Ch20 and 21 DNA, Synthesis and Repair 阮雪芬NTU April 29, 2003

What is Nucleic Acids?

Ribose and deoxyribose

Purines and Pyrimidines

Backbones of DNA and RNA sugar phosphate

Hydrogen Bond in Watson-Crick Base Pairs

Structure of a DNA Chain

Spontaneous Hybridization

DNA Structure X

Watson-Crick Model of Double- Helical DNA

Minor and Major Groove

Two Antiparallel Strands of DNA

How is the DNA Packaged Into A Nucleus Winding around nucleosomes Nucleosomes packed The fibres form loops The loops form other coils and/or folds

EM of a 30 nm Fibre of Chromatin

Electron Micrographs of Circular DNA from Mitochondria

Chromosome

DNA Synthesis and Repair

Semiconservative DNA Replication

Bidirectional Replication of E. coli Chromosome Very rich in A-T pairs helicase

Multiple Bidirectional Replicative Forks in a Eukaryotic Chromosome

The Eukaryotic Cell Cycle

Supercoil Formation

The Reaction Catalysed by Type I Topoisomerase Breaks one strand of a supercoiled double helix Does not require ATP

The Reaction Catalysed by Type II Topoisomerase Breaks two strand of a supercoiled double helix Require ATP

Summary of E. coli and Eukaryote Topoisomerases TypeAction on DNAEffect on supercoiling Topoisomerase I E. coliCuts one DNA strand Relaxes negative supercoils EukaryotesCuts one DNA strand Relaxes positive and negative supercoils Topoisomerase II E. coliCuts two DNA strand; ATP- dependent Relaxes positive supercoils; insert negative supercoils EukaryotesCuts two DNA strand; ATP- dependent Relaxes positive supercoils but cannot insert negative supercoils

The Balancing Act of Gyrase and Topoisomerase I in E. coli

A Mechanism by which Eukaryotic DNA Becomes Negatively Supercoiled

Polymerization Reaction Catalyzed by DNA Polymerases

DNA Replication

How Does a New Strand Get Started

RNA Polymerase RNA polymerase requires the following components: –A template –Activated precursors –A divalent metal ions RNA polymerase does not require a primer

The Polarity Problem in DNA Replication

Diagram of A Replicative Fork

The Loop Model for Okazaki Fragment Synthesis

The Loop Model ATP-driven Single-strand binding protein

Ribbon Representations of the Yeast and E. coli Sliding “Clamps” Yeas: trimer E. coli:dimer Sliding clamp is a ring shaped protein structure surrounding the DNA

The Step in Loading a Sliding Clamp for DNA Synthesis in E. coli

Polymerase I Actions In Processing Okazaki Fragments

Events involved in DNA Replication in E. coli and in Eukaryotes

Methyl-directed Pathway for Mismatch Repair

Repair of DNA Damage in E. coli Direct repair Nucleotide excision repair Base excision repair and AP site repair

The Pathway of Nucleotide Excision Repair in E. coli

AP Site Formation and Repair

The Machinery in the Eukaryotic Replicative Fork In E. coli, polymerase I, II, and III. In eukaryotes, polymerase . –Polymerase  and  : exonuclease –Polymerase  and  : have repair function –Polymerase  : for mitochondrial DNA replication

The Shortening of Linear Chromosomes by Replication Telomeric DNA: no informational content

Mechanism by Which Telomerase Synthesizes Telomeric DNA TTAGGG sequence Reverse transcriptase Telomerase

Tansposons or Jumping Genes

Transposon

Homologous Recombination

Single-strand Invasion This reaction is a test tube model; not involved in the cellular process

A Cross-over Junction

Meiosis