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Biology Definitions

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Cancer CellsCancer cells are cells that can grow constitutively. That is, they grow inspite of the absence of growth factors or the presence of growth inhibitors. A cell that has been converted from a normal cell into a cancer cell is said to be "transformed". Cancer cells often can be identified by the following phenotypes (traits): they change their shape, they can grow and divide with far less growth factors present, they have anchorage independence, and they exhibit tumorigenicity.Cancer, Growth Factor, Transformation, Anchorage Independence, Tumorigenicity
CapIn Eukaryotic cells, a special molecule is added to the 5' end of a messenger RNA. This molecule is called a "cap" and is used to signal the Ribosome that this mRNA should be translated.Transcription, Translation, Ribosome, messenger RNA, Poly (A) Tail
CapsidThe protein shell which surrounds a virus particle, containing and protecting nucleic acids (DNA or RNA) which contain the genetic code of the virus. If the virus has an envelope, then instead of saying "capsid" we say that the virus has a nucleocapsid, consisting of the protein and nucleic acid.Protein, Lipid, Virus, DNA, RNA, Nucleic Acid, Helical Symmetry, Viral Envelope
CarcinogenA chemical or physical agent that can cause cancer. Most Carcinogens are also Mutagens (cause mutations in DNA). Ultraviolet radiation, X-Rays, and tobacco smoke are examples of carcinogens. When plotting a graph of the Mutagenic vs.Carcinogenic attributes of a substance on a log-log scale, one usually discovers a linear relationship, indicating that mutations in DNA cause cancer.Mutagen, Carcinogen, Cancer Cells
CarcinomaA Tumor of an epithelial (skin or tissue that lines or covers other organs) tissue. 30 or 40% of all carcinomas are caused by overexpressed EGF receptors. Note that when two EGF receptors are brought together by a (dimer) ligand, they initiate a biochemcial pathway that causes the cell to grow and divide. If there are too many GF receptors, two receptors may come together by chance (without a ligand), and cause the cell to continuously grow and divide, ignoring the presence of Growth Inhibitors and absence of growth factors in the extra-cellular space.Cancer, Epithelial, Overexpressed
Catalytic CleftThe part of an enzyme which interacts with another molecule, called the substrate, enabling the substrate to more easily participate in a particular chemical reaction.Protein, Enzyme, Catalyst, Substrate
CD4A receptor on T Helper Cells which can recognize and bind to MHC Class II receptors (on B Cells, Macrophages, and Dendritic Cells). HIV infects T cells by first attaching to the CD4 receptor.B Cell, MHC Class II Receptor, Macrophage, Dendritic Cell, AIDS
cDNA"copied" DNA. A biochemist can use reverse transcriptase to convert an mRNA sequence into DNA, creating a single-stranded DNA sequence, called cDNA, which contains only the exons, and none of the introns, in the original DNA sequence. A biochemist may create cDNA, so that a gene from a eukaryote can be grown in a bacterial cell, even though the bacterial cell cannot do mRNA splicing.Cloning, mRNA, Eukaryote, Prokaryote
Cell Cycle PhasesThere are four cell cycle phases: 1.) M or Mitosis Phase (when the cell divides) 2.) G1 Phase 3.) S or Synthetic Phase (when cells replicate DNA) 4.) G2 Phase. The G phases are "Gap" phases. The human body has approximately 10^16 cell divisions in its lifetime, and 3 x 10^13 cells.Mitosis Phase, Synthetic Phase,
Cell Cycle SignalingMethods by which the cell "decides" whether or not to proceed through cell cycle phases (M->G1->S->G2). Note that when problems occur in cell cycle signaling, the body may produce too many cells (cancer). In a healthy organism, cells use inter-cellular communication mechanisms to decide when a cell should divide into two daughter cells. A cell which ignores these signals, and replicates autonomously, acts as a cancerous cell.Cell Cycle Phases, Growth Factor
Cellular ImmunityA component of the immune system which uses Cytotoxic T Cells which destroy cells which exhibit certain antigens on their surface. Important for anti-viral defenses (humoral immunity also plays a role in anti-viral defense).Humoral Immunity, T Cell
Central Dogma of Molecular BiologyStates that DNA replicates its information, that RNA is transcribed from the information in DNA, and that RNA translation creates proteins. Francis Crick used the word "Dogma" because at the time he and Watson postulated this theory, they had no scientific proof of its validity. DNA, RNA, Protein, Replication, Transcription, Translation
CentrifugeA machine used in the laboratory to separate out materials of different densities, often resulting in a "pellet" at the bottom of the tubes, and a liquid "supernatent".Supernatent, Pellet
CentromereDuring Mitosis, the two identical Chromatin produced during the Synthesis (S) Phase are attached to each other via Centromeres. The spindle fibers attach to the centromere, and pull the individual chromatin to opposite sides of the cell during the mitosis phase.Mitosis, Sythesis Phase
ChaperoneA protein whose function is to correctly (sometimes temporarily) fold or prevent folding of another protein (e.g. for transport to another part of the cell).Protein
Chargaff's RulesStates that in any DNA molecule, the number of the Adenine (A) molecules equals the number of Thymine molecules, and the number of Cytosine molecules equals the number of Guanine molecules. It can also be stated that the amount of purines (A and G) equals the amount of pyrimidines (T and C). This observation helped Watson and Crick determine the structure of DNA.Base Pairing, Purine, Pyrimidine, DNA, Adenine, Thymine, Cytosine, Guanine
Charged tRNAA tRNA with its associated Amino Acid attached.tRNA
ChiasmaPlural of chiasmata. Sometimes, during meiosis (cell division which produces haploid gametes), homologous chromosomes temporarily overlap each other. This temporary intersection is called a chiasmata. Chiasma can sometimes be viewed in a microscope during meiosis, and this led to the theory that homologous chromosomes sometimes exchange genetic material (Genetic Recombination).Meiosis, Chromosome, Genetic Recombination
ChiasmataSingular of Chiasma. Sometimes, during meiosis (cell division which produces haploid cells), homologous chromatids temporarily overlap each other. This temporary intersection is called a chiasmata. Chiasma can sometimes be viewed in a microscope during meiosis. This led to the theory that homologous chromosomes sometimes exchange genetic material (Genetic Recombination).Meiosis, Chromosome, Genetic Recombination
ChromatinA complex of DNA, proteins, and some RNA which condense into Chromosomes during Mitosis. Chromatin are dispersed througout the nuclues, and are thus harder to see than Chromosomes, which are visible under the microscope during mitosis.Cell Cycle Phases, DNA, Protein, RNA, Chromosome, Mitosis
Chromosomal Theory of InheritanceThe theory that genes are carried on chromosomes, and that inheritance patterns may be explained by the locations of genes in chromosomes. This theory is an improvement on Mendel's Law of Independent Assortment, as it explains how inheritance of genes on the same chromosome are more correlated with each other if the genes are closer to each other on the chromosome. If the genes are further appart, random recombinations of homologous parental chromosomes can produce offspring which have non-parental phenotypes which are combinations of both parent's traits.Chromosome, Gene, Mendelian Genetics, Law of Independent Assortment
Chromosomal WalkingA scientist is trying to find the gene that causes a disease. Using genetic linkage mapping, she finds that members of the affected individual's family who have the disease also have genetic markers (unusual genetic sequences) which have a low recombination rate (there is little recombination, because the "marker" sequence and disease sequence are near each other on the chromosome). Knowing the marker sequence, the scientist can find the next sequence, and the next one until she finds the disease sequence by using the process of "Chromosomal" Walking.Genetic Linkage Mapping, DNA Cloning
ChromosomeA structure in a cell that carries the genes (hereditary information) of the organism. In Eukaryotes, the chromosomes are made up of DNA and proteins, are thread-like single strands, are found in the nucleus, and are visible under the microscope during cell division as they coil up. In Prokaryotes, the chromosomes are circular strands of DNA, and appear in the nucleoid of the cell.Prokaryote, Eukaryote, Gene, Nucleus, Nucleoid, DNA, protein, Chromosomal Theory of Inheritance
Class SwitchingWhen the immune system creates B Cells (which eventually create Plasma Cells), the DNA is modified to create different antigen recognizing sites. These mRNA of these antigen recognizing sites is then spliced to different "constant" regions. During clonal expansion (A B Cell encounters its antigen, and begins creating daughter cells which may undergo more hypermutation), the constant region may be changed, changing the function of the antibody in a process known as "class switching". For example, one "constant region" may turn the antibody into a receptor which sits on the surface of the B Cell, waiting to bind to its antigen. Another constant region may result in an antibody which is secreted by the cell.Hypermutation, B Cell, Antibody, V Segment, J Segment, D Segment
Clonal ExpansionThe production of daughter cells from a single parent cells. For example, when a B Cell encounters its matching antigen, the antigen acts as a mitogen, telling the cell to grow and divide. Subsequently, these B Cells will produce Antibodies to fight these invaders. In this way, the immune system creates more B Cells to fight a foreign invader it has detected.B Cell, Mitogen, Antigen, Antibody
Cloning by ComplementationOnce a scientist has created a "library" from a genome, she is presented with the problem of finding genes of interest in the library. If she has a gene "library" of a bacterial wild-type genome, and she is studying a mutant bacteria which is auxotrophic, she can add plasmids from the dna "library" to the auxotrophic mutant, and find the gene of interest by finding out which bacteria are "saved" by the DNA from the library.DNA Cloning, Cloning by Hybridization, Cloning by Position
Cloning by HybridizationOnce a scientist has created a "library" from a genome, she is presented with the problem of finding genes of interest in the library. If she knows the protein which the gene of interest produces, she can use "Cloning by Hybridization" to find this gene by using Olignonucleotide Probes to find which colonies in the library contain the gene of interest. Also known as "Cloning by Sequence".Oligonucleotide Probe, DNA Cloning, Cloning by Complementation, Cloning by Position
Cloning by PositionA scientist has a DNA "library", but wants to find out which colony in the library contains the gene that causes a specific disease or phenotype. Using Genetic Linkage Mapping, the scientist finds "genetic markers" which are inherited along with the disease gene, and then uses Chromsomal Walking to find sequences which overlap the disease gene, thus finding the sequence of the disease-causing gene.Genetic Linkage Mapping, Chromsomal Walking, Cloning by Complementation, Cloning by Hybridization
Co-ReceptorCells have receptors on their surface which, when bound to their ligand, can initiate various biochemical pathways (e.g. bringing a molecule into the cell, or initiating mitosis). Sometimes, a cell requires a second receptor to initiate a biochemical pathway (e.g. one receptor may not be enough to import a particle into a cell). This second receptor is called a "co-receptor". Some viruses, like AIDS, require a co-receptor to infect the cell.Virus, AIDS
Coding StrandDuring RNA Transcription, the DNA strand that is opposite the strand being read during RNA Polymerization is called the "Coding Strand". Note that the sequence of bases in the RNA actually match the bases in the coding strand (except that Uracil replaces Thymine). The Coding Strand is also called the "Non-Transcribed Strand".RNA, RNA Polymerization, Non-Coding Strand
CodonDuring Translation, triplet codes of 3 bases are read sequentially from a single-stranded RNA molecule to determine which Amino Acid should be added to the protein which is being created (polymerized). There are 64 codons, 3 of them are stop codons.Translation, Amino Acid, Protein, Stop Codon, tRNA
Complementation GroupsAfter performing a Test of Complementation, the Complementation Groups are the sets of mutants who did not complement each other. The mutants in a particular complementation group have mutations in the same gene, so crossing them does not change the phenotype (assuming the phenotype is recessive).Test of Complementation, Test of Recessivity, Test of Epistasis
Condensation ReactionA chemical reaction in which two molecules are joined together and a water molecule (H20) is released. Also known as a dehydration reaction.Esterification, Dehydration Reaction, Peptide Bond, Polymer
Conditional LethalA mutant that can grow under some circumstances, but not others. Useful to geneticists to test whether or not a colony of bacteria has a certain mutation. These types of mutations usually result from a missense mutation (substitution of one amino acid for another). For example, the mutated strain may be able to grow at high temperatures, but not at lower temperatures where the mutation manifests by not correctly folding the protein.Genetics, Missense Mutation
Constitutive MutantA mutant which has lost the ability to regulate one of its genes, and instead expresses that gene under all conditions.Gene Regulation
Contact InhibitionIf a scientist puts healthy cells into a petri dish with growth factors, the cells will grow until they cover the bottom of the dish, forming a monolayer, and then stop growing and dividing due to "contact inhibition". Conversely, cancer cells will continue to grow, as they are uninhibited by contact inhibition, creating layer after layer of cells on top of each other.Cancer
Cooking TipsDuring a DNA Cloning procedure, a scientist may use many "cooking tips" to ensure that the resultant DNA library has all possible genomic sequences, and does not have any non-transformed cells which do not contain segments of the DNA genome of interest. For example, the scientist may remove phophates from the cleaved ends of a plasmid vector (to prevent the plasmid from ligating before a sequence has been attached), or the scientist may use "partial digestion" to ensure that the restriction enzyme doesn't cleave the middle of a particular gene.DNA Cloning, Partial Digestion
Covalent BondA chemical bond between atoms in which one or more electrons are shared between the nuclei of two atoms. Covalent bonds are stronger than Ionic bonds which are stronger than Hydrogen bonds which are stronger than Van der Waals forces. The DNA molecule uses strong covalent bonds to hold each strand together, and weak hydrogen bonds to hold the two strands together, making it easy to separate the strands during transcription of RNA and also replication of DNA. Proteins use both covalent and hydrogen bonds for both intra-molecular and inter-molecular bonds.Hydrogen Bond, Ionic Bond, Van der Waals Force, DNA, RNA, Transcription
CrypticIn a pedigree of a phenotype that is incompletely penetrant, an individual who has the genotype of interest but does not exhibit the phenotype is called "cryptic".Pedigree, Incompletely Penetrant, Genotype, Phenotype
Cytidine DeaminaseAn enzyme that creates deliberate mutations in DNA. It removes the amine group from a cytidine effectively converting it into a thymidine, thus changing the genetic code. The immune system stochastically modifies genes in different immune 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 in a processes known as "Hypermutation".Hypermutaion, D Segment, J Segment, V Segment
CytokineAny of several regulatory proteins, which are released by cells of the immune system, and that act as signalling molecules, mediating immune responses. Interleukins and lymphokines are two examples of cytokines.Protein
CytologyThe study of cells. Cytologists study the structure and function of cells using microscopes. Robert Hooke was the first cytologist in 1665.Cell, Robert Hooke
Cytotoxic T Cells (Tc)T Cells with receptors that can recognize infected cells which they subsequently kill. These cells are activated by T Helper Cells.T Helper Cells

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