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Transcription: DNA RNA
• Copy a particular portion of its DNA nucleotide sequence a gene into an RNA nucleotide sequence.
• The information in RNA, although copied into another chemical form, is still written in essentially the same language as it is in DNA the language of a nucleotide sequence.
DNA polymerase vs. RNA polymerase
• RNA polymerase is more error prone: 10-4 vs 10-7
• A modest proofreading mechanism
• RNA stores information only temporarily
Principal Types of RNAs Produced in Cells
mRNAs messenger RNAs, code for proteins
rRNAs ribosomal RNAs, form the basic structure of the ribosome and catalyze protein synthesis
tRNAs transfer RNAs, central to protein synthesis as adaptors between mRNA and amino acids
snRNAs small nuclear RNAs, function in a variety of nuclear processes, including the splicing of pre-mRNA
snoRNAs small nucleolar RNAs, used to process and chemically modify rRNAs
Other noncoding RNAs function in diverse cellular processes, including telomere synthesis, X-chromosome inactivation, and the transport of proteins into the ER
Types of RNA polymerases in eukaryotic cells
TYPE OF POLYMERASE GENES TRANSCRIBED
RNA polymerase I 5.8S, 18S, and 28S rRNA genes
RNA polymerase II all protein-coding genes, plus snoRNA genes and some snRNA genes
RNA polymerase III tRNA genes, 5S rRNA genes, some snRNA genes and genes for other small RNAs
Several important differences between the bacterial and eucaryotic RNA polymerases.
• .While bacterial RNA polymerase (with factor as one of its subunits) is able to initiate transcription on a DNA template in vitro without the help of additional proteins, eucaryotic RNA polymerases cannot. They require the help of a large set of proteins called general transcription factors, which must assemble at the promoter with the polymerase before the polymerase can begin transcription.
• Eucaryotic transcription initiation must deal with the packing of DNA into nucleosomes and higher order forms of chromatin structure, features absent from bacterial chromosomes.
Additional factors for correct splicing site recognition
• RNA factory concept
• Exon definition hypothesis