Homologous Recombination Repair and Transposable Elements: Mechanisms and Genetic Impact

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Homologous Recombination Repair (HRR)

Mechanism of Homologous Recombination Repair

Homologous recombination repair (HRR) is a critical pathway for repairing double-strand breaks (DSBs) in DNA, ensuring genomic stability. This process utilizes a homologous DNA sequence, typically a sister chromatid, as a template for accurate repair.

Double-strand break: Both DNA strands are severed, creating a DSB.
End resection: The broken ends are processed to generate single-stranded 3' overhangs.
Strand invasion: The single-stranded DNA invades a homologous region on the sister chromatid, forming a displacement loop (D-loop).
DNA synthesis: DNA polymerase extends the invading strand using the homologous template.
Resolution: The resulting Holliday junctions are resolved, leading to either crossover or non-crossover products.

Steps of homologous recombination repair: double-strand break, end processing, strand exchange Continuation of homologous recombination repair: strand exchange, DNA synthesis, resolution and ligation

Example: HRR is essential during the S and G2 phases of the cell cycle when sister chromatids are available.

Synthesis-Dependent Strand Annealing (SDSA)

SDSA is a sub-pathway of HRR that results in non-crossover repair. After strand invasion and DNA synthesis, the newly synthesized strand dissociates and anneals to the other processed end, avoiding the formation of stable Holliday junctions.

Strand displacement and annealing: The newly synthesized strand returns to its original DNA molecule.
Non-crossover outcome: No exchange of chromosome arms occurs.

SDSA pathway: double-strand break, end resection, strand invasion and DNA synthesis SDSA pathway: strand displacement, annealing, DNA synthesis, ligation, non-crossover outcome

Double-Strand Break Repair (DSBR) and Holliday Junctions

DSBR involves the formation of two Holliday junctions, which are cross-shaped DNA structures. The resolution of these junctions determines whether crossover or non-crossover products are formed.

Holliday junction: A four-stranded DNA structure formed during strand exchange.
Branch migration: The junction can move along the DNA, extending the region of heteroduplex.
Resolution: Enzymatic cleavage of the junctions produces either recombinant (crossover) or non-recombinant (non-crossover) chromosomes.

DSBR pathway: double-strand break, strand invasion, D-loop formation, gap repair synthesis, branch migration, resolution Holliday junction formation and branch migration Resolution of Holliday junctions: crossover and non-crossover outcomes

Transposable Elements (TEs)

Definition and Significance

Transposable elements (TEs) are DNA sequences capable of moving within and between chromosomes. They are found in all organisms and constitute a significant portion of many genomes, including humans.

Mutagenic potential: TEs can disrupt gene function by inserting into coding or regulatory regions.
Chromosomal rearrangements: TE activity can cause inversions, translocations, and double-strand breaks.
Evolutionary impact: TEs provide raw material for genetic variation and adaptation.

Categories of Transposable Elements

TEs are classified into two main categories based on their mechanism of movement:

DNA Transposons (Class II): Move directly as DNA via a "cut-and-paste" mechanism, mediated by the enzyme transposase.
Retrotransposons (Class I): Move via an RNA intermediate using "copy-and-paste" mechanism, increasing their copy number through reverse transcription.

DNA Transposons: Structure and Mechanism

DNA transposons contain an open reading frame (ORF) encoding transposase and are flanked by inverted terminal repeats (ITRs). Upon excision, they leave behind direct repeats (DRs) at the insertion site.

Autonomous elements: Encode their own transposase and can move independently.
Non-autonomous elements: Lack transposase and rely on autonomous elements for movement.

Structure of a DNA transposon: DR, ITR, transposase ORF Transposon movement: excision and insertion Transposase transcription and translation Steps of DNA transposon integration: cleavage, staggered cuts, insertion, gap filling Insertion element structure: DR, IR, transposase gene

Retrotransposons: Structure and Mechanism

Retrotransposons move via an RNA intermediate and are classified as LTR (long terminal repeat) or non-LTR elements. They encode reverse transcriptase and integrase, facilitating their integration into the genome.

LTR retrotransposons: Contain LTRs similar to retroviruses.
Non-LTR retrotransposons: Include LINEs (autonomous) and SINEs (non-autonomous).

Structure of LTR retrotransposon Structure of non-LTR retrotransposon Retrotransposon transcription and translation Retrotransposon integration mechanism

Impact of Transposable Elements on Genes and Genomes

TE insertions can cause mutations, alter gene expression, and promote chromosomal rearrangements. They are a major source of genetic diversity and have been repurposed for genetic engineering applications.

Gene disruption: Insertion into coding regions may cause frameshifts or premature stop codons.
Regulatory effects: Insertion into promoters or enhancers can alter gene expression.
Evolutionary adaptation: TE-derived sequences may be co-opted for beneficial functions.

Examples of TE-Induced Mutations

TEs are responsible for a variety of mutations in different organisms. For example, the insertion of a LINE-1 element into the F8 gene causes hemophilia in humans, while copia elements in Drosophila affect eye color.

Copia insertion causing mutant eye color in Drosophila Wild-type eye color in Drosophila Revertant wild-type eye color in Drosophila Pedigree showing X-linked mutation due to retrotransposon insertion

Summary Table: Types of Transposable Elements

Category	TE Element	Example	Organism
LTR, autonomous	Ty1-copia group	copia	Plants, animals, algae
LTR, non-autonomous	Large retrotransposon derivatives (LARDS)	Dasheng	Plants
Non-LTR, autonomous	LINE elements	L1	Humans, other organisms
Non-LTR, non-autonomous	SINE elements	Alu	Humans, other organisms

Conclusion

Homologous recombination repair and transposable elements are fundamental to understanding genetic stability, mutation, and evolution. Their mechanisms and impacts are central topics in genetics, with broad implications for genome engineering and disease research.