29: DNA replication Flashcards
which subunit is responsible for processivity of DNA-polymerase III
B2 subunit
what enzyme unwounds DNA in e-coli
DnaB (helicase)
what is the function of topoisomerase II in e-coil? and Why?
Introduce right-handed negative supercoils.
To avoid a topological crisis
What steps are necessary for the preparation for the replication of DNA in e.coli?
1- DnaA proteins binding to DNA.
2-Single DNA-strands exposed in the prepriming complex
3- Polymerase holoenzyme assembles
Which structure marks the origin of replication
hexamer structure of DnaA-molecules bound to each other and to the DNA
in which base-pair rich regions is DNA-separation favoured?
AT rich regions
How many nucleotides are synthesised in lagging strand before polymerase III let go of the lagging strand template? which subunit is reponsible
1000 nucleotides
sliding clamp B2
what proofreading mechanism does polymerase III have?
3’-5’ exonuclease activity
Why is it called trombone model?
because loops lengthen and shorten
What is the primase called in e,coli
DnaG
how are additional rounds of replication are avoided
by the dissociation of DnaA-assembly from origin of replication
binding affinity of DnaA for each other and for DNA
weak, strong
How many sites are there in OriC that bind DnaA
5 sites
Which family is DnaA a part of?
p-loop NTPases
how is replication terminated in e.coli
Ter-sites recognised by termination utilisation sequence (Tus)
Tus-tur complexes and replication fork
If it is approached from one side, nothing happens.
approached from the opposite side, replication fork bllocked
How many Ter-sites are there in E-coli genome
10 termination sites
which chromosomes are subject to shortening
linear chromosomes
How many origins of replications are there in a chromosome
several hundreds
what are replication units called
replicons
what is the hexameric helix in eukaryotes called?
single stranded binding protein
Sliding clamp
Mcm2-7
replication protein A
(PCNA) proliferating cell nuclear antigen
how many and what polymerases are used in eukaryotic replication
Polymerase a (primer and polymerase) adds a primer + 20 deoxynucleotides and displaced by polymerase delta
how many and what polymerases are used in eukaryotic replication
Polymerase a (primer and polymerase) adds a primer + 20 deoxynucleotides and displaced by polymerase delta
why is linear chromosome shortened after each round of replication
because of the removal of RNA-primer leaving an incomplete 5’end
G-rich at which end is found in telomeric DNA?
3’end
what is the repeating G-rich sequence in telomeric DNA in humans?
AGGGTT
How are telomeres replicated? and what is special about this enzyme
telomerases
has an RNA -template + a reverse transcriptase activity
what is polymerase switching
one polymerase displaces another in a process
mutagenic mismatches
mismatches that result in permanent changes in the DNA sequence
what can damage bases
reactive oxygen species
deamination
alkylation
why is oxoguanin problematic
binds to adenin
why is hypoxanthine problematic
binds C
how does ultraviolet light affect DNA
covalently links two adjacent pyrimdin residues
doesnot fit in DNA-helix and replication and gene expression are blocked
how can x-ray damage DNA
by inducing breaks
and by producing high concentrations of reactive species
which subunit in polymerasse III core enzyme is responsible for 3-5’ exonuclease activity
epsilon
what distinguishes uracil from thymine
the presence of a methyl-group at C5 in thymine
uracil is recognised by Uracil DNA glycosylase?
what distinguishes uracil from thymine
the presence of a methyl-group at C5 in thymine
uracil is recognised by Uracil DNA glycosylase
sporadic mutations
Mutations occuring in somatic cells rather than being inheirted
what is salmonella test used for?
used to test the mutagenic and carcinogenic risks of a certain chemical
when is recombination of DNA used?
1- repairing of DNA when double stranded breaks are introduced
2- antibodies
3-Vira integrating their DNA in the host genome
4-when replication stalls , they help the replication continue
5- in knockout mice
when is recombination of DNA most efficient
when the DNA- strands have very similar sequences
which protein initiates dna recombination and what is the process called?
RecA in e.coli. strand invasion
rad51 in human cells
In recombination pathways for meiosis, certain recombination structures are formed? which enzymes are used? figure 29.46 page 977
Holliday junctions
recombinases
how does branch migration affect DNA-recombination
It affects which DNA-segments are used for the DNA-exchange
Recombination of DNA can also occur during Cell-division between non-homologous chromosomes and they are called…..
translocations
most helicases move in ……direction
3’-5’ direction
