Molecular Recombination

Department of Biological and Environmental Sciences
Genetics
Dr. David A. Johnson
Biol 333 4 Credits Spring 2017 MWF 8:00-9:05 AM PH 204

Molecular Recombination
pp. 286-288; 145-158

How Does Recombination Occur at the DNA Level? As the genetic material, we know that a DNA must be able to break and rejoin with another DNA molecule, as occurs during crossing over. Study of this process has revealed that a similar process occurs in all organisms involving several steps and the formation of an intermediate called a Holliday Junction with a region of heteroduplex.The resolution of this junction molecule by endonuclease nicking can result in reciprocal exchange (a crossover). However, alternate resolution can create a molecule with with heteroduplex but no reciprocal exchange. Repair of any mismatched bases may lead to the phenomenon of gene conversion (non-reciprocal exchange). Tracts of potential gene conversion also may occur at the site of reciprocal exchange due to the presence of heteroduplex and subsequent repair of mismatched bases. In an organisms with an ordered tetrad of 8 spores, if a mismatch is not repaired, mitotic segregation may occur resulting in 5:3 (or other strange ordered) tetrads, while gene conversion can lead to such segregations as 6:2.

Eukaryotic Recombination: The above mechanism is at the core of crossing over as seen in higher organisms.
Prokaryotic Recombination: Bacteria may exchange genes by various methods, most of the examples below coming from E. coli.

Conjugation: F⁺ ("male") E. coli are capable of transferring genetic material to F^- cells. F⁺ cells have a small episome (plasmid) called an F factor, which has no genes essential for survival of E. coli. F^- cells lack the F factor. F⁺ cells extend conjugation tubes through which they transfer the F factor to F^- cells, turning them into F⁺ cells. Replication of the F factor occurs as it is being transferred (rolling-circle replication).
Hfr Conjugation: If the F factor becomes integrated into the bacterial chromosome, then these Hfr cells attempts to transfer the entire chromosome into an F^- cell. Since the F factor is the last part to be transferred, the recipient cells does not become "male" but does receive DNA from the Hfr cells. This DNA can recombine with its own DNA resulting in a new genotype. By disrupting Hfr conjugation, the E. coli chromosome can be mapped.
Sexduction: Sometimes the F factor excises from the chromosome and takes a piece of the chromosome with it to form an F' factor. Transfer of an F' factor creates a merozyote (partially diploid cell--also caused a merodiploid cell). This is sexduction.
Other Plasmids: R factors have genes that convey antibiotic resistance and may be transferred, thus spreading the resistance genes.)(Antibiotic Resistance in Bacteria video)(New MRSA treatment)(Search for new antibiotics)

Transduction: After infecting a bacterium a bacteriophage may simply replicate and produce more phage (released by lysis of the bacterium) or it may integrate into the DNA of the bacterium and remain latent. (These cells are called lysogenic bacteria.) Lysogenic bacteria may divide (replicating its DNA) and thereby pass the prophage DNA to the progeny. This prophage may later excise (when it is induced) and replicate and produce more phage, which are released by lysis. When these phage infect another bacterium, they may deliver a piece of the lysed bacterium's DNA and recombination may occur.)

Generalized Transduction: Some phages, like the T-even phages, may "accidentally" package a piece of the bacterial DNA into a virus protein particle. When this particle infects a new bacterium, it delivers the donors DNA which may recombine with the recipient DNA (but no phage will be produced). (Any DNA sequence can be transferred.)
Specialized Transduction: Some phages, like λ, integrate at a specific site and when they excise, accidentally pick up a piece of adjacent DNA and package it into the virus particle. When that phage infects a new bacterium, if it integrates, it will insert the other bacterium's DNA. (Only sequences adjacent to the integration site can be transferred.)

Transformation: As Griffith originally demonstrated, bacteria may be capable of picking up "naked" DNA and integrating it into its genome.

Phage Recombination: If two strains of phage infect the same bacterium, their DNAs may recombine producing a recombinant phage.

Things I Learned at the Movies:
Rather than wasting bullets, megalomaniacs prefer to kill their archenemies using complicated machinery involving fuses, pulley systems, deadly gasses, lasers, and man-eating sharks, which will allow their captives at least 20 minutes to escape.