lec24 Flashcards
Nucleosome Digestion & Histone Dissociation
nuclease cna break down the linker dna (the string between the beads) and then the seperated nucleosome bead can be dissacioated into each histone protein and the dna (~146 bp long) by a high conc of salt which disrupts the electrostatic interactions.
what histones are there
Core histone proteins (H2A, H2B, H3, H4) assemble into an octamer.
Forms the nucleosome, the basic unit of chromatin.
🔬 Assembly Process:
H3-H4 Tetramer → First, two H3/H4 dimers come together.
H2A-H2B Dimers → Two H2A/H2B dimers then associate with the tetramer.
h1 clamps dna wrapped around the histone
how do we know n terminal tails of the core histones are flexible. and what are they useful for
they stick out na ddont show up on the crystal x-ray.
n terminals are important for assembly of 30nm fiber and gene regulation. the + charge interacts w -charged backbone of neighbouring nucleosomes, these interactions help pack the 11nm into the more compact 30nm fibre
what form are non dividing cellls dna in
euchromatin, in 30nm chromatin fiber/solenoids
30-nm Fiber vs. 11-nm Fiber (Transcription Access)
–30-nm Fiber (Solenoid):
Held by histone H1
No transcription (DNA too packed)
Nucleosomes pulled tightly together
Low DNA accessibility
–11-nm Fiber (Beads-on-a-String):
Less H1, more open structure
Highly acetylated histones (H3/H4) → less + charge → DNA looser
N-terminal tails help organize
Transcription possible (DNA more accessible)
🧬 Higher-Order Chromatin Loops & Nuclear Scaffold
Key Points:
DNA forms loops attached to a nuclear scaffold
Loops may contain related genes (e.g. histone genes in Drosophila)
Scaffold attachment sites anchor loops
Nuclear scaffold includes:
Histone H1 (helps compact DNA)
Topoisomerase II (supercoils DNA during assembly)
Seen in partially unraveled chromosomes as loops coming off a central core
Level:
➡️ Higher-order chromatin structure (above 30-nm fiber)
telomere
–contains seq repeats at the end ofchromatids, this protects from nuclease degrading dna, also solves 5’ end replication problem as ends dotn get fully repicated.
centromere
-site for kinetochore to attach and seperate sis chromatids and move aorund.
what are the three things necessray for chromosome trasmission/replication in eukaryotes cuz rmb proksdont have chromosomes
- telomere
- centromere
- ori (multiple sites of replication)
🧬 RNA Primers & End-Replication Problem
ok and so since theres a primer beginning the 5’ end of the newly synthesized strand polymerase cant add anything there cuz theres nothing infront of it right right and so thats why we have repeated seq called telomeres right right. so 5’ end of chromosomes have telomers
🧬 Telomere Extension & Protection
Q: What happens to the extended telomere and how is it protected?
The template strand of the telomere is longer than the newly synthesized strand, leaving a single-stranded 3’ overhang.
This extended single-stranded DNA (ssDNA) is vulnerable to exonucleases.
To protect it, the ssDNA forms a T-loop, where it loops back and is shielded. telomerase extends the overhang before T-loop formation
Proteins TRF1, TRF2 bind to the T-loop to protect the telomere from degradation. “T ROOP, TO RMB TRF1 AND TRF2 FOR T LOOP”
