Lecture 6 - DNA Replication Flashcards
What are nucleic acids made of?
Nucleic acids (DNA and RNA) are biopolymers made of nucleotide monomers, which consist of a sugar, phosphate group, and nitrogenous base.
What sugars are found in nucleotides?
DNA contains deoxyribose, while RNA contains ribose.
How are nucleotides linked in DNA and RNA?
Nucleotides are linked through phosphodiester bonds between the 5’ carbon of one sugar and the 3’ carbon of the next.
What gives DNA and RNA strands polarity?
The 5’ end (with a free phosphate) and the 3’ end (with a free hydroxyl group) create polarity.
What is the base-pairing rule in DNA?
Adenine pairs with thymine (A-T) and cytosine pairs with guanine (C-G) via hydrogen bonds.
What is the role of DNA polymerase?
DNA polymerase synthesises DNA by adding nucleotides to the 3’ end of a growing strand, using a DNA template.
Where does the energy for DNA synthesis come from?
Energy is provided by the high-energy phosphate bonds in deoxyribonucleotide triphosphates (dNTPs).
What is a key limitation of DNA polymerase?
DNA polymerase cannot initiate synthesis on its own; it requires a primer with a free 3’ end.
What are helicases?
Helicases are enzymes that unwind DNA strands by breaking hydrogen bonds between base pairs.
What are the roles of topoisomerases during replication?
Topoisomerases relieve supercoiling stress ahead of the replication fork.
How is DNA replication initiated in prokaryotes?
Prokaryotic replication begins at a single origin of replication, such as oriC in E. coli, which includes an AT-rich region and DNA-binding sites for initiator proteins.
Why do eukaryotes have multiple origins of replication?
Eukaryotic genomes are much larger and organised into linear chromosomes, necessitating multiple origins for efficient replication.
What enzymes synthesise DNA on the leading and lagging strands in eukaryotes?
DNA polymerase epsilon synthesises the leading strand, and DNA polymerase delta synthesises the lagging strand.
What is the role of telomerase in eukaryotes?
Telomerase extends telomeres to solve the end-replication problem, preventing progressive chromosome shortening.
How does DNA polymerase ensure fidelity during replication?
DNA polymerase has proofreading activity, allowing it to remove mismatched bases using exonuclease activity.
What is the significance of Okazaki fragments?
Okazaki fragments are short DNA segments synthesised on the lagging strand, later joined by DNA ligase.
How does replication differ on the leading and lagging strands?
The leading strand is synthesised continuously, while the lagging strand is synthesised discontinuously in Okazaki fragments.
How is bacterial DNA replication unique?
Bacteria have circular chromosomes with a single origin of replication, enabling bidirectional synthesis.
What is a replicon?
DNA molecule containing an origin of replication region essential for initating replication. can replictae with its own initiator and termination sequence.
What are telomeres?
Telomeres are repetitive, non-coding DNA sequences at chromosome ends, protecting genetic material from degradation.
How does telomerase function?
Telomerase extends telomeres by adding repetitive sequences using its RNA template, preventing chromosome shortening.
What does “semi-conservative replication” mean?
Each daughter DNA molecule contains one parental strand and one newly synthesised strand.
Why is DNA replication in eukaryotes tightly regulated?
Replication is regulated to ensure that each part of the genome is replicated once per cell cycle, avoiding errors or duplications.
How do bacteria prevent over-replication?
Bacteria regulate DNA binding proteins (like DnaA) at the origin of replication, ensuring initiation occurs only once per cycle.
How is helicase loaded onto DNA?
Helicase is loaded at the replication origin by initiator proteins, which open up the AT-rich region to begin unwinding.
What is the function of DNA ligase?
DNA ligase seals nicks in the sugar-phosphate backbone, joining Okazaki fragments on the lagging strand.
What causes supercoiling during DNA replication?
The unwinding of DNA strands by helicase generates supercoils ahead of the replication fork, relieved by topoisomerases.
What are the roles of clamping proteins in DNA replication?
Clamping proteins increase the processivity of DNA polymerase, helping it stay attached to the DNA strand during synthesis.
What is the role of the clamping loader in DNA replication?
Clamping loaders load and unload the sliding clamps onto DNA, ensuring proper functioning of DNA polymerase.
What happens to bacterial DNA replication if DnaA protein binding is impaired?
Replication initiation at the oriC region would fail, as DnaA binding is essential for melting the AT-rich region and helicase loading.
Why is telomere shortening a problem for linear chromosomes?
During replication, the RNA primers at the ends of chromosomes cannot be fully replaced by DNA, leading to progressive loss of genetic material.
How does the bacterial replication fork differ from the eukaryotic fork?
In bacteria, the helicase moves on the lagging strand, while in eukaryotes, it moves on the leading strand.
Why do eukaryotes require sequential firing of replication origins?
Sequential firing ensures efficient replication of large genomes while coordinating replication timing with the cell cycle.
What is the role of primase in DNA replication?
Primase synthesises short RNA primers needed for DNA polymerase to initiate synthesis.
How do eukaryotic cells coordinate lagging strand synthesis?
DNA polymerase delta removes RNA primers and synthesises the full Okazaki fragment, with ligase sealing nicks in the backbone.
