All Study Guides General Genetics Unit 7
👨👩👦👦 General Genetics Unit 7 – DNA Replication and RepairDNA replication and repair are fundamental processes that maintain genetic integrity. Replication ensures accurate duplication of genetic material before cell division, while repair mechanisms fix DNA damage and errors. These processes involve complex molecular machinery and enzymes working together to preserve genomic stability.
Understanding DNA replication and repair is crucial for grasping how genetic information is passed on and protected. These processes play vital roles in evolution, disease development, and potential therapeutic targets. Exploring their mechanisms provides insights into the intricate workings of life at the molecular level.
Key Concepts and Terminology
DNA (deoxyribonucleic acid) stores genetic information in living organisms
Nucleotides consist of a sugar (deoxyribose), phosphate group, and nitrogenous base (adenine, thymine, guanine, or cytosine)
Complementary base pairing (A-T and G-C) enables accurate replication and transcription
Antiparallel orientation of DNA strands refers to the opposite 5' to 3' directionality of the two strands
Semi-conservative replication produces two identical copies of DNA, each with one original and one newly synthesized strand
Demonstrated by the Meselson-Stahl experiment using heavy isotope labeling
Replication fork forms as DNA unwinds and separates during replication
Okazaki fragments are short DNA segments synthesized on the lagging strand during replication
DNA Structure and Function
Double helix structure discovered by Watson and Crick in 1953
Two antiparallel polynucleotide strands coiled around a central axis
Stabilized by hydrogen bonds between complementary base pairs and base stacking interactions
Genetic information encoded in the sequence of nucleotide bases (A, T, G, C)
DNA packaging involves histones and chromatin to fit within the nucleus
Nucleosomes are the basic unit of chromatin, consisting of DNA wrapped around a histone octamer
DNA serves as a template for RNA synthesis (transcription) and self-replication
Mutations in DNA can lead to genetic variation and, in some cases, genetic disorders
Point mutations involve single nucleotide changes (substitutions, insertions, or deletions)
Chromosomal mutations affect larger segments of DNA (translocations, inversions, or duplications)
DNA Replication Process
Occurs during the S phase of the cell cycle, before cell division
Initiation begins at specific sites called origins of replication
Eukaryotes have multiple origins, while prokaryotes typically have a single origin
DNA helicase unwinds and separates the double helix, forming the replication fork
Single-stranded binding proteins (SSBs) stabilize the separated DNA strands
Primase synthesizes short RNA primers to initiate DNA synthesis
DNA polymerases extend the primers, adding nucleotides in the 5' to 3' direction
DNA polymerase III is the main replicative polymerase in prokaryotes
DNA polymerases α, δ, and ε are involved in eukaryotic replication
Leading strand synthesized continuously in the 5' to 3' direction
Lagging strand synthesized discontinuously as Okazaki fragments
Okazaki fragments are later joined by DNA ligase to form a continuous strand
Telomeres are repetitive DNA sequences at the ends of linear eukaryotic chromosomes
Telomerase, an RNA-dependent DNA polymerase, maintains telomere length during replication
Enzymes and Proteins Involved
DNA helicase unwinds the double helix and separates the DNA strands
Single-stranded binding proteins (SSBs) bind to and stabilize single-stranded DNA
Topoisomerases relieve tension and supercoiling caused by DNA unwinding
Type I topoisomerases create single-strand breaks, while Type II topoisomerases create double-strand breaks
Primase synthesizes short RNA primers (8-12 nucleotides) to initiate DNA synthesis
DNA polymerases catalyze the addition of nucleotides to the growing DNA strand
Require a template strand and a primer to initiate synthesis
Proofread and correct errors during replication
DNA ligase seals nicks between Okazaki fragments on the lagging strand
Sliding clamp (β-clamp in prokaryotes, PCNA in eukaryotes) encircles DNA and enhances polymerase processivity
Clamp loader (γ-complex in prokaryotes, RFC in eukaryotes) loads the sliding clamp onto DNA
Replication Errors and Mutations
Replication errors can occur due to base misincorporation, slippage, or damage to the DNA template
DNA polymerases have proofreading activity to correct most errors during replication
Spontaneous mutations arise from DNA damage caused by endogenous factors
Deamination converts cytosine to uracil and adenine to hypoxanthine
Depurination results in the loss of a purine base (adenine or guanine)
Induced mutations result from exposure to exogenous mutagens
UV radiation causes pyrimidine dimers and 6-4 photoproducts
Chemical mutagens (alkylating agents, intercalating agents) modify DNA structure
Translesion synthesis (TLS) polymerases bypass DNA lesions, but are error-prone
Mismatch repair corrects base mismatches and small insertion/deletion loops
MutS, MutL, and MutH proteins involved in prokaryotic mismatch repair
MSH and MLH proteins involved in eukaryotic mismatch repair
DNA Repair Mechanisms
Base excision repair (BER) corrects small, non-helix-distorting lesions
DNA glycosylases remove damaged bases, creating an apurinic/apyrimidinic (AP) site
AP endonuclease cleaves the phosphodiester backbone at the AP site
DNA polymerase and ligase fill the gap and seal the nick
Nucleotide excision repair (NER) corrects bulky, helix-distorting lesions
Recognition of the lesion by XPC-RAD23B or transcription-coupled repair factors
Excision of the damaged strand by XPF-ERCC1 and XPG endonucleases
Gap filling by DNA polymerase and ligation by DNA ligase
Double-strand break repair occurs through homologous recombination (HR) or non-homologous end joining (NHEJ)
HR uses the sister chromatid as a template for accurate repair
NHEJ directly ligates the broken ends, which can introduce errors
Mismatch repair (MMR) corrects base mismatches and small insertion/deletion loops
Recognition of the mismatch by MutS (prokaryotes) or MSH (eukaryotes) proteins
Recruitment of MutL (prokaryotes) or MLH (eukaryotes) proteins to coordinate repair
Excision of the mismatched region and resynthesis by DNA polymerase and ligase
Clinical and Research Applications
Mutations in DNA repair genes can lead to genetic instability and cancer predisposition
Xeroderma pigmentosum (XP) results from defects in nucleotide excision repair
Hereditary nonpolyposis colorectal cancer (HNPCC) is caused by mutations in mismatch repair genes
Inhibition of DNA repair pathways can sensitize cancer cells to chemotherapy and radiation
PARP inhibitors target base excision repair and are used to treat BRCA-deficient cancers
DNA damage response (DDR) pathways are potential targets for cancer therapy
ATM and ATR kinases are central regulators of the DDR and cell cycle checkpoints
Studying DNA replication and repair in model organisms (Escherichia coli, Saccharomyces cerevisiae) provides insights into conserved mechanisms
High-throughput sequencing technologies enable genome-wide analysis of replication origins, mutation patterns, and DNA damage
CRISPR-Cas9 gene editing relies on precise DNA repair mechanisms to introduce targeted modifications
Review and Practice Questions
What are the four nitrogenous bases found in DNA, and which bases pair with each other?
Describe the semi-conservative model of DNA replication and how it was demonstrated experimentally.
What is the role of DNA helicase in the replication process, and how does it contribute to the formation of the replication fork?
Explain the difference between continuous and discontinuous DNA synthesis on the leading and lagging strands.
What are Okazaki fragments, and how are they processed to form a continuous lagging strand?
List three key enzymes involved in DNA replication and describe their specific functions.
Distinguish between spontaneous and induced mutations, providing examples of each.
How do DNA polymerases ensure the accuracy of DNA replication, and what happens when errors occur?
Compare and contrast the mechanisms of base excision repair and nucleotide excision repair.
Discuss the clinical implications of defects in DNA repair pathways, such as xeroderma pigmentosum and hereditary nonpolyposis colorectal cancer.