Lecture 7

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25 Mar 2012
Lecture 7: Transcription 1
1. Molecular definition of a gene
2. Transcription and RNA polymerase in prokaryotes
Molecular definition of a gene
oInvokes the whole idea of coded information and turning it into something that
the cell can use
oThe entire nucleic acid sequence (usually DNA) that is necessary for the
synthesis of a protein (and its variants) or RNA. In other words, genes are
segments of DNA that are transcribed into RNA
The resulting RNA encodes a protein
The end product simply functions as RNA
oTranscription is a controlled mechanism which is used to modify patterns of
expression – it allows the cell to determine what level of expression is going to
occur for that encoded information
Schematic of RNA polymerase
oIt must be transcribed in one of the strands in 5’ to 3’
oThere are 4 different types of subunits
5 in total because there’s the newly duplicated subunit
Catalyzes the sequential addition of nucleotides
oRNA nucleotides are linked by phosphodiester bonds through ribose phosphate
oRibose sugars instead of deoxyribose sugars
oReplacement of uracil with thymine
oBut the chemistry is same
oIt doesn’t need primer to initiate chain synthesis. It can start simply by
recognizing complimentary strand of DNA
Generation of RNA transcript
oTemplate is single stranded DNA
oRNA (NTP’s) is linked in a linear chain by phosphodiester bonds through the
ribos phosphate backbone
oDNA-RNA hybrid is held together by base pairing
The transcription cycle
oSigma factor is the protein that binds to the DNA and recognizes specific
promoter sequences on the DNA strand, and it provides the specificity by
which RNA polymerase says ‘I have to now recognize this site as a place for a
gene to be transcribed.’
oDifferent sigma factors in bacterial cells position the RNA polymerase complex
at different genes
oSigma factor starts to dissociate away from RNA polymerase when the
transcription starts and RNA polymerase goes through abortive steps of
transcription where it tries to move down the template strand quite a few
times before it successfully engages. However once it passes this initiative
elongation phase, it then proceeds quite happily down the template
proceeding along to generate a complete transcript
oSo the sigma factor binds to the promoter and it facilitates localized unwinding
of DNA
Initial RNA is synthesized
Sigma factor is released
At the end of the template gene, RNA polymerase stops transcribing
The secondary structure of RNA (hairpin loop) that is being formed is what
tells the RNA to stop transcribing. The end of the gene that is transcribed
has series of G-C rich regions followed by A-T rich regions
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