tmRNAs typically contain four pseudoknots, one (pk1) upstream of the tag peptide CDS, and the other three pseudoknots (pk2 to pk4) downstream of the CDS. The pseudoknot regions, although generally conserved, are evolutionarily plastic. For example, in the (one-piece) tmRNAs of cyanobacteria, pk4 is substituted with two tandemly arranged smaller pseudoknots. This suggests that tmRNA folding outside the TLD can be important, yet the pseudoknot region lacks conserved residues and pseudoknots are among the first structures to be lost as ssrAsequences diverge in plastid and endosymbiont lineages. Base pairing in the three-pseudoknot region of E. colitmRNA is disrupted durig trans- translation

RNA-Seq data of Phytophthora sojae show an expression level similar to that of neighboring mitochondrial tRNAs, and four major processing sites confirm the predicted termini of mature mt-tmRNA.[33] The tmRNA precursor molecule is likely processed by RNase P and a tRNA 3’ processing endonuclease (see Figure 2); the latter activity is assumed to lead to the removal of the intervening sequence. Following the addition of CCA at the 3’ discriminator nucleotide, the tmRNA can be charged by alanyl-tRNA synthetase with alanine.

Processing of two-piece mt-tmRNA. The four major RNA processing sites are numbered (1-4). Processing at sites 1 and 4 is thought to occur by a tmRNA-specific activity, site 2 by RNase P and site 3 by a 3’ tRNA endonuclease processing. Nucleotides cleaved from the precursor are in gray; the post-transcriptionally added CCA is boxed.


As an example, tRNAAla encodes four different tRNA isoacceptors (AGC, UGC, GGC and CGC).

tRNA-derived fragments (or tRFs) are short molecules that emerge after cleavage of the mature tRNAs or the precursor transcript.[31][32][33][34] Both cytoplasmic and mitochondrial tRNAs can produce fragments.[35] There are at least four structural types of tRFs believed to originate from mature tRNAs, including the relatively long tRNA halves and short 5’-tRFs, 3’-tRFs and i-tRFs.[31][35][36] The precursor tRNA can be cleaved to produce molecules from the 5’ leader or 3’ trail sequences. Cleavage enzymes include Angiogenin, Dicer, RNase Z and RNase P.[31][32] Especially in the case of Angiogenin, the tRFs have a characteristically unusual cyclic phosphate at their 3’ end and a hydroxyl group at the 5’ end.[37]

tRFs have multiple dependencies and roles. They exhibit significant changes between sexes, among races and disease status.[35] Functionally, they can be loaded on Ago and act through RNAi pathways,[33][35][36][38] participate in the formation of stress granules,[39] displace mRNAs from RNA-binding proteins[40] or inhibit translation.[41]At the system or the organismal level, the four types of tRFs have a diverse spectrum of activities. Functionally, tRFs are associated with viral infection,[42] cancer,[35][36]cell proliferation [37] and also with epigenetic transgenerational regulation of metabolism.[43]

Structure looks like cross


However, in 1986, Noguchi made a gracefully sculptural adaptation from Fuller's energetic-synergetic geometry, which viewed the tetrahedron, a solid having four triangular sides, as the basic building block of the cosmos. Noguchi's "Challenger 7 Memorial," installed in the Noguchi-Sadao-designed Bayfront Park in Miami, is a 100-foot-high tetrahelix, a spiraling tower composed of 31 tetrahedrons joined to one another in the twisting helical pattern of DNA genetic material (as discovered by J. D. Watson and Francis Crick in 1953).

In Noguchi's view the tetrahelix symbolizes man's exploration of the mysteries of nature, and his sculpture, represented in the show by a model and a large photograph, is a tribute to the astronauts who died in the explosion of the Challenger space shuttle in 1986 while taking part in that exploration.


Periodic Modification of the Boerdijk-Coxeter Helix (Tetrahelix)

Garrett Sadler, Fang Fang, Julio Kovacs, Klee Irwin (2013)

The Boerdijk-Coxeter helix is a helical structure of tetrahedra which possesses no non-trivial translational or rotational symmetries. In this paper we develop a procedure by which this structure is modified to obtain both translational and rotational (upon projection) symmetries along/about its central axis. The findings of several distinct periodic structures are reported and focus on two particular forms related to the pentagonal and icosahedral aggregates of tetrahedra, as well as Buckminster Fuller’s “jitterbug transformation”.

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In the same forest, four separate species of arboreal marsupial folivores reacted differently to a secondary metabolite in eucalypts.

Also, there are four extensions from the body of the buccal fat pad: the sublevator, the melolabial, the buccal, and the pterygoid. The nomenclature of these extensions derives from their location and proximal muscles.[4]

The gland has four surfaces — superficial or lateral, superior, anteromedial, and posteromedial. The gland has three borders — anterior, medial, and posterior. The parotid gland has two ends — superior end in the form of small superior surface and an inferior end (apex).

FOUR GLANDS  Salivary glands are glands associated with the mouth or oral cavity and produce secretions (saliva) that are mixed with the food during feeding and are ingested along with the food. Among insects, there are four pairs of glands associated with the mouth or oral cavity, although all four generally are not present in the same insect. Each of these is associated with and is named after its associated mouthpart: the mandibular, maxillary, hypopharyngeal, and labial glands. The presence or absence of each of these glands varies among insect species, and in holometabolous insects, a given gland may be present in only one life stage. For example, in Lepidoptera, mandibular glands occur in the larval stage, but not in the adult. The four glands serve a variety of functions, including some non-salivary functions, and the same gland may serve different functions in different species or even in different life stages of the same species. Within a given species, one or more of these four pairs of glands usually function as salivary glands. the tobacco hornworm, Manduca sexta, adults have a pair of tubular salivary glands that are divided into four regions (Fig. 1C). In the apical region, proteinaceous material is synthesized in the extensive rough endoplasmic reticulum and Golgi bodies, and is stored in large vacuoles before eventual release into the lumen of the gland. The second region transports water from the hemolymph to the lumen. Cells in this region have the characteristic structure of water-secreting cells to accommodate what is believed to be the primary function of this region; but in addition, the presence of rough endoplasmic reticulum and Golgi bodies suggests that these cells secrete more than just water. The third and fourth regions, called the thin duct and bulbous duct, both seem to have a reabsorptive function, moving ions back from the saliva and into the hemolymph. Cells in these two regions differ in the structure of their surface adjacent to the lumen, but the reason for the difference is unknown. After the fourth region, the right and left glands fuse forming the common duct. Cells of the common duct are unspecial-ized and probably play no role in saliva production.

Salivary glands are glands associated with the mouth or oral cavity and produce secretions (saliva) that are mixed with the food during feeding and are ingested along with the food. Among insects, there are four pairs of glands associated with the mouth or oral cavity, although all four generally are not present in the same insect. Each of these is associated with and is named after its associated mouthpart: the mandibular, maxillary, hypopharyngeal, and labial glands. The presence or absence of each of these glands varies among insect species, and in holometabolous insects, a given gland may be present in only one life stage. For example, in Lepidoptera, mandibular glands occur in the larval stage, but not in the adult. The four glands serve a variety of functions, including some non-salivary functions, and the same gland may serve different functions in different species or even in different life stages of the same species. Within a given species, one or more of these four pairs of glands usually function as salivary glands.

Salivary glands 

These are mucous glands situated on the palate and tongue. Besides these mucous glands, there are also four pairs of salivary glands, situated around the buccal cavity. These are 

i. Infraorbitals situated below the eye, and their ducts open near the upper molar. 

ii. Parotids are at the base of external ears and through stensons duct open behind the upper incisor. 

iii. Sublinguals are situated under the tongue and their short ducts open below the free part of the tongue, 

iv. Submandibular or submaxillary glands lie on the inner side of the angles of lower jaws, through whartons duct open behind the lower incisors.

Where are the 4 minor salivary glands?

submuccous layer of:
lips - labial
cheeks - buccal
palate - palatine
tongue - lingual

A lot of times only three pairs of salivary glands are mentioned, but there is a transcendent fourth that is different

There are four pairs of salivary glands:

  • The parotid glands are located between the ear and the jaw.

  • The submandibular glands are located under the jaw.

  • The sublingual glands are located on the floor of the mouthunder the tongue.

  • The buccal glands are not shown in this picture. They are located in the mucous membrane lining the cheeks and mouth. These glands produce only a small amount of saliva.


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A salivary acinar cell contains four ion transporters, the Na+/K+ adenosine triphosphatase (ATPase), a Na+-K+-2Cl- cotransporter and a Ca2+-activated K+ channel, all located in the basolateral membrane, and a Ca2+-activated Cl- channel located in the apical membrane. Fluid secretion is thought to arise from the concerted actions of these four transporters.Increased salivary secretion in response to taste stimulation is, like chewing, induced reflexly. Taste sensation is traditionally divided into the four classical basic taste modalities: sweet, salty, sour, and bitter. 

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How are Botox injections done?

The interventional radiologist uses an ultrasound machine to find the glands in the face that produce saliva. Using the ultrasound image as a guide, a tiny needle is then inserted through the skin into the glands. Once the needle is in place, the radiologist will inject a small amount of Botox into the space. Typically four salivary glands (two submandibular glands and two parotoid glands) are injected.

Salivary Botox injections take one hour to be completed.

The four salivary glands commonly used for botox injection

Shh secreted from the floor plate creates a gradient along the ventral neural tube. Shh functions in a concentration-dependent manner to specify ventral neuronal fates. V0-V3 represent four different classes of ventral interneurons, and MN indicates motor neurons.


In the mouse's dorsal alar plate, six progenitor domains give rise to dI1-dI6 neurons and two classes of dorsal interneurons.[6] In addition, in the ventral half of the neural tube, four classes of (CPG) interneurons known as V0V1V2, and V3 neurons are generated.[6] V0 neurons are commissural neurons that extend their axons rostrally for 2-4 spinal cord regions in the embryonic spinal cord.[6] V3 neurons are excitatory commissural interneurons that extend caudally projecting primary axons.[6] The V1 neurons are inhibitory interneurons with axons that project ipsilaterraly and rostrally.[6] V2 neurons, which include a population of glutamatergic V2a neurons and inhibitory V2b neurons, project ispilaterally and caudally across multiple spinal cord regions.[6] The class V1 neurons give rise to two local circuit inhibitory neurons known as Renshaw cells and Ia inhibitory interneurons.[6]

CPG interneuronsTypeAxon projection in embryonic cord


V1Inhibitory (Renshaw cells and Ia interneurons)Rostrally and ipsilaterally

V2Glutamatergic V2a and Inhibitory V2bIpsilaterally and caudally

V3Excitatory CommissuralCaudally

Archicoretx and Allocortex four layers

The allocortex has just three or four layers of neuronal cell bodies in contrast to the six layers of the neocortex. There are three subtypes of allocortex, the paleocortex, archicortex and periallocortex.[3]

Paleocortex is a type of thin, primitive cortical tissue that consists of three cortical laminae (layers of neuronal cell bodies).[4][5] The two granular layers II and IV of neocortex are absent in paleocortex. The main areas of paleocortex are the olfactory bulbolfactory tubercle and piriform cortex.

Archicortex is a type of cortical tissue that consists of four laminae (layers of neuronal cell bodies).[6] The main areas of archicortex are the hippocampus and dentate gyrus


The cerebral cortex is derived from the pallium, a layered structure found in the forebrain of all vertebrates. The basic form of the pallium is a cylindrical layer enclosing fluid-filled ventricles. Around the circumference of the cylinder are four zones, the dorsal pallium, medial pallium, ventral pallium, and lateral pallium, which are thought respectively to give rise to the neocortexhippocampusamygdala, and olfactory cortex

Four domains In basal vertebrates the pallium encompassing 3-4 histogenetically distinct domains, plus the olfactory bulb


Complex eversion model and topological organization of the zebrafish pallium. (A) The teleostean pallium consists of the same four pallial divisions present in mammals. Adult location and topological origin are obscured by three major developmental events....

This displacement of Dc by Dl alters our understanding of the sulcus ypsiloniformis (Y) as well (fig. 3B). The sulcus ypsiloniformis (Y) has been interpreted as an independent and stable anatomical landmark between Dm and Dl. Our results, however, indicated that this is true only for posterior levels of the telencephalon. At most anterior levels, the sulcus ypsiloniformis (Y) marked the border between the medial (Dm) zone and the central (Dc) zone (fig. 3B). This is because here the central (Dc) zone, including its germinative proliferative layer, reaches the dorsal top of the telencephalon. A dorsal (Dd) zone is absent in zebrafish.

Parvalbumin Immunoreactivity Confirms Four Pallial Units

To confirm that the zebrafish pallium comprises only four histogenetic units, we examined the distribution of parvalbumin. And, indeed, parvalbumin differentially marked the same four pallial zones in the adult zebrafish as NADPHd (figs. 4A-D). For example, the lateral (Dl) zone displayed many parvalbumin-positive cells ventrally adjacent to the dorsal proliferative layer, parvalbumin-positive cells in the periphery of Dl, as well as strongly labeled parvalbumin-positive fibers (nicely displayed in fig. 5E). The central (Dc) zone, in contrast, markedly differed from the overlying lateral (Dl) zone through the absence of parvalbumin-positive cells in both, its proliferative layer and in its periphery. The presence of many parvalbumin fibers in Dc sets this pallial unit apart from the adjacent medial (Dm) and the posterior (Dp) zones, which both lack expression of parvalbumin. Note, also the lateral olfactory tract (lot), which is known to send olfactory projections to the posterior (Dp) zone as well as to the nucleus taeniae (Levine and Dethier, 1985Northcutt, 2008) is defined by the absence of parvalbumin-positive fibers (fig. 4A).


Fig. 4

Coronal sections of a zebrafish telencephalon stained against parvalbumin. (A-D) The differential distribution of parvalbumin-positive cells and fibers reveals all four pallial divisions in the anterior zebrafish telencephalon. The lateral (Dl) zone of ...


Fig. 5

Distribution of Parvalbumin in consecutive sections of the zebrafish pallium. (A-E) Parvalbumin is the second marker that allows discerning all four true histogenetic units in the zebrafish pallium. The central (Dc) zone is defined by many parvalbumin-positive ...

Furthermore, several research papers referred to four genes, dystrobrevin binding protein 1 (DTNBP1), neuregulin 1 (NRG1), disrupted in schizophrenia 1 (DISC1), and neuregulin 1 receptor (ERBB4), as being possibly responsible for this deficit in the normal regeneration of neurons.[45

Neural stem cells and neurons[edit]


Structure and features of the neurogenic niche. Adapted from a paper by Ilias Kazanis, et al., 2008.

The brain comprises many different types of neurons, but the SGZ generates only one type: granule cells—the primary excitatory neurons in the dentate gyrus (DG)--which are thought to contribute to cognitive functions such as memory and learning. The progression from neural stem cell to granule cell in the SGZ can be described by tracing the following lineage of cell types:[5][6]

  1. Radial glial cells. Radial glial cells are a subset of astrocytes, which are typically thought of as non-neuronal support cells. The radial glial cells in the SGZ have cell bodies that reside in the SGZ and vertical (or radial) processes that extend into the molecular layer of the DG. These processes act as a scaffold upon which newly formed neurons can migrate the short distance from the SGZ to the granule cell layer. Radial glia are astrocytic in their morphology, their expression of glial markers such as GFAP, and their function in regulating the NSC microenvironment. However, unlike most astrocytes, they also act as neurogenic progenitors; in fact, they are widely considered to be the neural stem cells that give rise to subsequent neuronal precursor cells. Studies have shown that radial glia in the SGZ express nestin and Sox2, biomarkers associated with neural stem cells, and that isolated radial glia can generate new neurons in vitro.[7] Radial glial cells often divide asymmetrically, producing one new stem cell and one neuronal precursor cell per division. Thus, they have the capacity for self-renewal, enabling them to maintain the stem cell population while simultaneously producing the subsequent neuronal precursors known as transiently amplifying cells.[8]

  2. Transiently amplifying progenitor cells. Transiently amplifying (or transit-amplifying) progenitor cells are highly proliferative cells that frequently divide and multiply via mitosis, thus "amplifying" the pool of available precursor cells. They represent the beginning of a transitory stage in NSC development in which NSCs begin to lose their glial characteristics and assume more neuronal traits. For instance, cells in this category may initially express glial markers like GFAP and stem cell markers such as nestin and Sox2, but eventually, they lose these characteristics and begin expressing markers specific to granule cells such as NeuroD and Prox1. It is thought that the formation of these cells represents a fate-choice in neural stem cell development.

  3. Neuroblasts. Neuroblasts represent the last stage of precursor cell development before cells exit the cell cycle and assume their identity as neurons. Proliferation of these cells is more limited, although cerebral ischemia can induce proliferation at this stage.

  4. Postmitotic neurons. At this point, after exiting the cell cycle, cells are considered immature neurons. The large majority of postmitotic neurons undergo apoptosis, or cell death. The few that survive begin developing the morphology of hippocampal granule cells, marked by the extension of dendrites into the molecular layer of the DG and the growth of axons into the CA3 region, and subsequently the formation of synaptic connections. Postmitotic neurons also pass through a late maturation phase characterized by increased synaptic plasticity and a decreased threshold for long-term potentiation. Eventually, the neurons are integrated into the hippocampal circuitry as fully matured granule cells.

Four component areas have been described:[3] parasubiculum (adjacent to the parahippocampal gyrus), presubiculum, subiculum, and prosubiculum.


As mitochondrial DNA (mtDNA) is uniparentally inherited, it undergoes negligible recombination at the population level, and mutations acquired over time have subdivided the human population into several discrete haplogroups. The major haplogroups arose 40,000–150,000 years before present (YBP) and have defined different human populations as they migrated out of Africa and populated the globe. The African root was the source of four lineages specific for sub-Saharan Africa: L0, L1, L2 and L3 (130,000–200,000 YBP). Two more haplogroups, M and N, arose from the African haplogroup L3 65,000–70,000 YBP to populate the rest of the world125. As humans migrated, haplogroup N was directed to Eurasia and haplogroup M lineages moved to Asia, giving rise to the haplogroups A, B, C, D, G and F. In Europe, haplogroup N led to haplogroup R, which is the root of the European haplogroups H, J, T, U and V, which emerged 39,000–51,000 YBP126. Haplogroups S, P, and Q are found in Australasia and were formed ~48,000 YBP, and haplogroups A, B, C and D arose <20,000 YBP and populated East Asia and the Americas. These low-resolution single-nucleotide variant studies have been superseded by massive whole-mtDNA-genome-sequencing studies, which have identified many different sub-haplogroups that define the contemporary mtDNA phylogenetic tree. For example, haplogroup H, the most common in Europe, is comprised of almost 90 different sub-haplogroups127. Adapted with permission from MITOMAP (original authors Lott, M. T. et al.;


Phylogenetic analysis revealed that most Native American lineages are classified into four major distinct clusters. Individuals belonging to each cluster share at least two specific polymorphic sites that are nearly absent in other human populations, indicating a unique phylogenetic position of Native Americans. A phylogenetic tree of 193 individuals including Africans, Europeans, Asians, and Native Americans indicated that the four Native American clusters are distinct and dispersed in the tree. These clusters almost exclusively consist of Native Americans--with only a few Asians, if any. We postulate that four ancestral populations gave rise to different waves of migration to the New World. From the estimated coalescence time of the Asian and Native American lineages, we infer that the first migration across the Bering landbridge took place approximately 14,000-21,000 years ago. Furthermore, sequence differences in all pairwise comparisons of Native Americans showed a bimodal distribution that is significantly different from Poisson. These results suggest that the ancestral Native American population underwent neither a severe bottleneck nor rapid expansion in population size, during the migration of people into the Americas.


NEW YORK — About 3.5 million of today’s Ashkenazi Jews — 40 percent of the total Ashkenazi population — are descended from just four women, a genetic study indicates.

Those women apparently lived somewhere in Europe within the last 2,000 years, but not necessarily in the same place or even the same century, said lead author Dr. Doron Behar of the Rambam Medical Center in Haifa, Israel.

He did the work with Karl Skorecki of the Technion-Israel Institute of Technology and others.

Each woman left a genetic signature that shows up in their descendants today, he and colleagues say in a report published online by the American Journal of Human Genetics. Together, their four signatures appear in about 40 percent of Ashkenazi Jews, while being virtually absent in non-Jews and found only rarely in Jews of non-Ashkenazi origin, the researchers said.


  2.  Lordkipanidze, David; Vekua, Abesalom; Ferring, Reid; et al. (November 2006). "A fourth hominin skull from Dmanisi, Georgia". The Anatomical Record Part A: Discoveries in Molecular, Cellular, and Evolutionary Biology. New York: Wiley-Liss. 288A (11): 1146–1157. ISSN 1552-4884PMID 17031841doi:10.1002/ar.a.20379.

  3. Jump up ^


Anthropologists in the 1980s were divided regarding some details of reproductive barriers and migratory dispersals of the Homo genus. Subsequently, genetics has been used to investigate and resolve these issues. According to the Sahara pump theory evidence suggests that genus Homohave migrated out of Africa at least three and possibly four times (e.g. Homo erectus, Homo heidelbergensis and two or three times for Homo sapiens). Recent evidence suggests these dispersals are closely related to fluctuating periods of climate change.[63]


Current dating of human fossils indicates that modern humans migrating Out of Africa between 125 - 18 kya (thousand years ago) moved into lands occupied by at least four known homo species. These are (species/place/most recent date alive): h.erectus, Eurasia, 27kya; h.neanderthalensis, Europe 30kya/Central Asia 40kya; h.sp.altai (or Denisovans), Siberia/Asia/SE Asia, 30kya), h. floresiensis (or 'Hobbits', note controversy whether this is a homo species), SE Asia, 18kya. There is genetic evidence from Melanesian and Australian Aboriginal DNA, of another unidentified homo speciesfrom around 400kya to a time when interbreeding with modern humans migrating OoA could have occurred (70-30kya?). Plus, early humans could have interacted with any number of hybrid groups that became extinct, as indicated by examples such as: Lapedo Child, Europe, 24kya; Red Deer Cave People, China, 11kya; h.tsaichangensis, Taiwan, 10kya.


Egon Freiherr von Eickstedt's Racial Definitions

for Europe

Racial classification of Europeans by the

20th century



Alpine race


Osteuropid race


Nordic race


Mediterranean race

Returning to Pasadena in search of Elaine, Benjamin breaks into the Robinson home but encounters Mrs. Robinson. She tells him he will not be able to stop the wedding and then calls the police claiming that her house is being burgled. Benjamin visits Carl’s fraternity brothers who tell him that the wedding is in Santa Barbara, Californiathat very morning. He rushes to the church and arrives just as Elaine is married. He bangs on the glass at the back of the church and screams out "Elaine!" repeatedly. After a brief hesitation, Elaine screams out "Ben!" and starts to run toward him. A brawl ensues as guests try to stop Elaine and Benjamin from leaving together. Elaine manages to break free from her mother, who then slaps her. Benjamin manages to keep the guests at bay by using a large cross and jamming it into the doors of the church. Both he and Elaine then run into the street to flag down a passing bus and take the back seat. Although initially elated at their victory, the pair become increasingly uncomfortable as they journey towards an uncertain future.