BIOL 118 Chapter 17: Chapter 17

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BIOL 118
Dennis Mc Gee, Steven Tammariello

An Overview of Transcription - in BACTERIA - RNA polymerase synthesize an mRNA version of the instructions is stored in DNA - Only one strand of DNA is the template - Called template strand - Other strand is the non-template or coding strand - Matches the sequence of the mRNA (except U or T) FIG 17.1: RNA transcription - Catalyzed by RNA polymerase - formation of phosphodiester linkages between base pairs of the RNA. ● Like DNA polymerase ○ RNA polymerase perform template directed synthesis in the 5’ to 3’ direction ● Unlike DNA polymerase ○ RNA polymerase DO NOT require a primer to begin transcription ● Bacteria have 1 RNA polymerase ● Eukaryotes have at least 3 distinct types ○ RNA polymerase I,II, III RNA polymerase I - Ribosomal RNAs RNA polymerase II - Messenger RNAa RNA polymerase III - Transfer RNAs Initiation: How does Transcription begin in bacteria ● Initiation is the first phase of transcription ● RNA polymerase cannot initiate transcription on its own ○ In bacteria, sigma protein must bind to it first * doesnt need primer but uses sigma protein as a coenzyme ● RNA polymerase and sigma form a holoenzyme ● RNA polymerase is the core enzyme because it has the active site ● Sigma binds to DNA segments called promoters that promote the start of transcription ○ Different sigma proteins bind to promoters with different DNA base sequences ○ Allows organisms to activate certain genes in response to environmental change FIGURE 17.2 ** promoters - 40-50 bp long, same series of bases on one strand of DNA “TATAAT” -10 box centered about 10 bases from the porint of transcription. Transcription begins at +1, sigma factors recognize -10 and -35 and recruits RNA polymerase so that transcription can start. Inhibit sigma from binding if you change sequences and will not bind and gene won't be turned on, called promoter bashing. -35 box is 35 bp upstream, sigma binds to both the -10 and -35 boxes. Events inside the Holoenzyme ** Exergonic, NTPs have much energy due to # phosphate groups ● RNA polymerase opens DNA double helix ○ Template strand is threaded through the RNA polymerase active site ● Incoming NTPs enter a channel in the enzyme and diffuse to the active site ● Complementary NTPs pair with complementary DNA bases, and polymerization begins. FIGURE 17.3: Initiationg transcription in bacteria sigma factor bound to -10 and -35 boxes - which will recruit an RNA polymerase. Initiation will continue an RNA polymerase will open DNA helix. Pulls DNA strand in so that +1 site will be in middle. Now, RNA polymerase parts will keep DNA sep, Sigma factor released, polymerase free to move “downstream” to do transcription FIGURE 17.4: Ending Transcription in bacteria - Continues, specific seq for hairpin loop RNA segments. Must be complementary sequences aka G and C. Termination signals from these hairpin loops or termination signal for transcription. Table 17.1 - tons of info Process Bacteria Eukarya Transcription: RNA polymerase(s) One 3: each different RNA type Promoter Structure: -35 and - 10 box TATA box; -30 from start site Proteins specific to promoter: Sigma; diff versions for diff Many basal transcription promoters factors RNA Processing: Rare Extensive; many processing steps in nucleus first before RNA sent out. 1) addition of 5’ cap on mRNAs 2) intron removal 3) enzyme calalyzed addition of 3’ on mRNA Translation (initiation, Initiation less complex, Initiation more complex, elongation, termination) elongation and termination elongation and termination simialr similar RNA Processing in Eukaryotes (transcription is done) - In bacteria, mRNAs are translated immediately, sometimes even before transcription is complete - In eukaryotes, the initial product of transcription is an immature primary transcript or pre-mRNA - Primary Transcripts must undergo RNA processing before they can be translated FIGURE 17.6 - read 1 2 3 and 4 and will underst
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