3.1-3.2 DNA and RNA Organization Flashcards
what does DNA fold to compress itself into?
a supercoil
how does circular prokaryotic DNA fold?
folds in on itself
how does linear eukaryotic DNA fold?
wraps around positively charged histone proteins
what do right handed DNA twists produce?
a negative supercoil
what do left handed DNA twists produce?
a positive supercoil
what is a linking number (Lk)?
a constant, the number of time a strand of DNA winds in the right handed direction around the helix axis
how is Lk calculated? in terms of Tw and Wr
Lk = Tw + Wr
linking number is twists plus writhes
what is Tw?
twist, the winding of DNA strands around each other
what is Wr?
writhes, the crossover of the double helix
relate changes in Tw and changes in Wr and why
changes in Tw = changes in Wr so that Lk stays constant
how is Lk calculated in terms of avergae base pairs per turn and total base pairs?
Lk = total base pairs/ average base pairs per turn
how is Lk shifted?
by DNA cleavage, the added or removing turns
what does removing turns make?
a negative supercoil
what does adding turns make?
a positive supercoil
what are topoisomers?
DNA that differ only in Lk
what is a nucleosome?
DNA wrapped around histone proteins
when DNA is not cleaved, describe Lk
constant
what do topoisomerases do?
cleave and reseal DNA to change Lk
what kind of supercoils aid in replication and transcription?
negative supercoils
describe why replication and transcription is easer with negative supercoils
- DNA must be unwound and separated in this process
- this is easier with fewer turns (like negative supercoils have)
describe most genomic DNA
negative supercoiled
why is positive supercoiled DNA not good for replication and transcription? how does topoisomerase help?
positive supercoiled DNA creates tension down the line, making the processes harder; topoisomerases release positive supercoils
during transcription, where does RNA polymerase enter and what does it form?
enters at the unwound region, forming a transcription bubble
when RNA polymerase enters at the unwound region forming a transcription bubble, what does it form? (2)
- negative supercoils upstream of transcription bubble
- positive supercoils downstream of transcription bubble
what do topoisomerases do?
relieve supercoiling
how many classes of topoisomerases are there?
2
describe type I topoisomerases (3)
- breakone strand of DNA
- change twists (Tw)
- not ATP-dependent
describe type II toposiomerases (3)
- break both strands of DNA
- change writhes (Wr)
- ATP-dependent
describe the structure of type I topoisomerases
monomeric
describe the structure and function of the structure of type II topoisomerases
dimeric; each monomer binds a strand of DNA
what are type II topoisomerases super important for? why? (2)
replication and transcription
1. negative supercoiling facilitates the start of transcription
2. positive supercoiling may inhibit replication of transcription
(topoisomerases relieve supercoiling)
give the 8 steps of the topoisomerase II mechanism
- topoisomerase in initial state
- enzyme opens and binds DNA strand
- enzyme breaks the first DNA strand
- enzyme then breaks the second strand
- enzyme crosses second strand through first strand using energy from ATP
- enzyme repairs break is 1st DNA strand
- enzyme releases second strand
- cycle repeats
what can inhibition of topoisomerases cause?
cell death
give 4 substances that inhibit type II topoisomerases and how
- aclarubicin: prevents binding of DNA
- ICRF-187: prevents DNA cleavage
- merbarone: prevents crossover
- etoposide: prevents release of DNA
(just one of these substances can inhibit, don’t need all)