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66-523: In the schematic above, the rectangular blocks represent each of the repeated DNA sequences at a particular VNTR location. The repeats are in tandem – i.e. they are clustered together and oriented in the same direction. Individual repeats can be removed from (or added to) the VNTR via recombination or replication errors, leading to alleles with different numbers of repeats. Flanking regions are segments of repetitive sequence (shown here as thin lines), allowing

132-405: A microtubular spindle during nuclear division, in the distinctively eukaryotic process of mitosis . Eukaryotes differ from prokaryotes in multiple ways, with unique biochemical pathways such as sterane synthesis. The eukaryotic signature proteins have no homology to proteins in other domains of life, but appear to be universal among eukaryotes. They include the proteins of the cytoskeleton,

198-518: A "symbiosis-based phylogeny", giving the description "Eukarya (symbiosis-derived nucleated organisms)". By 2014, a rough consensus started to emerge from the phylogenomic studies of the previous two decades. The majority of eukaryotes can be placed in one of two large clades dubbed Amorphea (similar in composition to the unikont hypothesis) and the Diphoda (formerly bikonts), which includes plants and most algal lineages. A third major grouping,

264-519: A benefit to pathogenic bacteria by allowing repair of DNA damage, particularly damages that occur in the inflammatory, oxidizing environment associated with infection of a host. When two or more viruses, each containing lethal genomic damages, infect the same host cell, the virus genomes can often pair with each other and undergo HRR to produce viable progeny. This process, referred to as multiplicity reactivation, has been studied in lambda and T4 bacteriophages , as well as in several pathogenic viruses. In

330-430: A body, with its cells dividing by mitosis , and at some stage produce haploid gametes through meiosis , a division that reduces the number of chromosomes and creates genetic variability . There is considerable variation in this pattern. Plants have both haploid and diploid multicellular phases . Eukaryotes have lower metabolic rates and longer generation times than prokaryotes, because they are larger and therefore have

396-479: A bundle of microtubules arising from a centriole , characteristically arranged as nine doublets surrounding two singlets. Flagella may have hairs ( mastigonemes ), as in many stramenopiles . Their interior is continuous with the cell's cytoplasm . Centrioles are often present, even in cells and groups that do not have flagella, but conifers and flowering plants have neither. They generally occur in groups that give rise to various microtubular roots. These form

462-449: A chromosome if they know the frequency of the crossovers. Geneticists can also use this method to infer the presence of certain genes. Genes that typically stay together during recombination are said to be linked . One gene in a linked pair can sometimes be used as a marker to deduce the presence of the other gene. This is typically used to detect the presence of a disease-causing gene. The recombination frequency between two loci observed

528-481: A form of recombination. Recombination also occurs in the reoviridae (dsRNA)(e.g. reovirus), orthomyxoviridae ((-)ssRNA)(e.g. influenza virus ) and coronaviridae ((+)ssRNA) (e.g. SARS ). Recombination in RNA viruses appears to be an adaptation for coping with genome damage. Switching between template strands during genome replication, referred to as copy-choice recombination, was originally proposed to explain

594-406: A primary component of the cytoskeleton, and are often assembled over the course of several cell divisions, with one flagellum retained from the parent and the other derived from it. Centrioles produce the spindle during nuclear division. The cells of plants, algae, fungi and most chromalveolates , but not animals, are surrounded by a cell wall. This is a layer outside the cell membrane , providing

660-495: A repeat sequence of about ten to one hundred nucleotides, and the number of times the sequence repeats varies from about five to fifty times. The sequences of minisatellites are larger than those of microsatellites , in which the repeat sequence is generally 1 to 6 nucleotides. The two types of repeat sequences are both tandem but are specified by the length of the repeat sequence. VNTRs, therefore, because they have repeat sequences of ten to one hundred nucleotides in which every repeat

726-510: A smaller surface area to volume ratio. The evolution of sexual reproduction may be a primordial characteristic of eukaryotes. Based on a phylogenetic analysis, Dacks and Roger have proposed that facultative sex was present in the group's common ancestor. A core set of genes that function in meiosis is present in both Trichomonas vaginalis and Giardia intestinalis , two organisms previously thought to be asexual. Since these two species are descendants of lineages that diverged early from

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792-417: A system of domains rather than kingdoms as top level rank being put forward by Carl Woese , Otto Kandler , and Mark Wheelis in 1990, uniting all the eukaryote kingdoms in the domain "Eucarya", stating, however, that " 'eukaryotes' will continue to be an acceptable common synonym". In 1996, the evolutionary biologist Lynn Margulis proposed to replace kingdoms and domains with "inclusive" names to create

858-542: A variety of exogenous agents (e.g. UV light , X-rays , chemical cross-linking agents) can be repaired by homologous recombinational repair (HRR). These findings suggest that DNA damages arising from natural processes , such as exposure to reactive oxygen species that are byproducts of normal metabolism, are also repaired by HRR. In humans, deficiencies in the gene products necessary for HRR during meiosis likely cause infertility In humans, deficiencies in gene products necessary for HRR, such as BRCA1 and BRCA2 , increase

924-574: Is a common mechanism used in DNA repair . Gene conversion – the process during which homologous sequences are made identical also falls under genetic recombination. Genetic recombination and recombinational DNA repair also occurs in bacteria and archaea , which use asexual reproduction . Recombination can be artificially induced in laboratory ( in vitro ) settings, producing recombinant DNA for purposes including vaccine development. V(D)J recombination in organisms with an adaptive immune system

990-596: Is a small probability of recombination at any location along a chromosome, the frequency of recombination between two locations depends on the distance separating them. Therefore, for genes sufficiently distant on the same chromosome, the amount of crossover is high enough to destroy the correlation between alleles. Tracking the movement of genes resulting from crossovers has proven quite useful to geneticists. Because two genes that are close together are less likely to become separated than genes that are farther apart, geneticists can deduce roughly how far apart two genes are on

1056-411: Is a type of site-specific genetic recombination that helps immune cells rapidly diversify to recognize and adapt to new pathogens . During meiosis, synapsis (the pairing of homologous chromosomes) ordinarily precedes genetic recombination. Genetic recombination is catalyzed by many different enzymes . Recombinases are key enzymes that catalyse the strand transfer step during recombination. RecA ,

1122-461: Is altered. Gene conversion has often been studied in fungal crosses where the 4 products of individual meioses can be conveniently observed. Gene conversion events can be distinguished as deviations in an individual meiosis from the normal 2:2 segregation pattern (e.g. a 3:1 pattern). Recombination can occur between DNA sequences that contain no sequence homology . This can cause chromosomal translocations , sometimes leading to cancer. B cells of

1188-555: Is called recombinant DNA . A prime example of such a use of genetic recombination is gene targeting , which can be used to add, delete or otherwise change an organism's genes. This technique is important to biomedical researchers as it allows them to study the effects of specific genes. Techniques based on genetic recombination are also applied in protein engineering to develop new proteins of biological interest. Examples include Restriction enzyme mediated integration , Gibson assembly and Golden Gate Cloning . DNA damages caused by

1254-610: Is closer in structure to bacterial RNA than to eukaryote RNA. Some eukaryotes, such as the metamonads Giardia and Trichomonas , and the amoebozoan Pelomyxa , appear to lack mitochondria, but all contain mitochondrion-derived organelles, like hydrogenosomes or mitosomes , having lost their mitochondria secondarily. They obtain energy by enzymatic action in the cytoplasm. Plants and various groups of algae have plastids as well as mitochondria. Plastids, like mitochondria, have their own DNA and are developed from endosymbionts , in this case cyanobacteria . They usually take

1320-457: Is exactly the same, are considered minisatellites. However, while all VNTRs are minisatellites, not all minisatellites are VNTRs. VNTRs can vary in number of repeats from individual to individual, as where some non-VNTR minisatellites have repeat sequences that repeat the same number of times in all individuals containing the tandem repeats in their genomes. Genetic recombination Genetic recombination (also known as genetic reshuffling )

1386-643: Is much larger than that of prokaryotes. The eukaryotes seemingly emerged within the Asgard archaea , and are closely related to the Heimdallarchaeia . This implies that there are only two domains of life , Bacteria and Archaea, with eukaryotes incorporated among the Archaea. Eukaryotes first emerged during the Paleoproterozoic , likely as flagellated cells. The leading evolutionary theory

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1452-536: Is that their cells have nuclei . This gives them their name, from the Greek εὖ ( eu , "well" or "good") and κάρυον ( karyon , "nut" or "kernel", here meaning "nucleus"). Eukaryotic cells have a variety of internal membrane-bound structures, called organelles , and a cytoskeleton which defines the cell's organization and shape. The nucleus stores the cell's DNA , which is divided into linear bundles called chromosomes ; these are separated into two matching sets by

1518-406: Is the crossing-over value . It is the frequency of crossing over between two linked gene loci ( markers ), and depends on the distance between the genetic loci observed. For any fixed set of genetic and environmental conditions, recombination in a particular region of a linkage structure ( chromosome ) tends to be constant, and the same is then true for the crossing-over value which is used in

1584-785: 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) int er chromosomal recombination, occurring through independent assortment of alleles whose loci are on different but homologous chromosomes (random orientation of pairs of homologous chromosomes in meiosis I); & (2) int ra chromosomal recombination, occurring through crossing over. During meiosis in eukaryotes , genetic recombination involves

1650-868: Is they were created by symbiogenesis between an anaerobic Asgard archaean and an aerobic proteobacterium , which formed the mitochondria . A second episode of symbiogenesis with a cyanobacterium created the plants, with chloroplasts . Eukaryotic cells contain membrane-bound organelles such as the nucleus , the endoplasmic reticulum , and the Golgi apparatus . Eukaryotes may be either unicellular or multicellular . In comparison, prokaryotes are typically unicellular. Unicellular eukaryotes are sometimes called protists . Eukaryotes can reproduce both asexually through mitosis and sexually through meiosis and gamete fusion ( fertilization ). Eukaryotes are organisms that range from microscopic single cells , such as picozoans under 3 micrometres across, to animals like

1716-601: Is well documented in male Drosophila melanogaster . The "Haldane-Huxley rule" states that achiasmy usually occurs in the heterogametic sex . Heterochiasmy occurs when recombination rates differ between the sexes of a species. In humans, each oocyte has on average 41.6 ± 11.3 recombinations, 1.63-fold higher than sperms. This sexual dimorphic pattern in recombination rate has been observed in many species. In mammals, females most often have higher rates of recombination. Numerous RNA viruses are capable of genetic recombination when at least two viral genomes are present in

1782-873: The Diamond Princess cruise, two mutations, 29736G > T and 29751G > T (G13 and G28) were located in Coronavirus 3′ stem-loop II-like motif (s2m) of SARS-CoV-2. Although s2m is considered an RNA motif highly conserved in 3' untranslated region among many coronavirus species, this result also suggests that s2m of SARS-CoV-2 is RNA recombination /mutation hotspot. SARS-CoV-2's entire receptor binding motif appeared, based on preliminary observations, to have been introduced through recombination from coronaviruses of pangolins . However, more comprehensive analyses later refuted this suggestion and showed that SARS-CoV-2 likely evolved solely within bats and with little or no recombination. Nowak and Ohtsuki noted that

1848-897: The CODIS database. When removed from surrounding DNA by the PCR or RFLP methods, and their size determined by gel electrophoresis or Southern blotting , they produce a pattern of bands unique to each individual. When tested with a group of independent VNTR markers, the likelihood of two unrelated individuals' having the same allelic pattern is extremely low. VNTR analysis is also being used to study genetic diversity and breeding patterns in populations of wild or domesticated animals. As such, VNTRs can be used to distinguish strains of bacterial pathogens. In this microbial forensics context, such assays are usually called Multiple Loci VNTR Analysis or MLVA . In analyzing VNTR data, two basic genetic principles can be used: Repetitive DNA , representing over 40% of

1914-518: The archaea —having a volume of around 10,000 times greater. Eukaryotes represent a small minority of the number of organisms , but, as many of them are much larger, their collective global biomass (468 gigatons) is far larger than that of prokaryotes (77 gigatons), with plants alone accounting for over 81% of the total biomass of Earth . The eukaryotes are a diverse lineage, consisting mainly of microscopic organisms . Multicellularity in some form has evolved independently at least 25 times within

1980-583: The blue whale , weighing up to 190 tonnes and measuring up to 33.6 metres (110 ft) long, or plants like the coast redwood , up to 120 metres (390 ft) tall. Many eukaryotes are unicellular; the informal grouping called protists includes many of these, with some multicellular forms like the giant kelp up to 200 feet (61 m) long. The multicellular eukaryotes include the animals, plants, and fungi , but again, these groups too contain many unicellular species . Eukaryotic cells are typically much larger than those of prokaryotes —the bacteria and

2046-558: The domain of Eukaryota or Eukarya , organisms whose cells have a membrane-bound nucleus . All animals , plants , fungi , and many unicellular organisms are eukaryotes. They constitute a major group of life forms alongside the two groups of prokaryotes : the Bacteria and the Archaea . Eukaryotes represent a small minority of the number of organisms, but given their generally much larger size, their collective global biomass

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2112-560: The genomes of an asexual population tend to accumulate more deleterious mutations over time than beneficial or reversing mutations. Chromosomal crossover involves recombination between the paired chromosomes inherited from each of one's parents, generally occurring during meiosis . During prophase I (pachytene stage) the four available chromatids are in tight formation with one another. While in this formation, homologous sites on two chromatids can closely pair with one another, and may exchange genetic information. Because there

2178-420: The immune system perform genetic recombination, called immunoglobulin class switching . It is a biological mechanism that changes an antibody from one class to another, for example, from an isotype called IgM to an isotype called IgG . In genetic engineering , recombination can also refer to artificial and deliberate recombination of disparate pieces of DNA, often from different organisms, creating what

2244-512: The poliovirus RNA-dependent RNA polymerase (RdRp) is able to carry out recombination. Recombination appears to occur by a copy choice mechanism in which the RdRp switches (+)ssRNA templates during negative strand synthesis. Recombination by RdRp strand switching also occurs in the (+)ssRNA plant carmoviruses and tombusviruses . Recombination appears to be a major driving force in determining genetic variability within coronaviruses, as well as

2310-514: The taxonomic rank of kingdom by Linnaeus in the 18th century. Though he included the fungi with plants with some reservations, it was later realized that they are quite distinct and warrant a separate kingdom. The various single-cell eukaryotes were originally placed with plants or animals when they became known. In 1818, the German biologist Georg A. Goldfuss coined the word Protozoa to refer to organisms such as ciliates , and this group

2376-453: The VNTR blocks to be extracted with restriction enzymes and analyzed by RFLP , or amplified by the polymerase chain reaction (PCR) technique and their size determined by gel electrophoresis . VNTRs were an important source of RFLP genetic markers used in linkage analysis (mapping) of diploid genomes. Now that many genomes have been sequenced , VNTRs have become essential to forensic crime investigations, via DNA fingerprinting and

2442-512: The ability of coronavirus species to jump from one host to another and, infrequently, for the emergence of novel species, although the mechanism of recombination in is unclear. In early 2020, many genomic sequences of Australian SARS‐CoV‐2 isolates have deletions or mutations (29742G>A or 29742G>U; "G19A" or "G19U") in the s2m, suggesting that RNA recombination may have occurred in this RNA element. 29742G("G19"), 29744G("G21"), and 29751G("G28") were predicted as recombination hotspots. During

2508-403: The arms of the chromosomes flanking the recombination event remain in the parental configuration. Thus, explanations for the adaptive function of meiosis that focus exclusively on crossing-over are inadequate to explain the majority of recombination events. Achiasmy is the phenomenon where autosomal recombination is completely absent in one sex of a species. Achiasmatic chromosomal segregation

2574-701: The bacterial RecA protein is RadA. Bacteria regularly undergo genetic recombination in three main ways: Sometimes a strand of DNA is transferred into the target cell but fails to be copied as the target divides. This is called an abortive transfer . In eukaryotes , recombination during meiosis is facilitated by chromosomal crossover . The crossover process leads to offspring having different combinations of genes from those of their parents, and can occasionally produce new chimeric alleles . The shuffling of genes brought about by genetic recombination produces increased genetic variation . It also allows sexually reproducing organisms to avoid Muller's ratchet , in which

2640-400: The case of pathogenic viruses, multiplicity reactivation may be an adaptive benefit to the virus since it allows the repair of DNA damages caused by exposure to the oxidizing environment produced during host infection. See also reassortment . A molecular model for the mechanism of meiotic recombination presented by Anderson and Sekelsky is outlined in the first figure in this article. Two of

2706-709: The cell to move, change shape, or transport materials. The motor structures are microfilaments of actin and actin-binding proteins , including α- actinin , fimbrin , and filamin are present in submembranous cortical layers and bundles. Motor proteins of microtubules, dynein and kinesin , and myosin of actin filaments, provide dynamic character of the network. Many eukaryotes have long slender motile cytoplasmic projections, called flagella , or multiple shorter structures called cilia . These organelles are variously involved in movement, feeding, and sensation. They are composed mainly of tubulin , and are entirely distinct from prokaryotic flagella. They are supported by

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2772-404: The cell with structural support, protection, and a filtering mechanism. The cell wall also prevents over-expansion when water enters the cell. The major polysaccharides making up the primary cell wall of land plants are cellulose , hemicellulose , and pectin . The cellulose microfibrils are linked together with hemicellulose, embedded in a pectin matrix. The most common hemicellulose in

2838-508: The chief recombinase found in Escherichia coli , is responsible for the repair of DNA double strand breaks (DSBs). In yeast and other eukaryotic organisms there are two recombinases required for repairing DSBs. The RAD51 protein is required for mitotic and meiotic recombination, whereas the DNA repair protein, DMC1 , is specific to meiotic recombination. In the archaea, the ortholog of

2904-412: The class of clustered tandem repeats that exhibit allelic variation in their lengths. VNTRs are a type of minisatellite in which the size of the repeat sequence is generally ten to one hundred base pairs. Minisatellites are a type of DNA tandem repeat sequence , meaning that the sequences repeat one after another without other sequences or nucleotides in between them. Minisatellites are characterized by

2970-427: The complex transcription machinery, the membrane-sorting systems, the nuclear pore , and some enzymes in the biochemical pathways. Eukaryote cells include a variety of membrane-bound structures, together forming the endomembrane system. Simple compartments, called vesicles and vacuoles , can form by budding off other membranes. Many cells ingest food and other materials through a process of endocytosis , where

3036-622: The cytoplasm. Mitochondria are organelles in eukaryotic cells. The mitochondrion is commonly called "the powerhouse of the cell", for its function providing energy by oxidising sugars or fats to produce the energy-storing molecule ATP . Mitochondria have two surrounding membranes , each a phospholipid bilayer , the inner of which is folded into invaginations called cristae where aerobic respiration takes place. Mitochondria contain their own DNA , which has close structural similarities to bacterial DNA , from which it originated, and which encodes rRNA and tRNA genes that produce RNA which

3102-406: The eukaryotes. Complex multicellular organisms, not counting the aggregation of amoebae to form slime molds , have evolved within only six eukaryotic lineages: animals , symbiomycotan fungi , brown algae , red algae , green algae , and land plants . Eukaryotes are grouped by genomic similarities, so that groups often lack visible shared characteristics. The defining feature of eukaryotes

3168-484: The eukaryotic evolutionary tree, core meiotic genes, and hence sex, were likely present in the common ancestor of eukaryotes. Species once thought to be asexual, such as Leishmania parasites, have a sexual cycle. Amoebae, previously regarded as asexual, may be anciently sexual; while present-day asexual groups could have arisen recently. In antiquity , the two lineages of animals and plants were recognized by Aristotle and Theophrastus . The lineages were given

3234-624: The first months of the COVID-19 pandemic, such a recombination event was suggested to have been a critical step in the evolution of SARS-CoV-2's ability to infect humans. Linkage disequilibrium analysis confirmed that RNA recombination with the 11083G > T mutation also contributed to the increase of mutations among the viral progeny. The findings indicate that the 11083G > T mutation of SARS-CoV-2 spread during Diamond Princess shipboard quarantine and arose through de novo RNA recombination under positive selection pressure. In three patients on

3300-646: The form of chloroplasts which, like cyanobacteria, contain chlorophyll and produce organic compounds (such as glucose ) through photosynthesis . Others are involved in storing food. Although plastids probably had a single origin, not all plastid-containing groups are closely related. Instead, some eukaryotes have obtained them from others through secondary endosymbiosis or ingestion. The capture and sequestering of photosynthetic cells and chloroplasts, kleptoplasty , occurs in many types of modern eukaryotic organisms. The cytoskeleton provides stiffening structure and points of attachment for motor structures that enable

3366-546: The four chromatids present early in meiosis (prophase I) are paired with each other and able to interact. Recombination, in this model, is initiated by a double-strand break (or gap) shown in the DNA molecule (chromatid) at the top of the figure. Other types of DNA damage may also initiate recombination. For instance, an inter-strand cross-link (caused by exposure to a cross-linking agent such as mitomycin C) can be repaired by HRR. Two types of recombinant product are produced. Indicated on

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3432-494: The human genome, is arranged in a bewildering array of patterns. Repeats were first identified by the extraction of Satellite DNA , which does not reveal how they are organized. The use of restriction enzymes showed that some repeat blocks were interspersed throughout the genome. DNA sequencing later showed that other repeats are clustered at specific locations, with tandem repeats being more common than inverted repeats (which may interfere with DNA replication). VNTRs are

3498-420: The origin of life ( abiogenesis ) is also the origin of biological evolution . They pointed out that all known life on earth is based on biopolymers and proposed that any theory for the origin of life must involve biological polymers that act as information carriers and catalysts. Lehman argued that recombination was an evolutionary development as ancient as the origins of life. Smail et al. proposed that in

3564-449: The outer membrane invaginates and then pinches off to form a vesicle. Some cell products can leave in a vesicle through exocytosis . The nucleus is surrounded by a double membrane known as the nuclear envelope , with nuclear pores that allow material to move in and out. Various tube- and sheet-like extensions of the nuclear membrane form the endoplasmic reticulum , which is involved in protein transport and maturation. It includes

3630-617: The pairing of homologous chromosomes . This may be followed by information transfer between the chromosomes. The information transfer may occur without physical exchange (a section of genetic material is copied from one chromosome to another, without the donating chromosome being changed) (see SDSA – Synthesis Dependent Strand Annealing pathway in Figure); or by the breaking and rejoining of DNA strands, which forms new molecules of DNA (see DHJ pathway in Figure). Recombination may also occur during mitosis in eukaryotes where it ordinarily involves

3696-626: The positive correlation of recombination events over short distances in organisms with a DNA genome (see first Figure, SDSA pathway). Recombination can occur infrequently between animal viruses of the same species but of divergent lineages. The resulting recombinant viruses may sometimes cause an outbreak of infection in humans. Especially in coronaviruses, recombination may also occur even among distantly related evolutionary groups (subgenera), due to their characteristic transcription mechanism, that involves subgenomic mRNAs that are formed by template switching. When replicating its (+)ssRNA genome ,

3762-417: The primary cell wall is xyloglucan . Eukaryotes have a life cycle that involves sexual reproduction , alternating between a haploid phase, where only one copy of each chromosome is present in each cell, and a diploid phase, with two copies of each chromosome in each cell. The diploid phase is formed by fusion of two haploid gametes, such as eggs and spermatozoa , to form a zygote ; this may grow into

3828-480: The primordial Earth, recombination played a key role in the expansion of the initially short informational polymers (presumed to be RNA ) that were the precursors to life. [REDACTED]  This article incorporates public domain material from Science Primer . NCBI . Archived from the original on 2009-12-08. Eukaryote The eukaryotes ( / j uː ˈ k ær i oʊ t s , - ə t s / yoo- KARR -ee-ohts, -⁠əts ) constitute

3894-413: The production of genetic maps . In gene conversion, a section of genetic material is copied from one chromosome to another, without the donating chromosome being changed. Gene conversion occurs at high frequency at the actual site of the recombination event during meiosis . It is a process by which a DNA sequence is copied from one DNA helix (which remains unchanged) to another DNA helix, whose sequence

3960-427: The right side is a "crossover" (CO) type, where the flanking regions of the chromosomes are exchanged, and on the left side, a "non-crossover" (NCO) type where the flanking regions are not exchanged. The CO type of recombination involves the intermediate formation of two "Holliday junctions" indicated in the lower right of the figure by two X-shaped structures in each of which there is an exchange of single strands between

4026-426: The risk of cancer (see DNA repair-deficiency disorder ). In bacteria, transformation is a process of gene transfer that ordinarily occurs between individual cells of the same bacterial species. Transformation involves integration of donor DNA into the recipient chromosome by recombination. This process appears to be an adaptation for repairing DNA damages in the recipient chromosome by HRR. Transformation may provide

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4092-498: The rough endoplasmic reticulum, covered in ribosomes which synthesize proteins; these enter the interior space or lumen. Subsequently, they generally enter vesicles, which bud off from the smooth endoplasmic reticulum. In most eukaryotes, these protein-carrying vesicles are released and further modified in stacks of flattened vesicles ( cisternae ), the Golgi apparatus . Vesicles may be specialized; for instance, lysosomes contain digestive enzymes that break down biomolecules in

4158-430: The same host cell. Recombination is largely responsible for RNA virus diversity and immune evasion. RNA recombination appears to be a major driving force in determining genome architecture and the course of viral evolution among picornaviridae ( (+)ssRNA ) (e.g. poliovirus ). In the retroviridae ((+)ssRNA)(e.g. HIV ), damage in the RNA genome appears to be avoided during reverse transcription by strand switching,

4224-585: The two participating chromatids. This pathway is labeled in the figure as the DHJ (double-Holliday junction) pathway. The NCO recombinants (illustrated on the left in the figure) are produced by a process referred to as "synthesis dependent strand annealing" (SDSA). Recombination events of the NCO/SDSA type appear to be more common than the CO/DHJ type. The NCO/SDSA pathway contributes little to genetic variation, since

4290-509: The two sister chromosomes formed after chromosomal replication. In this case, new combinations of alleles are not produced since the sister chromosomes are usually identical. In meiosis and mitosis, recombination occurs between similar molecules of DNA ( homologous sequences ). In meiosis, non-sister homologous chromosomes pair with each other so that recombination characteristically occurs between non-sister homologues. In both meiotic and mitotic cells, recombination between homologous chromosomes

4356-503: Was expanded until Ernst Haeckel made it a kingdom encompassing all single-celled eukaryotes, the Protista , in 1866. The eukaryotes thus came to be seen as four kingdoms: The protists were at that time thought to be "primitive forms", and thus an evolutionary grade , united by their primitive unicellular nature. Understanding of the oldest branchings in the tree of life only developed substantially with DNA sequencing , leading to

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