chapter 13 part 2 Flashcards
(65 cards)
what does positron effect variegation (PEV) is drosophila illustrate?
the effect of chromatin compaction on gene expression
what on the flies with variegated eye color had happened?
the X chromosomes had undergone chromosomal inversion
-the white gene had been moved from its normal position near the telomere to a region near the centromeric heterochromatin
what does the occurrence of PEV show?
gene expression can be silenced by the gene’s chromosomal position
-silencing is a feature of chromatin structure that can be transmitted from one cell generation to the next
what do E(var) mutations do
these enhance mutant phenotypes by encouraging spread of heterochromatin = more white & less red
what do Sur(var) mutations do?
restrict heterochromatin spread or interfere with its formation; this suppresses mutant phenotypes so more wild-type cells are seen = more red & less white
what are some Sur(var) mutations caused by?
defective expression of heterochromatin protein-1 (HP-1)
what is HP-1
it is a nucleosome binding protein that targets lysine in position 9 of histone H3 (H3K9me) if they carry a methyl group
what happens in the absence of HP-1
interferes with heterochromatin formation & suppresses variegation (more euchromatin = more red eye cells)
what does a second group of su(var) genes encode?
enzymes that add methyl groups to histone proteins
-called histone methyltransferases (HMTs)
-target lysine & arginine residues like (H3K9me)
what are the five features of epigenetic modifications
- epigenetic modification patterns alter chromatin structure
- they are transmissible during cell division
- they are reversible
- they are directly associated with gene transcription
- they do not alter DNA sequence
what is the defining feature of eukaryotic DNA
is its packing into chromatin
what are chromatin remodeling and modifying enzymes recruited to specific sites by?
trans-acting factors that bind target DNA sequences
what do chromatin remodelers & chromatin modifiers mediate?
the reversible transition from inactive, heterochromatin DNA to active, euchromatic DNA
what are epigenetic marks (like PEV) maintaned?
through cell division cycles
how are is the original epigenetic state quickly reestablished after replication?
based on the epigenetic marks present on the old histones to modify the newly deposited histones
what are open promoters?
they are constitutively expressed genes
about open promoters…!
they have a nucleosome-depleted region (NDR) of 100-150 bp immediately upstream of the start site
-do not have a TATA box
-have an A/T tract
what do open promoters have instead of a TATA box?
a poly A/T tract
what does the poly A/T tract of open promoters contain?
binding sequences (BS) that attract transcriptional activators (ACT)
what is the binding region of open promoters flanked by? ??? binding region of transcriptional activators?
flanked by sequences that help position one nucleosome upstream (-1) & one downstream (+1) of the NDR (nucleosome depleted region)
what does the downstream nucleosome from the ACT contain?
variant histone H2A called H2AZ
-that is more easily modified for removal from the transcription start site
about H2AZ
it is a variant histone in a nucleosome downstream the NDR
-it is easier to modify for removal from the transcription start site
what are covered promoters?
characterize genes whose transcription is regulated
-transcription is blocked until nucleosomes are displaced or removed from the promoter
transcription is blocked until what?
nucleosomes are displaced or removed from the promoter
-these promoters contain TATA boxes & other transcription-factor binding sequences
