DNA replication Flashcards
Semiconservative:
DNA replication uses each parental strand as a template for the daughter strands. Half
the parental DNA is conserved in each daughter strand.
bidirectional:
replication begins at a site of origin and simultaneously moves out in both directions from this point
Okasaki fragments:
the short fragments formed by the synthesis of the lagging strand begins from another
primer as the helix unwinds
Origin
sites beginning replication (single on prokaryotes and multiple on eukaryotes)
replication fork
sites where dan synthesis occurs
Origin binding proteins
recognize and bind to origins of replication, which are AT rich sequences.
-Helicases: unwind the double helix
Helicase
unwind the double helix
single strand binding proteins (SSB):
bind to each single strand of DNA and hold it in a single stranded conformation
Primase
DNA dependent RNA poly that cat. the rxn forming RNA primer (10 nucleotides) but copying the parental DNA strand
-DNA polymerase I and III:
Prokaryotic DNA replication is carried out by two DNA polymerases: DNA Pol I and Pol
III.
DNA poly III
DNA Pol III is the major replicative enzyme because it has a sliding clamp that keeps it attached to the DNA template over a long distance. Thus, DNA Pol III has much higher processivity than DNA Pol I.
DNA poly I
DNA Pol I performs clean-up function during DNA replication and repair. DNA Pol I mediates replacement of RNA primers with DNA through its 5’-to-3’ exonuclease activity and 5’-to-3’ DNA polymerase activity.
DNA ligase
an enzyme that catalyzes formation of phosphodiester bonds between a 3’-hydroxyl group and a 5’-phosphate group of two polynucleotide chains; join the Okazaki fragments
Sliding clamp
keeps DNA poly 3 attached to DNA template over long distance
topoisomerases
acts to prevent the extreme supercoiling of parental helix that would result as a consequence of unwinding at a replication fork
breaks and rejoins DNA chains
DNA gyrase
a topoisomerase inhibited by quinolones, is found mostly in prokaryotes
Telomerase
has a reverse transcriptase activity and carries its own RNA template, in order to restore the ends of chromosomes (telomeres) in human cancer and stem cells
Reverse Transcriptase
catalyzes synthesis of DNA from an RNA template (Retroviruses contain RNA as their genetic material. The retroviral RNA serves as a template for synthesis of DNA by reverse transcriptase)
leading strand
continuous
lagging strand
discontinuous
How does DNA Poly create phosphodiester bonds
during addition of dNTP
DNA polymerase catalyze nucleotide addition at the 3’ hydroxyl end of RNA primer and growing DNA chain . This means that new strands can only grow/elongate in the 5’ - 3’ direction
DNA synthesis occurs in
3’ to 5’ direction
errors during replication are corrected
3’ to 5’ exonuclease proofreading
end replication problem
leading strand can be synthesized until the end, but the lagging strand cannot.
An RNA primer is needed to begin synthesis of each lagging strand, but there is nothing for this piece to attach to at the end of DNA. Therefore, the last section of the lagging strand cannot be synthesized and the telomeres get shorter as the cell replicates its genomes and divides. The cells eventually become so short that they signal for cell death (which is a normal process)
Solution to end replication problem
Telomerase is RNA dependent DNA poly that maintains chromosomal ends by copying the telomeric repeat sequence from an RNA template. Telomerase activity is repressed in normal somatic cells.
Replication origins are:
- uniqueDNA segments with multiple short repeats
- recognized by multimeric origin binding proteins
- usually rich in A=T base pairs
SSB in bacteria= ______ in humans
function?
RPA
protect the exposed single stranded DNA
DNA Poly I has _____
5’ to 3’ endonuclease activity that removes the RNA primer
and then fills the gap by copying DNA 5’ to 3’
distributive polymerase
dissociates from DNA easily
Processive Polymerase
holds on to DNA via sliding clamp
eukaryotic DNA Pol δ sliding clamp
PCNA
______ is the sliding clamp of prokaryotic DNA Pol III
β-subunit
Eukaryotic origin binding protein
ORC
eukaryotic Helicase
MCM
eukarytic single stranded DNA binding protein
RPA
eukaryotic sliding clamp
PCNA
eukaryotic clamp loader
RFC
eukaryotic Polymerases
DNA Pol δ & Pol ε
DNA Pol α
Eukaryotic primase
Pri-S
Pri L
AZT targets
HIV reverse trasncriptase
Acyclovir targets
Viral DNA polymerase
Herpes Simplex Virus
Varicella Zoster Virus
Quinolones targets
bacterial DNA gyrase
stem cells and cancer cells have
increased telomerase activity
