CSB349H1 Lecture Notes - Lecture 8: Variegation, Heterochromatin, Euchromatin
14 May 2018
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Lecture 8(b): Heterochromatin & Positioning Effect
Genomic Insulators:
• Active and inactive genome regions must be insulated to prevent aberrant gene expression
o Prevent the inactive domain from mistakenly inhibiting genes in the active gene &
prevent active domain from activating mistakenly activating genes in inactive domain
ð Genomic insulators is a DNA sequence;
o CTCF protein is capable of binding to the insulator
that functions to block the active and inactive domain
from interacting with each other
§
!
Cohesin proteins: involved in the separation
of chromatids in cell division
Position Effect Variegation (PEV): - understanding the silencing effects of heterochromatin
• PEV is the phenomenon when a gene that is normally expressed in euchromatin is artificially
relocated to a heterochromatic region and is silenced
Drosophila Gene:
• Chromosome consists of two regions near each other:
o Heterochromatin (silenced) and euchromatin (white gene) – barrier is found between
them to prevent the heterochromatin region from moving into the euchromatin region
§ White gene controls pigmentation in the drosophila eye:
• Wild-Type = red color | Mutated = white color
• If a rare chromosomal inversion occurs; euchromatin region
(white gene) can be found near the heterochromatic region –
barrier is no longer separating the two regions
o When white gene is placed in a region close to
heterochromatin, the expression is silenced in
specific clones of cells => results in eyes with red
and white patches
Heterochromatinization – Genetic Screens:
• PEV in drosophila led to genetic screens to identify chromatin
modifies, and a mechanism for heterochromatinization
Genetic Screens:
• WT: white gene separated from heterochromatin = red eye
• M1 inversion: white gene close to heterochromatin = mix
• M1 flies are mutagenized and the next generation will consist
of two classes of mutants:
o (1) Suppressor of Variegation: Su(var)
§ Red > White: Genes weaken heterochromatin formation (less spread)
o (2) Enhancers of Variegation: E(var)
M1 = Mutant
‘spread’: we are looking at
the spread of
heterochromatin across the
white gene
Document Summary
Genomic insulators is a dna sequence: ctcf protein is capable of binding to the insulator that functions to block the active and inactive domain from interacting with each other. Cohesin proteins: involved in the separation of chromatids in cell division. Position effect variegation (pev): - understanding the silencing effects of heterochromatin: pev is the phenomenon when a gene that is normally expressed in euchromatin is artificially relocated to a heterochromatic region and is silenced. Drosophila gene: chromosome consists of two regions near each other, heterochromatin (silenced) and euchromatin (white gene) barrier is found between them to prevent the heterochromatin region from moving into the euchromatin region. White gene controls pigmentation in the drosophila eye: wild-type = red color | mutated = white color. Heterochromatinization genetic screens: pev in drosophila led to genetic screens to identify chromatin modifies, and a mechanism for heterochromatinization. Red > white: genes weaken heterochromatin formation (less spread) (2) enhancers of variegation: e(var)
