Nucleic Acids
Macromolecules that store genetic information and direct protein synthesis (DNA & RNA).
Central Dogma of Molecular Biology
DNA → RNA → Protein explains genetic information flow.
Nucleotide
Basic unit of DNA/RNA, consisting of a sugar, phosphate, and nitrogenous base.
Nucleoside
A molecule containing a sugar and nitrogenous base, but lacking a phosphate group.
Chargaff’s Rule
A = T and G = C in DNA, ensuring base-pair complementarity.
DNA vs RNA
DNA: Double-stranded, deoxyribose sugar, thymine (T).
RNA: Single-stranded, ribose sugar, uracil (U).
Purines
Adenine (A), Guanine (G) - Double-ring structures.
Pyrimidines
Cytosine (C), Thymine (T), Uracil (U) - Single-ring structures.
Phosphodiester Bonds
Link nucleotides via 3’-5’ bonds, forming a sugar-phosphate backbone.
Watson-Crick DNA Model
Double-helix, antiparallel strands, complementary base pairing, hydrogen bonding.
Hydrogen Bonds
A-T (2 bonds), G-C (3 bonds), making GC-rich DNA more stable.
DNA Melting Temperature (Tm)
Higher GC content increases Tm, requiring more energy to denature.
Denaturation vs Renaturation
Denaturation: DNA unwinding by heat/chemicals.
Renaturation: Cooling allows reformation of complementary strands.
Semi-Conservative Model
Each daughter DNA contains one original and one new strand.
Origin of Replication (OriC)
The AT-rich sequence where replication begins.
Replication Fork
The Y-shaped region where DNA is unwound and synthesized.
Leading Strand
Synthesized continuously in the 5' to 3' direction.
Lagging Strand
Synthesized discontinuously, forming Okazaki fragments.
Helicase
Unwinds the DNA double helix.
Topoisomerase (DNA Gyrase)
Relieves supercoiling during DNA unwinding.
Single-Strand Binding Proteins (SSBPs)
Stabilize single-stranded DNA, preventing premature reannealing.
DNA Primase
Synthesizes RNA primers for DNA polymerase to initiate replication.
DNA Polymerase
Adds nucleotides in the 5' to 3' direction.
Exonuclease (Proofreading)
Removes mispaired nucleotides (3’-5’ exonuclease activity).
DNA Ligase
Seals nicks between Okazaki fragments.
Prokaryotic DNA Polymerases
Pol I: Removes primers, fills gaps.
Pol II: DNA repair.
Pol III: Main replication enzyme.
Eukaryotic DNA Polymerases
Pol α: Initiates replication.
Pol δ: Synthesizes lagging strand.
Pol ε: Synthesizes leading strand.
Pol Îł: Replicates mitochondrial DNA.
Mitochondrial DNA (mtDNA)
Circular DNA, maternally inherited, essential for electron transport chain.
Telomeres
Repetitive sequences (TTAGGG in humans) protecting chromosome ends.
Telomerase
Extends telomeres, active in stem cells and cancer cells.
RNA Primer Removal
In prokaryotes: DNA polymerase I removes primers.
In eukaryotes: RNase H and FEN1 remove primers.
Okazaki Fragment Processing
DNA polymerase fills gaps; ligase seals the final bond.
Point Mutation
Single nucleotide substitution (e.g., sickle cell anemia).
Frameshift Mutation
Insertion or deletion of nucleotides, disrupting reading frame.
Mismatch Repair
Corrects errors made by DNA polymerase.
Base Excision Repair (BER)
Removes damaged bases (e.g., oxidative damage).
Nucleotide Excision Repair (NER)
Repairs thymine dimers (damage from UV light).
Telomerase & Cancer
Increased telomerase activity in cancer cells, allowing uncontrolled division.
DNA Intercalating Drugs
Actinomycin D binds minor groove, inhibiting replication/transcription.
Topoisomerase Inhibitors
Ciprofloxacin (bacteria), Etoposide (cancer) target topoisomerase activity.