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Lecture 7

Biology 1002B Lecture Notes - Lecture 7: Stem-Loop, Start Codon, Release Factor

Course Code
BIOL 1002B
Tom Haffie

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Lecture 14 Outcomes:
Relative location of such DNA sequence signals as promoter, 5 and 3 UTR, SD box,
start codon, stop codon, transcription terminator etc.
- Promoters are located before the gene that is transcribed because they are not transcribed
they function to attract the RNA polymerase (& protein factor)
- the 5’ UTR (untranslated region) is the sequence that is upstream (before) the start codon
(therefore it does not get translated) it contains the SD box
- 3’ UTR is the sequence after the gene (also not translated)
- SD box is a region of DNA (in the 5’ UTR) that when transcribed, the sequence on the mRNA
base pairs with corresponding sequence on the rRNA (in the small subunit) and helps with
the initiation of translation by allowing the small subunit to dock onto the mRNA (docking
box) because it is present on the mRNA it must also be present on the DNA (upstream of
the first start codon, but downstream of the promoter)
- Transcription terminators are sequences included in the mRNA that can pair with itself to a
make loop & causes the RNA polymerase to fall off the double stranded DNA helix (the
sequence on the mRNA is complementary when read in opposite directions)
Mechanism by which each signal is interpreted, or understood, by the cell
- the promoter sequence is upstream of the gene, and where the polymerase binds
- SD box is a region in the DNA that when transcribed, the mRNA base pairs with the rRNA
and helps the initiation of translation
- Start codons are understood by rRNA and interpreted as the place to start translation by
pairing with a Met tRNA (AUG sequence)
- Stop codons are understood by rRNA as the place to stop translation by attracting a release
factor protein in the A site that signals the end of translation (release factor is always trying to
bind with the mRNA but is always outcompeted by the tRNA, but since there is no tRNA to
compete at the stop codon, the release factor binds and signals the end)
- Transcription terminator is transcribed and base pairs with itself to form an mRNA hairpin
loop which signals the polymerase to stop transcription
Relationship between DNA sequence of signals and their function (ie. how would low
efficiency promoters be different than high efficiency promoters?
- mutations in the promoters causes them to be more or less attractive to RNA polymerase
- high efficiency promoters will be more successful in allowing RNA polymerase to bind,
therefore allowing more of the signals of transcription to be made into RNA, and more
functioning of these signals to produce more protein
- there might be a selective advantage for having a constitutively low expressed gene, if its
product is unnecessary or even detrimental in high concentrations
Characteristics of promoters that require a particular position and direction
- either strand of the DNA can function as the promoter, promoters are generally located on the
5’ side of the strand therefore RNA polymerase binds & functions in the 5’ to 3’ direction
- Promoters dictate the genes that get transcribed because they are the initiation point for
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