Gene Transcription and RNA ModificationLec1617NEW

Gene Transcription and RNA ModificationLec1617NEW - RNA...

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5 RNA polymerase reaches the terminator and a stem-loop causes RNA polymerase to pause. The ! protein catches up to the open complex and separates the RNA-DNA hybrid. ! -independent termination (also called intrinsic termination – see fig. 12.11) does not require ! . ! -independent termination is facilitated by two sequences in the RNA: a uracil-rich sequence located at the 3 end of the RNA, and a stem-loop structure upstream of the uracils. The stem-loop causes the RNA polymerase to pause. While RNA polymerase pauses, the uracil-rich sequence is not able to hold the RNA-DNA hybrid together and termination occurs. Transcription in Eukaryotes Many of the basic features of gene transcription are very similar in bacteria and eukaryotes; however, gene transcription in eukaryotes is more complex because of: Larger organisms and cells. Cellular complexity such as organelles (added complexity means more genes). Multicellularity, which requires increased regulation so that the correct genes are expressed in the right cells at the right time. Eukaryotes have multiple RNA polymerases that are structurally similar to the bacterial enzyme. RNA polymerase I transcribes all rRNA genes (except for the 5S rRNA). RNA polymerase II transcribes all structural genes – all mRNAs and some snRNA genes. RNA polymerase III transcribes all tRNA genes and the 5S rRNA gene. All three are very similar structurally and are composed of many subunits. There is also a remarkable similarity between the bacterial RNA polymerase and its eukaryotic counterparts (see fig. 12.12a). Mechanism of transcription in eukaryotes (see fig. 12.12b). The double-stranded DNA enters the polymerase along a bridge surface that is between the jaw and clamp.
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6 At a region termed the wall, the RNA-DNA hybrid is forced to make a right angle turn, which enables nucleotides to bind to the template strand. Mg 2+ is located at the catalytic site; nucleoside triphosphates enter the catalytic site via a pore region, bind to the template strand, and nucleotides are covalently attached to the 3 end of the RNA. As RNA polymerase slides down the template, a small region of the protein termed the rudder separates the RNA-DNA hybrid and the single-stranded RNA exits under a small lid. Eukaryotic structural genes have a core promoter and regulatory elements. Eukaryotic promoter sequences are more variable and often more complex than those of bacteria; for structural genes, at least three features are found in most promoters (see fig. 12.13): Regulatory elements TATA box Transcriptional start site The core promoter is relatively short; it consists of the TATA box, which is important in determining the precise start point for transcription, and the transcriptional start site. The core promoter by itself produces a low level of transcription, termed basal transcription.
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Gene Transcription and RNA ModificationLec1617NEW - RNA...

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