Caenorhabditis elegans is a free-living, transparent nematode, about 1 mm in length, that lives in temperate soil environments. It is the type species of its genus; the name is rhabditis and Latin elegans. In 1900, Maupas named it Rhabditides elegans, Osche placed it in the subgenus Caenorhabditis in 1952, in 1955, Dougherty raised Caenorhabditis to the status of genus. C. Elegans lacks respiratory or circulatory systems. Most of these nematodes are hermaphrodites and a few are males. Males have specialised tails for mating. In 1963, Sydney Brenner proposed research into C. elegans in the area of neuronal development. In 1974, he began research into the molecular and developmental biology of C. elegans, which has since been extensively used as a model organism. It was the first multicellular organism to have its whole genome sequenced, as of 2012, is the only organism to have its connectome completed. C. elegans is unsegmented and bilaterally symmetrical. It has a cuticle, four main epidermal cords, a fluid-filled pseudocoelom.
It has some of the same organ systems as larger animals. About one in a thousand individuals is male and the rest are hermaphrodites; the basic anatomy of C. elegans includes a mouth, intestine and collagenous cuticle. Like all nematodes, they have neither a respiratory system; the four bands of muscles that run the length of the body are connected to a neural system that allows the muscles to move the animal's body only as dorsal bending or ventral bending, but not left or right, except for the head, where the four muscle quadrants are wired independently from one another. When a wave of dorsal/ventral muscle contractions proceeds from the back to the front of the animal, the animal is propelled backwards; when a wave of contractions is initiated at the front and proceeds posteriorly along the body, the animal is propelled forwards. Because of this dorsal/ventral bias in body bends, any normal living, moving individual tends to lie on either its left side or its right side when observed crossing a horizontal surface.
A set of ridges on the lateral sides of the body cuticle, the alae, is believed to give the animal added traction during these bending motions. In relation to lipid metabolism, C. elegans does not have any specialized adipose tissues, a pancreas, a liver, or blood to deliver nutrients compared to mammals. Neutral lipids are instead stored in the intestine and embryos; the epidermis corresponds to the mammalian adipocytes by being the main triglyceride depot. The pharynx is a muscular food pump in the head of C. elegans, triangular in cross-section. This transports it directly to the intestine. A set of "valve cells" connects the pharynx to the intestine, but how this valve operates is not understood. After digestion, the contents of the intestine are released via the rectum, as is the case with all other nematodes. No direct connection exists between the pharynx and the excretory canal, which functions in the release of liquid urine. Males have a single-lobed gonad, a vas deferens, a tail specialized for mating, which incorporates spicules.
Hermaphrodites have two ovaries and spermatheca, a single uterus. Numerous gut granules are present in the intestine of C. elegans, the functions of which are still not known, as are many other aspects of this nematode, despite the many years that it has been studied. These gut granules are found in all of the Rhabditida orders, they are similar to lysosomes in that they feature an acidic interior and the capacity for endocytosis, but they are larger, reinforcing the view of their being storage organelles. A remarkable feature of the granules is that when they are observed under ultraviolet light, they react by emitting an intense blue fluorescence. Another phenomenon seen is termed'death fluorescence'; as the worms die, a dramatic burst of blue fluorescence is emitted. This death fluorescence takes place in an anterior to posterior wave that moves along the intestine, is seen in both young and old worms, whether subjected to lethal injury or peacefully dying of old age. Many theories have been posited on the functions of the gut granules, with earlier ones being eliminated by findings.
They are thought to store zinc as one of their functions. Recent chemical analysis has identified the blue fluorescent material they contain as a glycosylated form of anthranilic acid; the need for the large amounts of AA the many gut granules contain is questioned. One possibility is. Another possibility is; this is seen a possible link to the melanin–containing melanosomes. The hermaphroditic worm is considered to be a specialized form of self-fertile female, as its soma is female; the hermaphroditic germline produces male gametes first, lays eggs through its uterus after internal fertilization. Hermaphrodites produce all their sperm in the L4 stage and produce only oocytes; the hermaphroditic gonad acts as an ovotestis with sperm cells being stored in the same area of the gonad as the oocytes until the first oocyte pushes the sperm into the spermatheca. The male can inseminate the hermaphrodite; the sperm of
A chromosome is a deoxyribonucleic acid molecule with part or all of the genetic material of an organism. Most eukaryotic chromosomes include packaging proteins which, aided by chaperone proteins, bind to and condense the DNA molecule to prevent it from becoming an unmanageable tangle. Chromosomes are visible under a light microscope only when the cell is undergoing the metaphase of cell division. Before this happens, every chromosome is copied once, the copy is joined to the original by a centromere, resulting either in an X-shaped structure if the centromere is located in the middle of the chromosome or a two-arm structure if the centromere is located near one of the ends; the original chromosome and the copy are now called sister chromatids. During metaphase the X-shape structure is called a metaphase chromosome. In this condensed form chromosomes are easiest to distinguish and study. In animal cells, chromosomes reach their highest compaction level in anaphase during chromosome segregation.
Chromosomal recombination during meiosis and subsequent sexual reproduction play a significant role in genetic diversity. If these structures are manipulated incorrectly, through processes known as chromosomal instability and translocation, the cell may undergo mitotic catastrophe; this will make the cell initiate apoptosis leading to its own death, but sometimes mutations in the cell hamper this process and thus cause progression of cancer. Some use the term chromosome in a wider sense, to refer to the individualized portions of chromatin in cells, either visible or not under light microscopy. Others use the concept in a narrower sense, to refer to the individualized portions of chromatin during cell division, visible under light microscopy due to high condensation; the word chromosome comes from the Greek χρῶμα and σῶμα, describing their strong staining by particular dyes. The term was coined by von Waldeyer-Hartz, referring to the term chromatin, introduced by Walther Flemming; some of the early karyological terms have become outdated.
For example and Chromosom, both ascribe color to a non-colored state. The German scientists Schleiden, Virchow and Bütschli were among the first scientists who recognized the structures now familiar as chromosomes. In a series of experiments beginning in the mid-1880s, Theodor Boveri gave the definitive demonstration that chromosomes are the vectors of heredity, it is the second of these principles, so original. Wilhelm Roux suggested. Boveri was able to confirm this hypothesis. Aided by the rediscovery at the start of the 1900s of Gregor Mendel's earlier work, Boveri was able to point out the connection between the rules of inheritance and the behaviour of the chromosomes. Boveri influenced two generations of American cytologists: Edmund Beecher Wilson, Nettie Stevens, Walter Sutton and Theophilus Painter were all influenced by Boveri. In his famous textbook The Cell in Development and Heredity, Wilson linked together the independent work of Boveri and Sutton by naming the chromosome theory of inheritance the Boveri–Sutton chromosome theory.
Ernst Mayr remarks that the theory was hotly contested by some famous geneticists: William Bateson, Wilhelm Johannsen, Richard Goldschmidt and T. H. Morgan, all of a rather dogmatic turn of mind. Complete proof came from chromosome maps in Morgan's own lab; the number of human chromosomes was published in 1923 by Theophilus Painter. By inspection through the microscope, he counted 24 pairs, his error was copied by others and it was not until 1956 that the true number, 46, was determined by Indonesia-born cytogeneticist Joe Hin Tjio. The prokaryotes – bacteria and archaea – have a single circular chromosome, but many variations exist; the chromosomes of most bacteria, which some authors prefer to call genophores, can range in size from only 130,000 base pairs in the endosymbiotic bacteria Candidatus Hodgkinia cicadicola and Candidatus Tremblaya princeps, to more than 14,000,000 base pairs in the soil-dwelling bacterium Sorangium cellulosum. Spirochaetes of the genus Borrelia are a notable exception to this arrangement, with bacteria such as Borrelia burgdorferi, the cause of Lyme disease, containing a single linear chromosome.
Prokaryotic chromosomes have less sequence-based structure than eukaryotes. Bacteria have a one-point from which replication starts, whereas some archaea contain multiple replication origins; the genes in prokaryotes are organized in operons, do not contain introns, unlike eukaryotes. Prokaryotes do not possess nuclei. Instead, their DNA is organized into a structure called the nucleoid; the nucleoid occupies a defined region of the bacterial cell. This structure is, dynamic and is maintained and remodeled by the actions of a range of histone-like proteins, which associate with the bacterial chromosome. In archaea, the DNA in chromosomes is more organized, with the DNA packaged within structures similar to eukaryotic nucleosomes. Certain bacteria contain plasmids or other extrachromosomal DNA; these are circular structures in the cytoplasm that contain cellular DNA and play a role in horizontal gene transfer. In prokaryotes and viruses, the DNA is densely packed and organized.
Mammals are vertebrate animals constituting the class Mammalia, characterized by the presence of mammary glands which in females produce milk for feeding their young, a neocortex, fur or hair, three middle ear bones. These characteristics distinguish them from reptiles and birds, from which they diverged in the late Triassic, 201–227 million years ago. There are around 5,450 species of mammals; the largest orders are the rodents and Soricomorpha. The next three are the Primates, the Cetartiodactyla, the Carnivora. In cladistics, which reflect evolution, mammals are classified as endothermic amniotes, they are the only living Synapsida. The early synapsid mammalian ancestors were sphenacodont pelycosaurs, a group that produced the non-mammalian Dimetrodon. At the end of the Carboniferous period around 300 million years ago, this group diverged from the sauropsid line that led to today's reptiles and birds; the line following the stem group Sphenacodontia split off several diverse groups of non-mammalian synapsids—sometimes referred to as mammal-like reptiles—before giving rise to the proto-mammals in the early Mesozoic era.
The modern mammalian orders arose in the Paleogene and Neogene periods of the Cenozoic era, after the extinction of non-avian dinosaurs, have been among the dominant terrestrial animal groups from 66 million years ago to the present. The basic body type is quadruped, most mammals use their four extremities for terrestrial locomotion. Mammals range in size from the 30–40 mm bumblebee bat to the 30-meter blue whale—the largest animal on the planet. Maximum lifespan varies from two years for the shrew to 211 years for the bowhead whale. All modern mammals give birth to live young, except the five species of monotremes, which are egg-laying mammals; the most species-rich group of mammals, the cohort called placentals, have a placenta, which enables the feeding of the fetus during gestation. Most mammals are intelligent, with some possessing large brains, self-awareness, tool use. Mammals can communicate and vocalize in several different ways, including the production of ultrasound, scent-marking, alarm signals and echolocation.
Mammals can organize themselves into fission-fusion societies and hierarchies—but can be solitary and territorial. Most mammals are polygynous. Domestication of many types of mammals by humans played a major role in the Neolithic revolution, resulted in farming replacing hunting and gathering as the primary source of food for humans; this led to a major restructuring of human societies from nomadic to sedentary, with more co-operation among larger and larger groups, the development of the first civilizations. Domesticated mammals provided, continue to provide, power for transport and agriculture, as well as food and leather. Mammals are hunted and raced for sport, are used as model organisms in science. Mammals have been depicted in art since Palaeolithic times, appear in literature, film and religion. Decline in numbers and extinction of many mammals is driven by human poaching and habitat destruction deforestation. Mammal classification has been through several iterations since Carl Linnaeus defined the class.
No classification system is universally accepted. George Gaylord Simpson's "Principles of Classification and a Classification of Mammals" provides systematics of mammal origins and relationships that were universally taught until the end of the 20th century. Since Simpson's classification, the paleontological record has been recalibrated, the intervening years have seen much debate and progress concerning the theoretical underpinnings of systematization itself through the new concept of cladistics. Though field work made Simpson's classification outdated, it remains the closest thing to an official classification of mammals. Most mammals, including the six most species-rich orders, belong to the placental group; the three largest orders in numbers of species are Rodentia: mice, porcupines, beavers and other gnawing mammals. The next three biggest orders, depending on the biological classification scheme used, are the Primates including the apes and lemurs. According to Mammal Species of the World, 5,416 species were identified in 2006.
These were grouped into 153 families and 29 orders. In 2008, the International Union for Conservation of Nature completed a five-year Global Mammal Assessment for its IUCN Red List, which counted 5,488 species. According to a research published in the Journal of Mammalogy in 2018, the number of recognized mammal species is 6,495 species included 96 extinct; the word "mammal" is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the Latin mamma. In an influential 1988 paper, Timothy Rowe defined Mammalia phylogenetically as the crown group of mammals, the clade consisting of the most recent common ancestor of living monotremes and therian m
Drosophila melanogaster is a species of fly in the family Drosophilidae. The species is known as the common fruit fly or vinegar fly. Starting with Charles W. Woodworth's proposal of the use of this species as a model organism, D. melanogaster continues to be used for biological research in genetics, microbial pathogenesis, life history evolution. As of 2017, eight Nobel prizes had been awarded for research using Drosophila. D. Melanogaster is used in research because it can be reared in the laboratory, has only four pairs of chromosomes and lays many eggs, its geographic range includes all continents, including islands. D. melanogaster is a common pest in homes and other places where food is served. Flies belonging to the family Tephritidae are called "fruit flies"; this can cause confusion in the Mediterranean and South Africa, where the Mediterranean fruit fly Ceratitis capitata is an economic pest. Wildtype fruit flies are yellow-brown, with brick-red eyes and transverse black rings across the abdomen.
They exhibit sexual dimorphism. Males are distinguished from females based on colour differences, with a distinct black patch at the abdomen, less noticeable in emerged flies, the sexcombs. Furthermore, males have a cluster of spiky hairs surrounding the reproducing parts used to attach to the female during mating. Extensive images are found at FlyBase. Under optimal growth conditions at 25 °C, the D. melanogaster lifespan is about 50 days from egg to death. The developmental period for D. melanogaster varies with temperature, as with many ectothermic species. The shortest development time, 7 days, is achieved at 28 °C. Development times increase at higher temperatures due to heat stress. Under ideal conditions, the development time at 25 °C is 8.5 days, at 18 °C it takes 19 days and at 12 °C it takes over 50 days. Under crowded conditions, development time increases. Females lay some 400 eggs, about five at a time, into rotting fruit or other suitable material such as decaying mushrooms and sap fluxes.
The eggs, which are about 0.5 mm long, hatch after 12–15 hours. The resulting larvae grow for about 4 days while molting twice, at about 48 h after hatching. During this time, they feed on the microorganisms that decompose the fruit, as well as on the sugar of the fruit itself; the mother puts feces on the egg sacs to establish the same microbial composition in the larvae's guts that has worked positively for herself. The larvae encapsulate in the puparium and undergo a 4-day-long metamorphosis, after which the adults eclose; the female fruit fly prefers a shorter duration. Males, prefer it to last longer. Males perform a sequence of five behavioral patterns to court females. First, males orient themselves while playing a courtship song by horizontally extending and vibrating their wings. Soon after, the male positions himself at the rear of the female's abdomen in a low posture to tap and lick the female genitalia; the male curls his abdomen and attempts copulation. Females can reject males by moving away and extruding their ovipositor.
Copulation lasts around 15–20 minutes, during which males transfer a few hundred long sperm cells in seminal fluid to the female. Females store the sperm in two mushroom-shaped spermathecae. A last male precedence is believed to exist; this precedence was found to occur through both incapacitation. The displacement is attributed to sperm handling by the female fly as multiple matings are conducted and is most significant during the first 1–2 days after copulation. Displacement from the seminal receptacle is more significant than displacement from the spermathecae. Incapacitation of first male sperm by second male sperm becomes significant 2–7 days after copulation; the seminal fluid of the second male is believed to be responsible for this incapacitation mechanism which takes effect before fertilization occurs. The delay in effectiveness of the incapacitation mechanism is believed to be a protective mechanism that prevents a male fly from incapacitating his own sperm should he mate with the same female fly repetitively.
Sensory neurons in the uterus of female D. melanogaster respond to a male protein, sex peptide, found in sperm. This protein makes the female reluctant to copulate for about 10 days after insemination; the signal pathway leading to this change in behavior has been determined. The signal is sent to a brain region, a homolog of the hypothalamus and the hypothalamus controls sexual behavior and desire. Gonadotropic hormones in Drosophila maintain homeostasis and govern reproductive output via a cyclic interrelationship, not unlike the mammalian estrous cycle. Sex Peptide perturbs this homeostasis and shifts the endocrine state of the female by inciting juvenile hormone synthesis in the corpus allatum. D. Melanogaster is used for life extension studies, such as to identify genes purported to increase lifespan when mutated. Females become receptive to courting males about 8–12 hours after emergence. Specific neuron groups in females have been found to affect copulation behavior a
In the nervous system, a synapse is a structure that permits a neuron to pass an electrical or chemical signal to another neuron or to the target effector cell. Santiago Ramón y Cajal proposed that neurons are not continuous throughout the body, yet still communicate with each other, an idea known as the neuron doctrine; the word "synapse" – from the Greek synapsis, meaning "conjunction", in turn from συνάπτεὶν – was introduced in 1897 by the English neurophysiologist Charles Sherrington in Michael Foster's Textbook of Physiology. Sherrington struggled to find a good term that emphasized a union between two separate elements, the actual term "synapse" was suggested by the English classical scholar Arthur Woollgar Verrall, a friend of Foster; some authors generalize the concept of the synapse to include the communication from a neuron to any other cell type, such as to a motor cell, although such non-neuronal contacts may be referred to as junctions. Synapses are essential to neuronal function: neurons are cells that are specialized to pass signals to individual target cells, synapses are the means by which they do so.
At a synapse, the plasma membrane of the signal-passing neuron comes into close apposition with the membrane of the target cell. Both the presynaptic and postsynaptic sites contain extensive arrays of a molecular machinery that link the two membranes together and carry out the signaling process. In many synapses, the presynaptic part is located on an axon and the postsynaptic part is located on a dendrite or soma. Astrocytes exchange information with the synaptic neurons, responding to synaptic activity and, in turn, regulating neurotransmission. Synapses are stabilized in position by synaptic adhesion molecules projecting from both the pre- and post-synaptic neuron and sticking together where they overlap. There are two fundamentally different types of synapses: In a chemical synapse, electrical activity in the presynaptic neuron is converted into the release of a chemical called a neurotransmitter that binds to receptors located in the plasma membrane of the postsynaptic cell; the neurotransmitter may initiate an electrical response or a secondary messenger pathway that may either excite or inhibit the postsynaptic neuron.
Chemical synapses can be classified according to the neurotransmitter released: glutamatergic, GABAergic and adrenergic. Because of the complexity of receptor signal transduction, chemical synapses can have complex effects on the postsynaptic cell. In an electrical synapse, the presynaptic and postsynaptic cell membranes are connected by special channels called gap junctions or synaptic cleft that are capable of passing an electric current, causing voltage changes in the presynaptic cell to induce voltage changes in the postsynaptic cell; the main advantage of an electrical synapse is the rapid transfer of signals from one cell to the next. Synaptic communication is distinct from an ephaptic coupling, in which communication between neurons occurs via indirect electric fields. An autapse is a chemical or electrical synapse that forms when the axon of one neuron synapses onto dendrites of the same neuron. Synapses can be classified by the type of cellular structures serving as the pre- and post-synaptic components.
The vast majority of synapses in the mammalian nervous system are classical axo-dendritic synapses, however, a variety of other arrangements exist. These include but are not limited to axo-axonic, dendro-dendritic, axo-secretory, somato-dendritic, dendro-somatic, somato-somatic synapses; the axon can synapse onto a dendrite, onto a cell body, or onto another axon or axon terminal, as well as into the bloodstream or diffusely into the adjacent nervous tissue. It is accepted that the synapse plays a role in the formation of memory; as neurotransmitters activate receptors across the synaptic cleft, the connection between the two neurons is strengthened when both neurons are active at the same time, as a result of the receptor's signaling mechanisms. The strength of two connected neural pathways is thought to result in the storage of information, resulting in memory; this process of synaptic strengthening is known as long-term potentiation. By altering the release of neurotransmitters, the plasticity of synapses can be controlled in the presynaptic cell.
The postsynaptic cell can be regulated by altering the number of its receptors. Changes in postsynaptic signaling are most associated with a N-methyl-d-aspartic acid receptor -dependent long-term potentiation and long-term depression due to the influx of calcium into the post-synaptic cell, which are the most analyzed forms of plasticity at excitatory synapses. For technical reasons, synaptic structure and function have been studied at unusually large model synapses, for example: Squid giant synapse Neuromuscular junction, a cholinergic synapse in vertebrates, glutamatergic in insects Ciliary calyx in the ciliary ganglion of chicks Calyx of Held in the brainstem Ribbon synapse in the retina Schaffer collateral synapse in the hippocampus The function of neurons depends upon cell polarity; the distinctive structure of nerve cells allows action potentials to travel directionally, for these signals to be received and carried on by post-synaptic neurons or received by effector cells. Nerve cells have long been used as models f
Chromosomal crossover is the exchange of genetic material between two homologous chromosomes non-sister chromatids that results in recombinant chromosomes during sexual reproduction. 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 occurs when matching regions on matching chromosomes break and reconnect to the other chromosome. Crossing over was described, in theory, by Thomas Hunt Morgan, he relied on the discovery of Frans Alfons Janssens who described the phenomenon in 1909 and had called it "chiasmatypie". The term chiasma is linked, to chromosomal crossover. Morgan saw the great importance of Janssens' cytological interpretation of chiasmata to the experimental results of his research on the heredity of Drosophila; the physical basis of crossing over was first demonstrated by Harriet Creighton and Barbara McClintock in 1931.
The linked frequency of crossing over between two gene loci is the crossing-over value. For fixed set of genetic and environmental conditions, recombination in a particular region of a linkage structure tends to be constant and the same is true for the crossing-over value, used in the production of genetic maps. There are two popular and overlapping theories that explain the origins of crossing-over, coming from the different theories on the origin of meiosis; the first theory rests upon the idea that meiosis evolved as another method of DNA repair, thus crossing-over is a novel way to replace damaged sections of DNA. The second theory comes from the idea that meiosis evolved from bacterial transformation, with the function of propagating diversity. In 1931, Barbara McClintock discovered a triploid maize plant, she made key findings regarding corn's karyotype, including the shape of the chromosomes. McClintock used the prophase and metaphase stages of mitosis to describe the morphology of corn's chromosomes, showed the first cytological demonstration of crossing over in meiosis.
Working with student Harriet Creighton, McClintock made significant contributions to the early understanding of codependency of linked genes. Crossing over and DNA repair are similar processes, which utilize many of the same protein complexes. In her report, “The Significance of Responses of the Genome to Challenge”, McClintock studied corn to show how corn's genome would change itself to overcome threats to its survival, she used 450 self-pollinated plants. She used modified patterns of gene expression on different sectors of leaves of her corn plants show that transposable elements hide in the genome, their mobility allows them to alter the action of genes at different loci; these elements can restructure the genome, anywhere from a few nucleotides to whole segments of chromosome. Recombinases and primases lay a foundation of nucleotides along the DNA sequence. One such particular protein complex, conserved between processes is RAD51, a well conserved recombinase protein, shown to be crucial in DNA repair as well as cross over.
Several other genes in D. melanogaster have been linked as well to both processes, by showing that mutants at these specific loci cannot undergo DNA repair or crossing over. Such genes include mei-41, mei-9, spnA, brca2; this large group of conserved genes between processes supports the theory of a close evolutionary relationship. Furthermore, DNA repair and crossover have been found to favor similar regions on chromosomes. In an experiment using radiation hybrid mapping on wheat's 3B chromosome, crossing over and DNA repair were found to occur predominantly in the same regions. Furthermore, crossing over has been correlated to occur in response to stressful, DNA damaging, conditions The process of bacterial transformation shares many similarities with chromosomal cross over in the formation of overhangs on the sides of the broken DNA strand, allowing for the annealing of a new strand. Bacterial transformation itself has been linked to DNA repair many times; the second theory comes from the idea that meiosis evolved from bacterial transformation, with the function of propagating genetic diversity..
Thus, this evidence suggests that it is a question of whether cross over is linked to DNA repair or bacterial transformation, as the two do not appear to be mutually exclusive. It is that crossing over may have evolved from bacterial transformation, which in turn developed from DNA repair, thus explaining the links between all three processes. Meiotic recombination may be initiated by double-stranded breaks that are introduced into the DNA by exposure to DNA damaging agents or the Spo11 protein. One or more exonucleases digest the 5’ ends generated by the double-stranded breaks to produce 3’ single-stranded DNA tails; the meiosis-specific recombinase Dmc1 and the general recombinase Rad51 coat the single-stranded DNA to form nucleoprotein filaments. The recombinases catalyze invasion of the opposite chromatid by the single-stranded DNA from one end of the break. Next, the 3’ end of the invading DNA primes DNA synthesis, causing displacement of the complementary strand, which subsequently anneals to the single-stranded DNA generated from the other end of the initial double-stranded break.
The structure that results is a cross-strand exchange known as a Holliday junction. The contact between two chromatids that will soon undergo crossing-over is known as a chiasma; the Holliday junction is a