Illegitimate recombination, or nonhomologous recombination, is the process by which two unrelated double stranded segments of DNA are joined. This insertion of genetic material which is not meant to be adjacent tends to lead to genes being broken causing the protein which they encode to not be properly expressed. One of the primary pathways by which this will occur is the repair mechanism known as non-homologous end joining (NHEJ). [1]
Illegitimate recombination is a natural process which was first found to be present within E. coli . A 700-1400 base pair segment of DNA was found to have inserted itself into the gal and lac operons resulting in a strong polar mutation. [2] This mechanism was then found to have the ability to insert other short genetic sequences into other locations within the bacterial genome often leading to a change in the expression of neighboring genes. Oftentimes it leads to the neighboring genes to simply shut off. However some of these segments also had strong start and stop signals which changed the regulation of neighboring genes leading in changes in the amount of transcription. What differentiated this form of genetic recombination from those dependent of genetic homology was that the process observed as illegitimate did not require the use of homologous segments of DNA. While not being entirely understood at the time, it was recognized to hold potential in generating changes in the chromosomal evolution. [3]
In prokaryotes, illegitimate recombination results in a mutation of the genetic sequence of the prokaryote. This process takes different forms in eukaryotes one of which is deletions. In a deletion mutation the prokaryotic organism undergoes illegitimate recombination resulting in the removal of a continuous segment of genetic code. However this form of mutation occurs infrequently among mutants of natural origin rather than those that have been induced. Another form of illegitimate recombination in prokaryotes is that of a duplication mutations of a genome. In this case a portion of the parental genome is inserted multiple times into the genome. This duplication either inserts the genetic material in the same orientation or opposite of the original parental segments as it is non-homology driven. [3]
The mechanism of Illegitimate recombination is that of non-homologous end joining in which two strands of DNA not sharing homology will be joined together by the gene repair machinery. Upon recognition of a double strand break a protein complex will keep the two strands within close enough proximity in order to allow for repair of the strands. Next the ends of the DNA are repaired such that any incorrect or damaged nucleotides are removed. Once this happens the strands are able to be ligated together such that they are a single strand of DNA which previously had not been adjacent. This process is common for eukaryotic cells and tends to act as a repair mechanism, but can lead to these mutations if illegitimate recombination occurs. The illegitimate recombination will often take the form of large chromosomal aberrations within a eukaryotic organism as it has much larger segments of DNA than prokaryotic cells. As of such non-homologous end joining can cause illegitimate recombination which creates insertion and deletion mutations in chromosomes as well as translocation of one chromosomal segment to that of another chromosome. These large scale changes in the chromosome in eukaryotic organisms tend to have deleterious effects on the organism rather than conferring a type of genetic advantage. [4]
Illegitimate recombination oftentimes has deleterious effects on an organism as it results in a large scale change on the genetic sequence of an organism. These changes will result in mutations as the joining of DNA not based on homology will most often place genetic elements in locations in which they previously had not been placed. This can disrupt the function of genes which may be essential to the function of an organism. In the case of cancer it has been found that tumors can be a result of illegitimate recombination resulting in hairpin formation which alters the gene function within the genome of tumor cells. [5]
Illegitimate recombination is a tool which can be used in the laboratory as well as it is a useful research tool. Illegitimate recombination can generate random mutagenesis in order to generate a random alteration of the genetic sequence of an organism. [6] The induction of this mutagenesis allows for the study of a genetic sequence by creating a mutation in a genetic segment altering the function of that genetic segment. This allows for the study of gene function through the analysis of differences between mutants and natural organisms to interpret what process a gene is linked to.
Mutagenesis is a process by which the genetic information of an organism is changed by the production of a mutation. It may occur spontaneously in nature, or as a result of exposure to mutagens. It can also be achieved experimentally using laboratory procedures. A mutagen is a mutation-causing agent, be it chemical or physical, which results in an increased rate of mutations in an organism's genetic code. In nature mutagenesis can lead to cancer and various heritable diseases, and it is also a driving force of evolution. Mutagenesis as a science was developed based on work done by Hermann Muller, Charlotte Auerbach and J. M. Robson in the first half of the 20th century.
Chromosomal crossover, or crossing over, is the exchange of genetic material during sexual reproduction between two homologous chromosomes' non-sister chromatids that results in recombinant chromosomes. It is one of the final phases of genetic recombination, which occurs in the pachytene stage of prophase I of meiosis during a process called synapsis. Synapsis begins before the synaptonemal complex develops and is not completed until near the end of prophase I. Crossover usually occurs when matching regions on matching chromosomes break and then reconnect to the other chromosome.
Gene knockouts are a widely used genetic engineering technique that involves the targeted removal or inactivation of a specific gene within an organism's genome. This can be done through a variety of methods, including homologous recombination, CRISPR-Cas9, and TALENs.
Genetic recombination is the exchange of genetic material between different organisms which leads to production of offspring with combinations of traits that differ from those found in either parent. In eukaryotes, genetic recombination during meiosis can lead to a novel set of genetic information that can be further passed on from parents to offspring. Most recombination occurs naturally and can be classified into two types: (1) interchromosomal recombination, occurring through independent assortment of alleles whose loci are on different but homologous chromosomes ; & (2) intrachromosomal recombination, occurring through crossing over.
DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encodes its genome. In human cells, both normal metabolic activities and environmental factors such as radiation can cause DNA damage, resulting in tens of thousands of individual molecular lesions per cell per day. Many of these lesions cause structural damage to the DNA molecule and can alter or eliminate the cell's ability to transcribe the gene that the affected DNA encodes. Other lesions induce potentially harmful mutations in the cell's genome, which affect the survival of its daughter cells after it undergoes mitosis. As a consequence, the DNA repair process is constantly active as it responds to damage in the DNA structure. When normal repair processes fail, and when cellular apoptosis does not occur, irreparable DNA damage may occur, including double-strand breaks and DNA crosslinkages. This can eventually lead to malignant tumors, or cancer as per the two-hit hypothesis.
Non-homologous end joining (NHEJ) is a pathway that repairs double-strand breaks in DNA. It is called "non-homologous" because the break ends are directly ligated without the need for a homologous template, in contrast to homology directed repair (HDR), which requires a homologous sequence to guide repair. NHEJ is active in both non-dividing and proliferating cells, while HDR is not readily accessible in non-dividing cells. The term "non-homologous end joining" was coined in 1996 by Moore and Haber.
Gene conversion is the process by which one DNA sequence replaces a homologous sequence such that the sequences become identical after the conversion event. Gene conversion can be either allelic, meaning that one allele of the same gene replaces another allele, or ectopic, meaning that one paralogous DNA sequence converts another.
Homologous recombination is a type of genetic recombination in which genetic information is exchanged between two similar or identical molecules of double-stranded or single-stranded nucleic acids.
Genetic variability is either the presence of, or the generation of, genetic differences. It is defined as "the formation of individuals differing in genotype, or the presence of genotypically different individuals, in contrast to environmentally induced differences which, as a rule, cause only temporary, nonheritable changes of the phenotype". Genetic variability in a population is important for biodiversity.
Copy number variation (CNV) is a phenomenon in which sections of the genome are repeated and the number of repeats in the genome varies between individuals. Copy number variation is a type of structural variation: specifically, it is a type of duplication or deletion event that affects a considerable number of base pairs. Approximately two-thirds of the entire human genome may be composed of repeats and 4.8–9.5% of the human genome can be classified as copy number variations. In mammals, copy number variations play an important role in generating necessary variation in the population as well as disease phenotype.
Exon shuffling is a molecular mechanism for the formation of new genes. It is a process through which two or more exons from different genes can be brought together ectopically, or the same exon can be duplicated, to create a new exon-intron structure. There are different mechanisms through which exon shuffling occurs: transposon mediated exon shuffling, crossover during sexual recombination of parental genomes and illegitimate recombination.
Gene targeting is a biotechnological tool used to change the DNA sequence of an organism. It is based on the natural DNA-repair mechanism of Homology Directed Repair (HDR), including Homologous Recombination. Gene targeting can be used to make a range of sizes of DNA edits, from larger DNA edits such as inserting entire new genes into an organism, through to much smaller changes to the existing DNA such as a single base-pair change. Gene targeting relies on the presence of a repair template to introduce the user-defined edits to the DNA. The user will design the repair template to contain the desired edit, flanked by DNA sequence corresponding (homologous) to the region of DNA that the user wants to edit; hence the edit is targeted to a particular genomic region. In this way Gene Targeting is distinct from natural homology-directed repair, during which the ‘natural’ DNA repair template of the sister chromatid is used to repair broken DNA. The alteration of DNA sequence in an organism can be useful in both a research context – for example to understand the biological role of a gene – and in biotechnology, for example to alter the traits of an organism.
Delitto perfetto is a genetic technique for in vivo site-directed mutagenesis in yeast. This name is the Italian term for "perfect murder", and it refers to the ability of the technique to create desired genetic changes without leaving any foreign DNA in the genome.
Microhomology-mediated end joining (MMEJ), also known as alternative nonhomologous end-joining (Alt-NHEJ) is one of the pathways for repairing double-strand breaks in DNA. As reviewed by McVey and Lee, the foremost distinguishing property of MMEJ is the use of microhomologous sequences during the alignment of broken ends before joining, thereby resulting in deletions flanking the original break. MMEJ is frequently associated with chromosome abnormalities such as deletions, translocations, inversions and other complex rearrangements.
The origin and function of meiosis are currently not well understood scientifically, and would provide fundamental insight into the evolution of sexual reproduction in eukaryotes. There is no current consensus among biologists on the questions of how sex in eukaryotes arose in evolution, what basic function sexual reproduction serves, and why it is maintained, given the basic two-fold cost of sex. It is clear that it evolved over 1.2 billion years ago, and that almost all species which are descendants of the original sexually reproducing species are still sexual reproducers, including plants, fungi, and animals.
Genome evolution is the process by which a genome changes in structure (sequence) or size over time. The study of genome evolution involves multiple fields such as structural analysis of the genome, the study of genomic parasites, gene and ancient genome duplications, polyploidy, and comparative genomics. Genome evolution is a constantly changing and evolving field due to the steadily growing number of sequenced genomes, both prokaryotic and eukaryotic, available to the scientific community and the public at large.
No-SCAR genome editing is an editing method that is able to manipulate the Escherichia coli genome. The system relies on recombineering whereby DNA sequences are combined and manipulated through homologous recombination. No-SCAR is able to manipulate the E. coli genome without the use of the chromosomal markers detailed in previous recombineering methods. Instead, the λ-Red recombination system facilitates donor DNA integration while Cas9 cleaves double-stranded DNA to counter-select against wild-type cells. Although λ-Red and Cas9 genome editing are widely used technologies, the no-SCAR method is novel in combining the two functions; this technique is able to establish point mutations, gene deletions, and short sequence insertions in several genomic loci with increased efficiency and time sensitivity.
Bacterial recombination is a type of genetic recombination in bacteria characterized by DNA transfer from one organism called donor to another organism as recipient. This process occurs in three main ways:
A double-strand break repair model refers to the various models of pathways that cells undertake to repair double strand-breaks (DSB). DSB repair is an important cellular process, as the accumulation of unrepaired DSB could lead to chromosomal rearrangements, tumorigenesis or even cell death. In human cells, there are two main DSB repair mechanisms: Homologous recombination (HR) and non-homologous end joining (NHEJ). HR relies on undamaged template DNA as reference to repair the DSB, resulting in the restoration of the original sequence. NHEJ modifies and ligates the damaged ends regardless of homology. In terms of DSB repair pathway choice, most mammalian cells appear to favor NHEJ rather than HR. This is because the employment of HR may lead to gene deletion or amplification in cells which contains repetitive sequences. In terms of repair models in the cell cycle, HR is only possible during the S and G2 phases, while NHEJ can occur throughout whole process. These repair pathways are all regulated by the overarching DNA damage response mechanism. Besides HR and NHEJ, there are also other repair models which exists in cells. Some are categorized under HR, such as synthesis-dependent strain annealing, break-induced replication, and single-strand annealing; while others are an entirely alternate repair model, namely, the pathway microhomology-mediated end joining (MMEJ).
This glossary of cell and molecular biology is a list of definitions of terms and concepts commonly used in the study of cell biology, molecular biology, and related disciplines, including genetics, microbiology, and biochemistry. It is split across two articles: