DNA Packaging (6) Flashcards
Are chromosomes wound or unwound when in a defined territory in interphase?
Unwound
What holds c’somes in their territories?
nuclear matrix
Nuclear lamina
(both PRO frameworks)
Where is heterochromatin located in the nucleus? & where is it attached
Periphery of the nucleus
attached to nuclear lamina
Where is euchromatin located? & what does it contain?
toward interior of nucleus
where transcription machinery is located
Why do c’somes overlap territories?
to share transcriptional machinery
Nucleosomes
grp of positively charged highly conserved PROs
How is DNA held by histones?
ionic bonds b/w negatively charged sugar phosphate backbone & positively charged histones
What are the levels of organization of histones in DNA packaging?
o Level 1 10nm: histones, DNA wraps around nucleosome
o Level 2 30nm: assembly of adjacent nucleosomes
o Level 3 looped domains: MARs have an affinity for the scaffold
o Level 4 c’somes have looped domain structure
Cohesin of looped domains
ring shaped PRO may contain loops
Matrix Assoc Region
DNA sequence that has a high affinity for the nuclear matrix, DNA loop can be moved to facilitate transcription
What is MAR composed of?
alpha satellite
non-coding DNA
What is the function of MARs?
provides structure to DNA to anchor it to the PRO (no function provided)
What is MAR’s role in initiating transcription?
Transcription is initiated by the RNA polymerase on the nuclear matrix by drawing the loop towards it
What is a nuclear matrix & what are the major organizing structures?
PRO framework
o RNA polymerase (makes mRNA)
o RNA processing PRO
o DNA replication PRO
Topoisomerase II
un-tangling PRO (for unwinding DNA for replication)
Insulator PRO
keeps loops separate & enables transcription machinery to regulate separate loops at a time
Nuclear lamina
lamins bind telomeres & alpha satellite DNA of the centromere
How can the nuclear scaffold be viewed?
Digesting DNA & removing histones
What are the 4 scaffold PROs?
nuclear matrix
topoisomerase II
insulator PROs
nuclear lamina
What activates condensin?
phosphorylation by MPF
Describe mitotic c’somes
• Have looped domain structure
o Chromosome in proper structure for replication
o No nucleus present when chromosomes are seen in this state
o DNA loops on condensin PRO
o More compact than interphase chromatin
o Condensin is activated by phosphorylation via MPF
How does the cell make DNA accessible to PROs for transcription?
modifying histones
What are the mechanisms of histone modification?
o Replacement with modified types of histone
o Chemical modification of the histones
H2A.X
o ATM PRO must signal to the cell to get checkpoint going to stop mitosis from occurring when DNA is damaged
o Replaces H2A less than 10% of nucleosomes receptive for ATM PRO
o ATM phosphorylates a target … H2AX signaling where repair is necessary
macroH2A
barr body –> keeps chromatin condensed & inactive
What are the 3 chemical modifications of histones?
• Acetylated histones looser DNA binding
o Acetyl grp is negatively charged by weakening some of its own positive charge
Decreases the affinity for the positively charged DNA
o HAT adds acetyl grp onto histone & opp is down with HDAC (more condensed wrapping)
• Phosphorylated histones can increase or decrease DNA winding of histones
o Can weaken or tighten interaction b/w DNA & histones
What are the 2 types of heterochromatin?
constitutive
facultative
How does heterochromatin work?
covered in histones
making DNA unavailable to machinery
Constitutive Heterochromatin & an ex
o condensed always
AT rich alpha satellite DNA at the centromere
Telomeres (ends of chromosomes)
Facultative Heterochromatin
o transient condensation, contains temporary active genes
Can be condensed or opened
Signals sent when genes needed & it unwraps
How does the X inactivation occur?
o XIST RNA accumulates along the chromosome & inactivates genes barr body
barr body
highly condensed X c’some
What events does XIST induce? & how?
Deacetylation of DNA of H3
Methylation of H3
Ubiquitination of H2A
How –> recruitment of enzymes
