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)
what does the hydroxyl at the 2’ carbon of RNA facilitate? how?
facilitates spontaneous degradation; in alkaline conditions, the OH- can deprotonate and cause intramolecular cleavage at the phosphodiester bond
why is the spontaneous degradation capability of RNA advantageous for mRNA translation?
you can get a quick burst of translation with no lasting protein expression as the mRNA is degraded after time
when auto-cleavage is prevented, what can RNA form?
secondary structures
give an example of the secondary structures formed by RNA and its function
ribozymes: RNA molecules with catalytic activity
give an example of a ribozyme and its function
Ribonuclease P; a ribozyme that cleaves nucleic acids
what can RNA secondary structures form?
3-D structures from modified bases
what is an example of a 3-D structure formed from secondaary RNA structures and what is the advantage?
tRNA; has specificity for amino acids
histones bind DNA to form what?
nucleosomes
what does a nucleosome contain?
DNA + 8 histones
where does DNA connect on histones?
to the positively charged sections, since the DNA itself is negatively charged
what is the point of nucleosomes?
keeps DNA organized and protected
what do single stranded DNA binding proteins do?
protect single stranded DNA from nucleases, premature annealing, and formation of secondary structures
what do SS DNA binding proteins contain that allows them to bind DNA?
contain a positively charged region that binds to negatively charged DNA
what are SS DNA binding proteins selective for?
single stranded DNA
what is a genome?
the complete set of genes of an organism
is genome size proportional to the number of genes of an organism?
no!
what is necessary for storage of the genome?
compaction
what kind of chromosomes are more compacted?
mitotic
give the 6 levels of organization from DNA up to chromosomes
- DNA
- nucleosomes
- chromatin
- looped chromatin
- condensed coils of chromatin
- chromosomes
do humans have the most base pairs per chromosome?
nope
is all DNA destined for transcription/translation?
nope
what are the 2 types of sequences that DNA contains?
coding and noncoding sequences
what is heterochromatin?
mostly noncoding portions of chromatin that are highly condensed
what is heterochromatin important for?
chromosomal division and maintenance
what is euchromatin?
gene-rich portions of chromatin that are less compacted; becomes YOU
what are sister chromatids?
identical copies of DNA in a mitotic chromosome
what is a kinetochore?
protein complex needed for separation of chromosomes
how does the chromosome divide?
mitotic spindles attach to the kinetochore and pull sister chromatids apart
what is a telomere?
heterochromatin that protects chromosomal ends from degradation
what do telomeres contain? what do these form?
thousands of repetitive G-rich sequences that form the G-quadruplex structure
what do proteins do after binding telomeres?
proteins bind telomeres to stabilize loop structures
what is a promoter?
DNA sequence that occurs upstream of coding sequence
what does a promoter do?
binds transcription factors that recruit RNA ploymerase to start transcription
what is a coding sequence?
the actual gene to be transcribed
what is a monocistronic gene in prokaryotes?
a promoter followed by a single coding region
what is a polycistronic gene in prokaryotes?
a promoter followed by multiple coding regions
how is a polycistronic gene transcribed? what does it form?
transcribed as a single RNA transcript but forms multiple proteins whose functions are usually related
what does a polycistronic gene contain in terms of start and stop points for translation?
contains multiple start and stop points for translation
what are operons?
polycistronic genes that contain coding sequences for proteins in a single pathway
what are exons in DNA?
coding regions
what are introns in DNA?
noncoding regions
what do regulatory sequences bind in transcription of eukaryotic DNA?
regulatory sequences bind transcriptional regulatory proteins
list and describe 2 regulatory sequences in eukaryotic DNA
- 5’ UTR: the 5’ untranslation region that facilitates interaction with translational proteins
- 3’ UTR: the 3’ untranslated region that facilitates termination of transcription by RNA polymerase
what adds stability and translational efficiency to RNA? (2)
- the 5’ cap
- polyadenylation
what does genetic recombination result in?
new proteins from the same gene sequence
what do exons often encode?
functional domains
what is exon shuffling?
when exons are deleted or inserted across genes
what does exon shuffling allow for?
evolution of novel proteins
what does alternative splicing result in?
produces different protein products with alternative structures and functions
what does Hutchinson-Gilford progeria syndrome (HGPS) cause? how common is it?
rapid aging and death; occurs in 1 in 4 million
what causes Hutchinson-Gilford progeria syndrome (HGPS)?
a mutation in the lamin A gene
describe the HGPS mechanism of disease
- prelamin precursor is modified by the ICMT enzyme, which causes methylation of prelamin A
- this methylation is normally removed later by protease, forming the functional lamin
- but the mutation in the lamin A gene prevents demethylation
- toxic lamin accumulates in the nuclear membrane and damages nuclei leading to cell death
