Chapter 10: DNA Structure

Overview of DNA Structure

This section covers the fundamental components and architecture of DNA, the molecule responsible for genetic information in living organisms.

• Nucleotide Structure: A nucleotide consists of three parts: a phosphate group, a five-carbon sugar (either ribose or deoxyribose), and a nitrogenous base. The phosphate and sugar form the backbone, while the base encodes genetic information.

• Phosphodiester vs. Hydrogen Bonds: Phosphodiester bonds link nucleotides in a DNA strand, while hydrogen bonds connect complementary bases between strands.

• Ribose vs. Deoxyribose: Ribose (in RNA) has a hydroxyl group at the 2' carbon, while deoxyribose (in DNA) lacks this group.

• Pyrimidines and Purines: Purines (adenine, guanine) have two rings; pyrimidines (cytosine, thymine, uracil) have one. Thymine is found in DNA, uracil in RNA.

• DNA vs. RNA: DNA contains deoxyribose and thymine; RNA contains ribose and uracil.

• 5' and 3' Ends: The 5' end has a phosphate group; the 3' end has a hydroxyl group. DNA synthesis proceeds 5' to 3'.

• Complementary Base Pairing: Adenine pairs with thymine (A-T), guanine with cytosine (G-C) via hydrogen bonds.

• Griffith and Hershey-Chase Experiments: Demonstrated that DNA is the genetic material through transformation and bacteriophage experiments.

Chapter 11: DNA Replication

Mechanisms and Enzymes of DNA Replication

DNA replication is the process by which a cell duplicates its DNA before cell division. It is semi-conservative, meaning each new DNA molecule contains one old and one new strand.

• Meselson and Stahl Experiment: Provided evidence for semi-conservative replication using isotopic labeling.

• Replication Origins: Replication begins at specific sites called origins. Prokaryotes typically have one origin; eukaryotes have many.

• Replication Fork: The Y-shaped region where DNA is unwound and new strands are synthesized.

• Key Enzymes:

  • Helicase: Unwinds the DNA double helix.

  • SSBPs (Single-Strand Binding Proteins): Stabilize unwound DNA.

  • Primase: Synthesizes RNA primers.

  • DNA Polymerase: Adds nucleotides to the growing DNA strand.

  • Ligase: Joins Okazaki fragments on the lagging strand.

• Continuous vs. Discontinuous Synthesis: The leading strand is synthesized continuously; the lagging strand is synthesized in Okazaki fragments.

• Bidirectional Replication: DNA synthesis proceeds in both directions from the origin.

• Polymerase Functions: DNA polymerases have 5' to 3' polymerization and 3' to 5' exonuclease (proofreading) activity.

Chapter 13: Transcription and the Genetic Code

From DNA to RNA: The Central Dogma

Transcription is the process by which genetic information in DNA is copied into RNA. The genetic code specifies how nucleotide sequences are translated into amino acids.

• Central Dogma: Information flows from DNA to RNA to protein.

• Genetic Code: The code is a triplet (codon), unambiguous, degenerate (multiple codons per amino acid), and nearly universal.

• Reading Frame: The way nucleotides are grouped into codons. Frameshift mutations can alter the reading frame.

• Promoters and Initiation: Promoters are DNA sequences where RNA polymerase binds to start transcription. In E. coli, the -10 (Pribnow box) and -35 regions are important.

• RNA Polymerases: Eukaryotes have three main RNA polymerases (I, II, III), each transcribing different types of genes.

• Termination: Transcription ends at specific sequences (terminators).

• RNA Processing: Eukaryotic pre-mRNA undergoes capping, polyadenylation, and splicing to become mature mRNA.

• Alternative Splicing: Allows a single gene to code for multiple proteins by including/excluding different exons.

Chapter 14: Translation

Protein Synthesis from mRNA

Translation is the process by which ribosomes synthesize proteins using mRNA as a template.

• tRNA Structure: tRNA molecules have an anticodon (binds mRNA codon) and an amino acid attachment site. Aminoacyl-tRNA synthetases attach amino acids to tRNAs.

• Ribosome Structure: Ribosomes are composed of rRNA and proteins. Prokaryotic and eukaryotic ribosomes differ in size and composition.

• Translation Initiation: Involves recognition of the start codon (AUG) and assembly of the ribosome on the mRNA. Shine-Dalgarno (prokaryotes) and Kozak (eukaryotes) sequences help position the ribosome.

• Sites on Ribosome: A (aminoacyl), P (peptidyl), and E (exit) sites facilitate tRNA binding and peptide bond formation.

• Polysomes: Multiple ribosomes translating a single mRNA simultaneously.

• Post-Translational Modifications: Proteins may be modified after translation (e.g., phosphorylation, glycosylation).

Chapter 15: Mutations and DNA Repair

Types, Causes, and Repair of Genetic Mutations

Mutations are changes in DNA sequence that can affect gene function. Cells have multiple mechanisms to repair DNA damage.

• Types of Mutations:

  • Base Substitution: Replacement of one base with another (can be silent, missense, or nonsense).

  • Frameshift Mutation: Insertion or deletion of bases that alters the reading frame.

  • Gain/Loss/Conditional Mutations: Affect gene function in different ways.

• Transition vs. Transversion: Transition: purine to purine or pyrimidine to pyrimidine; Transversion: purine to pyrimidine or vice versa.

• Spontaneous vs. Induced Mutations: Spontaneous arise naturally; induced are caused by mutagens (e.g., chemicals, UV light).

• Mutagens: Agents that cause mutations, such as alkylating agents, intercalating agents, and radiation.

• DNA Repair Mechanisms:

  • Mismatch Repair: Corrects errors missed by proofreading.

  • Photoreactivation: Repairs thymine dimers caused by UV light.

  • Excision Repair (BER and NER): Removes and replaces damaged bases or nucleotides.

  • SOS Repair: Emergency repair system in bacteria.

  • Double-Strand Break Repair: Includes homologous recombination and nonhomologous end joining.

• Transposable Elements: DNA sequences that can move within the genome, sometimes causing mutations.

Summary Table: Types of DNA Mutations

Key Equations

• Chargaff's Rule:

• Central Dogma:

Additional info: Some explanations and examples were expanded for clarity and completeness based on standard genetics curricula.