Bio111 Exam 2- Module 14- TAMU- Fletcher

🧬 MOLECULAR GENETICS – STUDY NOTES

Part 1: Key Experiments and Discoveries

🧪 Griffith (1928) – Transformation

• Studied Streptococcus pneumoniae in mice.

• Found that harmless (R) bacteria could be transformed into deadly (S) bacteria when mixed with heat-killed S strain.

• Concluded: a “transforming principle” (later found to be DNA) transferred genetic information.

🧬 Avery, MacLeod, & McCarty (1944)

• Repeated Griffith’s work in vitro (in test tubes).

• Treated samples with enzymes that destroyed proteins, RNA, or DNA.

• Only when DNA was destroyed did transformation not occur.

• Conclusion: DNA is the genetic material.

☢ Hershey & Chase (1952)

• Used bacteriophages (viruses that infect bacteria).

• Labeled DNA with radioactive phosphorus (³²P) and protein with radioactive sulfur (³⁵S).

• Only ³²P (DNA) entered bacteria.

• Proved: DNA, not protein, carries genetic info.

🧮 Chargaff (1950)

• Analyzed DNA composition across species.

• Found that A = T and G = C (base pairing rule).

• Known as Chargaff’s Rule.

💎 Wilkins & Franklin (1952)

• Used X-ray crystallography to photograph DNA.

• Franklin’s photo showed an X-shaped pattern, indicating a double helix structure.

🧩 Watson & Crick (1953)

• Built the first 3D model of DNA.

• Combined Chargaff’s rules and Franklin’s X-ray data.

• Determined the double helix structure: sugar-phosphate backbone on the outside, base pairs inside (A-T, G-C).

Part 2: DNA Structure & Organization

Structure of Double-Stranded DNA

• Components: Sugar (deoxyribose), phosphate group, nitrogenous base (A, T, G, C).

• Arrangement: Two antiparallel strands forming a double helix.

• Base pairing: A–T (2 hydrogen bonds), G–C (3 hydrogen bonds).

• Importance: Enables accurate replication and information storage.

Nucleotide Percentages

• A = T, G = C, and A + T + G + C = 100%.
  Example: If A = 30%, then T = 30%, G = 20%, C = 20%.

DNA Packaging

• Histones: Positively charged proteins that DNA wraps around.

• Nucleosomes: “Beads on a string” (DNA + histone).

• Chromatin: DNA-protein complex that folds into higher-order structures to fit in the nucleus.

• Importance: Compacts DNA and regulates gene expression.

Euchromatin vs. Heterochromatin

• Euchromatin: Loosely packed, active genes, seen during interphase.

• Heterochromatin: Densely packed, inactive genes, often near centromeres/telomeres.

• Dynamic — can switch forms to control gene activity.

Part 3: DNA Replication

Main Enzymes & Their Roles

Leading vs. Lagging Strand

• Leading strand: Synthesized continuously (5’ → 3’).

• Lagging strand: Synthesized discontinuously in short Okazaki fragments due to opposite direction (3’ → 5’).

• DNA polymerase can only add nucleotides to the 3’ end.

End Replication Problem

• Eukaryotic chromosomes are linear, and lagging strand synthesis leaves a small gap at the 3’ end.

• Telomeres: Repetitive, noncoding DNA sequences at chromosome ends that protect genes.

• Telomerase: Enzyme that extends telomeres, active in stem cells and cancer cells.

Part 4: Mutation Prevention & DNA Repair

Mutation Prevention

1. Proofreading (DNA Polymerase):

   • During replication, fixes mismatched bases instantly.

2. Nucleotide Excision Repair (NER):

   • After replication, removes and replaces damaged DNA sections (e.g., from UV light).

Telomeres & Telomerase

• Telomeres: Repetitive DNA protecting chromosome ends.

• Telomerase: Enzyme that adds telomere repeats.

• Active in germ cells, stem cells, and cancer cells, allowing continuous division.

• Most somatic cells have inactive telomerase → aging and cell death.

Consequences of Unfixed Errors

• Mutations: permanent DNA sequence changes.

• Can be beneficial, neutral, or harmful.

• Some cause diseases or cancer.

Types of Mutations