DNA Replication Flashcards
DNA replication is bidirectional and semiconservative - define these terms
Bidirectional - replication begins in the interior of a DNA molecule and proceeds in both directions (replicated in 2 directions at the same time - after both strands are separated)
Semiconservative - after replication, each copy of the DNA contains one strand from the original template and one newly synthesized strand
In terms of number of origins of replication, how to prokaryotes and eukaryotes differ?
Prokaryotes only have one origin of replication; eukaryotes have multiple
PROKARYOTIC DNA REPLICATION:
- Origin of replication sequences are almost exclusively composed of ____
- Origin of replication needs to be ___; what accomplishes this and how?
- A-T bases
2. Melted; dnaA proteins help melt the DNA by separating the strands
PROKARYOTIC DNA REPLICATION:
3 main steps in DNA replication?
- Separation of 2 complementary DNA strands
- Origin of replication needs to be melted
- Formation of replication fork
PROKARYOTIC DNA REPLICATION:’
In formation of replication fork step, what is the function of single stranded binding proteins (SSBs)?
Once DNA helicase unwinds the double helix, SSBs bind to the single strand to keep the two strands from reannealing and protect DNA from nuclease degradation
PROKARYOTIC DNA REPLICATION:
Since DNA is a helix, what happens when helicase separates the strands?
What alleviates this?
Supercoiling ahead of the replication fork will occur
Topoisomerase (2 types)
PROKARYOTIC DNA REPLICATION:
Difference between type I and type II topoisomerase?
Type I - creates nick in one strand so dna swivels around the intact strand
Type II - cuts both strands then re-ligates the two strands
PROKARYOTIC DNA REPLICATION:
DNA gyrase is a special type __ topoisomerase
Function?
Required for?
Inhibited by?
II
Introduces super coils
Prokaryotic cell division
Quinolones
___ strand is replicated continuously, ___ strand is replicated discontinously
Leading; Lagging (Okazaki fragments)
DNA polymerases require a free 3’ OH group to begin synthesis, what solves this problem and how?
Primase - it is an RNA polymerase that does not require a free 3’ OH group so it copies the first 10 nucleotides to “prime” synthesis; DNA polymerases then begin synthesis from the free 3’ OH group of the RNA primer
What does DNA polymerase III do in prokaryotes?
It’s the enzyme that elongates the leading and lagging strands AND it has proofreading activity (checks each added nucleotide and ensures correct base pairing)
When you add a nucleotide to a growing DNA strand, what is released?
What happens to this product?
Pyrophosphate; which is further cleaved to inorganic phosphate (Pi) to make the reaction irreversible and drive the reaction forward
If DNA polymerase III recognizes a mistake what will it do? What is this called?
It will remove the nucleotide that is incorrect; this is called exonuclease activity
- RNA primer is elongated by ___?
- RNA primer is excised by ___?
- Gap is filled by ___?
- Remaining nick is sealed by ___?
- DNA polymerase III
- DNA polymerase I (replaces rNTPs with correct dNTPs
- DNA polymerase I
- DNA ligase
~moving onto eukaryotic cell cycle~
What are the 4 phases?
G1 phase (G0 is part of this) S phase (replication of DNA) G2 phase (cell prepares to divide) M phase (mitosis)
Eukaryotic DNA Polymerases:
- Pol. Alpha contains __ + __ activity and does what?
- Pol. delta contains __ + ___ activity and does what?
- Pol beta and epsilon function?
- Pol gamma?
- Primase + DNA polymerase activity; begins strand synthesis
- DNA polymerase + proofreading; extends strands
- DNA repair enzymes
- Mitochondrial DNA polymerase
How do eukaryotes solve the problem of having a gap once the RNA primer is removed?
By the action of enzymes called telomerases
What are telomeres?
Telomeres are repeated sequences at the ends of eukaryotic chromosomes (6 nucleotide repeats)
4 step process of telomerase and telomeres?
- Telomerase extends the 3’ end of the DNA
- RNA primer is synthesized by primase
- The 3’-end of the RNA serves as a primer for DNA polymerase
- RNA primer is removed
Even as primer is removed there is always a section of DNA left single stranded - why?
The 3’ overhang assumes a special structure with the dsDNA and certain proteins to protect the end of the DNA
Telomerase is not expressed in all cells; its expressed in cells that___?
Characteristics of cells that do not express telomerase?
Continually divide and are not terminally differentiated
They have their chromosomes shortened at cell division, a finite # of cell divisions, and cellular equivalent of aging
