Covers site-specific recombination and the integration of small DNA molecules into host chromosomes, contrasting integration with and without homologous sequences, and the role of integrases. It then moves through transposons (cut-and-paste DNA transposons and copy-and-paste retrotransposons), oncogenic DNA viruses, retroviruses, processed pseudogenes, and recombination in V(D)J immune diversity with its 12/23 spacer rule. The note ends with the dark genome, medical implications such as cancer and SCID, the Sleeping Beauty system, and a list of outcomes. Several mnemonics and a comparison of retrotransposons with DNA transposons are included.
Topic 17 of 105 in the MU Sofia syllabus for Biology (Medicine, Year 1). The free AceMed note covers it in 16 sections, about 1,362 words (10 min of reading), with 26 flashcards.
In this note
Site - specific recombination
General Overview of Site-Specific Recombination
Two Main Mechanisms of Integration
Jumping Genes and Mobile Elements
Transposition Mechanisms
Key Elements Involved in Site-Specific Recombination
DNA Transposons
Retrotransposons
Differences between Retrotransposons and DNA Transposons
Oncogenic DNA Viruses
Retroviruses
Processed Pseudogenes
Immune System and Site-Specific Recombination
The Dark Genome
Medical Implications of Site-Specific Recombination
Sleeping Beauty Transposon System
General Overview of Site-Specific Recombination
Site-specific recombination is a precise genetic process used by small DNA molecules to integrate into or exit from host chromosomes.
This mechanism is essential for genome rearrangements, allowing DNA elements to move, insert, or excise themselves at specific sites in the genome.
It plays a critical role in bacterial adaptation, viral life cycles, immune diversity, and genome evolution.
Specialized enzymes called integrases are responsible for this recombination. These enzymes function as endonucleases, transferases, and ligases.
Two Main Mechanisms of Integration
1. Integration with Homologous Sequences
Occurs when the foreign DNA and host chromosome share short homologous sequences.
A classic example is the integration of lambda phage DNA into the E. coli chromosome.
Both the phage and bacterial DNA contain specific attachment sites that align via base pairing.
Integrases introduce single strand breaks similar to the Holliday model, forming and resolving two Holliday junctions.
This process is often used by bacterial episomes and lysogenic phages.
The integrated DNA can remain dormant during the lysogenic cycle until conditions trigger its activation.
2. Integration without Homologous Sequences
Seen in retroviruses and oncogenic DNA viruses, where integration is random and does not rely on sequence similarity.
The host DNA is nicked, and the foreign DNA (usually blunt-ended) is cut and inserted.
Gaps are filled by host DNA polymerases.
Integration is random, and the inserted DNA is known as a provirus.
This randomness can disrupt host genes and potentially cause cancer.
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