Rucete ✏ Campbell Biology In a Nutshell
Unit 3 GENETICS — Concept 16.2 Many Proteins Work Together in DNA Replication and Repair
DNA replication is a highly coordinated and precise process involving many enzymes and proteins. It ensures accurate transmission of genetic material from one generation to the next while also incorporating systems to repair damage and preserve genome integrity.
Basic Principle of DNA Replication
- Each DNA strand serves as a template for the creation of a complementary strand
- Base pairing (A with T, G with C) enables accurate copying
- The semiconservative model: each new DNA molecule consists of one parental strand and one newly made strand
- Confirmed by the Meselson-Stahl experiment using isotope-labeled DNA in E. coli
Origins of Replication
- Replication begins at specific sites called origins of replication
- Bacteria typically have a single circular origin; eukaryotes have many origins per chromosome
- A replication bubble forms, and replication proceeds in both directions
- At each end of a bubble is a replication fork, where the DNA is unwound and copied
Enzymes and Proteins Involved
- Helicase: unwinds the double helix at the replication fork
- Single-strand binding proteins: stabilize unwound DNA strands
- Topoisomerase: relieves strain ahead of the fork by cutting and rejoining DNA
- Primase: synthesizes short RNA primers that start DNA synthesis
- DNA polymerase III: extends DNA from the RNA primer in 5′→3′ direction
- DNA polymerase I: replaces RNA primers with DNA
- DNA ligase: joins fragments into continuous strands
Leading vs. Lagging Strand
- Leading strand:
- Synthesized continuously in the direction of the replication fork
- Requires only one primer
- Lagging strand:
- Synthesized in short Okazaki fragments in the opposite direction
- Each fragment needs a new RNA primer
- DNA polymerase I replaces RNA with DNA; DNA ligase joins fragments
DNA Replication Complex
- The replication machinery acts as a multi-enzyme complex
- May remain stationary while DNA "threads through" the complex
- Synthesis on both strands occurs simultaneously at each fork
- The lagging strand loops back in a “trombone model” to coordinate synthesis
Proofreading and DNA Repair
- DNA polymerases proofread as they add nucleotides, removing errors
- Mismatch repair: enzymes fix mispaired bases after replication
- Nucleotide excision repair:
- A damaged DNA segment is cut out by a nuclease
- DNA polymerase fills in the gap
- DNA ligase seals the strand
- Defects in repair enzymes can lead to diseases like xeroderma pigmentosum (XP) and cancer
Telomeres and Replication of DNA Ends
- Linear eukaryotic chromosomes face end-replication problems
- Repeated rounds of replication shorten DNA molecules
- Telomeres: noncoding, repetitive sequences at chromosome ends protect genes
- In most somatic cells, telomeres shorten with age
- Telomerase:
- Enzyme that extends telomeres in germ cells and some stem cells
- In cancer cells, telomerase is abnormally active, allowing unlimited division
In a Nutshell
DNA replication is a fast, accurate process requiring many specialized proteins. It uses a semiconservative mechanism to copy DNA and includes built-in proofreading and repair systems. Telomeres protect chromosome ends, and their regulation is key to aging, cancer, and genome stability.