Key Terms
- 7-methylguanosine cap
- modification added to the 5′ end of pre-mRNAs to protect mRNA from degradation and assist translation
- aminoacyl tRNA synthetase
- enzyme that “charges” tRNA molecules by catalyzing a bond between the tRNA and a corresponding amino acid
- anticodon
- three-nucleotide sequence in a tRNA molecule that corresponds to an mRNA codon
- CAAT box
- (GGCCAATCT) essential eukaryotic promoter sequence involved in binding transcription factors
- central dogma
- states that genes specify the sequence of mRNAs, which in turn specify the sequence of proteins
- codon
- three consecutive nucleotides in mRNA that specify the insertion of an amino acid or the release of a polypeptide chain during translation
- colinear
- in terms of RNA and protein, three “units” of RNA (nucleotides) specify one “unit” of protein (amino acid) in a consecutive fashion
- consensus
- DNA sequence that is used by many species to perform the same or similar functions
- core enzyme
- prokaryotic RNA polymerase consisting of α, α, β, and β‘ but missing σ; this complex performs elongation
- degeneracy
- (of the genetic code) describes that a given amino acid can be encoded by more than one nucleotide triplet; the code is degenerate, but not ambiguous
- downstream
- nucleotides following the initiation site in the direction of mRNA transcription; in general, sequences that are toward the 3′ end relative to a site on the mRNA
- exon
- sequence present in protein-coding mRNA after completion of pre-mRNA splicing
- FACT
- complex that “facilitates chromatin transcription” by disassembling nucleosomes ahead of a transcribing RNA polymerase II and reassembling them after the polymerase passes by
- GC-rich box
- (GGCG) nonessential eukaryotic promoter sequence that binds cellular factors to increase the efficiency of transcription; may be present several times in a promoter
- hairpin
- structure of RNA when it folds back on itself and forms intramolecular hydrogen bonds between complementary nucleotides
- holoenzyme
- prokaryotic RNA polymerase consisting of α, α, β, β‘, and σ; this complex is responsible for transcription initiation
- initiation site
- nucleotide from which mRNA synthesis proceeds in the 5′ to 3′ direction; denoted with a “+1”
- initiator tRNA
- in prokaryotes, called tRNAMetftRNAfMet; in eukaryotes, called tRNAi; a tRNA that interacts with a start codon, binds directly to the ribosome P site, and links to a special methionine to begin a polypeptide chain
- intron
- non–protein-coding intervening sequences that are spliced from mRNA during processing
- Kozak’s rules
- determines the correct initiation AUG in a eukaryotic mRNA; the following consensus sequence must appear around the AUG: 5’-GCC(purine)CCAUGG-3’; the bolded bases are most important
- nonsense codon
- one of the three mRNA codons that specifies termination of translation
- nontemplate strand
- strand of DNA that is not used to transcribe mRNA; this strand is identical to the mRNA except that T nucleotides in the DNA are replaced by U nucleotides in the mRNA
- Octamer box
- (ATTTGCAT) nonessential eukaryotic promoter sequence that binds cellular factors to increase the efficiency of transcription; may be present several times in a promoter
- peptidyl transferase
- RNA-based enzyme that is integrated into the 50S ribosomal subunit and catalyzes the formation of peptide bonds
- plasmid
- extrachromosomal, covalently closed, circular DNA molecule that may only contain one or a few genes; common in prokaryotes
- poly-A tail
- modification added to the 3′ end of pre-mRNAs to protect mRNA from degradation and assist mRNA export from the nucleus
- polysome
- mRNA molecule simultaneously being translated by many ribosomes all going in the same direction
- preinitiation complex
- cluster of transcription factors and other proteins that recruit RNA polymerase II for transcription of a DNA template
- promoter
- DNA sequence to which RNA polymerase and associated factors bind and initiate transcription
- reading frame
- sequence of triplet codons in mRNA that specify a particular protein; a ribosome shift of one or two nucleotides in either direction completely abolishes synthesis of that protein
- rho-dependent termination
- in prokaryotes, termination of transcription by an interaction between RNA polymerase and the rho protein at a run of G nucleotides on the DNA template
- rho-independent
- termination sequence-dependent termination of prokaryotic mRNA synthesis; caused by hairpin formation in the mRNA that stalls the polymerase
- RNA editing
- direct alteration of one or more nucleotides in an mRNA that has already been synthesized
- Shine-Dalgarno sequence
- (AGGAGG); initiates prokaryotic translation by interacting with rRNA molecules comprising the 30S ribosome
- signal sequence
- short tail of amino acids that directs a protein to a specific cellular compartment
- small nuclear RNA
- molecules synthesized by RNA polymerase III that have a variety of functions, including splicing pre-mRNAs and regulating transcription factors
- splicing
- process of removing introns and reconnecting exons in a pre-mRNA
- start codon
- AUG (or rarely, GUG) on an mRNA from which translation begins; always specifies methionine
- TATA box
- conserved promoter sequence in eukaryotes and prokaryotes that helps to establish the initiation site for transcription
- template strand
- strand of DNA that specifies the complementary mRNA molecule
- transcription bubble
- region of locally unwound DNA that allows for transcription of mRNA
- upstream
- nucleotides preceding the initiation site; in general, sequences toward the 5′ end relative to a site on the mRNA