells which have reached the Synthesis Phase of Mitosis, giving the biochemist data on how long the different phases of Mitosis last.Mitosis Phase, Synthesis Phase, Cell Cycle Phases
tRNAAn abbreviation for Transfer RNA. As a Ribosome "reads" a messenger RNA (mRNA), it uses tRNAs to map the current codon (3 letter nucleotide sequence of the mRNA) to the next amino acid it needs to add to the protein. tRNA supplies this mapping between nucleotide sequences and protein sequences because it has both a 3-letter nucleotide sequence (which is complementary to the current mRNA codon) and also a specific amino acid. The mapping between 3-letter nucleotide sequences and amino acids provides the code of life.mRNA, Nucleotide, Amino Acid, Protein
Tumor Suppressor GenesGenes that work in an opposite fashion to proto-oncogenes and oncogenes. Tumor Supporessor Genes create proteins which try to inhibit the growth and division of the cell. Mutations in these genes can cause cancer (e.g. Retinoblastoma), as these proteins inhibit the growth and division cycle of the cell.Oncogene, Proto-Oncogene, Retinoblastoma
TumorigenicityThe ability of a cell to grow into a tumor. A phenotype of cancer cells.Cancer Cells
Type 1 DiabetesA type of autoimmune disease in which the immune system recognizes (attacks) antigens on islet cells in the pancreas which make insulin.Autoimmune Disease, B Cell
TyrosineAn amino acid; one of the 20 amino acids used for building proteinsAmino Acid, Residue, Protein, Tyrosine Kinase
Tyrosine KinaseA kinase that adds a phophate group (taken from an ATP moledule's Gamma Phosphate) to a tyrosine amino acid, thus modifying the protein which contains that tyrosine. Tyrosine Kinases are often used to enable a biochemical pathway. For example, they may signal a cell to begin dividing (mitosis).Tyrosine, Protein, Residue, Amino Acid, Kinase, Mitosis

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