Transcription and RNA Modification

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37 Terms

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promoter

signals beginning of transcription

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regulatory sequence

site for the binding of regulatory proteins; influence rate of transcription

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transcribed region

part of this region contains the information that specifies an amino acid sequence

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terminator

signals the end of transcription

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3 stages of transcription

  1. intitiation

  2. elongation

  3. termination

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TATA box

portion of the promotor region about 25 nucleotides upstream of the start site (eukaryotes)

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sigma factor (bacteria)

part of RNA polymerase that recognizes and facilitates binding to the promoter

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initiation

transcription factors meditate the binding of RNA polymerase and initiation of transcription

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completed transcription initiation complex

transcription factors and RNA polymerase

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Elongation

RNA polymerase moves along the DNA strand

  • untwists double helix about 20 bases at a time

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what direction is RNA made?

5’ to 3’

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how are the RNA strands made

reading the template strand and adding in complementary RNA nucleotides

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termination

end of transcription; signaled by terminator in bacteria

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termination in bacteria

terminator sequence signals the end of transcription

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termination in eukaryotes

polyadenylation sequence; RNA transcript released 10-35 nucleotides past sequence

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directionality of transcription

  • DNA template read 3’ to 5’

  • RNA transcript synthesized 5’ to 3’

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RNA polymerase I

transcribes rRNA

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RNA polymerase II

transcribes mRNA

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RNA polymerase III

transcribes tRNA

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prokaryotic transcription to translation

immediate; mRNA begins to be translated BEFORE transcription is done

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eukaryotic transcription and translation timing

occurs in separate compartments

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where does transcription occur in eukaryotes?

nucleus

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where does translation occur in eukaryotes?

cytoplasm

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pre-mRNA

product of transcription; longer RNA

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mature mRNA

mRNA after undergoing modifications before exiting nucleus

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3 common mRNA modifications

capping, Poly A tail, splicing

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capping

modified guanosine added to 5’ end; added after first 20-25 nucleotides

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5’ cap functions

  • needed to exit nucleus

  • prevents degradation

  • enables mRNA to bind to ribosome

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Poly A Tail

100-200 adenine nucleotides added to 3’ end

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how is the Poly A Tail added?

enzymatically by poly-A polymerase (NOT encoded by gene)

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Poly A tail function

increases stability and lifespan in cytosol

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shared functions of capping and Poly A Tail

  • facilitating export of mRNA into cytoplasm

  • protecting mRNA from hydrolytic enzymes

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splicing

removal of introns, linking of exons

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introns

transcribed but NOT translated, non-coding sequences

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exon

encoding sequence found in mature mRNA; EXpressed

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spliceosome

composed of snRNP (small nuclear RNA and set of proteins); removes introns from pre-mRNA

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alternative splicing

creates different proteins from same gene