6 Flashcards
Why is DNA replication important?
DNA replication is crucial for cell division, allowing one cell to divide into many, and for maintaining genetic information across generations
How many nucleotides are copied during each cell division?
Approximately 3 billion nucleotides are copied during each cell division
What is the error rate in DNA replication?
About 1-2 errors per cell division
What is semiconservative replication?
Semiconservative replication means each new DNA molecule consists of one original strand and one newly synthesized strand
How does base-pairing enable DNA replication?
Each strand of the DNA double helix serves as a template, where complementary nucleotides pair (A-T and G-C) to form new strands
Where does DNA synthesis begin?
DNA synthesis begins at replication origins, which are AT-rich regions where the DNA strands are separated
What is the replication fork?
The replication fork is the area where the two strands of DNA are separated and new strands are synthesized
Why is the replication fork asymmetrical?
The replication fork is asymmetrical because DNA polymerase synthesizes DNA only in the 5’ to 3’ direction, creating a leading strand (continuous) and a lagging strand (discontinuous)
What is the role of DNA polymerase in replication?
DNA polymerase synthesizes new DNA strands by adding nucleotides to the 3’ end of the growing strand
How does DNA polymerase ensure the accuracy of replication?
DNA polymerase is self-correcting; it has a proofreading function that checks for correct base-pairing and removes mismatched nucleotides
What are the two main sites on DNA polymerase?
The polymerization site (P) for adding nucleotides and the error-correcting (E) site for proofreading
What is the role of RNA primers in DNA replication?
Short RNA primers are synthesized by DNA primase and provide a starting point for DNA polymerase to begin adding nucleotides
Why are RNA primers needed on the lagging strand?
The lagging strand is synthesized discontinuously in short fragments (Okazaki fragments), each of which requires an RNA primer to initiate DNA synthesis
How are RNA primers removed and replaced?
Nuclease breaks down the RNA primers, repair polymerase replaces them with DNA, and DNA ligase joins the fragments
What is the role of helicase in DNA replication?
Helicase unwinds the DNA double helix, separating the strands so they can be copied
What does single-strand binding protein (SSB) do?
SSB binds to the unwound single strands of DNA to prevent them from re-annealing or forming secondary structures
What is the function of the sliding clamp?
The sliding clamp holds DNA polymerase in place as it moves along the DNA template during replication
What is the role of DNA ligase in DNA replication?
DNA ligase seals the nicks in the sugar-phosphate backbone of the lagging strand, joining the Okazaki fragments into a continuous strand
What is the difference between leading and lagging strand synthesis?
The leading strand is synthesized continuously in the 5’ to 3’ direction, while the lagging strand is synthesized discontinuously in short Okazaki fragments
Why can DNA polymerase only synthesize DNA in the 5’ to 3’ direction?
DNA polymerase requires a free 3’-OH group to add nucleotides, so it can only synthesize DNA in the 5’ to 3’ direction
How does DNA polymerase correct errors during replication?
DNA polymerase proofreads the newly synthesized DNA, and if it detects a mismatch, it removes the incorrect nucleotide and replaces it with the correct one
What happens if DNA polymerase fails to correct an error?
If an error is not corrected, it can result in a mutation, which may have consequences for cell function or cause diseases like cancer
What are the key proteins involved in the DNA replication “machine”? (8)
- Helicase
- Single-strand binding proteins
- DNA primase
- DNA polymerase
- Sliding clamp
- Nuclease
- Repair polymerase
- DNA ligase
Role of Helicase
Unwinds the DNA
Role of SSBs
Stabilize the unwound strands
Role of DNA Primase
Synthesizes RNA primers
DNA Polymerase
Synthesizes new DNA strands and proofreads
Role of Sliding Clamp
Hold DNA polymerase in place
Role of Nuclease
Removes RNA primers
Role of Repair Polymerase
Replaces RNA primers with DNA
Role of DNA Ligase
Seals the gap between Okazaki Fragments on the lagging strand
What feature of DNA origins makes them easier to open for replication?
Replication origins are rich in adenine (A) and thymine (T) base pairs because A-T pairs have only two hydrogen bonds, making them easier to separate
What is the role of topoisomerase in DNA replication?
Topoisomerase helps relieve the tension that builds up ahead of the replication fork as the DNA double helix is unwound
Why does the replication fork move in opposite directions?
Replication forks move in opposite directions from each replication origin, allowing for the simultaneous replication of the leading and lagging strands
How does DNA replication differ between prokaryotes and eukaryotes?
In eukaryotes, replication begins at multiple origins of replication along the DNA, while in prokaryotes, there is typically a single origin of replication
What is the difference between Okazaki fragments in prokaryotes and eukaryotes?
Okazaki fragments are shorter in eukaryotes than in prokaryotes due to higher complexity of DNA packaging and organization in eukaryotic cells
What happens if DNA polymerase encounters a damaged template?
replication may stall
and repair enzymes must correct the damage before replication can continue
How do all the proteins involved in DNA replication work together?
The proteins at the replication fork form a coordinated “replication machine” that ensures continuous and efficient DNA synthesis, proofreading, and correction
What is the role of chromatin remodeling during DNA replication?
Chromatin remodeling complexes may be required to temporarily open chromatin structure so that the DNA is accessible for replication, particularly in eukaryotic cells
Why is the 5’ to 3’ direction critical for DNA polymerase activity?
DNA polymerase can only add nucleotides to the 3’ end of the growing DNA strand
- which requires the presence of a free hydroxyl group (-OH) at the 3’ position for the polymerization reaction to proceed
How is energy supplied for the polymerization of new nucleotides during DNA replication?
The energy for adding new nucleotides comes from breaking off two phosphate groups from the incoming nucleoside triphosphate, driving the polymerization reaction
What are the three steps that occur after the removal of the RNA primer on the lagging strand?
- Nuclease removes the RNA primer
- Repair polymerase replaces it with DNA
- DNA ligase seals the nicks in the DNA backbone
