Ch9: The Chemistry Of Heredity And Gene Expression

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totipotent

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For a plant cell, having the potential to develop into an entire plant

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genetic code

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The system of nucleotide triplets (codons) in DNA and RNA that dictate the amino acid sequence in proteins; except for three "stop" signals, each codon specifies one of 20 amino acids

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transcription

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The synthesis of messenger RNA, which is a copy of a portion of one strand of the double-stranded DNA helix

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thymine

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A pyrimidine occurring in DNA but not in RNA. See also uracil

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heterochromatin

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Regions of chromosomes that remain highly condensed throughout the cell cycle and appear to be devoid of transcription; for example, the chromatin located in the centromere region of each chromosome and at the end sequences, called telomeres

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cytosine

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One of the four pyrimidine bases found in the nucleic acids DNA and RNA

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guanine

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[Sp. from Quechua, huanu, dung] A purine base found in DNA and RNA; name derived from "guano," because is abundant as a white crystalline base in guano and other kinds of animal excrement

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genome

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The totality of genetic information contained in the nucleus, plastid, or mitochondrion

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template

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A pattern or mold guiding the formation of a negative or complement; a term applied especially to DNA duplication, which is explained in terms of a template hypothesis

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anticodon

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In a tRNA molecule, the three-nucleotide sequence that base-pairs with the mRNA codon for the amino acid carried by that particular tRNA; the anticodon is complementary to the mRNA codon

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replication fork

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In DNA synthesis, the Y-shaped structure formed at the point where the two strands of the original molecule are being separated and the complementary strands are being synthesized

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ribonucleic acid (RNA)

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Type of nucleic acid formed on chromosomal DNA and involved in protein synthesis; composed of chains of phosphate, sugar molecules (ribose), and purines and pyrimidines. RNA is the genetic material of many kinds of viruses

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pyrimidine

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The smaller of the two kinds of nucleotide bases found in DNA and RNA: a nitrogenous base with a single-ring structure, such as cytosine, thymine, or uracil

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codon

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Sequence of three adjacent nucleotides in a molecule of DNA or mRNA that form the code for a single amino acid or for the termination of a polypeptide chain

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Okazaki fragments

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[after R. Okazaki, Japanese geneticist] In DNA replication, the discontinuous segments in which the 3' to 5' strand (the lagging strand) of the DNA double helix is synthesized; typically 1000 to 2000 nucleotides long in prokaryotes, and 100 to 200 nucleotides long in eukaryotes

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polysome or polyribosome

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An aggregation of ribosomes actively involved in the translation of the same mRNA molecule, one after another

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ligase

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An enzyme that joins together (ligates) two molecules in an energy-dependent process; DNA ligase, for example, is essential for DNA replication, catalyzing the covalent bonding of the 3' end of a new DNA fragment to the 5' end of a growing chain

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promoter

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A specific segment of DNA to which RNA polymerase attaches to initiate transcription of mRNA from an operon

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transfer RNA (tRNA)

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Low-molecular-weight RNA that becomes attached to an amino acid and guides it to the correct position on the ribosome for protein synthesis; there is at least one tRNA molecule for each amino acid

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replicate

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Produce a facsimile or a very close copy; term used to indicate the production of a second molecule of DNA exactly like the first molecule or the production of a sister chromatid

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polypeptide

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A molecule composed of amino acids linked together by peptide bonds; not as complex as a protein

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telomere

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The end of a chromosome; has repetitive DNA sequences that help counteract the tendency of the chromosome, without telomeres, to shorten with each round of replication

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messenger RNA (mRNA)

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The class of RNA that carries genetic information from the gene to the ribosomes, where it is translated into protein

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purine

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The larger of the two kinds of nucleotide bases found in DNA and RNA; a nitrogenous base with a double-ring structure, such as adenine or guanine

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transcription factors

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Proteins that directly or indirectly affect the initiation of transcription

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adenine

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A purine base present in DNA, RNA, and nucleotide derivatives, such as ADP and ATP

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euchromatin

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Regions of chromosomes that undergo condensation and decondensation in the cell cycle; regions capable of gene transcripton

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deoxyribonucleic acid (DNA)

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Carrier of genetic information in cells; composed of chains of phosphate, sugar molecules (deoxyribose), and purines and pyrimidines; capable of self-replication and determines RNA synthesis

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ribosomal RNA (rRNA)

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Any of a number of specific molecules that form part of the structure of a ribosome and participate in the synthesis of proteins

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structural gene

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Any gene that codes for a protein; as opposed to a regulatory gene

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intron

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[L. intra, within] In eukaryotes, a portion of mRNA, transcribed from DNA, that is removed by enzymes before the mRNA is translated into protein. See also exon

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exon

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[Gk. exo, outside] A segment of DNA that is both transcribed into RNA and translated into protein; exons are characteristic of eukaryotes. See also intron

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noncoding RNA

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An RNA molecule that is not translated into protein

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translation

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The synthesis of a polypeptide directed by the nucleotide sequence of messenger RNA

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uracil

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A pyrimidine found in RNA but not in DNA. See also thymine

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transposon

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[L. transponere, to change the position of something] A DNA sequence that carries one or more genes and is flanked by sequences of nucleotides that confer the ability to move from one DNA molecule to another; an element capable of transposition, which is the changing of a chromosomal location