DNA Replication Flashcards
What is the origin of replication? How many origin of replications do eukaryotes and prokaryotes have?
A particular sequence in a genome at which replication is initiated. Prokaryotes have only one. Eukaryotes have many.
How does DNA replication of the bacterial chromosome begin?
Begins with binding of a protein called DnaA at the origin of replication. DnaA uses ATP to unwind the double helix at the origin (uses helicase)
How are single strands kept apart after being unwound?
By single-stranded binding proteins (Ssb).
What is primase? What does it do?
It’s an RNA polymerase that synthesizes a short stretch of RNA that is complementary to the DNA template. They can synthesize a polynucleotide chain de novo. This serves as the primer for DNA polymerase/
How is the DNA processive? What does that mean?
It means that it can carry out multiple rounds of nucleotide addition and it won’t fall off the template until it reaches the end. It is processive because of the B subunit. It clamps DNA polymerase onto the template
Which strand is synthesized in the same direction as the replication fork? What are Okazaki fragments?
The leading strand.
They are the discontinuous fragments of the lagging strand. Each fragment is initiated by primase and elongated by DNA pol.
What does DNA ligase do? What does DNA polymerase I do?
Once synthesis is complete, DNA ligase seals the nick between the Okasaki fragments together.
It removes the RNA primers from the Okasaki fragments
What does topoisomerase do?
It prevents overwinding or unwinding of the DNA. Helps to release tension too.
How does DNA polymerase proof read?
May remove incorrect base and replace with correct base.Or could do strand slipping, always in the 3’ to 5’ direction.
What are differences between DNA polymerases versus RNA polymerases?
RNA polymerase does not require a primer (primase is RNA pol).
RNA polymerase has no 3’ to 5’ exonuclease activity and therefore cannot do any proofreading… So it has a higher rate of error.
DNA Replication in Eukaryotes, name differences
- Licensing: replication is regulated by the cell cycle
- Eukaryotes have multiple origins of replication
- Chromatin: needs to be unpackaged and repackaged
- Mitochondrial DNA replication
- Telomeres: eukaryotic chromosomes are linear not circular
What are the major differences in mtDNA replication?
- There is a specialized mtDNA polymerase
- DNA is circular (no telomeres)
- replication fork is unidirectional (so no Okasaki fragments)
- no licensing, not cell cycle regulated
What are telomeres?
Repeats of a simple sequence (GGGGTTTT) at the ends of chromosomes. They protect the ends of chromosomes.
The 3’ end of the telomere is bounded by proteins
It is added by telomerase
- Telomerase binds to incomplete strand, bases are added using RNA as template, telomerase relocates and bases are added using RNA, DNA pol will then complement the lagging strand
Can a DNA polymerase start a new chain de novo?
No, it can only elongate the 3’ end of a pre-existing polynucleotide chain
