BIOL 311 Lecture Notes - Lecture 25: Regulatory Sequence, Chromatin, Tata Box

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Published on 13 Dec 2017
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Goin g from Pro karyo te s to Euka ryote s
TURNING ON GENE EXP RES S ION
- Turning on gene e xpre ssion in e uka ryotes is much more difficult than that in prokaryotes for
two importa nt re a s ons:
- DNA is wra ppe d a round his tone s (protein) to form chroma tin. This prevents tra nscriptiona l
machine ry from e a s ily a cce s sing DNA regula tory s e que nces tha t are important for
tra nscription to occur. Thus, chroma tin must be remodelle d for transcription to ta ke pla ce
- This packing s ys tem of genetic information in e uka ryotes makes it difficult to acce s s
genes , a nd there fore a llow their expre s s ion
- How its packe d:
- Short region of DNA double helix is bound to nucle osome s (the bas ic unit of
chromatin). Bunches of pa cked nucleosomes form the chroma tin fibre
- Ma ny different re gula tory prote ins mus t bind to many different DNA regula tory s equence s,
an d to each other, to help RNA polyme ra se bind to the promote r. This forms a n
enha nce osome
- Chroma tin re mode lling allows previous ly ina cce ss ible DNA re gula tory s eque nce s to be bound
by tra nscriptiona l ma chine ry
- Two methods for chroma tin remodelling to occur:
- SWI-S NF
- Use s ATP to “nudge” over histone core s , s o that DNA ca n be e xpose d enough to be
able to get tra ns cribed
- SNF (suga r non-fermenting) found because yea s t cha nge d their carbon source from
sugar to a nother a lternative
- Acetyla tion of histone tails
- Acetyla tion is done by e nzymes calle d HAT’s (e .g. GCN5)
- The process of a cetylation helps to re move the negative cha rge on tail, therefore, DNA
doesn’t ha ve a ny attra ctive forces towards the pos itively cha rged histone s
- Histone ta ils are normally pos itive ly cha rge d, DNA is negative ly cha rged, this is why
DNA a nd histone s can be bound so tightly together
- Acetyla ted his tone s open up chroma tin ma king the DNA tra ns criptiona lly a ctive , a nd
making it e as ier to nudge his tones
- Dea cetyla te d histone s re s ult in clos e d chroma tin, thus making DNA tra ns criptiona lly
inactive
- Forming e nha nce osome s
- An e nha nceosome is a n a ss embly of diffe re nt re gula tory proteins (known a s Genera l
Transcription Fa ctors or GTF’s) bound to s pecific DNA re gula tory s eque nce s (e nha ncer,
PPE’s, promoter) that work toge ther to help re cruit RNA polymera s e a nd other prote ins
nee ded for tra nscription to occur
- Transcription only happens when an e nha nceosome forms
- Ke y points:
- Enhance osomes have two key properties :
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