88-1071: 1E1G , 1E1J , 1E1P , 1E1S , 1E1U , 1E1W , 1FKC , 1FO7 , 1H0L , 1HJM , 1HJN , 1I4M , 1OEH , 1OEI , 1QLX , 1QLZ , 1QM0 , 1QM1 , 1QM2 , 1QM3 , 2IV4 , 2IV5 , 2IV6 , 2K1D , 2KUN , 2LBG , 2LEJ , 2LFT , 2LSB , 2LV1 , 2OL9 , 2W9E , 3HAF , 3HAK , 3HEQ , 3HER , 3HES , 3HJ5 , 3HJX , 3MD4 , 3MD5 , 3NHC , 3NHD , 3NVF , 4DGI , 4E1H , 4E1I ,%%s 1E1G , 1E1J , 1E1P , 1E1S , 1E1U , 1E1W , 1FKC , 1FO7 , 1H0L , 1HJM , 1HJN , 1I4M , 1OEH , 1OEI , 1QLX , 1QLZ , 1QM0 , 1QM1 , 1QM2 , 1QM3 , 2IV4 , 2IV5 , 2IV6 , 2K1D , 2KUN , 2LBG , 2LEJ , 2LFT , 2LSB , 2LV1 , 2M8T , 2W9E , 3HAF , 3HAK , 3HEQ , 3HER , 3HES , 3HJ5 , 3HJX , 3MD4 , 3MD5 , 3NHC , 3NVF , 4DGI , 4E1H , 4E1I , 4KML , 4N9O 5621 19122 ENSG00000171867 ENSMUSG00000079037 P04156 P04925 NM_001271561 NM_001278256 NM_011170 NP_898902 NP_000302.1 NP_001073590.1 NP_001073591.1 NP_001073592.1 NP_898902.1 NP_001265185 NP_035300 The major prion protein ( PrP )
176-588: A promoter sequence. The promoter is recognized and bound by transcription factors that recruit and help RNA polymerase bind to the region to initiate transcription. The recognition typically occurs as a consensus sequence like the TATA box . A gene can have more than one promoter, resulting in messenger RNAs ( mRNA ) that differ in how far they extend in the 5' end. Highly transcribed genes have "strong" promoter sequences that form strong associations with transcription factors, thereby initiating transcription at
264-445: A continuous messenger RNA , referred to as a polycistronic mRNA . The term cistron in this context is equivalent to gene. The transcription of an operon's mRNA is often controlled by a repressor that can occur in an active or inactive state depending on the presence of specific metabolites. When active, the repressor binds to a DNA sequence at the beginning of the operon, called the operator region , and represses transcription of
352-498: A double-helix run in opposite directions. Nucleic acid synthesis, including DNA replication and transcription occurs in the 5'→3' direction, because new nucleotides are added via a dehydration reaction that uses the exposed 3' hydroxyl as a nucleophile . The expression of genes encoded in DNA begins by transcribing the gene into RNA , a second type of nucleic acid that is very similar to DNA, but whose monomers contain
440-488: A few genes and are transferable between individuals. For example, the genes for antibiotic resistance are usually encoded on bacterial plasmids and can be passed between individual cells, even those of different species, via horizontal gene transfer . Whereas the chromosomes of prokaryotes are relatively gene-dense, those of eukaryotes often contain regions of DNA that serve no obvious function. Simple single-celled eukaryotes have relatively small amounts of such DNA, whereas
528-434: A gene - surprisingly, there is no definition that is entirely satisfactory. A gene is a DNA sequence that codes for a diffusible product. This product may be protein (as is the case in the majority of genes) or may be RNA (as is the case of genes that code for tRNA and rRNA). The crucial feature is that the product diffuses away from its site of synthesis to act elsewhere. The important parts of such definitions are: (1) that
616-573: A gene corresponds to a transcription unit; (2) that genes produce both mRNA and noncoding RNAs; and (3) regulatory sequences control gene expression but are not part of the gene itself. However, there's one other important part of the definition and it is emphasized in Kostas Kampourakis' book Making Sense of Genes . Therefore in this book I will consider genes as DNA sequences encoding information for functional products, be it proteins or RNA molecules. With 'encoding information', I mean that
704-410: A gene may be split across chromosomes but those transcripts are concatenated back together into a functional sequence by trans-splicing . It is also possible for overlapping genes to share some of their DNA sequence, either on opposite strands or the same strand (in a different reading frame, or even the same reading frame). In all organisms, two steps are required to read the information encoded in
792-404: A gene's DNA and produce the protein it specifies. First, the gene's DNA is transcribed to messenger RNA ( mRNA ). Second, that mRNA is translated to protein. RNA-coding genes must still go through the first step, but are not translated into protein. The process of producing a biologically functional molecule of either RNA or protein is called gene expression , and the resulting molecule
880-578: A gene: that of bacteriophage MS2 coat protein. The subsequent development of chain-termination DNA sequencing in 1977 by Frederick Sanger improved the efficiency of sequencing and turned it into a routine laboratory tool. An automated version of the Sanger method was used in early phases of the Human Genome Project . The theories developed in the early 20th century to integrate Mendelian genetics with Darwinian evolution are called
968-439: A gene; however, members of a population may have different alleles at the locus, each with a slightly different gene sequence. The majority of eukaryotic genes are stored on a set of large, linear chromosomes. The chromosomes are packed within the nucleus in complex with storage proteins called histones to form a unit called a nucleosome . DNA packaged and condensed in this way is called chromatin . The manner in which DNA
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#17327978803621056-504: A globular domain with three α-helices and a two-strand antiparallel β-sheet , an NH 2 -terminal tail, and a short COOH-terminal tail. A glycophosphatidylinositol (GPI) membrane anchor at the COOH-terminal tethers PrP to cell membranes , and this proves to be integral to the transmission of conformational change; secreted PrP lacking the anchor component is unaffected by the infectious isoform. The primary sequence of PrP
1144-448: A high rate. Others genes have "weak" promoters that form weak associations with transcription factors and initiate transcription less frequently. Eukaryotic promoter regions are much more complex and difficult to identify than prokaryotic promoters. Additionally, genes can have regulatory regions many kilobases upstream or downstream of the gene that alter expression. These act by binding to transcription factors which then cause
1232-458: A large body of research has developed on candidates and their interaction with the PrP. Copper , zinc , manganese , and nickel are confirmed PrP ligands that bind to its octarepeat region. Ligand binding causes a conformational change with unknown effect. Heavy metal binding at PrP has been linked to resistance to oxidative stress arising from heavy metal toxicity . The precise function of PrP
1320-572: A new expanded definition that includes noncoding genes. However, some modern writers still do not acknowledge noncoding genes although this so-called "new" definition has been recognised for more than half a century. Although some definitions can be more broadly applicable than others, the fundamental complexity of biology means that no definition of a gene can capture all aspects perfectly. Not all genomes are DNA (e.g. RNA viruses ), bacterial operons are multiple protein-coding regions transcribed into single large mRNAs, alternative splicing enables
1408-400: A process known as RNA splicing . Finally, the ends of gene transcripts are defined by cleavage and polyadenylation (CPA) sites , where newly produced pre-mRNA gets cleaved and a string of ~200 adenosine monophosphates is added at the 3' end. The poly(A) tail protects mature mRNA from degradation and has other functions, affecting translation, localization, and transport of the transcript from
1496-419: A protein-coding gene consists of many elements of which the actual protein coding sequence is often only a small part. These include introns and untranslated regions of the mature mRNA. Noncoding genes can also contain introns that are removed during processing to produce the mature functional RNA. All genes are associated with regulatory sequences that are required for their expression. First, genes require
1584-412: A single genomic region to encode multiple district products and trans-splicing concatenates mRNAs from shorter coding sequence across the genome. Since molecular definitions exclude elements such as introns, promotors, and other regulatory regions , these are instead thought of as "associated" with the gene and affect its function. An even broader operational definition is sometimes used to encompass
1672-421: A stack of PrP molecules glued together by parallel in-register intermolecular beta sheets. This refolding renders the PrP isoform extremely resistant to proteolysis . The propagation of PrP is a topic of great interest, as its accumulation is a pathological cause of neurodegeneration . Based on the progressive nature of spongiform encephalopathies, the predominant hypothesis posits that the change from normal PrP
1760-475: A strict definition of the word "gene" with which nearly every expert can agree. First, in order for a nucleotide sequence to be considered a true gene, an open reading frame (ORF) must be present. The ORF can be thought of as the "gene itself"; it begins with a starting mark common for every gene and ends with one of three possible finish line signals. One of the key enzymes in this process, the RNA polymerase, zips along
1848-409: A true gene, by this definition, one has to prove that the transcript has a biological function. Early speculations on the size of a typical gene were based on high-resolution genetic mapping and on the size of proteins and RNA molecules. A length of 1500 base pairs seemed reasonable at the time (1965). This was based on the idea that the gene was the DNA that was directly responsible for production of
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#17327978803621936-428: Is 253 amino acids long before post-translational modification . Signal sequences in the amino - and carboxy - terminal ends are removed posttranslationally, resulting in a mature length of 208 amino acids. For human and golden hamster PrP, two glycosylated sites exist on helices 2 and 3 at Asn 181 and Asn197. Murine PrP has glycosylation sites as Asn180 and Asn196. A disulfide bond exists between Cys 179 of
2024-530: Is accompanied by a strong up-regulation of PrP, though it is not requisite. The lack of immunoresponse to transmissible spongiform encephalopathies (TSE), neurodegenerative diseases caused by prions, could stem from the tolerance for PrP. PrP-null mice provide clues to a role in muscular physiology when subjected to a forced swimming test , which showed reduced locomotor activity. Aging mice with an overexpression of PRNP showed significant degradation of muscle tissue. Though present, very low levels of PrP exist in
2112-738: Is associated with a variety of cognitive disorders and neurodegenerative diseases such as in animals: ovine scrapie , bovine spongiform encephalopathy (BSE, mad cow disease), feline spongiform encephalopathy , transmissible mink encephalopathy (TME), exotic ungulate encephalopathy , chronic wasting disease (CWD) which affects deer ; and in humans: Creutzfeldt–Jakob disease (CJD), fatal familial insomnia (FFI), Gerstmann–Sträussler–Scheinker syndrome (GSS), kuru , and variant Creutzfeldt–Jakob disease (vCJD). Similarities exist between kuru, thought to be due to human ingestion of diseased individuals, and vCJD, thought to be due to human ingestion of BSE-tainted cattle products. The human PRNP gene
2200-418: Is associated with axon and dendritic outgrowth with a series of kinases. Though most attention is focused on PrP's presence in the nervous system, it is also abundant in immune system tissue. PrP immune cells include hematopoietic stem cells, mature lymphoid and myeloid compartments, and certain lymphocytes ; also, it has been detected in natural killer cells , platelets , and monocytes . T cell activation
2288-454: Is attributable to Doppel gene expression. However, spatial learning , a predominantly hippocampal-function, is decreased in the null mice and can be recovered with the reinstatement of PrP in neurons; this indicates that loss of PrP function is the cause. The interaction of hippocampal PrP with laminin (LN) is pivotal in memory processing and is likely modulated by the kinases PKA and ERK1/2. Further support for PrP's role in memory formation
2376-456: Is called a gene product . The nucleotide sequence of a gene's DNA specifies the amino acid sequence of a protein through the genetic code . Sets of three nucleotides, known as codons , each correspond to a specific amino acid. The principle that three sequential bases of DNA code for each amino acid was demonstrated in 1961 using frameshift mutations in the rIIB gene of bacteriophage T4 (see Crick, Brenner et al. experiment ). Additionally,
2464-511: Is caused by the presence and interaction with PrP. Strong support for this is taken from studies in which PRNP -knockout mice are resistant to the introduction of PrP. Despite widespread acceptance of the conformation conversion hypothesis, some studies mitigate claims for a direct link between PrP and cytotoxicity . Polymorphisms at sites 136, 154, and 171 are associated with varying susceptibility to ovine scrapie . (These ovine sites correspond to human sites 133, 151, and 168.) Polymorphisms of
2552-551: Is derived from several population studies. A test of healthy young humans showed increased long-term memory ability associated with an MM or MV genotype when compared to VV. Down syndrome patients with a single valine substitution have been linked to earlier cognitive decline. Several polymorphisms in PRNP have been linked with cognitive impairment in the elderly as well as earlier cognitive decline. All of these studies investigated differences in codon 129, indicating its importance in
2640-470: Is encoded in the human body by the PRNP gene also known as CD230 ( cluster of differentiation 230). Expression of the protein is most predominant in the nervous system but occurs in many other tissues throughout the body. The protein can exist in multiple isoforms : the normal PrP form, and the protease -resistant form designated PrP such as the disease-causing PrP (scrapie) and an isoform located in mitochondria . The misfolded version PrP
2728-434: Is located on the short (p) arm of chromosome 20 between the end (terminus) of the arm and position 13, from base pair 4,615,068 to base pair 4,630,233. PrP is highly conserved through mammals, lending credence to application of conclusions from test animals such as mice. Comparison between primates is especially similar, ranging from 92.9 to 99.6% similarity in amino acid sequences . The human protein structure consists of
Major prion protein - Misplaced Pages Continue
2816-871: Is most closely associated with Alzheimer's disease. Variant V allele carriers (VV and MV) show a 13% decreased risk with respect to developing Alzheimer's compared to the methionine homozygote (MM). However, the protective effects of variant V carriers have been found exclusively in Caucasians . The decreased risk in V allele carriers is further limited to late-onset Alzheimer's disease only (≥ 65 years). PRNP can also functionally interact with polymorphisms in two other genes implicated in Alzheimer's, PSEN1 and APOE , to compound risk for both Alzheimer's and sporadic Creutzfeldt–Jakob disease . A point mutation on codon 102 of PRNP at least in part contributed to three separate patients' atypical frontotemporal dementia within
2904-400: Is nearly the same for all known organisms. The total complement of genes in an organism or cell is known as its genome , which may be stored on one or more chromosomes . A chromosome consists of a single, very long DNA helix on which thousands of genes are encoded. The region of the chromosome at which a particular gene is located is called its locus . Each locus contains one allele of
2992-429: Is not yet known. It may play a role in the transport of ionic copper into cells from the surrounding environment. Researchers have also proposed roles for PrP in cell signaling or in the formation of synapses . PrP attaches to the outer surface of the cell membrane by a glycosylphosphatidylinositol anchor at its C-terminal Ser 231. Prion protein contains five octapeptide repeats with sequence PHGGGWGQ (though
3080-475: Is one of several cellular receptors of soluble amyloid beta (Aβ) oligomers, which are canonically implicated in causing Alzheimer's disease . These oligomers are composed of smaller Aβ plaques, and are the most damaging to the integrity of a neuron . The precise mechanism of soluble Aβ oligomers directly inducing neurotoxicity is unknown, and experimental deletion of PRNP in animals has yielded several conflicting findings. When Aβ oligomers were injected into
3168-403: Is still part of the definition of a gene in most textbooks. For example, The primary function of the genome is to produce RNA molecules. Selected portions of the DNA nucleotide sequence are copied into a corresponding RNA nucleotide sequence, which either encodes a protein (if it is an mRNA) or forms a 'structural' RNA, such as a transfer RNA (tRNA) or ribosomal RNA (rRNA) molecule. Each region of
3256-399: Is stored on the histones, as well as chemical modifications of the histone itself, regulate whether a particular region of DNA is accessible for gene expression . In addition to genes, eukaryotic chromosomes contain sequences involved in ensuring that the DNA is copied without degradation of end regions and sorted into daughter cells during cell division: replication origins , telomeres , and
3344-417: Is the proposed cause. Some PRNP mutations lead to a change in single amino acids (the building-blocks of proteins) in the prion protein. Others insert additional amino acids into the protein or cause an abnormally short protein to be made. These mutations cause the cell to make prion proteins with an abnormal structure. The abnormal protein PrP accumulates in the brain and destroys nerve cells, which leads to
3432-470: The N-terminus . PrP is a conformational isoform of PrP, but this orientation tends to accumulate in compact, protease -resistant aggregates within neural tissue. The abnormal PrP isoform has a different secondary and tertiary structure from PrP, but identical primary sequence. Whereas PrP has largely alpha helical and disordered domains, PrP has no alpha helix and an amyloid fibril core composed of
3520-443: The PRNP gene have been identified in people with inherited prion diseases , which include the following: The conversion of PrP to PrP conformation is the mechanism of transmission of fatal, neurodegenerative transmissible spongiform encephalopathies (TSE). This can arise from genetic factors, infection from external source, or spontaneously for reasons unknown. Accumulation of PrP corresponds with progression of neurodegeneration and
3608-511: The aging process. The centromere is required for binding spindle fibres to separate sister chromatids into daughter cells during cell division . Prokaryotes ( bacteria and archaea ) typically store their genomes on a single, large, circular chromosome . Similarly, some eukaryotic organelles contain a remnant circular chromosome with a small number of genes. Prokaryotes sometimes supplement their chromosome with additional small circles of DNA called plasmids , which usually encode only
Major prion protein - Misplaced Pages Continue
3696-401: The central dogma of molecular biology , which states that proteins are translated from RNA , which is transcribed from DNA . This dogma has since been shown to have exceptions, such as reverse transcription in retroviruses . The modern study of genetics at the level of DNA is known as molecular genetics . In 1972, Walter Fiers and his team were the first to determine the sequence of
3784-419: The centromere . Replication origins are the sequence regions where DNA replication is initiated to make two copies of the chromosome. Telomeres are long stretches of repetitive sequences that cap the ends of the linear chromosomes and prevent degradation of coding and regulatory regions during DNA replication . The length of the telomeres decreases each time the genome is replicated and has been implicated in
3872-761: The cerebellum results in decreased motor coordination. However, this effect is not a direct result of PrP's absence, and rather arises from increased Doppel gene expression. Other observed differences include reduced stress response and increased exploration of novel environments. Circadian rhythm is altered in null mice. Fatal familial insomnia is thought to be the result of a point mutation in PRNP at codon 178, which corroborates PrP's involvement in sleep-wake cycles. In addition, circadian regulation has been demonstrated in PrP mRNA, which cycles regularly with day-night. While null mice exhibit normal learning ability and short-term memory , long-term memory consolidation deficits have been demonstrated. As with ataxia , this
3960-562: The cerebral ventricles of a mouse model of Alzheimer's, PRNP deletion did not offer protection, only anti-PrP antibodies prevented long-term memory and spatial learning deficits. This would suggest either an unequal relation between PRNP and Aβ oligomer-mediated neurodegeneration or a site-specific relational significance. In the case of direct injection of Aβ oligomers into the hippocampus , PRNP -knockout mice were found to be indistinguishable from control with respect to both neuronal death rates and measurements of synaptic plasticity . It
4048-554: The modern synthesis , a term introduced by Julian Huxley . This view of evolution was emphasized by George C. Williams ' gene-centric view of evolution . He proposed that the Mendelian gene is a unit of natural selection with the definition: "that which segregates and recombines with appreciable frequency." Related ideas emphasizing the centrality of Mendelian genes and the importance of natural selection in evolution were popularized by Richard Dawkins . The development of
4136-475: The neutral theory of evolution in the late 1960s led to the recognition that random genetic drift is a major player in evolution and that neutral theory should be the null hypothesis of molecular evolution. This led to the construction of phylogenetic trees and the development of the molecular clock , which is the basis of all dating techniques using DNA sequences. These techniques are not confined to molecular gene sequences but can be used on all DNA segments in
4224-750: The operon ; when the repressor is inactive transcription of the operon can occur (see e.g. Lac operon ). The products of operon genes typically have related functions and are involved in the same regulatory network . Though many genes have simple structures, as with much of biology, others can be quite complex or represent unusual edge-cases. Eukaryotic genes often have introns that are much larger than their exons, and those introns can even have other genes nested inside them . Associated enhancers may be many kilobase away, or even on entirely different chromosomes operating via physical contact between two chromosomes. A single gene can encode multiple different functional products by alternative splicing , and conversely
4312-508: The phenotype of the individual. Most biological traits occur under the combined influence of polygenes (a set of different genes) and gene–environment interactions . Some genetic traits are instantly visible, such as eye color or the number of limbs, others are not, such as blood type , the risk for specific diseases, or the thousands of basic biochemical processes that constitute life . A gene can acquire mutations in its sequence , leading to different variants, known as alleles , in
4400-449: The population . These alleles encode slightly different versions of a gene, which may cause different phenotypical traits. Genes evolve due to natural selection or survival of the fittest and genetic drift of the alleles. There are many different ways to use the term "gene" based on different aspects of their inheritance, selection, biological function, or molecular structure but most of these definitions fall into two categories,
4488-530: The synaptic cleft . In this role, the protein could serve as either a copper homeostasis mechanism, a calcium modulator, or a sensor for copper or oxidative stress. Loss of PrP function has been linked to long-term potentiation (LTP). This effect can be positive or negative and is due to changes in neuronal excitability and synaptic transmission in the hippocampus . Some research indicates PrP involvement in neuronal development, differentiation, and neurite outgrowth. The PrP-activated signal transduction pathway
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#17327978803624576-404: The DNA helix that produces a functional RNA molecule constitutes a gene. We define a gene as a DNA sequence that is transcribed. This definition includes genes that do not encode proteins (not all transcripts are messenger RNA). The definition normally excludes regions of the genome that control transcription but are not themselves transcribed. We will encounter some exceptions to our definition of
4664-450: The DNA sequence is used as a template for the production of an RNA molecule or a protein that performs some function. The emphasis on function is essential because there are stretches of DNA that produce non-functional transcripts and they do not qualify as genes. These include obvious examples such as transcribed pseudogenes as well as less obvious examples such as junk RNA produced as noise due to transcription errors. In order to qualify as
4752-766: The DNA to loop so that the regulatory sequence (and bound transcription factor) become close to the RNA polymerase binding site. For example, enhancers increase transcription by binding an activator protein which then helps to recruit the RNA polymerase to the promoter; conversely silencers bind repressor proteins and make the DNA less available for RNA polymerase. The mature messenger RNA produced from protein-coding genes contains untranslated regions at both ends which contain binding sites for ribosomes , RNA-binding proteins , miRNA , as well as terminator , and start and stop codons . In addition, most eukaryotic open reading frames contain untranslated introns , which are removed and exons , which are connected together in
4840-514: The Mendelian gene or the molecular gene. The Mendelian gene is the classical gene of genetics and it refers to any heritable trait. This is the gene described in The Selfish Gene . More thorough discussions of this version of a gene can be found in the articles Genetics and Gene-centered view of evolution . The molecular gene definition is more commonly used across biochemistry, molecular biology, and most of genetics —
4928-542: The PrP-VRQ form and PrP-ARQ form are associated with increased susceptibility, whereas PrP-ARR is associated with resistance. The National Scrapie Plan of the UK aims to breed out these scrapie polymorphisms by increasing the frequency of the resistant allele. However, PrP-ARR polymorphisms are susceptible to atypical scrapie, so this may prove unfruitful. The strong association to neurodegenerative diseases raises many questions of
5016-433: The adenines of one strand are paired with the thymines of the other strand, and so on. Due to the chemical composition of the pentose residues of the bases, DNA strands have directionality. One end of a DNA polymer contains an exposed hydroxyl group on the deoxyribose ; this is known as the 3' end of the molecule. The other end contains an exposed phosphate group; this is the 5' end . The two strands of
5104-402: The complexity of these diverse phenomena, where a gene is defined as a union of genomic sequences encoding a coherent set of potentially overlapping functional products. This definition categorizes genes by their functional products (proteins or RNA) rather than their specific DNA loci, with regulatory elements classified as gene-associated regions. The existence of discrete inheritable units
5192-445: The deletion of PRNP in both APPswe and SEN1dE9, two other transgenic models of Alzheimer's, attenuated the epilepsy-induced death phenotype seen in a subset of these animals. Taken collectively, recent evidence suggests PRNP may be important for conducing the neurotoxic effects of soluble Aβ-oligomers and the emergent disease state of Alzheimer's. In humans, the methionine / valine polymorphism at codon 129 of PRNP (rs1799990)
5280-524: The distinction between a heterozygote and homozygote , and the phenomenon of discontinuous inheritance. Prior to Mendel's work, the dominant theory of heredity was one of blending inheritance , which suggested that each parent contributed fluids to the fertilization process and that the traits of the parents blended and mixed to produce the offspring. Charles Darwin developed a theory of inheritance he termed pangenesis , from Greek pan ("all, whole") and genesis ("birth") / genos ("origin"). Darwin used
5368-490: The diversity of interactions, effects, and distribution, PrP has been proposed as dynamic surface protein functioning in signaling pathways. Specific sites along the protein bind other proteins, biomolecules, and metals. These interfaces allow specific sets of cells to communicate based on level of expression and the surrounding microenvironment. The anchoring on a GPI raft in the lipid bilayer supports claims of an extracellular scaffolding function. More than 20 mutations in
SECTION 60
#17327978803625456-410: The early 1950s the prevailing view was that the genes in a chromosome acted like discrete entities arranged like beads on a string. The experiments of Benzer using mutants defective in the rII region of bacteriophage T4 (1955–1959) showed that individual genes have a simple linear structure and are likely to be equivalent to a linear section of DNA. Collectively, this body of research established
5544-522: The fact that both protein-coding genes and noncoding genes have been known for more than 50 years, there are still a number of textbooks, websites, and scientific publications that define a gene as a DNA sequence that specifies a protein. In other words, the definition is restricted to protein-coding genes. Here is an example from a recent article in American Scientist. ... to truly assess the potential significance of de novo genes, we relied on
5632-407: The first repeat has the slightly modified, histidine -deficient sequence PQGGGGWGQ). This is thought to generate a copper- binding domain via nitrogen atoms in the histidine imidazole side-chains and deprotonated amide nitrogens from the 2nd and 3rd glycines in the repeat. The ability to bind copper is, therefore, pH -dependent. NMR shows copper binding results in a conformational change at
5720-414: The full functional significance of PRNP remains unclear, as PRNP deletion has been prophylactically implemented by the cattle industry without apparent harm. In mice, this same deletion phenotypically varies between Alzheimer's mouse lines, as hAPPJ20 mice and TgCRND8 mice show a slight increase in epileptic activity, contributing to conflicting results when examining Alzheimer's survival rates. Of note,
5808-426: The function of PrP in the brain. A common approach is using PrP-knockout and transgenic mice to investigate deficiencies and differences. Initial attempts produced two strains of PrP-null mice that show no physiological or developmental differences when subjected to an array of tests. However, more recent strains have shown significant cognitive abnormalities. As the null mice age, a marked loss of Purkinje cells in
5896-413: The functional product. The discovery of introns in the 1970s meant that many eukaryotic genes were much larger than the size of the functional product would imply. Typical mammalian protein-coding genes, for example, are about 62,000 base pairs in length (transcribed region) and since there are about 20,000 of them they occupy about 35–40% of the mammalian genome (including the human genome). In spite of
5984-630: The gene that is described in terms of DNA sequence. There are many different definitions of this gene — some of which are misleading or incorrect. Very early work in the field that became molecular genetics suggested the concept that one gene makes one protein (originally 'one gene - one enzyme'). However, genes that produce repressor RNAs were proposed in the 1950s and by the 1960s, textbooks were using molecular gene definitions that included those that specified functional RNA molecules such as ribosomal RNA and tRNA (noncoding genes) as well as protein-coding genes. This idea of two kinds of genes
6072-421: The genome. The vast majority of organisms encode their genes in long strands of DNA (deoxyribonucleic acid). DNA consists of a chain made from four types of nucleotide subunits, each composed of: a five-carbon sugar ( 2-deoxyribose ), a phosphate group, and one of the four bases adenine , cytosine , guanine , and thymine . Two chains of DNA twist around each other to form a DNA double helix with
6160-421: The genomes of complex multicellular organisms , including humans, contain an absolute majority of DNA without an identified function. This DNA has often been referred to as " junk DNA ". However, more recent analyses suggest that, although protein-coding DNA makes up barely 2% of the human genome , about 80% of the bases in the genome may be expressed, so the term "junk DNA" may be a misnomer. The structure of
6248-477: The intermediate template for the synthesis of a protein. The transmission of genes to an organism's offspring , is the basis of the inheritance of phenotypic traits from one generation to the next. These genes make up different DNA sequences, together called a genotype , that is specific to every given individual, within the gene pool of the population of a given species . The genotype, along with environmental and developmental factors, ultimately determines
6336-655: The liver and could be associated with liver fibrosis. Presence in the pituitary has been shown to affect neuroendocrine function in amphibians, but little is known concerning mammalian pituitary PrP. Varying expression of PrP through the cell cycle has led to speculation on involvement in development. A wide range of studies has been conducted investigating the role in cell proliferation, differentiation, death, and survival. Engagement of PrP has been linked to activation of signal transduction . Modulation of signal transduction pathways has been demonstrated in cross-linking with antibodies and ligand-binding (hop/STI1 or copper). Given
6424-424: The mental and behavioral features of prion diseases. Several other changes in the PRNP gene (called polymorphisms) do not cause prion diseases but may affect a person's risk of developing these diseases or alter the course of the disorders. An allele that codes for a PRNP variant, G127V, provides resistance to kuru . In addition, some prion diseases can be transmitted from external sources of PrP. PrP protein
6512-413: The nucleus. Splicing, followed by CPA, generate the final mature mRNA , which encodes the protein or RNA product. Many noncoding genes in eukaryotes have different transcription termination mechanisms and they do not have poly(A) tails. Many prokaryotic genes are organized into operons , with multiple protein-coding sequences that are transcribed as a unit. The genes in an operon are transcribed as
6600-410: The overall functionality of PrP, in particular with regard to memory. PrP is present in both the pre- and post-synaptic compartments, with the greatest concentration in the pre-synaptic portion. Considering this and PrP's suite of behavioral influences, the neural cell functions and interactions are of particular interest. Based on the copper ligand, one proposed function casts PrP as a copper buffer for
6688-431: The phosphate–sugar backbone spiralling around the outside, and the bases pointing inward with adenine base pairing to thymine and guanine to cytosine. The specificity of base pairing occurs because adenine and thymine align to form two hydrogen bonds , whereas cytosine and guanine form three hydrogen bonds. The two strands in a double helix must, therefore, be complementary , with their sequence of bases matching such that
6776-465: The same family, suggesting a new phenotype for Gerstmann–Sträussler–Scheinker syndrome . The same study proposed sequencing PRNP in cases of ambiguously diagnosed dementia, as the various forms of dementia can prove challenging to differentially diagnose . In 2006 the production of cattle lacking PrP form of the major prion protein (PrP) protein was reported which were resistant to prion propagation with no apparent developmental abnormalities. Besides
6864-401: The second helix and Cys214 of the third helix (human PrP numbering). PrP messenger RNA contains a pseudoknot structure ( prion pseudoknot ), which is thought to be involved in regulation of PrP protein translation . The mechanism for conformational conversion to the scrapie isoform is speculated to be an elusive ligand -protein, but, so far, no such compound has been identified. However,
6952-467: The strand of DNA like a train on a monorail, transcribing it into its messenger RNA form. This point brings us to our second important criterion: A true gene is one that is both transcribed and translated. That is, a true gene is first used as a template to make transient messenger RNA, which is then translated into a protein. This restricted definition is so common that it has spawned many recent articles that criticize this "standard definition" and call for
7040-424: The study of bovine products free of prion proteins another use could be so that human pharmaceuticals can be made in their blood without the danger that those products might get contaminated with the infectious agent that causes mad cow. A strong interaction exists between PrP and the cochaperone Hop ( Hsp70 / Hsp90 organizing protein; also called STI1 (Stress-induced protein 1)). Gene In biology ,
7128-461: The sugar ribose rather than deoxyribose . RNA also contains the base uracil in place of thymine . RNA molecules are less stable than DNA and are typically single-stranded. Genes that encode proteins are composed of a series of three- nucleotide sequences called codons , which serve as the "words" in the genetic "language". The genetic code specifies the correspondence during protein translation between codons and amino acids . The genetic code
7216-809: The term gemmule to describe hypothetical particles that would mix during reproduction. Mendel's work went largely unnoticed after its first publication in 1866, but was rediscovered in the late 19th century by Hugo de Vries , Carl Correns , and Erich von Tschermak , who (claimed to have) reached similar conclusions in their own research. Specifically, in 1889, Hugo de Vries published his book Intracellular Pangenesis , in which he postulated that different characters have individual hereditary carriers and that inheritance of specific traits in organisms comes in particles. De Vries called these units "pangenes" ( Pangens in German), after Darwin's 1868 pangenesis theory. Twenty years later, in 1909, Wilhelm Johannsen introduced
7304-436: The term gene , he explained his results in terms of discrete inherited units that give rise to observable physical characteristics. This description prefigured Wilhelm Johannsen 's distinction between genotype (the genetic material of an organism) and phenotype (the observable traits of that organism). Mendel was also the first to demonstrate independent assortment , the distinction between dominant and recessive traits,
7392-412: The term "gene" (inspired by the ancient Greek : γόνος, gonos , meaning offspring and procreation) and, in 1906, William Bateson , that of " genetics " while Eduard Strasburger , among others, still used the term "pangene" for the fundamental physical and functional unit of heredity. Advances in understanding genes and inheritance continued throughout the 20th century. Deoxyribonucleic acid (DNA)
7480-472: The word gene has two meanings. The Mendelian gene is a basic unit of heredity . The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA . There are two types of molecular genes: protein-coding genes and non-coding genes. During gene expression (the synthesis of RNA or protein from a gene), DNA is first copied into RNA . RNA can be directly functional or be
7568-450: Was first suggested by Gregor Mendel (1822–1884). From 1857 to 1864, in Brno , Austrian Empire (today's Czech Republic), he studied inheritance patterns in 8000 common edible pea plants , tracking distinct traits from parent to offspring. He described these mathematically as 2 combinations where n is the number of differing characteristics in the original peas. Although he did not use
7656-620: Was further found that Aβ-oligomers bind to PrP at the postsynaptic density , indirectly overactivating the NMDA receptor via the Fyn enzyme, resulting in excitotoxicity . Soluble Aβ oligomers also bind to PrP at the dendritic spines , forming a complex with Fyn and excessively activating tau , another protein implicated in Alzheimer's. As the gene FYN codes for the enzyme Fyn, FYN-knockout mice display neither excitotoxic events nor dendritic spine shrinkage when injected with Aβ oligomers. In mammals,
7744-430: Was shown to be the molecular repository of genetic information by experiments in the 1940s to 1950s. The structure of DNA was studied by Rosalind Franklin and Maurice Wilkins using X-ray crystallography , which led James D. Watson and Francis Crick to publish a model of the double-stranded DNA molecule whose paired nucleotide bases indicated a compelling hypothesis for the mechanism of genetic replication. In
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