Lecture 16

Repair mechanisms within the cell occur in real time or post-replication, primarily focusing on recombination, which promotes genetic diversity. Recombination can happen in three ways:

1. Using homologous sequences: This utilizes similar DNA regions within the same chromosome or homologous chromosomes for accurate repair.

2. Using exogenous DNA: External DNA introduced into the cell can initiate recombination, whether through natural processes or genetic engineering.

3. Nonhomologous sequences: This random method occurs when precise sequences are unavailable, potentially resulting in less accurate repairs.

Homologous and Nonhomologous Recombination

Homologous recombination ensures precise DNA repair and maintains genetic integrity, while nonhomologous recombination is random, risking mutations that can disrupt gene functions.

Site-Specific Recombination

This controlled mechanism is used in genetic engineering to insert or replace DNA at specific locations, relying on sequences recognized by recombinases, such as Cre/lox and Flp/FRT systems.

Initiation of Recombination

Recombination often starts with a double-strand break (DSB) in DNA, caused by cellular processes or external factors like radiation. Repairing DSBs is essential for genetic stability, as unresolved breaks can lead to chromosomal aberrations or cell death.

DNA Strand Processing and Recombinases

Exonucleases remove bases from broken DNA strands, creating single-stranded regions. Recombinase enzymes facilitate strand invasion for homologous pairing, essential for accurate repair, and DNA polymerases help synthesize new strands.

The Holliday Structure and Crossover Events

The Holliday structure is crucial in homologous recombination, marking a crossover junction. Crossover events exchange genetic information, enhancing diversity, while non-crossover events preserve adjacent DNA segments' integrity.

Applications in Genetic Engineering and Site-Specific Recombination

Recombination principles are applied in genetic engineering for genome manipulation and gene replacement strategies for diseases like cystic fibrosis. Site-specific recombination systems allow precise modifications, such as the Cre-lox system for targeted DNA changes.

Implications and Risks

Recombination is vital for DNA repair; however, unintended events or error-prone joining can result in mutations or diseases like cancer. Understanding recombination mechanisms helps maximize benefits while addressing risks.

Conclusion

Understanding recombination is essential for appreciating cellular repair mechanisms and for advancements in biotechnology, including genetic engineering, supporting innovative treatments and genetic research advancements.