Lecture 6 - Recombination of DNA.pdf

MICROBIAL GENETICS Lecture 6 Recombination of DNA Prepared by: Dr. Abeer Aloufi Assistant professor in Microbiology Lecture 6 - Recombination of DNA ▪ Genetic recombination refers to the rearrangement of DNA sequences by the breakage and rejoining of chromosomes or chromosome segments ▪ It also...

MICROBIAL GENETICS Lecture 6 Recombination of DNA Prepared by: Dr. Abeer Aloufi Assistant professor in Microbiology Lecture 6 - Recombination of DNA ▪ Genetic recombination refers to the rearrangement of DNA sequences by the breakage and rejoining of chromosomes or chromosome segments ▪ It also describes the consequences of such rearrangements, that is, the inheritance of novel combinations of alleles in the offspring that carry recombinant chromosomes ▪ Because the frequency of recombination is approximately proportional to the physical distance between markers, it provides the basis for genetic mapping ▪ Recombination also serves as a mechanism to repair some types of potentially lethal damage to chromosomes Lecture 6 - Recombination of DNA In Bacteria: ▪ Bacterial recombination is a common feature of gene transfer between bacteria, is the requirement for the transferred piece of DNA to be inserted into the recipient chromosome by breaking both DNA molecules, crossing them over, and rejoining them ▪ There are three methods of genetic recombination that are utilized by bacteria, they are transformation, transduction, and conjugation ▪ This recombination process creates genetic diversity at the level of genes that reflects differences in the DNA sequences of different Bacteria Lecture 6 - Recombination of DNA In Bacteria: Lecture 6 - Recombination of DNA In Eukaryotic: ▪ Genetic recombination is of fundamental importance for a wide variety of biological processes in eukaryotic cells ▪ One of the major questions in recombination relates to the mechanism by which the exchange of genetic information is initiated ▪ In recent years, DNA double-strand breaks (DSBs) have emerged as an important lesion that can initiate and stimulate meiotic and mitotic homologous recombination, they are described in this review article (Meiotic versus mitotic recombination: Two different routes for double-strand break repair) Lecture 6 - Recombination of DNA In Eukaryotic: ▪ The models by which DSBs induce recombination, describe the types of recombination events that DSBs stimulate, and compare the genetic control of DSB-induced mitotic recombination in budding and fission yeasts was examined in this review article (Double-Strand BreakInduced Recombination in Eukaryotes) Lecture 6 - Recombination of DNA In Eukaryotic: BioEssays, Volume: 32, Issue: 12, Pages: 1058-1066, First published: 21 October 2010, DOI: (10.1002/bies.201000087) Lecture 6 - Recombination of DNA In Viruses: ▪ Genetic recombination of viruses is the process by which genetic material (either DNA or RNA) is exchanged between parental viral genomes ▪ The outcome of genetic recombination is a new genetic entity that carries genetic information in non-parental combinations ▪ Biochemically, recombination is a process of combining or substituting portions of nucleic acid molecules Lecture 6 - Recombination of DNA In Viruses: ▪ Recombination has been recognized as an important process leading to the genetic diversity of viral genomes upon which natural selection can function ▪ Depending on the category of viruses, recombination can occur at the RNA or DNA levels ▪ Since these processes are different for DNA and RNA viruses, they are described in this review article (Recombination) Lecture 6 - Recombination of DNA In Viruses: ▪ Recombination involves the exchange of genetic material between two related viruses during the coinfection of a host cell ▪ Recombination by Independent Assortment ▪ Recombination by independent assortment can occur among viruses with segmented genomes ▪ Genes that reside on different pieces of nucleic acid are randomly assorted Lecture 6 - Recombination of DNA In Viruses: ▪ This can result in the generation of viruses with new antigenic determinants and new host ranges ▪ The development of viruses with new antigenic determinants through independent assortment is called antigenic shift Lecture 6 - Recombination of DNA In Viruses: Lecture 6 - Recombination of DNA Types and Examples of Recombination: ▪ Four types of naturally occurring recombination have been identified in living organisms: 1- General or homologous recombination 2- Illegitimate or nonhomologous recombination 3- Site-specific recombination 4- Replicative recombination Lecture 6 - Recombination of DNA Types and Examples of Recombination: General or homologous recombination (Holliday Model) ▪ In general recombination (also known as homologous recombination), is the genetic exchange that takes place between a pair of homologous DNA sequences ▪ These are usually located on two copies of the same chromosome, although other types of DNA molecules that share the same nucleotide sequence can also participate ▪ The general recombination reaction is essential for every proliferating cell because accidents occur during nearly every round of DNA replication that interrupt the replication fork and require general recombination mechanisms to repair Lecture 6 - Recombination of DNA Types and Examples of Recombination: General or homologous recombination (Holliday Model) ▪ The details of the intimate interplay between replication and recombination are still incompletely understood, but they include using variations of the homologous end-joining reaction to restart replication forks that have run into a break in the parental DNA template ▪ It occurs between DNA molecules of very similar sequences, such as homologous chromosomes in diploid organisms, using one or a small number of common enzymatic pathways Lecture 6 - Recombination of DNA Types and Examples of Recombination: Two types of recombination are typically distinguished: Homologous recombination, where a fragment of a genome is replaced by the corresponding sequence from another genome, and nonhomologous recombination, which causes genetic additions of new material and is also called lateral gene transfer (LGT) Lecture 6 - Recombination of DNA Types and Examples of Recombination: Site-specific recombination ▪ Occurs between particular short sequences (about 12 to 24 bp) present on otherwise dissimilar parental molecules ▪ Site-specific recombination requires a special enzymatic machinery, basically one enzyme or enzyme system for each particular site ▪ Good examples are the systems for integration of some bacteriophage, such as λ, into a bacterial chromosome and the rearrangement of immunoglobulin genes in vertebrate Lecture 6 - Recombination of DNA Types and Examples of Recombination: What is the difference between homologous and site-specific recombination? Homologous recombination occurs between DNA with extensive sequence homology anywhere within the homology Site-specific recombination occurs between DNA with no extensive homology (although very short regions may be critical) only at special sites