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Virus

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A microscopic particle capable of infecting cells of living organisms and inserting its genetic material. They are generally organisms not considered to be alive because they do not display all of the characteristics associated with life. (181)

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Molecular Biology

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The study of the molecular basis of genes and gene expression; molecular genetics.(181)

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Bacteriophage/ Phage

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A virus that infects bacteria. (182)

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Nucleotide

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An organic monomer consisting of a five-carbon sugar covalently bonded to a nitrogenous base and a phosphate group. They are the building blocks of nucleic acids. (184)

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Polynucleotide

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A polymer made up of many nucleotides covalently bonded together. (184)

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Sugar-phosphate backbone

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The alternating chain of sugar and phosphate to which the DNA and RNA nitrogenous bases are attached. (184)

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Thymine (T)

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A single ring nitrogenous base found in DNA. (185)

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Cytosine (C)

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A single-ring nitrogenous base in DNA and RNA. (185)

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Adenine (A)

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A double-ringed nitrogenous base found in DNA and RNA. (185)

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Guanine (G)

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A double-ringed nitrogenous base found in DNA and RNA. (185)

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Uracil (U)

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A single-ringed nitrogenous base found in RNA. (185)

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Double Helix

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The form of native DNA, referring to its two adjacent polynucleotide strands wound into a spiral shape. (186)

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Semiconservative Model

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Type of DNA replication in which the replicated double helix consists of one old strand, derived from the old molecule, and one newly made strand. (188)

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DNA Polymerase

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An enzyme that assembles DNA nucleotides into polynucleotides using a preexisting strand of DNA as a template. (189)

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DNA Ligase

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An enzyme, essential for DNA replication, that catalyzes the covalent bonding of adjacent DNA strands; used in genetic engineering to paste a specific piece of DNA containing a gene of interest into a bacterial plasmid or other vector. (189)

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Transcription

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The synthesis of RNA on a DNA template. (190)

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Translation

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The synthesis of a polypeptide using the genetic information encoded in an mRNA molecule. There is a change of "language" from nucleotides to amino acids. (190)

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Triplet Code

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A set of three-nucleotide-long "words" that specify the amino acids for polypeptide chains. (191)

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Codon

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A three-nucleotide sequence in mRNA that specifies a particular amino acid or polypeptide termination signal; the basic unit of the genetic code. (191)

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Genetic Code

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The set of rules that dictates the correspondence between RNA codons in an mRNA molecule and amino acids in protein. (192)

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RNA Polymerase

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An enzyme that links together the growing chain of RNA molecules during transcription, using a DNA strand as a template. (193)

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Promoter

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A specific nucleotide sequence in DNA located at the start of a gene that is the binding site for RNA polymerase and the place where transcription begins. (193)

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Terminator

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A special sequence of nucleotides in DNA that marks the end of a gene. It signals RNA polymerase to release the newly made RNA molecules and then depart from the gene. (193)

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Messenger RNA (mRNA)

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The type of ribonucleic acid that encodes genetic information from DNA and conveys it to ribosomes, where the information is translated into amino acid sequences. (194)

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Introns

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In eukaryotes, a nonexpressed (noncoding) portion of a gene that is excised from the RNA transcript. (194)

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Exon

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In eukaryotes, a coding portion of a gene.(194)

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RNA Splicing

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The removal of introns and joining of exons in eukaryotic RNA, forming an mRNA molecule with a continuous coding sequence; occurs before mRNA leaves the nucleus. (194)

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Transfer RNA (tRNA)

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A type of ribonucleic acid that functions as an interpreter in translation. Each of these molecules has a specific anticodon, picks up a specific amino acid, and conveys the amino acid to the appropriate codon on mRNA. (194)

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Anticodon

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On a tRNA molecule, a specific sequence of three nucleotides that is complementary to a codon triplet on mRNA. (195)

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Ribosomal RNA (rRNA)

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The type of ribonucleic acid that, together with proteins, makes up ribosomes; the most abundant type of RNA in most cells. (196)

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Start Codon

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On mRNA, the specific three-nucleotide sequence (AUG) to which the initiator tRNA molecule binds, starting translation of genetic material. (196)

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Translocation

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(1) During protein synthesis, the movement of a tRNA molecule carrying a growing polypeptide chain from the A site to the P site on a ribosome. (The mRNA travels with it.) (2) A change in a chromosome resulting from a chromosomal fragment attaching to a nonhomologous chromosome; can occur as a result of an error in meiosis or from mutagenesis. (197)

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Stop Codon

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In mRNA, one of the three triplets (UAG, UAA, UGA) that signal gene translation to stop. (197)

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Mutation

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A change in the nucleotide sequence of an organism's DNA; this can also occur in the DNA or RNA of a virus; the ultimate source of genetic diversity. (199)

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Reading Frame

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The way in which a cell's mRNA -translating machinery groups the mRNA nucleotides into codons. (199)

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Mutagenesis

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The creation of a mutation. (199)

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Mutagen

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A chemical or physical agent that interacts with DNA and causes a mutation. (199)

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Capsid

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The protein shell that encloses a viral genome. It may be rod-shaped, polyhedral, or more complex in shape.(200)

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Lytic Cycle

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A type of viral replication cycle resulting in the release of new viruses by lysis (breaking open) of the host cell. (200)

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Lysogenic Cycle

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A type of bacteriophage replication cycle in which the viral genome is incorporated into the bacterial host chromosome as a prophage. New phages aren't produced, and the host cell is not killed or lysed unless the viral genome leaves the host chromosome. (200)

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Prophage

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Phage DNA that has inserted by genetic recombination into the DNA of a prokaryotic chromosome. (200)

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Emerging Virus

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A virus that has suddenly appeared or has recently come to the attention of medical scientists. (202)

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Reverse Transcriptase

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An enzyme used by retroviruses that catalyze the synthesis of DNA on an RNA template. (203)

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Retroviruses

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An RNA virus that reproduces by means of a DNA molecule. It reverse-transcribes its RNA into DNA, inserts the DNA into a cellular chromosome, and then transcribes more copies of the RNA from the viral DNA. (203)

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AIDS

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Acquired immunodeficiency syndrome; the late stages of HIV infection, characterized by a reduced number of T cells and the appearance of characteristic opportunistic infections. (203)

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HIV

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Human immunodeficiency virus; the retrovirus that attacks the immune system and causes AIDS. (203)

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Viroids

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A plant pathogen composed of molecules of naked, circular RNA several hundred nucleotides long. (203)

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Prions

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An infectious form of protein that may multiply by converting related proteins into more of these. These cause several related diseases in different animals, including scrapie in sheep and mad cow disease. (203)

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Transformation

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The incorporation of new genes into a cell from DNA that the cell takes up from the surrounding environment.(204)

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Transduction

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(1) The transfer of bacterial genes from one bacterial cell to another by a phage. (204)

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Conjunction

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The union (mating) of two bacterial cells or protist cells and the transfer of DNA between the two cells. (204)

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F Factor

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A piece of DNA that can exist as a bacterial plasmid. This carries genes for making sex pili and other structures needed for conjugation, as well as a site where DNA replication can start. F stands for fertility. (205)

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Plasmid

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A small ring of independently replicating DNA separate from the main chromosome(s). Are found in prokaryotes and yeast. (205)

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R Plasmid

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A bacterial plasmid that carries genes for enzymes that destroy particular antibiotics, thus making the bacterium resistant to the antibiotics. (205)

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