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Chapter 17

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gene expression
The process by which DNA directs the synthesis of proteins or, in some cases, just RNAs.
transcription
The synthesis of RNA using a DNA template.
messenger RNA (mRNA)
A type of RNA, synthesized using a DNA template, that attaches to ribosomes in the cytoplasm and specifies the primary structure of a protein.
translation
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.
ribosome
A complex of rRNA and protein molecules that functions as a site of protein synthesis in the cytoplasm; in eukaryotic cells, each subunit is assembled in the nucleolus. See also nucleolus.
primary transcript
An initial RNA transcript; also called pre-mRNA when transcribed from a protein-coding gene.
triplet code
A set of three-nucleotide-long words that specify the amino acids for polypeptide chains.
template strand
The DNA strand that provides the pattern for ordering the sequence of nucleotides in an RNA transcript.
codon
A three-nucleotide sequence of DNA or mRNA that specifies a particular amino acid or termination signal; the basic unit of the genetic code.
reading frame
On an mRNA, the triplet grouping of ribonucleotides used by the translation machinery during polypeptide synthesis.
RNA polymerase
An enzyme that links ribonucleotides into a growing RNA chain during transcription.
promoter
A specific nucleotide sequence in DNA that binds RNA polymerase, positioning it to start transcribing RNA at the appropriate place.
terminator
In bacteria, a sequence of nucleotides in DNA that marks the end of a gene and signals RNA polymerase to release the newly made RNA molecule and detach from the DNA.
transcription unit
A region of DNA that is transcribed into an RNA molecule.
transcription factor
A regulatory protein that binds to DNA and affects transcription of specific genes.
transcription initiation complex
the completed assembly of transcription factors and RNA polymerase bound to a promoter
TATA box
a DNA sequence in eukaryotic promoters crucial in forming the transcription iniation complex
RNA processing
Modifications of RNA transcripts, including splicing out of introns, joining together of exons, and alterations of the 5' and 3' ends
5' Cap
A modified form of guanine nucleotide added on to the nucleotide at the 5' end of a pre-mRNA molecule
poly-A tail
A sequence of 50 to 250 adenine nucleotides added on to the 3' end of a pre-mRNA molecule
RNA splicing
after synthesis of a eukaryotic primary RNA transcript, the removal of portions (introns) of the transcript that will not be included in the mRNA
intron
a noncoding, intervening sequence within a primary transcript that is removed from the transcript during RNA processing; also refers to the region of the DNA from which this sequence was transcribed
exons
a sequence within a primary transcript that remains in the RNA after RNA processing; also refers to the region of the DNA from which this sequence was transcribed
spliceosome
a large complex made up of proteins and RNA molecules that splices RNA by interacting with the ends of an RNA intron, releasing the intron and joining the adajcent exons
ribozyme
an RNA molecule that functions as an enzyme, catalyzing reactions during RNA splicing
alternative RNA splicing
a type of eukaryotic gene regulation at the RNA-processing level in which different mRNA molcules are produced from the same primary transcript depending on which RNA segments are treated as exons and which as introns
domains
an independently folding part of a protein
transfer RNA (tRNA)
an RNA molecule that functions as an interpreter between nucleic acid and protein language by picking up specific amino acids and recognizing the appropriate codons in the mRNA
anticodon
a nucleotide triplet at one end of a tRNA molecule that recognizes a particular complementary codon on an mRNA molecule
aminoacyl-tRNA synthetase
an enzyme that joins each amino acid to the appropriate tRNA
wobble
flexibility in the base-pairing rules in which the nucleotide at the 5' end of a tRNA anticodon can form hydrogen bonds with more than one kind of base in the third position (3' end) of a codon
ribosomal RNA (rRNA)
the most abundant type of RNA, which together with proteins makes up ribosomes
A site
one of a ribosome's three binding site for tRNA during translation. It holds the tRNA carrying the next amino acid to be added to the polypeptide chain
E site
one of a ribosome's three binding site for tRNA during translation. It is the place where discharged tRNAs leave the ribosome
P site
one of a ribosome's three binding sites for tRNA during translation. it holds the tRNA carrying the growing polypeptide chain.
polyribosome (polysome)
a group of several ribosomes attatched to, and translating, the same messenger RNA molecule
signal peptide
a sequence of about 20 amino acids at or near the leading amino end of a polypeptide that targets it to the endoplasmic reticulum or other organelles in a eukaryotic cell
signal-recognition particle (SRP)
a protein-RNA complex that recognizes a signal peptide as it emerges from a ribosome and helps direct the ribosome to the endoplasmic reticulum (ER) by binding to a receptor protein on the ER
mutation
a change in the nucleotide sequence of an organism's DNA, ultimately creating genetic diversity. It can also occur in the DNA or RNA of a virus
point mutation
a change in a gene at a single nucleotide pair
base-pair substitution
a type of point mutation; the replacement of one nucleotide and its partner in the complementary DNA strand by another pair of nucleotides
missense mutation
a base-pair subsitution that results in a codon that codes for a different amino acid
nonsense mutation
a mutation that changes an amino acid codon to one of the three stop codons, resulting in a shorter and usually nonfunctional protein
insertion
a mutation involving the addition of one or more nucleotide pairs to a gene
deletion
a mutational loss of one or more nucleotide pairs from a gene
frameshift mutation
a mutation occuring when the number of nucleotides inserted or deleted is not a multiple of three, resulting in the improper grouping of the subsequent nucleotides into codons
mutagen
a chemical or physical agent that interacts with DNA and causes a mutation