Elongation rna is formed from 5 prime to 3 prime 3

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-ELONGATION – RNA is formed from 5 prime to 3 prime (3 prime to 5 prime of DNA) -TERMINATION – once stop codon is reached, RNA is released INITIATION In Eukaryotic cells -promoter includes a specific sequence (called TATA box) -transcription factors bind to promor region -RNA polymerase binds to transcription factors so transcription can then begin -RNA polymerase plus Transcription factors = Transcription initiation complex -RNA polymerase II is main RNA polymerase -in prokaryotes, TATA box is not present and no transcription factors are necessary ELONGATION -RNA polymerase untwists DNA double helix 10 to 20 bases at a time -40 nucleotides per second in eukaryote
-nucleotides are added to the 3’ end of growing RNA molecule -a gene can be transcribed simultaneously by several RNA polymerases TERMINATION -bacteria have TERMINATOR SEUENCE in DNA -when this sequence in RNA signals polymerase to detach from DNA and release transcript -Eukaryotes have a POLYADENYLATION SIGNAL SEQUENCE in DNA -in the pre-mRNA this specifies a polyadenylation signal (AAUAAA) -this signal is then bound by special proteins in nucleus, and 10 to 35 nucleotides later, cut it from polymerase -pre-mRNA is released 10/29/13 Eukaryotic cells modify RNA after transcription: RNA PROCESSING -occurs before genetic messages are sent out of nucleus into cytoplasm -both ends of primary transcript (pre-mRNA molecule) are usually altered -also, usually some interior parts of the molecule are cut out and other parts spliced together Each end of a pre-mRNA molecule is modified in a particular way -the 5’ end receives a modified nucleotide 5’ CAP (modified Guanine) -FIG 17.10 -the 3’ end gets a poly-A tail (50-250 Adenine nucleotides) -functions of modification are to facilitate export of mRNA from nucleus to cytoplasm, protect mRNA from hydrolytic enzymes, and help ribosomes attach to the 5’ end -there are some untranslated regions , the polyadenylation signal is an example – not translated to amino acid RNA SPLICING removes large non-coding portions of pre-mRNA (portions that are not translated) -the noncoding regions are called intervening sequences, or INTRONS -the other regions are called EXONS because they are eventually expressed usually translated into amino acid sequences -FIG 17.11 -RNA SPLICING removes introns and connects the exons RNA splicing is generally carried out by SPLICEOSOMES -spliceosomes consist of various proteins and severall small nuclear ribonuleoproteins (snRNPs) that recognize the splice sites -FIG 17.12 -small RNAs in snRNPs not only help in recognition of introns but also catalyze the reactions Idea of catalytic role of snRNAs came from discovery of RIBOZYMES
-RIBOSYMES are RNA molecules that function as enzymes -three properties of RNA enable it to function as an enzyme: -it can form a 3D structure because of ability to base-pair with itself -some bases in RNA contain functional groups that may participate in catalysis -RNA may hydrogen-bond with other nucleic acid molecules (DNA or RNA) FIG 17.3 Molecular components of translation

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