DNA replication, mutation and repair Flashcards
Q1: What is the main difference between DNA replication in prokaryotes and eukaryotes?
A1: In prokaryotes, DNA replication begins at a single origin of replication (oriC) and proceeds bidirectionally. In eukaryotes, DNA replication starts at multiple origins of replication to compensate for the slower replication rate, allowing the entire genome to be replicated efficiently.
Q2: What are the key enzymes involved in DNA replication in prokaryotes?
A2: The key enzymes include:
Helicase (dnaB): Unwinds the DNA double helix.
Primase (dnaG): Synthesizes RNA primers.
DNA Polymerase III: The main enzyme responsible for DNA synthesis.
DNA Polymerase I: Removes RNA primers and fills in the gaps with DNA.
DNA Ligase: Seals nicks in the DNA backbone.
Topoisomerase (DNA gyrase): Relieves torsional stress by introducing negative supercoils.
Q3: How does DNA replication in eukaryotes differ in terms of polymerase usage?
A3: In eukaryotes, DNA replication involves multiple DNA polymerases:
DNA Polymerase α: Synthesizes RNA primers and extends them with DNA.
DNA Polymerase δ: Responsible for leading strand synthesis.
DNA Polymerase ε: Responsible for leading strand synthesis.
DNA Polymerase γ: Replicates mitochondrial DNA.
Q4: What is the role of the sliding clamp in DNA replication?
A4: The sliding clamp (e.g., PCNA in eukaryotes) keeps DNA polymerase attached to the DNA template, increasing the processivity of the enzyme. It allows the polymerase to add many nucleotides without dissociating from the DNA.
Q5: What is the end-replication problem?
A5: The end-replication problem refers to the inability of DNA polymerase to fully replicate the ends of linear chromosomes. This is because DNA polymerase requires a primer to start synthesis, and once the final RNA primer is removed, there is no way to fill in the gap at the 3’ end of the lagging strand.
Q6: How do eukaryotes solve the end-replication problem?
A6: Eukaryotes solve the end-replication problem using telomerase, an enzyme that adds repetitive nucleotide sequences (telomeres) to the ends of chromosomes. Telomerase extends the 3’ end of the chromosome, allowing DNA polymerase to complete the lagging strand.
Q7: What are telomeres, and why are they important?
A7: Telomeres are repetitive nucleotide sequences (e.g., TTAGGG in humans) at the ends of chromosomes. They protect the chromosome ends from degradation and prevent them from being recognized as broken DNA. Telomeres also play a role in cellular aging and cancer.
Q8: What happens if telomeres become too short?
A8: If telomeres become too short, cells enter a state called replicative senescence, where they can no longer divide. This is associated with aging and the loss of cellular function. In some cases, cells may undergo apoptosis (programmed cell death).
Q9: What are the main causes of DNA mutations?
A9: DNA mutations can be caused by:
Endogenous factors: Errors during DNA replication, spontaneous chemical changes (e.g., deamination of bases), and oxidative damage.
Exogenous factors: UV radiation, ionizing radiation, chemical mutagens (e.g., cigarette smoke), and certain viruses.
Q10: What are the different types of DNA mutations?
A10: The main types of DNA mutations include:
Point mutations: Single nucleotide changes (e.g., substitutions, insertions, deletions).
Frameshift mutations: Insertions or deletions that shift the reading frame of the gene.
Chromosomal mutations: Large-scale changes such as deletions, duplications, inversions, and translocations.
Q11: What are the potential effects of DNA mutations?
A11: The effects of DNA mutations can vary:
Silent mutations: No change in the protein product.
Missense mutations: A single amino acid change in the protein.
Nonsense mutations: Introduction of a premature stop codon, leading to a truncated protein.
Frameshift mutations: Can completely alter the protein sequence, often leading to nonfunctional proteins.
Chromosomal mutations: Can cause large-scale genomic rearrangements, leading to genetic disorders or cancer.
Q12: How do mutations contribute to cancer?
A12: Mutations in key genes, such as tumor suppressor genes (e.g., TP53) and oncogenes (e.g., RAS), can lead to uncontrolled cell growth and cancer. Mutations that disrupt DNA repair mechanisms can also increase the likelihood of cancer by allowing additional mutations to accumulate.
Q13: What are the main DNA repair mechanisms in cells?
A13: The main DNA repair mechanisms include:
Base Excision Repair (BER): Repairs small, non-helix-distorting base lesions (e.g., deaminated bases).
Nucleotide Excision Repair (NER): Repairs bulky, helix-distorting lesions (e.g., thymine dimers caused by UV light).
Mismatch Repair (MMR): Corrects base-pairing mismatches that occur during DNA replication.
Homologous Recombination (HR): Repairs double-strand breaks using a homologous sequence as a template.
Non-Homologous End Joining (NHEJ): Repairs double-strand breaks by directly ligating the broken ends, often with some loss of genetic material.
Q14: How does the mismatch repair system distinguish between the correct and incorrect base?
A14: In prokaryotes, the mismatch repair system distinguishes the correct strand from the incorrect strand by recognizing methylation patterns. The newly synthesized strand is not immediately methylated, so the system targets the unmethylated strand for repair. In eukaryotes, the mechanism is less well understood but involves recognizing nicks in the newly synthesized DNA.
Q15: What is the role of the enzyme photolyase in DNA repair?
A15: Photolyase is involved in the direct repair of UV-induced DNA damage, specifically thymine dimers. It uses light energy to cleave the dimer and restore the original bases.
Q16: How does homologous recombination repair double-strand breaks?
A16: Homologous recombination repairs double-strand breaks by using a homologous sequence (usually the sister chromatid) as a template to accurately repair the break. The process involves:
Resection of the DNA ends to create single-stranded overhangs.
Strand invasion, where the single-stranded DNA pairs with the homologous sequence.
DNA synthesis to fill in the missing sequence.
Resolution of the Holliday junction to complete the repair.
Q17: What is the difference between homologous recombination and non-homologous end joining?
A17: Homologous recombination (HR) uses a homologous sequence as a template to accurately repair double-strand breaks, while non-homologous end joining (NHEJ) directly ligates the broken DNA ends without a template. NHEJ is faster but more error-prone, often resulting in small deletions or insertions at the repair site.
Q18: What are the consequences of defective DNA repair mechanisms?
A18: Defective DNA repair mechanisms can lead to an increased mutation rate, genomic instability, and a higher risk of cancer. For example, defects in mismatch repair genes (e.g., MLH1, MSH2) are associated with hereditary non-polyposis colorectal cancer (HNPCC).
Q19: How are telomeres linked to aging?
A19: Telomeres shorten with each cell division due to the end-replication problem. When telomeres become critically short, cells enter replicative senescence and can no longer divide. This process is associated with aging and the decline in tissue function.
Q20: What role does telomerase play in cancer?
A20: Telomerase is often reactivated in cancer cells, allowing them to maintain telomere length and divide indefinitely. This contributes to the immortality of cancer cells. Inhibiting telomerase is a potential strategy for cancer therapy.
