Lecture 24 - Eukaryotic DNA Organization Flashcards
Describe the form of chromosomes during Interphase and mitosis
Interphase: 30nm fiber
mitosis: condensed chromosomes
Describe the 3 levels of chromatin packaging
1: Nucleosomes –> histone proteins with 1.7 turns of DNA wound around it with an 11nm diameter
2: solenoid –> packed nucleosomes with a 30nm diameter
3: wrapping of solenoid into loops around nuclear scaffolding
Describe the histone
Histone: octamer and H1. Octamer made of 4 different subunits, 2 copies of each subunit. H2A and H2B form two dimers through head-to-tail interaction. Wrap themselves around tetramer of H3/H4. H1 acts as a clamp to secure the DNA around the nucleosome. Histones have basic, positively charge N-termini tails with lots of Arginine or Lysine to interact with the negative sugar-phosphate backbone of adjacent nucleotides to help with solenoid formation
When does RNA transcription occur and how is it accomplished?
Transcription occurs during interphase, only the 11nm fiber is accessible to be read by proteins, therefore nucleosomes are remodelled. Histones are moved around to open up the solenoid to be read.
How is chromatin remodeled?
ATP-dependent chromatin remodeling complexes, histone acetylation (loss of positive charge)
How do telomeres work and how are they conserved? Why are they needed?
Telomeres prevent the DNA from being degraded by nucleases. There is a gap problem where the 5’ end of both strands will have a couple hundred of nucleotides not replicated because there is no primer available. Therefore there is a free 3’ end that can be degraded by nucleases. Telomeres are long sequences of short repeating DNA patterns that stabilize the ends. Telomerase is complementary to the telomere, extends the 3’ end. This allows a primer to be placed and the end of the chromosome to be replicated, and when the primer drops off, it isn’t a problem for the end of the chromosome to be degraded.
