Genetic recombination

Both catenanes and knots can bring together remote DNA sequences and may be important in transcription regulation and genetic recombination... 

Genetic recombination arises by exchange of homologous segments of DNA between viral genomes, most often during the replication process. The enzymes involved in recombination are DNA polymerases, endonucleases, and ligases, which also play a role in DNA repair and synthesis processes. 

A number of bacterial viruses have RNA genomes. The best-known bacterial RNA viruses have single-stranded RNA. Interestingly, the bacterial RNA viruses known in the enteric bacteria group infect only bacterial cells which behave as gene donors (males) in genetic recombination. This restriction to male bacterial cells arises because these viruses infect bacteria by attaching to male-specific pili. Since such pili are absent on female cells, these RNA viruses are unable to attach to the females, and hence do not initiate infection in females. 

Besides its problems in undermining putative macromolecular explanations of (PS), (G) and what (G) explains, antireductionism faces some problems in substantiating its claims that (PS) explains (G) and (G) explains individual cases of genetic recombination. The problems, of course, stem from the fact that neither (PS) nor (G) are laws, and therefore an account is owing of how statements like these can explain. This, in fact, is a problem that any revision of a thesis of reductionism must come to grips with as well. So, perhaps we should turn to this problem directly, and then reformulate and reassess both reductionism and antireductionism as explanatory theses in its light. 

Red algae and metaphytes Large cells. Endosymbiosis. Aerobic respiration. Meiosis. Genetic recombination 0.5%... 

Mismatch Repair. Mispairs that break the normal base-pairing rules can arise spontaneously due to DNA biosynthetic errors, events associated with genetic recombination and the deamination of methylated cytosine (Modrich, 1987). With the latter, when cytosine deaminates to uracil, an endonuclease enzyme, /V-uracil-DNA glycosylase (Lindahl, 1979), excises the uracil residue before it can pair with adenine at the next replication. However, 5-methyl cytosine deaminates to form thymine and will not be excised by a glycosylase. As a result, thymine exits on one strand paired with guanine on the sister strand, that is, a mismatch. This will result in a spontaneous point mutation if left unrepaired. For this reason, methylated cytosines form spontaneous mutation hot-spots (Miller, 1985). The cell is able to repair mismatches by being able to distinguish between the DNA strand that exists before replication and a newly synthesized strand. 

Linkage analysis by studying RFLP patterns in two or more generations of family members afflicted with a disease is useful for genetic counseling for the prediction of inherited disease, in spite of its limitations, such as the possibility of genetic recombination or the presence of more than one gene locus in different family members. 

Orotic acid readily forms dimers even when irradiated in liquid medium [582, 583]. 5-Bromouracil (5-BrU) in DNA is dehalogenated, rather than forming cyclobutane-type dimers. Such DNA derivatives are more sensitive to ultraviolet irradiation than normal DNAs [584-594], Irradiation of 5-bromo-uracil and derivatives in aqueous medium produces 5,5 -diuracil [590, 591]. However, derivatives such as 3-sbutyl-5-bromo-6-methyluracil have been reported to yield cyclobutane dimers either by irradiation of frozen aqueous solutions, or by catalysis with free radical initiators, such as aluminium chloride, ferric chloride, peroxides or azonitriles [595]. 5-Hydroxymethyluracil is reported to dimerize very slowly in frozen water at 2537 A [596]. The fundamental research in the photochemistry of the nucleic acids, the monomeric bases, and their analogues has stimulated new experiments in certain micro-organisms and approaches in such diverse fields as template coding and genetic recombination [597-616]. 

---