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Relaxed: 10 rotations
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Large amounts of DNA are packed into a cellWe also discuss several other topics like How are non-polar molecules characterized?
The structure of DNA can be considered at three hierarchical levels: the primary structure is its nucleotide sequence; its secondary structure is the double - stranded helix; and the tertiary structure refers to the higher ordered holding that allow DNA to be confined into a cellWe also discuss several other topics like Cite the three steps in chromosomal replication.
Supercoiling (around 10 rotation pairs = 100 nucleotides)
Coiled tertiary structure that forms when strain is placed on a DNA helix by overwinding or underwinding of the helix an overwound DNA exhibits positive supercoiling, an underwound exhibits negative supercoiling
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A molecule is in its relaxed state when there is no structural strain on it. Supercoiling is a partial solution to the cell’s packing problem because it occupies less space than relaxed DNA. it takes place when the strain of over rotating or under rotating cannot be compensated by the turning of the ends of the double helix (if DNA is circular - no free ends) if the chains can turn freely, they will turn as extra rotations or be removed, and the molecules will become relaxed
Supercoiling relies on topoisomerase
Negative vs. Non supercoiled DNA
The Bacterial Chromosome
Each eukaryotic chromosome consists of a single long molecule of DNA. Although the DNA in interphase is less tightly packed than miosis, H is still highly condensed in the course of the cell cycle, the level of packing changes: chromosomes go from highly packed, to extremely highly packed. DNA packing also changes locally in replication and transcription
The combination of DNA and proteins. Complex structure with levels of organization
Undergoes condensation and decondensation in the course of the cell cycle
Remains highly condensed throughout the cell cycle. Found at telomeres and centromeres it may occur during certain developmental stages (facultative developmental stages). H lacks crossing over, transcription and replication in this phase
Histones: the most abundant proteins in chromatin (+ charged). The five major types are H1, H2A, H2B,H3, and H4
Non histone chromosomal protein:
One of a heterogeneous assortment of non histone proteins in chromatin
Chromatin is not random association of proteins and DNA but a fundamental repeating structure the repeating core is called the nucleosome
The nucleosome consists of DNA wrapped about two times around an octamer of eight histone proteins (two copies of each H2A, H2B, H3, H4). the DNA is in contract with the histone octamer in around 145 and 147 bp in length
Each of the constituting histones has a tail (11-37 amino acids). These trails interact with helps in compacting nucleosomes