Comprehensive Study Notes on DNA Recombination

Fundamentals of Molecular Biology: DNA Recombination

Overview and Definition

DNA recombination is a process or a set of complex processes by which DNA molecules interact with one another to produce a rearrangement of the genetic information or content within an organism. This fundamental biological phenomenon occurs in both prokaryotic and eukaryotic cells, ensuring genetic plasticity and stability across different life forms.

1. Biological Roles for Recombination

Recombination serves several critical roles in biology:

1. DNA Repair: It is a vital mechanism for fixing double-strand breaks and other forms of genomic damage.

2. Integration of Genetic Elements: It allows for the integration of specific DNA elements or viruses (such as bacteriophages) into a host genome.

3. Generation of Genetic Diversity: It creates new gene/allele combinations, most notably during the process of crossing over during meiosis in eukaryotic organisms.

Recombination in Prokaryotic Systems

Common examples of recombination in prokaryotes include:

• The integration of the bacteriophage lambda ($\lambda$) prophage into the bacterial host chromosome.

• The recombination of bacterial DNA occurring after conjugation between bacteria.

• The formation of plasmid multimers.

Bacteriophage Lifecycle and Recombination Evidence

The replication cycle of bacteriophages provides clear evidence for recombination events. This typically involves the following stages:

The Lytic and Lysogenic Cycles

1. Adsorption and Penetration: The phage attaches to a receptor site on the bacterial cell wall and injects its DNA into the host.

2. Lysogenic Cycle (Integration):

   • Prophage Formation: The phage DNA inserts itself into the bacterial chromosome, becoming a prophage. This is a site-specific recombination event.

   • Replication: The prophage DNA is replicated along with the bacterial DNA prior to binary fission.

   • Indefinite Cell Division: Each daughter cell inherits the incorporated phage DNA. Under certain conditions, Induction occurs, pushing the cell into the lytic cycle.

3. Lytic Cycle (Biosynthesis and Release):

   • Biosynthesis: Phage DNA directs the host metabolism to produce viral components (proteins and DNA copies).

   • Maturation: Phage heads are packed with DNA. Collars, sheaths, and base plates are attached. Tail fibers are added last.

   • Release: The bacterial cell lyses (bursts), releasing completed, infective phages.

Historical Foundations: The Hershey and Delbrück Experiment (1947)

Al Hershey and Max Delbrück first demonstrated recombination in a bacterial system using E. coli and bacteriophages. They discovered that infecting a single E. coli cell simultaneously with two genetically distinct phages resulted in a population containing both original parental types and new recombinant phage types.

Experimental Components

Phage Loci:

• The $h$ locus: Determines host range.

  • $h^-$: Can infect all strains of E. coli used in the study.

  • $h^+$: Can only infect selected strains.

• The $r$ locus: Determines lysis speed.

  • $r^-$: Rapid lysis of host cells.

  • $r^+$: Slow lysis of host cells.

E. coli Strains:

• Strain 1: Can be killed by $h^-$ phage but not $h^+$ phage.

• Strain 2: Can be killed by both $h^-$ and $h^+$ phages.

Methodology and Plaque Assay

The experiment involved infecting a mixture of both E. coli strains with two parental phage strains: $h^- r^+$ (infects all, slow lysis) and $h^+ r^-$ (infects some, rapid lysis). The phages were then plated on a bacterial lawn containing both E. coli strains.

Interpretation of Plaque Phenotypes:

• Clear Plaque: Produced by $h^-$ phages because they infect and kill both strains in the local area.

• Turbid/Cloudy Plaque: Produced by $h^+$ phages because they only kill one strain, allowing the non-infected bacteria to continue growing.

• Small Plaque: Result of slow lysis ($r^+$).

• Large Plaque: Result of rapid lysis ($r^-$).

Results and Observed Phenotypes

Four types of plaques were observed, including two recombinant genotypes:

1. Clear and Small ($h^- r^+$): Parental phenotype.

2. Cloudy and Large ($h^+ r^-$): Parental phenotype.

3. Cloudy and Small ($h^+ r^+$): Recombinant phenotype.

4. Clear and Large ($h^- r^-$): Recombinant phenotype.

This provided physical proof of DNA recombination. For this and related work on viral replication and structure, Hershey, Delbrück, and Salvador Luria shared the 1969 Nobel Prize in Medicine & Physiology.

General Mechanisms and Types of Recombination

Recombination can be categorized by its molecular structure (intermolecular vs. intramolecular) or by the specific biological mechanism used.

Structural Categories

• Intermolecular Recombination:

  • Single Crossover

  • Double Crossover

• Intramolecular Recombination:

  • Direct Repeats

  • Inverted Repeats

The Four Functional Types of Recombination

1. Homologous Genetic Recombination: Occurs between DNA sequences that are identical or nearly identical (e.g., during meiosis or bacterial conjugation).

2. Site-Specific Recombination: Occurs at specific sequences with limited similarity; facilitated by recombinase proteins.

3. DNA Transposition: Movement of DNA elements from one site to another with little to no sequence similarity involved.

4. Illegitimate Recombination: Genetic exchanges that do not fit the other categories; often independent of the RecA protein.

Detailed Study of Homologous Recombination

Often called "general recombination," this involves the exchange of sequences between DNA molecules containing high levels of homology.

Essential Requirements

1. Homology: Two DNA sequences with similar or near-identical base pairs. The region can be as short as $50-100\,bp$ or as long as a whole chromosome. Increased homology length correlates with higher recombination frequency.

2. Hydrogen Bonding: The ability to form stable hydrogen bonds between complementary strands from different DNA molecules.

3. Enzymatic Machinery: Specific proteins (e.g., RecBCD, RecA, RuvABC) to catalyze the process.

The RecBCD Pathway in E. coli

This pathway is primarily used for DNA repair following double-strand breaks (caused by UV light or radiation).

Step 1: Nicking and Exchanging

1. Binding: The RecBCD enzyme complex binds to the blunt end of a double-strand break.

2. Unwinding/Degradation: RecBCD unwinds the dsDNA and preferentially degrades the $3'$ terminating strand using its nuclease activity.

3. Chi Site Recognition: Upon reaching the $\chi$ (Chi) site (sequence: $5'-GCTGGTGG-3'$), RecBCD's activity changes. The $3' \to 5'$ nuclease activity is attenuated, the weaker $5' \to 3'$ nuclease activity is activated, and RecA protein is loaded onto the $\chi$-containing single-stranded DNA (ssDNA).

4. Strand Invasion: The resulting RecA/ssDNA filament invades a homologous dsDNA molecule to find complementary sequences and promote base pairing.

Key Proteins in the RecBCD Pathway

• RecBCD Complex:

  • RecB: Acts as a nuclease (nicks dsDNA/ssDNA) and a helicase ($3' \to 5'$).

  • RecC: Recognizes and binds to the Chi sequence.

  • RecD: Acts as a helicase ($5' \to 3'$).

  • These subunits provide helicase activity to separate strands and ATPase activity to hydrolyze ATP.

• RecA: Binds preferentially to ssDNA to form a filament. It catalyzes strand invasion and exchange using ATP. It allows for the formation of a transient 3-stranded helix to ensure correct pairing. RecA homologs are found in nearly every species.

• Single-Strand Binding (SSB) Proteins: Assist RecA by protecting the ssDNA from degradation.

Step 2: Branch Migration and Resolution

After strand invasion, the structures must be processed and separated.

1. Holliday Junction (HJ) Formation: Once the nicks are sealed, a cross-shaped intermediate called a Holliday Junction or "chi form" is created.

2. Branch Migration:

   • RuvA: A tetramer that recognizes the HJ and binds to it, forcing it into a square planar conformation.

   • RuvB: A hexameric ring (helicase/ATPase) that loads onto opposite sides of the HJ to drive the movement of the junction along the DNA.

   • RuvAB Complex: Promotes branch migration in the direction of the RecA-mediated exchange.

3. Resolution:

   • RuvC: An endonuclease (resolvase) that binds as a dimer to the HJ (already bound by RuvAB). It cleaves the junction at specific consensus sequences: $5'-(A/T)TT(G/C)-3'$.

Possible Outcomes of Resolution

• Restore Parental Molecules: If the same strands are cleaved as the original nicks.

• Generate Recombinant Molecules: If the opposite strands are cleaved.

Physical Evidence: The Holliday Model (1964) and Potter-Dressler (1976)

Robin Holliday proposed the model in 1964. In 1976, David Dressler and Hunt Potter provided physical evidence using electron microscopy (EM).

The Experiment:

• Used E. coli containing pMB9 plasmids (normally 20 copies/cell).

• Treatment with chloramphenicol stopped chromosomal replication but allowed plasmid replication to reach 1000 copies/cell, increasing recombination frequency.

• EM showed "figure 8" structures.

• To prove these were recombination intermediates rather than interlocked circles, they used EcoRI digestion. EcoRI digestion of interlocked circles results in linear monomers, but digestion of a Holliday intermediate results in a unique chi-shaped (X-shaped) structure.

• Between $0.5\%$ and $3\%$ of molecules were chi-shaped. These structures were absent in recA- strains, confirming the biological nature of the intermediate.

Site-Specific Recombination (SSR)

SSR involves the exchange of genetic material at very specific nucleotide sequences. Unlike homologous recombination, it requires only limited stretches of similarity.

Key Characteristics of SSR

• Recombinase: A specific protein that recognizes and acts on specific sites present on both DNA segments.

• Conservative: No nucleotides are lost during the process.

• Efficiency: Does not require DNA replication or ATP.

• Rearrangements: Can cause Insertion (intermolecular), Deletion (intramolecular), or Inversion (intramolecular/inverted repeats).

Example: Lambda ($\lambda$) Phage Integration

Integration of the $\lambda$ genome into the E. coli chromosome requires:

• Specific sites: attP (on phage) and attB (on bacteria).

• $\lambda$ Int: The integrase protein.

• IHF (Integration Host Factor): An E. coli protein that binds DNA and promotes integration.

DNA Transposition

Transposition involves mobile genetic elements called transposons (or "jumping genes") moving from one site in the genome to another.

Impact on the Genome

• Transposons can make up large fractions of an organism's genome.

• They can disrupt gene function by inserting into coding or regulatory sequences.

• Transposase/Integrase: The enzyme responsible for the movement.

General Mechanisms

1. DNA Transposons: Move directly as DNA using a transposase enzyme (acts as both nuclease and ligase). Features include terminal inverted repeats and flanking direct repeats.

2. Retrotransposons: Move through an RNA intermediate. The DNA is transcribed into RNA, then reverse-transcribed back into DNA by Reverse Transcriptase (RT) before insertion. These often contain Long Terminal Repeats (LTRs).

Illegitimate Recombination

Illegitimate recombination refers to genetic exchanges that are RecA-independent. They often involve small repeated sequences termed Short Sequence Repeats (SSR) (usually $3-4\,bp$).

Characteristics and HFIR

• Includes spontaneous rearrangements like deletions, duplications, and specialized phage formation.

• Frequency increases exponentially as the length of the repeat increases (significant at $8\,bp$).

• Frequency decreases as the distance between repeats increases.

• Homology-Facilitated Illegitimate Recombination (HFIR): A process where homologous "anchor" sequences help integrate heterologous regions, even when branch migration is typically stopped by heterology.

Questions & Discussion

Sample Multiple Choice

1. Replication is when…

   • Answer: C. another copy of DNA is made.

2. What happens to homologous recombination if the RuvB protein is mutated?

   • Answer: C. The Holliday junctions is formed but cannot be resolved (because RuvB is required for the RuvAB complex to drive branch migration to reach the consensus sequence for RuvC cleavage).

Sample True/False

1. During DNA replication, the leading strand serves as template for replication of the lagging strand.

   • Answer: False. The lagging strand is synthesized using the other parental DNA strand as a template in the opposite direction of the replication fork.

Sample Structural Questions

• Replication Origins (Prokaryote vs. Eukaryote):

  • Single origin of replication: A (Prokaryotes only).

  • Primase synthesizes RNA primer: C (Both).

  • DNA ligase joins Okazaki fragments: C (Both).

  • SSB proteins stabilize the fork: C (Both).

  • Lagging strand requires repeated initiation: C (Both).

• Describing SSR: One example is the integration of Bacteriophage $\lambda$ into E. coli using $\lambda$ Int, IHF, and the attP/attB sites to form a prophage.