chromatin Flashcards
what is chromatin?
highly ordered DNA-protein-RNA complex containing RNA, non histone proteins and histones
what are the 5 types of histones?
H1 H2A H2B H3 H4
what is a nucleosome?
the fundamental subunit of chromatin, composed of a little less than 2 turns of DNA wrapped around a histone octamer
what were the results of experiments using chromatin and micrococcal nuclease?
a 200 bp fragment seen and multiples of this indicating a regular arrangement of nucleosomes
a 160 bp fragment
a 146 bp fragment which does not decrease in size, this is the dna wound around the nucelosome
where does H1 bind?
binds to linker DNA close to the nucleosome
what joins adjacent nucleosomes?
linker DNA
what is the rough rule about the length of linker DNA?
the more highly transcribed the gene sequence is the longer the linker region
what is a solenoid?
nucleosomes are then condensed into a 30nm fiber called a solenoid which loop to give further condensation
a chromosome scaffold is needed
what level of condensation is present at interphase?
loosely packaged euchromatin
what level of condensation is present at metaphase?
highly condensed heterochromatin
what sequences are most commonly methylated?
CpG sequences
what effect does methylation have on chromatin condensation?
methylation of cytosine leads to heterochromatin formation
what effect does acetylation have on chromatin condensation?
acetylation of lysine on N terminal tail leads to euchromatin condensation
what is euchromatin?
chromatin that is transcriptionally active and in an open configuration
H1 binds weakly and RNA polymerase can access the gene
extensive acetylation of core nucleosomal histones
stains poorly
what is heterochromatin?
chromatin that is transcriptionally inactive
H1 binds strongly
stains darkly
replicated in S phase
what are the two types of heterochromatin?
constitutive
facultative
what us constitutive heterochromatin?
permanently irreversibly condensed
genetically inactive in somatic cells
repetitive DNA sequences that could never be transcribed
what is facultative heterochromatin?
heterochromatin that can be decondensed
genes may be expressed elsewhere in the genome
can encompass and entire chromosome
what is X inactivation?
a mechanism for switching off an entire X chromosome
why does X inactivation occur?
to prevent a potentially toxic double dose of X linked genes
where is X inactivation initiated?
X-inactivation centre
what does the X-inactivation centre code for?
a large non coding RNA
X-inactivation specific transcript
how does X-inactivation occur?
XIST coats the whole chromosome
it recruits repressive proteins
the chromosome then becomes highly condensed and transcriptionally inactive
what is DNase I?
an enzyme used to degrade DNA
secreted by exocrine cells
found most abundantly in pancreas
what is a chromosome scaffold?
comprised of non-histone proteins in low abundance
help maintain the integrity of the chromosome
what is the nuclear matrix?
a filamentous structure seen on the inside face of the nuclear membrane
maintains the shape of the nucleus
