Paraphrase the paragraphs in your own words Transcription mi

Paraphrase the paragraphs in your own words? Transcription miRNA genes are usually transcribed by RNA polymerase II (Pol II).[50][59] The polymerase often binds to a promoter found near the DNA sequence, encoding what will become the hairpin loop of the pre-miRNA. The resulting transcript is capped with a specially modified nucleotide at the 5’ end, polyadenylated with multiple adenosines (a poly(A) tail),[50][54] and spliced. Animal miRNAs are initially transcribed as part of one arm of an 80 nucleotide RNA stem-loop that in turn forms part of a several hundred nucleotide-long miRNA precursor termed a primary miRNA (pri-miRNA).[50][54] When a stem-loop precursor is found in the 3\' UTR, a transcript may serve as a pri-miRNA and a mRNA.[54] RNA polymerase III (Pol III) transcribes some miRNAs, especially those with upstream Alu sequences, transfer RNAs (tRNAs), and mammalian wide interspersed repeat (MWIR) promoter units. [60] Nuclear processing A single pri-miRNA may contain from one to six miRNA precursors. These hairpin loop structures are composed of about 70 nucleotides each. Each hairpin is flanked by sequences necessary for efficient processing. The double-stranded RNA (dsRNA) structure of the hairpins in a pri-miRNA is recognized by a nuclear protein known as DiGeorge Syndrome Critical Region 8 (DGCR8 or \"Pasha\" in invertebrates), named for its association with DiGeorge Syndrome. DGCR8 associates with the enzyme Drosha, a protein that cuts RNA, to form the Microprocessor complex.[61][62] In this complex, DGCR8 orients the catalytic RNase III domain of Drosha to liberate hairpins from pri-miRNAs by cleaving RNA about eleven nucleotides from the hairpin base (one helical dsRNA turn into the stem).[63][64] The product resulting has a two-nucleotide overhang at its 3’ end; it has 3\' hydroxyl and 5\' phosphate groups. It is often termed as a pre-miRNA (precursor-miRNA). Sequence motifs downstream of the pre-miRNA that are important for efficient processing have been identified.[65][66][67] Pre-miRNAs that are spliced directly out of introns, bypassing the Microprocessor complex, are known as \"Mirtrons.\" Originally thought to exist only in Drosophila and C. elegans, mirtrons have now been found in mammals.[68] As many as 16% of pre-miRNAs may be altered through nuclear RNA editing.[69][70][71] Most commonly, enzymes known as adenosine deaminases acting on RNA (ADARs) catalyze adenosine to inosine (A to I) transitions. RNA editing can halt nuclear processing (for example, of pri-miR-142, leading to degradation by the ribonuclease Tudor-SN) and alter downstream processes including cytoplasmic miRNA processing and target specificity (e.g., by changing the seed region of miR-376 in the central nervous system).[69] Nuclear export Pre-miRNA hairpins are exported from the nucleus in a process involving the nucleocytoplasmic shuttler Exportin-5. This protein, a member of the karyopherin family, recognizes a two-nucleotide overhang left by the RNase III enzyme Drosha at the 3\' end of the pre-miRNA hairpin. Exportin-5-mediated transport to the cytoplasm is energy-dependent, using GTP bound to the Ran protein.[72] Cytoplasmic processing In the cytoplasm, the pre-miRNA hairpin is cleaved by the RNase III enzyme Dicer.[73] This endoribonuclease interacts with 5\' and 3\' ends of the hairpin[74] and cuts away the loop joining the 3\' and 5\' arms, yielding an imperfect miRNA:miRNA* duplex about 22 nucleotides in length.[73] Overall hairpin length and loop size influence the efficiency of Dicer processing. The imperfect nature of the miRNA:miRNA* pairing also affects cleavage.[73][75] Some of the G-rich pre-miRNAs can potentially adopt the G-quadruplex structure as an alternative to the canonical stem-loop structure. For example, human pre-miRNA 92b adopts a G-quadruplex structure which is resistant to the Dicer mediated cleavage in the cytoplasm.[76] Although either strand of the duplex may potentially act as a functional miRNA, only one strand is usually incorporated into the RNA-induced silencing complex (RISC) where the miRNA and its mRNA target interact. The mature miRNA is part of an active RNA-induced silencing complex (RISC) containing Dicer and many associated proteins.[78] RISC is also known as a microRNA ribonucleoprotein complex (miRNP);[79] RISC with incorporated miRNA is sometimes referred to as \"miRISC.\" Dicer processing of the pre-miRNA is thought to be coupled with unwinding of the duplex. Generally, only one strand is incorporated into the miRISC, selected on the basis of its thermodynamic instability and weaker base-pairing on the 5\' end relative to the other strand.[80][81][82] The position of the stem-loop may also influence strand choice.[83] The other strand, called the passenger strand due to its lower levels in the steady state, is denoted with an asterisk (*) and is normally degraded. In some cases, both strands of the duplex are viable and become functional miRNA that target different mRNA populations.[84] Members of the Argonaute (Ago) protein family are central to RISC function. Argonautes are needed for miRNA-induced silencing and contain two conserved RNA binding domains: a PAZ domain that can bind the single stranded 3’ end of the mature miRNA and a PIWI domain that structurally resembles ribonuclease-H and functions to interact with the 5’ end of the guide strand. They bind the mature miRNA and orient it for interaction with a target mRNA. Some argonautes, for example human Ago2, cleave target transcripts directly; argonautes may also recruit additional proteins to achieve translational repression.[85] The human genome encodes eight argonaute proteins divided by sequence similarities into two families: AGO (with four members present in all mammalian cells and called E1F2C/hAgo in humans), and PIWI (found in the germ line and hematopoietic stem cells).[79][85] Additional RISC components include TRBP [human immunodeficiency virus (HIV) transactivating response RNA (TAR) binding protein],[86] PACT (protein activator of the interferon-induced protein kinase), the SMN complex, fragile X mental retardation protein (FMRP), Tudor staphylococcal nuclease-domain-containing protein (Tudor-SN), the putative DNA helicase MOV10, and the RNA recognition motif containing protein TNRC6B.[72][87][88] Mode of silencing and regulatory loops Gene silencing may occur either via mRNA degradation or preventing mRNA from being translated. For example, miR16 contains a sequence complementary to the AU-rich element found in the 3\'UTR of many unstable mRNAs, such as TNF alpha or GM-CSF.[89] It has been demonstrated that given complete complementarity between the miRNA and target mRNA sequence, Ago2 can cleave the mRNA and lead to direct mRNA degradation. Absent complementarity, silencing is achieved by preventing translation.[43] The relation of miRNA and its target mRNA(s) can be based on the simple negative regulation of a target mRNA, but it seems that a common scenario is the use of a “coherent feed-forward loop” (Fig. 1C), “mutual negative feedback loop” (also termed double negative loop) and “positive feedback/feed-forward loop” Some miRNAs work as buffers of random gene expression changes arising due to stochastic events in transcription, translation and protein stability. Such regulation is typically achieved by the virtue of negative feedback loops or incoherent feed-forward loop uncoupling protein output from mRNA transcription.[46] miRNA turnover Turnover of mature miRNA is needed for rapid changes in miRNA expression profiles. During miRNA maturation in the cytoplasm, uptake by the Argonaute protein is thought to stabilize the guide strand, while the opposite (* or \"passenger\") strand is preferentially destroyed. In what has been called a \"Use it or lose it\" strategy, Argonaute may preferentially retain miRNAs with many targets over miRNAs with few or no targets, leading to degradation of the non-targeting molecules.[90] Decay of mature miRNAs in Caenorhabditis elegans is mediated by the 5´-to-3´ exoribonuclease XRN2, also known as Rat1p.[91] In plants, SDN (small RNA degrading nuclease) family members degrade miRNAs in the opposite (3\'-to-5\') direction. Similar enzymes are encoded in animal genomes, but their roles have not been described.[90] Several miRNA modifications affect miRNA stability. As indicated by work in the model organism Arabidopsis thaliana (thale cress), mature plant miRNAs appear to be stabilized by the addition of methyl moieties at the 3\' end. The 2\'-O-conjugated methyl groups block the addition of uracil (U) residues by uridyltransferase enzymes, a modification that may be associated with miRNA degradation. However, uridylation may also protect some miRNAs; the consequences of this modification are incompletely understood. Uridylation of some animal miRNAs has been reported. Both plant and animal miRNAs may be altered by addition of adenine (A) residues to the 3\' end of the miRNA. An extra A added to the end of mammalian miR-122, a liver-enriched miRNA important in hepatitis C, stabilizes the molecule and plant miRNAs ending with an adenine residue have slower decay rates.[90] MiRNAs Function The function of miRNAs appears to be in gene regulation. For that purpose, a miRNA is complementary to a part of one or more messenger RNAs (mRNAs). Animal miRNAs are usually complementary to a site in the 3\' UTR whereas plant miRNAs are usually complementary to coding regions of mRNAs.[93] Perfect or near perfect base pairing with the target RNA promotes cleavage of the RNA.[94] This is the primary mode of plant miRNAs.[95] In animals the match-ups are imperfect. For partially complementary microRNAs to recognise their targets, nucleotides 2–7 of the miRNA (its \'seed region\'[10][20]) must be perfectly complementary.[96] Animal miRNAs inhibit protein translation of the target mRNA[97] (this is present but less common in plants).[95] Partially complementary microRNAs can also speed up deadenylation, causing mRNAs to be degraded sooner.[98] While degradation of miRNA-targeted mRNA is well documented, whether or not translational repression is accomplished through mRNA degradation, translational inhibition, or a combination of the two is hotly debated. Recent work on miR-430 in zebrafish, as well as on bantam-miRNA and miR-9 in Drosophila cultured cells, shows that translational repression is caused by the disruption of translation initiation, independent of mRNA deadenylation.[99][100] miRNAs occasionally also cause histone modification and DNA methylation of promoter sites, which affects the expression of target genes.[101][102] Nine mechanisms of miRNA action are described and assembled in a unified mathematical model:[92] • Cap-40S initiation inhibition; • 60S Ribosomal unit joining inhibition; • Elongation inhibition; • Ribosome drop-off (premature termination); • Co-translational nascent protein degradation; • Sequestration in P-bodies; • mRNA decay (destabilisation); • mRNA cleavage; • Transcriptional inhibition through microRNA-mediated chromatin reorganization followed by gene silencing. It is often impossible to discern these mechanisms using experimental data about stationary reaction rates. Nevertheless, they are differentiated in dynamics and have different kinetic signatures.[92] Unlike plant microRNAs, the animal microRNAs target diverse genes.[20] However, genes involved in functions common to all cells, such as gene expression, have relatively fewer microRNA target sites and seem to be under selection to avoid targeting by microRNAs.[103] dsRNA can also activate gene expression, a mechanism that has been termed \"small RNA-induced gene activation\" or RNAa. dsRNAs targeting gene promoters can induce potent transcriptional activation of associated genes. This was demonstrated in human cells using synthetic dsRNAs termed small activating RNAs (saRNAs),[104] but has also been demonstrated for endogenous microRNA.[105] Interactions between microRNAs and complementary sequences on genes and even pseudogenes that share sequence homology are thought to be a back channel of communication regulating expression levels between paralogous genes. Given the name \"competing endogenous RNAs\" (ceRNAs), these microRNAs bind to \"microRNA response elements\" on genes and pseudogenes and may provide another explanation for the persistence of non-coding DNA.[106]

Solution

Discussing about the trascription of miRNA genes.This process is carried out by the help of RNA poymerase II.As a normal process the polymerase get bound to the promotor region of the DNA sequence and and transcription goes on.As a result of this the transcript has special sequence of neucleotides at the 5\'end and poly A tail in the 3\'end.The pri-miRNA which is the precursor contains hundrerds of nucleotides which are transcibed initially as in animals from an arm of 80 neucleotides RNA stem loop.When this stem loop is found at 3\'end then the precursor can do or it can act as pri-miRNA and mRNA.Now here comes the chance of RNA polymerase III.This comes to the field and transcibes MWIR (mammalian wide interspersed repeat promotor units),t RNAs, some miRNAs espically which have Alu sequencezs in their upstream.During the nuclear processing we came to know that one pri-miRNA contains atleast 1-6 precusors of miRNA.The hairpin loop contains 70 neucleotides each.The DiGeorge Syndrome Critical region 8(DGCR8) a nuclear protein is the indication of the hairpin loop oe we can say we can recognise the sequence in the pri-miRNA of dsRNA.nOW This DGCR8 combines with an enzyme called as DORSA and form a protein which cuts RNA and forms microprocessor complex.Now this complex with the help of RNA III cuts RNA at 11 neucleotides from the hairpin base and releases hairpins from the pri-miRNAs.Now at the 3\' end it has 3\'hydroxyl and 5\'phosphate group which came from the release of hairpins.It is known as precursor miRNA. Mitrons are the sequences/pri-miRNAs which are directly spliced from the introns from microprocessor complex.By the process of nuclear RNA editing 16% of pri-miRNAs.can be altered.Enzymes known as ADARs can catalyse tansitions.Nuclear processing and downstream processing can be halted by RNA editing.The enzyme called Exportin-5 an energy dependent protein helps in transporting pri-miRNAs from nucleus to cytoplasm.Now in cytoplasm the pri-miRNA hairpin are cleaved by the RNA III dicer. Dicer processing efficeincy is effected by the length and size of the loop of hairpin.It cuts away the loop and joins 3\' and 5\' arms.Some pri-miRNAs adopt G-quaduplex srtucture which is resistant to dicer mechanism.