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123-621: The first settlements in the region date from the Pre-Columbian era . Nearby are the ruins of the Mayan city of Xcalak, from which the town derives its name. In May 1900, the Mexican Navy established the villa of Xcalak as a control post for the region. There the first shipyard of the Mexican Caribbean was constructed. During the twentieth century, the area became a major population area, with

246-433: A DNA repair process, and that when it occurs during meiosis it is an adaptation for repairing the genomic DNA that is passed on to progeny. Experimental findings indicate that a substantial benefit of meiosis is recombinational repair of DNA damage in the germline , as indicated by the following examples. Hydrogen peroxide is an agent that causes oxidative stress leading to oxidative DNA damage. Treatment of

369-493: A barrel shaped spindle. In human oocytes spindle microtubule nucleation begins on the chromosomes, forming an aster that eventually expands to surround the chromosomes. Chromosomes then slide along the microtubules towards the equator of the spindle, at which point the chromosome kinetochores form end-on attachments to microtubules. Homologous pairs move together along the metaphase plate: As kinetochore microtubules from both spindle poles attach to their respective kinetochores,

492-465: A common intestinal parasite, was previously considered to have descended from a lineage that predated the emergence of meiosis and sex. However, G. intestinalis has now been found to possess a core set of meiotic genes, including five meiosis specific genes. Also evidence for meiotic recombination , indicative of sexual reproduction , was found in G. intestinalis . Another example of organisms previously thought to be asexual are parasitic protozoa of

615-591: A different social structure. Until the accurate dating of Watson Brake and similar sites, the oldest mound complex was thought to be Poverty Point , also located in the Lower Mississippi Valley . Built about 1500 BCE, it is the centerpiece of a culture extending over 100 sites on both sides of the Mississippi . The Poverty Point site has earthworks in the form of six concentric half-circles, divided by radial aisles, together with some mounds. The entire complex

738-433: A diploid cell called the zygote . The organism's diploid germ-line stem cells undergo meiosis to make haploid gametes (the spermatozoa in males and ova in females), which fertilize to form the zygote. The diploid zygote undergoes repeated cellular division by mitosis to grow into the organism. In the haplontic life cycle (with post-zygotic meiosis), the organism is haploid, by the proliferation and differentiation of

861-406: A diploid cell, which contains two copies of each chromosome, termed homologs . First, the cell undergoes DNA replication , so each homolog now consists of two identical sister chromatids. Then each set of homologs pair with each other and exchange genetic information by homologous recombination often leading to physical connections ( crossovers ) between the homologs. In the first meiotic division,

984-419: A male and a female will fuse to create a zygote , a cell with two copies of each chromosome again. Errors in meiosis resulting in aneuploidy (an abnormal number of chromosomes) are the leading known cause of miscarriage and the most frequent genetic cause of developmental disabilities . In meiosis, DNA replication is followed by two rounds of cell division to produce four daughter cells, each with half

1107-407: A new combination of maternal and paternal genetic information, resulting in offspring that are genetically distinct from either parent. Furthermore, an individual gamete can include an assortment of maternal, paternal, and recombinant chromatids. This genetic diversity resulting from sexual reproduction contributes to the variation in traits upon which natural selection can act. Meiosis uses many of

1230-400: A new diploid organism. The haplodiplontic life cycle can be considered a fusion of the diplontic and haplontic life cycles. Meiosis occurs in all animals and plants. The result, the production of gametes with half the number of chromosomes as the parent cell, is the same, but the detailed process is different. In animals, meiosis produces gametes directly. In land plants and some algae, there

1353-402: A pair of sister chromatids) to opposite poles. Nonkinetochore microtubules lengthen, pushing the centrosomes farther apart. The cell elongates in preparation for division down the center. Unlike in mitosis, only the cohesin from the chromosome arms is degraded while the cohesin surrounding the centromere remains protected by a protein named Shugoshin (Japanese for "guardian spirit"), what prevents

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1476-420: A period of rest known as interkinesis or interphase II. No DNA replication occurs during this stage. Meiosis II is the second meiotic division, and usually involves equational segregation, or separation of sister chromatids. Mechanically, the process is similar to mitosis, though its genetic results are fundamentally different. The result is the production of four haploid cells (n chromosomes; 23 in humans) from

1599-730: A population of over 20,000. Other chiefdoms were constructed throughout the Southeast, and its trade networks reached to the Great Lakes and the Gulf of Mexico. At its peak, between the 12th and 13th centuries, Cahokia was the most populous city in North America. (Larger cities did exist in Mesoamerica and the Andes.) Monks Mound , the major ceremonial center of Cahokia, remains the largest earthen construction of

1722-423: A prolonged G 2 -like stage known as meiotic prophase . During this time, homologous chromosomes pair with each other and undergo genetic recombination , a programmed process in which DNA may be cut and then repaired, which allows them to exchange some of their genetic information . A subset of recombination events results in crossovers , which create physical links known as chiasmata (singular: chiasma, for

1845-616: A significant production of coconut from coco palm plantations that reached up to 300 tons monthly. In 1955, the city was whipped by Hurricane Janet, which destroyed a large part of the town and scattered the population to other areas. Xcalak was the largest city in Quintana Roo until it was destroyed by Hurricane Janet in 1955. Hurricane Dean struck Majahual, only 50 kilometers north of Xcalak in 2007. 18°16′17″N 87°50′09″W  /  18.27139°N 87.83583°W  / 18.27139; -87.83583 Pre-Columbian era In

1968-399: A single haploid cell called the gamete . Two organisms of opposing sex contribute their haploid gametes to form a diploid zygote. The zygote undergoes meiosis immediately, creating four haploid cells. These cells undergo mitosis to create the organism. Many fungi and many protozoa utilize the haplontic life cycle. In the haplodiplontic life cycle (with sporic or intermediate meiosis),

2091-570: A single unified empire. The Mixtecs would eventually be conquered by the Aztecs until the Spanish conquest. The Mixtecs saw the Spanish conquest as an opportunity for liberation and established agreements with the conquistadors that allowed them to preserve their cultural traditions, though relatively few sections resisted Spanish rule. The Totonac civilization was concentrated in the present-day states of Veracruz and Puebla . The Totonacs were responsible for

2214-448: A total of four daughter cells, each with a haploid set of chromosomes. Meiosis is now complete and ends up with four new daughter cells. Meiosis appears to be a fundamental characteristic of eukaryotic organisms and to have been present early in eukaryotic evolution. Eukaryotes that were once thought to lack meiotic sex have recently been shown to likely have, or once have had, this capability. As one example, Giardia intestinalis ,

2337-400: Is an alternation of generations such that meiosis in the diploid sporophyte generation produces haploid spores instead of gametes. When they germinate, these spores undergo repeated cell division by mitosis, developing into a multicellular haploid gametophyte generation, which then produces gametes directly (i.e. without further meiosis). In both animals and plants, the final stage is for

2460-423: Is cleaved, allowing the sister chromatids to segregate. The sister chromatids by convention are now called sister chromosomes as they move toward opposing poles. The process ends with telophase II , which is similar to telophase I, and is marked by decondensation and lengthening of the chromosomes and the disassembly of the spindle. Nuclear envelopes re-form and cleavage or cell plate formation eventually produces

2583-673: Is currently divided into two general approaches. The first is the short chronology theory with the first movement beyond Alaska into the Americas occurring no earlier than 14,000–17,000 years ago, followed by successive waves of immigrants. The second belief is the long chronology theory , which proposes that the first group of people entered the hemisphere at a much earlier date, possibly 50,000–40,000 years ago or earlier. Artifacts have been found in both North and South America which have been dated to 14,000 years ago, and accordingly humans have been proposed to have reached Cape Horn at

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2706-520: Is divided into meiosis I and meiosis II which are further divided into Karyokinesis I, Cytokinesis I, Karyokinesis II, and Cytokinesis II, respectively. The preparatory steps that lead up to meiosis are identical in pattern and name to interphase of the mitotic cell cycle. Interphase is divided into three phases: Interphase is followed by meiosis I and then meiosis II. Meiosis I separates replicated homologous chromosomes, each still made up of two sister chromatids, into two daughter cells, thus reducing

2829-578: Is likely mediated by oxidative stress leading to increased DNA damage. Meiosis occurs in eukaryotic life cycles involving sexual reproduction , consisting of the cyclical process of growth and development by mitotic cell division, production of gametes by meiosis and fertilization. At certain stages of the life cycle, germ cells produce gametes. Somatic cells make up the body of the organism and are not involved in gamete production. Cycling meiosis and fertilization events results in alternation between haploid and diploid states. The organism phase of

2952-411: Is nearly a mile across. Mound building was continued by succeeding cultures, who built numerous sites in the middle Mississippi and Ohio River valleys as well, adding effigy mounds , conical and ridge mounds, and other shapes. The Woodland period of North American pre-Columbian cultures lasted from roughly 1000 BCE to 1000 CE. The term was coined in the 1930s and refers to prehistoric sites between

3075-522: Is reflected in the oral histories of the indigenous peoples, described by a wide range of traditional creation stories which often say that a given people have been living in a certain territory since the creation of the world. Throughout thousands of years, paleo-Indian people domesticated, bred, and cultivated many plant species, including crops that now constitute 50–60% of worldwide agriculture. In general, Arctic, Subarctic, and coastal peoples continued to live as hunters and gatherers, while agriculture

3198-578: Is the Olmec. This civilization established the cultural blueprint by which all succeeding indigenous civilizations would follow in Mexico. Pre-Olmec civilization began with the production of pottery in abundance, around 2300 BCE in the Grijalva River delta. Between 1600 and 1500 BCE, the Olmec civilization had begun, with the consolidation of power at their capital, a site today known as San Lorenzo Tenochtitlán near

3321-617: Is the determinant factor for the number of gene lineages and founding haplotypes present in today's Indigenous populations . Human settlement of the Americas occurred in stages from the Bering Sea coastline , with an initial 20,000-year layover on Beringia for the founding population . The microsatellite diversity and distributions of the Y lineage specific to South America indicate that certain Amerindian populations have been isolated since

3444-449: Is the stage at which all autosomal chromosomes have synapsed. In this stage homologous recombination, including chromosomal crossover (crossing over), is completed through the repair of the double strand breaks formed in leptotene. Most breaks are repaired without forming crossovers resulting in gene conversion . However, a subset of breaks (at least one per chromosome) form crossovers between non-sister (homologous) chromosomes resulting in

3567-678: The Americas via the Bering Land Bridge (Beringia), now the Bering Strait , and possibly along the coast. Genetic evidence found in Indigenous peoples ' maternally inherited mitochondrial DNA (mtDNA) supports the theory of multiple genetic populations migrating from Asia. After crossing the land bridge, they moved southward along the Pacific coast and through an interior ice-free corridor. Throughout millennia, Paleo-Indians spread throughout

3690-594: The Archaic Period , numerous archaeological cultures have been identified. The unstable climate led to widespread migration, with early Paleo-Indians soon spreading throughout the Americas, diversifying into many hundreds of culturally distinct tribes. The Paleo-Indians were hunter-gatherers , likely characterized by small, mobile bands consisting of approximately 20 to 50 members of an extended family. These groups moved from place to place as preferred resources were depleted and new supplies were sought. During much of

3813-658: The Archaic period and the Mississippian cultures . The Adena culture and the ensuing Hopewell tradition during this period built monumental earthwork architecture and established continent-spanning trade and exchange networks. This period is considered a developmental stage without any massive changes in a short period but instead has a continuous development in stone and bone tools, leatherworking, textile manufacture, tool production, cultivation, and shelter construction. Some Woodland people continued to use spears and atlatls until

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3936-672: The Cliff Palace of Mesa Verde National Park in Colorado and the Great Houses in Chaco Canyon , New Mexico . The Puebloans also constructed a road system that stretched from Chaco Canyon to Kutz Canyon in the San Juan Basin . The Ancestral Puebloans are also known as "Anasazi", though the term is controversial, as the present-day Pueblo peoples consider the term to be derogatory, due to

4059-670: The Maya script . Other accounts also suggest that the Huastecs migrated as a result of the Classic Maya collapse around the year 900 CE. The Zapotecs were a civilization that thrived in the Oaxaca Valley from the late 6th century BCE until their downfall at the hands of the Spanish conquistadors. The city of Monte Albán was an important religious center for the Zapotecs and served as the capital of

4182-651: The Olmec , Teotihuacan , Mayas , Zapotecs , Mixtecs , Huastecs , Purepecha , Toltecs , and Mexica / Aztecs . The Mexica civilization is also known as the Aztec Triple Alliance since they were three smaller kingdoms loosely united together. These Indigenous civilizations are credited with many inventions: building pyramid temples, mathematics , astronomy , medicine, writing, highly accurate calendars , fine arts , intensive agriculture, engineering , an abacus calculator, and complex theology . They also invented

4305-509: The Spanish conquest of El Salvador , Cuzcatlan was forced to surrender to conquistador Pedro de Alvarado in 1528. Meiosis This is an accepted version of this page Meiosis ( / m aɪ ˈ oʊ s ɪ s / ; from Ancient Greek μείωσις ( meíōsis )  'lessening', (since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces

4428-458: The Tlingit , Haida , Chumash , Mandan , Hidatsa , and others, and some established large settlements, even cities, such as Cahokia , in what is now Illinois . Mesoamerica is the region extending from central Mexico south to the northwestern border of Costa Rica that gave rise to a group of stratified, culturally related agrarian civilizations spanning an approximately 3,000-year period before

4551-440: The diakinesis stage, from Greek words meaning "moving through". This is the first point in meiosis where the four parts of the tetrads are actually visible. Sites of crossing over entangle together, effectively overlapping, making chiasmata clearly visible. Other than this observation, the rest of the stage closely resembles prometaphase of mitosis; the nucleoli disappear, the nuclear membrane disintegrates into vesicles, and

4674-435: The gametes , the sperm or egg cells . It involves two rounds of division that ultimately result in four cells, each with only one copy of each chromosome ( haploid ). Additionally, prior to the division, genetic material from the paternal and maternal copies of each chromosome is crossed over , creating new combinations of code on each chromosome. Later on, during fertilisation , the haploid cells produced by meiosis from

4797-632: The history of the Americas , the pre-Columbian era , also known as the pre-contact era , or as the pre-Cabraline era specifically in Brazil , spans from the initial peopling of the Americas in the Upper Paleolithic to the onset of European colonization , which began with Christopher Columbus 's voyage in 1492. This era encompasses the history of Indigenous cultures prior to significant European influence, which in some cases did not occur until decades or even centuries after Columbus's arrival. During

4920-474: The meiotic spindle begins to form. Unlike mitotic cells, human and mouse oocytes do not have centrosomes to produce the meiotic spindle. In mice, approximately 80 MicroTubule Organizing Centers (MTOCs) form a sphere in the ooplasm and begin to nucleate microtubules that reach out towards chromosomes, attaching to the chromosomes at the kinetochore . Over time, the MTOCs merge until two poles have formed, generating

5043-625: The Americas and oral histories. Other civilizations, contemporaneous with the colonial period, were documented in European accounts of the time. For instance, the Maya civilization maintained written records, which were often destroyed by Christian Europeans such as Diego de Landa , who viewed them as pagan but sought to preserve native histories. Despite the destruction, a few original documents have survived, and others were transcribed or translated into Spanish, providing modern historians with valuable insights into ancient cultures and knowledge. Before

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5166-495: The DNA of the germline . The repair process used appears to involve homologous recombinational repair Prophase I arrested oocytes have a high capability for efficient repair of DNA damage , particularly exogenously induced double-strand breaks. DNA repair capability appears to be a key quality control mechanism in the female germ line and a critical determinant of fertility . Genetic recombination can be viewed as fundamentally

5289-472: The Europeans arrived, Indigenous peoples of North America had a wide range of lifeways from sedentary, agrarian societies to semi-nomadic hunter-gatherer societies. Many formed new tribes or confederations in response to European colonization. These are often classified by cultural regions , loosely based on geography. These can include the following: Numerous pre-Columbian societies were sedentary, such as

5412-498: The Greek letter Chi , Χ) between the homologous chromosomes. In most organisms, these links can help direct each pair of homologous chromosomes to segregate away from each other during meiosis I, resulting in two haploid cells that have half the number of chromosomes as the parent cell. During meiosis II, the cohesion between sister chromatids is released and they segregate from one another, as during mitosis . In some cases, all four of

5535-648: The Hohokam, they constructed kivas and great houses as well as ballcourts . Several of the Sinagua ruins include Montezuma Castle , Wupatki , and Tuzigoot . The Salado resided in the Tonto Basin in southeastern Arizona from 1150 CE to the 15th century. Archaeological evidence suggests that they traded with far-away cultures, as evidenced by the presence of seashells from the Gulf of California and macaw feathers from Mexico. Most of

5658-707: The Maya city of Chichen Itza . The Toltecs established vast trading relations with other Mesoamerican civilizations in Central America and the Puebloans in present-day New Mexico . During the Post-Classic era, the Toltecs suffered a subsequent collapse in the early 12th century, due to famine and civil war. The Toltec civilization was so influential to the point where many groups such as the Aztecs claimed to be descended from. With

5781-450: The Maya civilization cannot be overstated: it transformed political power, artistic depictions, and the nature of economics. Within the city of Teotihuacan was a diverse and cosmopolitan population. Most of the regional ethnicities of Mexico were represented in the city, such as Zapotecs from the Oaxaca region. They lived in apartment communities where they worked their trades and contributed to

5904-802: The Mississippian groups had vanished, and vast swaths of their territory were virtually uninhabited. The Ancestral Puebloans thrived in what is now the Four Corners region in the United States. It is commonly suggested that the culture of the Ancestral Puebloans emerged during the Early Basketmaker II Era during the 12th century BCE. The Ancestral Puebloans were a complex Oasisamerican society that constructed kivas , multi-story houses, and apartment blocks made from stone and adobe, such as

6027-483: The Olmec resulted in a power vacuum in Mexico. Emerging from that vacuum was Teotihuacan, first settled in 300 BCE. By 150 CE, Teotihuacan had risen to become the first true metropolis of what is now called North America. Teotihuacan established a new economic and political order never before seen in Mexico. Its influence stretched across Mexico into Central America, founding new dynasties in the Maya cities of Tikal , Copan , and Kaminaljuyú . Teotihuacan's influence over

6150-732: The Olmecs, Teotihuacan, the Toltecs, the Mexica, and the Mayas. These civilizations (except for the politically fragmented Maya) extended their reach across Mesoamerica—and beyond—like no others. They consolidated power and distributed influence in matters of trade, art, politics, technology, and theology. Other regional power players made economic and political alliances with these civilizations over 4,000 years. Many made war with them, but almost all peoples found themselves within one of their spheres of influence. Regional communications in ancient Mesoamerica have been

6273-662: The Paleo-Indian period, bands are thought to have subsisted primarily through hunting now-extinct giant land animals such as mastodon and ancient bison . Paleo-Indian groups carried a variety of tools, including distinctive projectile points and knives, as well as less distinctive butchering and hide-scraping implements. The vastness of the North American continent, and the variety of its climates, ecology , vegetation , fauna , and landforms, led ancient peoples to coalesce into many distinct linguistic and cultural groups. This

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6396-459: The Spanish colonists. The Wichita people were a loose confederation that consisted of sedentary agriculturalists and hunter-gatherers who resided in the eastern Great Plains . They lived in permanent settlements and even established a city called Etzanoa , which had a population of 20,000 people. The city was eventually abandoned around the 18th century after it was encountered by Spanish conquistadors Jusepe Gutierrez and Juan de Oñate . When

6519-602: The Tlaxcalans for preserving their culture and for their assistance in defeating the Aztecs. The Tlaxcalans would once again assist to the Spaniards during the Mixtón War and the conquest of Guatemala . Cuzcatlan was a Pipil confederacy of kingdoms and city-states located in present-day El Salvador . According to legend, Cuzcatlan was established by Toltec migrants during the Classic Maya collapse in approximately 1200 CE. During

6642-409: The ability to reproduce by parthenogenesis . Meiosis does not occur in archaea or bacteria , which generally reproduce asexually via binary fission . However, a "sexual" process known as horizontal gene transfer involves the transfer of DNA from one bacterium or archaeon to another and recombination of these DNA molecules of different parental origin. Meiosis was discovered and described for

6765-547: The calendar, were bequest from the former inhabitants of Tula, the Toltecs. The Mexica-Aztecs were the rulers of much of central Mexico by about 1400 (while Yaquis , Coras, and Apaches commanded sizable regions of northern desert), having subjugated most of the other regional states by the 1470s. At their peak, the Valley of Mexico where the Aztec Empire presided, saw a population growth that included nearly one million people during

6888-535: The chromosome number by half. During meiosis II, sister chromatids decouple and the resultant daughter chromosomes are segregated into four daughter cells. For diploid organisms, the daughter cells resulting from meiosis are haploid and contain only one copy of each chromosome. In some species, cells enter a resting phase known as interkinesis between meiosis I and meiosis II. Meiosis I and II are each divided into prophase , metaphase , anaphase , and telophase stages, similar in purpose to their analogous subphases in

7011-596: The chromosomes at the first meiotic division. The paired and replicated chromosomes are called bivalents (two chromosomes) or tetrads (four chromatids ), with one chromosome coming from each parent. Prophase I is divided into a series of substages which are named according to the appearance of chromosomes. The first stage of prophase I is the leptotene stage, also known as leptonema , from Greek words meaning "thin threads". In this stage of prophase I, individual chromosomes—each consisting of two replicated sister chromatids—become "individualized" to form visible strands within

7134-507: The city's economic and cultural prowess. Teotihuacan's economic pull impacted areas in northern Mexico as well. It was a city whose monumental architecture reflected a monumental new era in Mexican civilization, declining in political power about 650 CE—but lasting in cultural influence for the better part of a millennium, to around 950 CE. Contemporary to Teotihuacan's greatness was that of the Maya civilization. The period between 250 CE and 650 CE

7257-516: The civilizations in its area, the Tarascan Empire was the most prominent in metallurgy, harnessing copper, silver, and gold to create items such as tools, decorations, and even weapons and armor. Bronze was also used. The great victories over the Aztecs by the Tarascans cannot be understated. Nearly every war they fought in resulted in a Tarascan victory. Because the Tarascan Empire had little links to

7380-712: The cliff dwellings constructed by the Salado are primarily located in Tonto National Monument . The Iroquois League of Nations or "People of the Long House" was a politically advanced, democratic society, which is thought by some historians to have influenced the United States Constitution , with the Senate passing a resolution to this effect in 1988. Other historians have contested this interpretation and believe

7503-404: The coast in southeast Veracruz . The Olmec influence extended across Mexico, into Central America , and along the Gulf of Mexico . They transformed many peoples' thinking toward a new way of government, pyramid temples, writing, astronomy, art, mathematics, economics, and religion. Their achievements paved the way for the Maya civilization and the civilizations in central Mexico. The decline of

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7626-541: The de Soto expedition wandered the American Southeast for four years, becoming more bedraggled, losing more men and equipment, and eventually arriving in Mexico as a fraction of its original size. The local people fared much worse though, as the fatalities of diseases introduced by the expedition devastated the populations and produced much social disruption. By the time Europeans returned a hundred years later, nearly all of

7749-656: The decline of the Toltec civilization came political fragmentation in the Valley of Mexico . Into this new political game of contenders to the Toltec throne stepped outsiders: the Mexica . They were also a desert people, one of seven groups who formerly called themselves "Azteca", in memory of Aztlán , but they changed their name after years of migrating. Since they were not from the Valley of Mexico , they were initially seen as crude and unrefined in

7872-507: The development of archaeology in the 19th century, historians of the pre-Columbian period mainly interpreted the records of the European conquerors and the accounts of early European travelers and antiquaries. It was not until the nineteenth century that the work of people such as John Lloyd Stephens , Eduard Seler , and Alfred Maudslay , and institutions such as the Peabody Museum of Archaeology and Ethnology of Harvard University , led to

7995-536: The empire from 700 BCE to 700 CE. The Zapotecs resisted the expansion of the Aztecs until they were subjugated in 1502 under Aztec emperor Ahuitzotl . After the Spanish conquest of the Aztec Empire , the Zapotecs resisted Spanish rule until King Cosijopii I surrendered in 1563. Like the Zapotecs, the Mixtecs thrived in the Oaxaca Valley. The Mixtecs consisted of separate independent kingdoms and city-states, rather than

8118-575: The end of the period when they were replaced by bows and arrows . The Mississippian culture was spread across the Southeast and Midwest of what is today the United States, from the Atlantic coast to the edge of the plains, from the Gulf of Mexico to the Upper Midwest, although most intensively in the area along the Mississippi River and Ohio River . One of the distinguishing features of this culture

8241-457: The establishment of cities, such as El Tajín as important commercial trading centers. The Totonacs would later assist in the Spanish conquest of the Aztec Empire as an opportunity to liberate themselves from Aztec military imperialism. The Toltec civilization was established in the 8th century CE. The Toltec Empire expanded its political borders to as far south as the Yucatán peninsula , including

8364-469: The exchange of genetic information. The exchange of information between the homologous chromatids results in a recombination of information; each chromosome has the complete set of information it had before, and there are no gaps formed as a result of the process. Because the chromosomes cannot be distinguished in the synaptonemal complex, the actual act of crossing over is not perceivable through an ordinary light microscope, and chiasmata are not visible until

8487-403: The first Cazonci, Tariacuri, united these communities and built them into one of the most advanced civilizations in Mesoamerica. Their capital at Tzintzuntzan was just one of the many cities—there were ninety more under its control. The Tarascan Empire was among the largest in Central America, so it is no surprise that they routinely came into conflict with the neighboring Aztec Empire . Out of all

8610-618: The first time in sea urchin eggs in 1876 by the German biologist Oscar Hertwig . It was described again in 1883, at the level of chromosomes , by the Belgian zoologist Edouard Van Beneden , in Ascaris roundworm eggs. The significance of meiosis for reproduction and inheritance, however, was described only in 1890 by German biologist August Weismann , who noted that two cell divisions were necessary to transform one diploid cell into four haploid cells if

8733-400: The former Toltec Empire , they were also quite independent in culture from their neighbors. The Aztecs, Tlaxcaltec , Olmec, Mixtec, Maya, and others were very similar to each other, however. This is because they were all directly preceded by the Toltecs, and they therefore shared almost identical cultures. The Tarascans, however, possessed a unique religion, as well as other things. Tlaxcala

8856-443: The genus Leishmania , which cause human disease. However, these organisms were shown to have a sexual cycle consistent with a meiotic process. Although amoeba were once generally regarded as asexual, evidence has been presented that most lineages are anciently sexual and that the majority of asexual groups probably arose recently and independently. Dacks and Rogers proposed, based on a phylogenetic analysis, that facultative sex

8979-518: The homologous chromosomes become much more closely (~100 nm) and stably paired (a process called synapsis) mediated by the installation of the transverse and central elements of the synaptonemal complex . Synapsis is thought to occur in a zipper-like fashion starting from a recombination nodule. The paired chromosomes are called bivalent or tetrad chromosomes. The pachytene stage ( / ˈ p æ k ɪ t iː n / PAK -i-teen ), also known as pachynema , from Greek words meaning "thick threads".

9102-414: The homologous chromosomes, forming inter-axis bridges, and resulting in the pairing/co-alignment of homologues (to a distance of ~400 nm in mice). Leptotene is followed by the zygotene stage, also known as zygonema , from Greek words meaning "paired threads", which in some organisms is also called the bouquet stage because of the way the telomeres cluster at one end of the nucleus. In this stage

9225-460: The homologs are segregated to separate daughter cells by the spindle apparatus . The cells then proceed to a second division without an intervening round of DNA replication. The sister chromatids are segregated to separate daughter cells to produce a total of four haploid cells. Female animals employ a slight variation on this pattern and produce one large ovum and three small polar bodies. Because of recombination, an individual chromatid can consist of

9348-402: The idiosyncratic rendering "maiosis": We propose to apply the terms Maiosis or Maiotic phase to cover the whole series of nuclear changes included in the two divisions that were designated as Heterotype and Homotype by Flemming . The spelling was changed to "meiosis" by Koernicke (1905) and by Pantel and De Sinety (1906) to follow the usual conventions for transliterating Greek . Meiosis

9471-592: The impact was minimal or did not exist, pointing to numerous differences between the two systems and the ample precedents for the constitution in European political thought. The Calusa were a complex paramountcy/kingdom that resided in southern Florida . Instead of agriculture, the Calusa economy relied on abundant fishing. According to Spanish sources, the "king's house" at Mound Key was large enough to house 2,000 people. The Calusa ultimately collapsed into extinction at around 1750 after succumbing to diseases introduced by

9594-687: The influence that astronomical activities had upon Mesoamerican people before the arrival of Europeans. Many of the later Mesoamerican civilizations carefully built their cities and ceremonial centers according to specific astronomical events. The biggest Mesoamerican cities, such as Teotihuacan , Tenochtitlan , and Cholula , were among the largest in the world. These cities grew as centers of commerce, ideas, ceremonies, and theology, and they radiated influence outwards onto neighboring cultures in central Mexico. While many city-states, kingdoms, and empires competed with one another for power and prestige, Mesoamerica can be said to have had five major civilizations:

9717-429: The initial colonization of the region. The Na-Dené , Inuit , and Indigenous Alaskan populations exhibit haplogroup Q-M242 (Y-DNA) mutations, however, and are distinct from other Indigenous peoples with various mtDNA mutations. This suggests that the earliest migrants into the northern extremes of North America and Greenland derived from later populations. Asian nomadic Paleo-Indians are thought to have entered

9840-472: The late Aztec period (1350–1519). Their capital, Tenochtitlan , is the site of modern-day Mexico City . At its peak, it was one of the largest cities in the world with population estimates of 200,000–300,000. The market established there was the largest ever seen by the conquistadores on arrival. Initially, the lands that would someday comprise the lands of the powerful Tarascan Empire were inhabited by several independent communities. Around 1300, however,

9963-607: The late twentieth century, archeologists have studied, analyzed, and dated these sites, realizing that the earliest complexes were built by hunter-gatherer societies, whose people occupied the sites on a seasonal basis. Watson Brake , a large complex of eleven platform mounds, was constructed beginning in 3400 BCE and added to over 500 years. This has changed earlier assumptions that complex construction arose only after societies had adopted agriculture, and become sedentary, with stratified hierarchy and usually ceramics. These ancient people had organized to build complex mound projects under

10086-417: The life cycle can occur either during the diploid state ( diplontic life cycle), during the haploid state ( haplontic life cycle), or both ( haplodiplontic life cycle), in which there are two distinct organism phases, one with haploid cells and the other with diploid cells. In the diplontic life cycle (with pre-gametic meiosis), as in humans, the organism is multicellular and diploid, grown by mitosis from

10209-458: The living organism alternates between haploid and diploid states. Consequently, this cycle is also known as the alternation of generations . The diploid organism's germ-line cells undergo meiosis to produce spores. The spores proliferate by mitosis, growing into a haploid organism. The haploid organism's gamete then combines with another haploid organism's gamete, creating the zygote. The zygote undergoes repeated mitosis and differentiation to produce

10332-455: The majority of the Y-chromosome is unique and does not recombine during meiosis . This has the effect that the historical pattern of mutations can easily be studied. The pattern indicates Indigenous peoples of the Americas experienced two very distinctive genetic episodes: first with the initial peopling of the Americas and second with European colonization of the Americas . The former

10455-556: The meiotic products form gametes such as sperm , spores or pollen . In female animals, three of the four meiotic products are typically eliminated by extrusion into polar bodies , and only one cell develops to produce an ovum . Because the number of chromosomes is halved during meiosis, gametes can fuse (i.e. fertilization ) to form a diploid zygote that contains two copies of each chromosome, one from each parent. Thus, alternating cycles of meiosis and fertilization enable sexual reproduction , with successive generations maintaining

10578-577: The metaphase plate during metaphase I and orientation of sister chromatids in metaphase II, this is the subsequent separation of homologs and sister chromatids during anaphase I and II, it allows a random and independent distribution of chromosomes to each daughter cell (and ultimately to gametes); and (2) Crossing Over . The physical exchange of homologous chromosomal regions by homologous recombination during prophase I results in new combinations of genetic information within chromosomes. However, such physical exchange does not always occur during meiosis. In

10701-704: The metaphase plate, with respect to the orientation of the other bivalents along the same equatorial line. The protein complex cohesin holds sister chromatids together from the time of their replication until anaphase. In mitosis, the force of kinetochore microtubules pulling in opposite directions creates tension. The cell senses this tension and does not progress with anaphase until all the chromosomes are properly bi-oriented. In meiosis, establishing tension ordinarily requires at least one crossover per chromosome pair in addition to cohesin between sister chromatids (see Chromosome segregation ). Kinetochore microtubules shorten, pulling homologous chromosomes (which each consist of

10824-431: The mitotic cell cycle. Therefore, meiosis includes the stages of meiosis I (prophase I, metaphase I, anaphase I, telophase I) and meiosis II (prophase II, metaphase II, anaphase II, telophase II). During meiosis, specific genes are more highly transcribed . In addition to strong meiotic stage-specific expression of mRNA , there are also pervasive translational controls (e.g. selective usage of preformed mRNA), regulating

10947-417: The next stage. During the diplotene stage, also known as diplonema , from Greek words meaning "two threads", the synaptonemal complex disassembles and homologous chromosomes separate from one another a little. However, the homologous chromosomes of each bivalent remain tightly bound at chiasmata, the regions where crossing-over occurred. The chiasmata remain on the chromosomes until they are severed at

11070-452: The nucleus. The chromosomes each form a linear array of loops mediated by cohesin , and the lateral elements of the synaptonemal complex assemble forming an "axial element" from which the loops emanate. Recombination is initiated in this stage by the enzyme SPO11 which creates programmed double strand breaks (around 300 per meiosis in mice). This process generates single stranded DNA filaments coated by RAD51 and DMC1 which invade

11193-485: The number of chromosomes as the original parent cell. The two meiotic divisions are known as meiosis I and meiosis II . Before meiosis begins, during S phase of the cell cycle , the DNA of each chromosome is replicated so that it consists of two identical sister chromatids , which remain held together through sister chromatid cohesion. This S-phase can be referred to as "premeiotic S-phase" or "meiotic S-phase". Immediately following DNA replication, meiotic cells enter

11316-474: The number of chromosomes had to be maintained. In 1911, the American geneticist Thomas Hunt Morgan detected crossovers in meiosis in the fruit fly Drosophila melanogaster , which helped to establish that genetic traits are transmitted on chromosomes. The term "meiosis" is derived from the Greek word μείωσις , meaning 'lessening'. It was introduced to biology by J.B. Farmer and J.E.S. Moore in 1905, using

11439-510: The only true writing system native to the Americas using pictographs and syllabic elements in the form of texts and codices inscribed on stone, pottery, wood, or perishable books made from bark paper. The Huastecs were a Maya ethnic group that migrated northwards to the Gulf Coast of Mexico. The Huastecs are considered to be distinct from the Maya civilization, as they separated from the main Maya branch at around 2000 BCE and did not possess

11562-417: The oocytes of the silkworm Bombyx mori , meiosis is completely achiasmate (lacking crossovers). Although synaptonemal complexes are present during the pachytene stage of meiosis in B. mori , crossing-over homologous recombination is absent between the paired chromosomes . Female mammals and birds are born possessing all the oocytes needed for future ovulations, and these oocytes are arrested at

11685-417: The paired homologous chromosomes align along an equatorial plane that bisects the spindle, due to continuous counterbalancing forces exerted on the bivalents by the microtubules emanating from the two kinetochores of homologous chromosomes. This attachment is referred to as a bipolar attachment. The physical basis of the independent assortment of chromosomes is the random orientation of each bivalent along with

11808-646: The people abandoned their settlements, likely due to drought. The Mogollon resided in the present-day states of Arizona , New Mexico, and Texas as well as Sonora and Chihuahua . Like most other cultures in Oasisamerica, the Mogollon constructed sophisticated kivas and cliff dwellings. In the village of Paquimé , the Mogollon are revealed to have housed pens for scarlet macaws , which were introduced from Mesoamerica through trade. The Sinagua were hunter-gatherers and agriculturalists who lived in central Arizona. Like

11931-399: The pinching of the cell membrane in animal cells or the formation of the cell wall in plant cells, occurs, completing the creation of two daughter cells. However, cytokinesis does not fully complete resulting in "cytoplasmic bridges" which enable the cytoplasm to be shared between daughter cells until the end of meiosis II. Sister chromatids remain attached during telophase I. Cells may enter

12054-730: The ploidy is reduced from diploid to haploid, meiosis I is referred to as a reductional division . Meiosis II is an equational division analogous to mitosis, in which the sister chromatids are segregated, creating four haploid daughter cells (1n, 1c). Prophase I is by far the longest phase of meiosis (lasting 13 out of 14 days in mice ). During prophase I, homologous maternal and paternal chromosomes pair, synapse , and exchange genetic information (by homologous recombination ), forming at least one crossover per chromosome. These crossovers become visible as chiasmata (plural; singular chiasma ). This process facilitates stable pairing between homologous chromosomes and hence enables accurate segregation of

12177-422: The pre-Columbian era, many civilizations developed permanent settlements, cities, agricultural practices, civic and monumental architecture, major earthworks , and complex societal hierarchies. Some of these civilizations had declined by the time of the establishment of the first permanent European colonies, around the late 16th to early 17th centuries, and are known primarily through archaeological research of

12300-416: The prehistoric Americas . The culture reached its peak in about 1200–1400 CE, and in most places, it seems to have been in decline before the arrival of Europeans. Many Mississippian peoples were encountered by the expedition of Hernando de Soto in the 1540s, mostly with disastrous results for both sides. Unlike the Spanish expeditions in Mesoamerica, which conquered vast empires with relatively few men,

12423-447: The prophase I stage of meiosis. In humans, as an example, oocytes are formed between three and four months of gestation within the fetus and are therefore present at birth. During this prophase I arrested stage ( dictyate ), which may last for decades, four copies of the genome are present in the oocytes. The arrest of ooctyes at the four genome copy stage was proposed to provide the informational redundancy needed to repair damage in

12546-613: The reconsideration and criticism of the early European sources. Now, the scholarly study of pre-Columbian cultures is most often based on scientific and multidisciplinary methodologies. The haplogroup most commonly associated with Indigenous Amerindian genetics is Y-chromosome haplogroup Q1a3a . Researchers have found genetic evidence that the Q1a3a haplogroup has been in South America since at least 18,000 BCE. Y-chromosome DNA , like mtDNA , differs from other nuclear chromosomes in that

12669-511: The rest of North and South America. Exactly when the first people migrated into the Americas is the subject of much debate. One of the earliest identifiable cultures was the Clovis culture , with sites dating from some 13,000 years ago. However, older sites dating back to 20,000 years ago have been claimed. Some genetic studies estimate the colonization of the Americas dates from between 40,000 and 13,000 years ago. The chronology of migration models

12792-401: The same mechanisms as mitosis , the type of cell division used by eukaryotes to divide one cell into two identical daughter cells. In some plants, fungi, and protists meiosis results in the formation of spores : haploid cells that can divide vegetatively without undergoing fertilization. Some eukaryotes, like bdelloid rotifers , do not have the ability to carry out meiosis and have acquired

12915-561: The same number of chromosomes, occurs in all organisms that utilize meiosis. Meiosis occurs in all sexually-reproducing single-celled and multicellular organisms (which are all eukaryotes ), including animals , plants and fungi . It is an essential process for oogenesis and spermatogenesis . Although the process of meiosis is related to the more general cell division process of mitosis , it differs in two important respects: usually occurs between identical sister chromatids and does not result in genetic changes Meiosis begins with

13038-465: The same number of chromosomes. For example, diploid human cells contain 23 pairs of chromosomes including 1 pair of sex chromosomes (46 total), half of maternal origin and half of paternal origin. Meiosis produces haploid gametes (ova or sperm) that contain one set of 23 chromosomes. When two gametes (an egg and a sperm) fuse, the resulting zygote is once again diploid, with the mother and father each contributing 23 chromosomes. This same pattern, but not

13161-405: The second meiotic division. In metaphase II , the centromeres contain two kinetochores that attach to spindle fibers from the centrosomes at opposite poles. The new equatorial metaphase plate is rotated by 90 degrees when compared to meiosis I, perpendicular to the previous plate. This is followed by anaphase II , in which the remaining centromeric cohesin, not protected by Shugoshin anymore,

13284-452: The sister chromatids from separating. This allows the sister chromatids to remain together while homologs are segregated. The first meiotic division effectively ends when the chromosomes arrive at the poles. Each daughter cell now has half the number of chromosomes but each chromosome consists of a pair of chromatids. The microtubules that make up the spindle network disappear, and a new nuclear membrane surrounds each haploid set. Cytokinesis,

13407-545: The southern tip of South America by this time. In that case, the Inuit would have arrived separately and at a much later date, probably no more than 2,000 years ago, moving across the ice from Siberia into Alaska. The North American climate was unstable as the ice age receded during the Lithic stage . It finally stabilized about 10,000 years ago; climatic conditions were then very similar to today's. Within this time frame, roughly about

13530-545: The subject of considerable research. There is evidence of trade routes starting as far north as the Mexico Central Plateau , and going down to the Pacific coast. These trade routes and cultural contacts then went on as far as Central America . These networks operated with various interruptions from pre-Olmec times and up to the Late Classical Period (600–900 CE). The earliest known civilization in Mesoamerica

13653-446: The transition to anaphase I to allow homologous chromosomes to move to opposite poles of the cell. In human fetal oogenesis , all developing oocytes develop to this stage and are arrested in prophase I before birth. This suspended state is referred to as the dictyotene stage or dictyate. It lasts until meiosis is resumed to prepare the oocyte for ovulation, which happens at puberty or even later. Chromosomes condense further during

13776-428: The two haploid cells (with n chromosomes, each consisting of two sister chromatids) produced in meiosis I. The four main steps of meiosis II are: prophase II, metaphase II, anaphase II, and telophase II. In prophase II , we see the disappearance of the nucleoli and the nuclear envelope again as well as the shortening and thickening of the chromatids. Centrosomes move to the polar regions and arrange spindle fibers for

13899-413: The ultimate meiotic stage-specific protein expression of genes during meiosis. Thus, both transcriptional and translational controls determine the broad restructuring of meiotic cells needed to carry out meiosis. Meiosis I segregates homologous chromosomes , which are joined as tetrads (2n, 4c), producing two haploid cells (n chromosomes, 23 in humans) which each contain chromatid pairs (1n, 2c). Because

14022-550: The visits to the Caribbean by Christopher Columbus. Mesoamerican is the adjective generally used to refer to that group of pre-Columbian cultures. This refers to an environmental area occupied by an assortment of ancient cultures that shared religious beliefs, art, architecture, and technology in the Americas for more than three thousand years. Between 2000 and 300 BCE, complex cultures began to form in Mesoamerica. Some matured into advanced pre-Columbian Mesoamerican civilizations such as

14145-496: The ways of the Nahua civilization. Through political maneuvers and ferocious martial skills, they managed to rule Mexico as the head of the 'Triple Alliance' which included two other Aztec cities, Tetxcoco and Tlacopan . Latecomers to Mexico's central plateau , the Mexica thought of themselves, nevertheless, as heirs of the civilizations that had preceded them. For them, arts, sculpture, architecture, engraving, feather-mosaic work, and

14268-420: The wheel, but it was used solely as a toy. In addition, they used native copper , silver , and gold for metalworking. Archaic inscriptions on rocks and rock walls all over northern Mexico (especially in the state of Nuevo León ) demonstrate an early propensity for counting. Their number system was base 20 and included zero . These early count markings were associated with astronomical events and underscore

14391-654: The word tracing its origins to a Navajo word meaning "ancestor enemies". The Hohokam thrived in the Sonoran desert in what is now the U.S. state of Arizona and the Mexican state of Sonora . The Hohokam were responsible for the construction of a series of irrigation canals that led to the successful establishment of Phoenix, Arizona via the Salt River Project . The Hohokam also established complex settlements such as Snaketown , which served as an important commercial trading center. After 1375 CE, Hohokam society collapsed and

14514-401: The yeast Schizosaccharomyces pombe with hydrogen peroxide increased the frequency of mating and the formation of meiotic spores by 4 to 18-fold. Volvox carteri , a haploid multicellular, facultatively sexual green algae, can be induced by heat shock to reproduce by meiotic sex. This induction can be inhibited by antioxidants indicating that the induction of meiotic sex by heat shock

14637-487: Was a Nahua republic and confederation in central Mexico. The Tlaxcalans fiercely resisted Aztec expansion during the Flower Wars ever since the Aztecs expelled them from Lake Texcoco . The Tlaxcalans would later ally with the Spanish conquistadors under Hernán Cortés as an opportunity to liberate them from the Aztecs and managed to successfully conquer the Aztecs with the help of the conquistadors. The Spaniards would reward

14760-416: Was a time of intense flourishing of Maya civilized accomplishments. While the many Maya city-states never achieved political unity on the order of the central Mexican civilizations, they exerted tremendous intellectual influence upon Mexico and Central America. The Maya built some of the most elaborate cities on the continent and made innovations in mathematics, astronomy, and calendrics. The Maya also developed

14883-573: Was adopted in more temperate and sheltered regions, permitting a dramatic rise in population. After the migration or migrations, it was several thousand years before the first complex societies arose, the earliest emerging about seven to eight thousand years ago. As early as 5500 BCE, people in the Lower Mississippi Valley at Monte Sano and other sites in present-day Louisiana , Mississippi , and Florida were building complex earthwork mounds , probably for religious purposes. Beginning in

15006-404: Was likely present in the common ancestor of eukaryotes. The new combinations of DNA created during meiosis are a significant source of genetic variation alongside mutation, resulting in new combinations of alleles , which may be beneficial. Meiosis generates gamete genetic diversity in two ways: (1) Law of Independent Assortment . The independent orientation of homologous chromosome pairs along

15129-538: Was the construction of complexes of large earthen mounds and grand plazas, continuing the mound-building traditions of earlier cultures. They grew maize and other crops intensively, participated in an extensive trade network, and had a complex stratified society. The Mississippians first appeared around 1000 CE, following and developing out of the less agriculturally intensive and less centralized Woodland period. The largest urban site of these people, Cahokia —located near modern East St. Louis, Illinois —may have reached

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