Lecture 9 DNA Replication 2025 FS(1) - Tagged (1)

Lecture Overview

• Topic: DNA Replication

• Date: February 29, 2025

• Resources: Slides, videos, animations for understanding DNA replication (e.g., HHMI, Meselson-Stahl experiment animations).

Origin of Replication

• 5' to 3' Directionality: Understanding strand orientation is crucial.

• Leading Strand: Synthesized continuously in the direction of fork movement.

• Lagging Strand: Synthesized discontinuously opposite to the fork movement using Okazaki fragments.

Structure of DNA

• 1953 Discovery by Watson and Crick:

  • Used models and data from Chargaff and Franklin.

  • DNA's structure is vital for its replication mechanism.

Components of Nucleotides

• Three Parts of a Nucleotide:

  1. Pentose Sugar:

     • DNA: Deoxyribose

     • RNA: Ribose

  2. Base:

     • DNA has Adenine (A), Cytosine (C), Guanine (G), Thymine (T)

     • RNA has A, C, G, Uracil (U)

  3. Phosphate Group (PO4):

• Polymers of Nucleotides: Form nucleic acids.

DNA Strands and Bonds

• Phosphodiester Bonds: Connect nucleotides, linking the 5' phosphate of one to the 3' hydroxyl of another.

• Polarity of DNA Strands: Each strand is oriented 5' to 3'.

• Double Helix Design:

  • Right-handed double helix structure with a sugar-phosphate backbone encasing nucleobases.

  • Bases are paired internally via hydrogen bonds; A pairs with T and G pairs with C.

Genetic Material Properties

• Replication Accuracy: To provide progeny with same genetic info as parents.

• Information Content: Controls cell structure, function, and behavior (DNA -> RNA -> Protein).

• Capable of Change: Allows for mutations, which enable natural selection.

Watson-Crick Model of Replication

• Template Role: Each strand acts as a template for replication by complementary base pairing.

• Mechanism Inquiry: What processes allow the replication to occur within cells?

Meselson-Stahl Experiment**

• Semi-conservative Model Confirmation: DNA replication retains one original and one new strand in daughter molecules.

• Nitrogen Isotopes: Using heavy (15N) and light (14N) nitrogen for DNA analysis to trace replication dynamics.

E. coli DNA Replication

• Origin (oriC): Circular chromosome initiation at the fixed site.

• Unwinding: Initiator proteins (DnaA) create a replication bubble; helicases unwind DNA strands, forming replication forks.

• Single-Stranded Binding Proteins: Prevent re-annealing of the unwound strands.

DNA Primase and Polymerase in Synthesis

• RNA Primer: Short compliment synthesized by primase starts the new strand.

• DNA Polymerase III: Extends from RNA primer, synthesizing DNA in a 5' to 3' direction.

• Leading vs. Lagging Strands:

  • Leading strand synthesized continuously, while lagging strand is made in fragments (Okazaki fragments).

• DNA Ligase: Joins Okazaki fragments to ensure continuity.

Proofreading Mechanism

• DNA Polymerase Activity: Includes proofreading to correct mismatched bases using 3' to 5' exonuclease activity after synthesize.

Eukaryotic DNA Replication

• Multiple Origins: Linear chromosomes have several origins of replication.

• Telomere Protection: Telomeres prevent chromosome degradation.

• Telomerase Role: Replicates telomeres using RNA as a template to add repeated sequences (e.g., TTAGGG).

Importance of Telomeres**

• Junk DNA Concept: Telomeres consist of non-coding DNA sequences that protect coding regions from loss during replication.