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DNA structure and replication
Fichas del conjunto (24)
- 1Why is DNA important?molecular basis for inheritance
contains the code for all other cellular molecules
its complementary structure allows it to be replicated and the code to be read
variations in DNA sequence lead to phenotypic differences and susceptibility to disease
defects in DNA replication and repair lead to many diseases - 2What is a DNA nucleotide made out of?phosphate
deoxyribose sugar
organic nitrogenous base (GCAT) - 3What can be said about the strands of DNA?double stranded DNA
strands in antiparallel directions linked through hydrogen bonding
A and T = 2 H bonds
C and G = 3 H bonds
A and G = 2 rings, purines
T and C = 3 rings, pyrimidines - 4What is the 5' end?free phosphate group at the 5' position on the sugar backbone
- 5What is the 3' end?unlinked OH group at 3' position on sugar
- 6Explain the compaction of DNADNA exists as double helix
wraps around histones
to form nucleosomes
undergoes further folding to form 30nm chromatin fibre
compacted further to form chromosome - 7Describe the differences in compaction at interphase and metaphaseeach cell has 2m of DNA
1,000x compaction at interphase
10,000x compaction at metaphase - 8What is the role of linker histones?histone H1
keeps DNA wrapped around the histones in place by binding nucleosome at exit and entry sites
binds to linker DNA region between nucleosomes, helping stabilize the zig-zagged 30nm chromatin fibre - 9Why are histones abundant in the nucleus?form the framework around which DNA is wrapped
- 10What are core histones?four different types
H2A H2B H3 H4
these all exist as dimers
form octameric nucleosome core
which DNA strands wrap around twice - 11With chromosomes in interphase nucleus what do you see?you don't see separate chromosomes but rather chromosome families
- 12What is the difference between the sense and anti-sense strand?sense - runs from 5' to 3'
antisense - runs from 3' to 5' - 13What is the difference between the lagging and leading strand?lagging - synthesized discontinuously
leading - synthesized continually - 14Explain the process of DNA replication in terms of leading and lagging strandsreplication starts at the point of origin
here DNA polymerase moves in opposite directions
continuous synthesis if DNA polymerase is moving in 5' to 3' direction (in terms of new strand) - leading strand
if polymerase is moving in the 3' to 5' direction simple replication not possible as no free OH 3' end for polymerase to bind
replication needs to happen in chunks thus forming okazaki fragments that are then fused together by DNA ligase - lagging strand - 15Explain the role of DNA topoisomerasesany protein that propels itself alone along a DNA strand of a double helix e.g. helicase tends to generate superhelical tension
DNA topoisomerase enzymes rapidly remove this tension
prevents DNA supercoiling - 16What targets topoisomerases? What is the result of this?quinolones
this leads to supercoiling of DNA
so double strand breaks - 17Some antibiotics interfere with DNA replication in bacteria. Give some examplesfluoroquinolones e.g. gemifloxacin target topoisomerases
in aerobic gram negative and positive species
trimethoprim targets nucleotide synthesis in aerobic gram negative and positive species - 18There are three ways to repair single strand defects. Explain base excision repaireach DNA glycosylases can recognise a specific type of altered base in DNA and catalyse its hydrolytic removal
flipping out of altered nucleotide base from helix
DNA glycosylase probes all faces of base for damage
once an enzyme finds damaged base it recognises it and removes base from sugar
sugar phosphate with missing base cut out by the sequential action of AP endonucleases and phosphodiesterase
DNA polymerase adds new nucleotide
DNA ligase seals nick - 19There are three ways to repair single strand defects. Explain nucleotide excision repairmultienzyme complex recognises pyrimidine dimer
one cut is made on each side of lesion
DNA helicase removes entire portion of damaged strand
leaves a gap of 12 nucleotides
gap in DNA helix repaired by DNA polymerase and DNA ligase - 20What is a pyrimidine dimer and what is it caused by?pyrimidines become chemically bonded together
e.g. T-T, C-T, C-C
caused by sunlight - 21What is the major concern with double strand breaks?means no intact template strand to enable accurate repair
if not repaired leads to breakdown of chromosomes into smaller fragments and loss of genes when the cell divides - 22Double strand breaks can be joined by end-joining. Explain how non-homologous end-joining worksbroken ends brought together and joined by DNA ligation
with a loss of nucleotides at sight of joining due to degradation from ends
common in mammalian somatic cells
although there's a change in DNA sequence alright for chromosomes as little of genome is essential for life - 23Double strand breaks can be joined by end-joining. Explain how homologous end-joining worksoccurs in newly replicated DNA
uses sister chromatid as template
only happens in S and G2 phases
(when SC available to serve as templates)
the copying process involves homologous recombination
a complete sequence restored by copying second chromosome - 24Draw a table depicting some examples of inherited DNA repair defects. Include name, phenotype and affected processesBRCA2
breast and ovarian cancer
repair by homologous recombination
MutS, MutL
colon cancer
mismatch repair
Xeroderma pigmentosum (XP)
skin cancer, cellular UV sensitivity, neurological abnormalities
nucleotide excision repair
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- University (Molecular Biology and Genetics) Fundamentals of Medicine Fichas sobre DNA structure and replication, creado por Ifeoma Ezepue el 05/11/2015.
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- Ifeoma Ezepue
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