Contents•Introduction•Discovery of transposable elements•Nomenclature•General characteristics• Types of transposable elements•Mechanism of transposition•Mutagenic effects of transposition•Regulation•Transposable Elements in Bacteria.•Transposable Elements in Fungi.•Transposable Elements in Eukaryotes.•Conclusion•References
INTRODUCTIONBarbara McClintock first discoveredtransposable elements in corn in the 1940. Transposable elements, transposons oreven jumping genes are regions of genomethat can move from one place to another. The first transposable element isdiscovered in bacteria is called insertionsequences or IS elements. It turns out thatthese are the simplest transposons.Comprises about 45% in humangenome. Inserts at many different locations.
Discovery of transposable elementsIn 1940s by McClintock Barbara in maizeFound genetic elements regularly jump to new location affect gene expressionMaize kernels show variation in colour.Later in 1960s bacteria & bacteriophages were shown to posses TE.Development of recombinant technology demonstrate TE exist in all organisms.
NomenclatureCampbell et al in 1977 described the nomenclature inprokaryotes.Initially named as insertion sequences- IS IS1, IS2, IS3etc.,In bacteria transposons containing genes for antibioticresistance are named as Tn like Tn1, Tn2, etc.The number distinguish different transposons, those arerepresented by standard genotypic designation such as Tn1[ampr], where ampr refers that the transposons carries thegenetic locus or Ampicilin resistanceIn eukaryotes named in nonstandard way ex: Drosophila- copia,497, p-elements yeast- Ty, Maize- Ds& Ac , Human-Alu
General characteristics of TEThey were found to be DNA sequences that code forenzymes, which bring about the insertion of anidentical copy of themselves into a new DNA site.Transposition events involve both recombination andreplication processes which frequently generate twodaughter copies of the original transposable elements.One copy remains at the parent site and anotherappears at the target site.A transposable element is not a replicon. Thus, Itcannot replicate apart from the host chromosome.
Types of transposable elementsDifferent types of transposable elements are presentin both prokaryotes and eukaryotes.There 3 types in prokaryotes a) Insertion sequences b) Transposons c) Bacteriophage µ
Insertion sequence:IS were the first transposableelements identified asspontaneous insertion in somebacterial operon.The IS are shorter (800 to 1500base pairs) and do not code forproteins.In fact, IS carry the geneticinformation necessary for theirtransposition (the gene for theenzyme transposase).There are different IS such asIS1, IS2, IS3 and IS4 and so on inE.coli.
Transposons:Transposons are similar to IS elements but carryadditional genes.Tn are several thousands base pair long and havegenes coding for one or more proteinsOn either side of a transposon is a short directrepeat. The sequence into which the transposableelement insert is called target sequence
Two types of transposona) Composite transposonb) Noncomposite transposonComposite transposon:Any segment of DNA that flanked by two copies of an IS andcentral coding region with antibiotic resistant gene and nomarker gene.Designated by the Tn.
Noncomposite transposon:Do not terminate with IS elements but contain terminalinverted repeats.Has three genes at central region1.bla-beta-lactamase-breaks amphicilin2.tnpA-Transposase-for insertion3.tnpB-resolvase-recombinational events
Bacteriophage MuThe longest transposon knows so far.Caries numerous genes for viral head and tailformation.The vegetative replication of mu produces about 100viral chromosomes in a cell arises from thetransposition of Mu to about 100 different target sites.Therefore considered as giant mutator transposon.
Mechanism of transpositionMovement of transposon occurs only when enzyme tansposaserecognizes and cleaves at either 5’ or 3’ of both ends oftransposon and catalysis at either 5’ or 3’ of both the ends oftransposon and catalysis staggered cut at the target site. Depending on transposon, a duplication of 3 to 12 bases of target DNA occurs at the site where insertion is to be done. One copy remains at each end of the transposon sequence. After attachment of both ends of transposon to the target site, two replication forks are immediately formed. This stage there starts two paths for carrying out onward.
TranspositionMechanism of movement of TE from onelocation to another.In the process staggered cuts are made in thetarget DNA.The TE is joined to single stranded ends of thetarget DNAFinally DNA is replicated in the single strandedgap.
Mutagenic effects of transposition• Sometimes it activate a gene or change the phenotype of the Transposon is generally mutagenic.• e cell in a beneficial way.• Mutagenic effect can be best studied in color varieties of grapes which come in red, white, and black.• White are resulted from the mutation in the black grapes due to transposition of Gret1 to gene which codes for anthocyanin.
Regulation of transposition Many cells regulate transposition by limiting the production of the transposase enzyme required for movement. some other regulatory mechanism directly inhibit the transposition event.
Transposable elements in Bacteria There are three main types: the insertion sequences or IS elements, composite transposons, and the Tn3 elements.IS Elements: IS elements are compactly organised. Typically, they consist of fewer than 2500 nucleotide pairs and contain only genes whose product is involved in promoting or regulating transposition
At least some IS elements encode a protein that is needed fortransposition. This protein, called transposase, seems to bind at or nearthe ends of the element, where it cuts both strands of the DNA. Cleavageof DNA at these sites excises the element from the chromosome orplasmid, so that it can be inserted at a new position in the same or adifferent DNA molecule. IS elements are therefore cut –and – pastemechanism.
Composite Transposons:Composite transposons, which arebacterial cut-and-paste transposonsdenoted by the symbol Tn, are createdwhen two IS elements insert near eachother. In Tn9, the flanking IS elementsare in the same orientation withrespect to each other, whereas in Tn5and Tn10, the orientation is inverted.The region between the IS elements ineach case of these transposonscontains gene that have nothing to dowith transposition.
Tn3 Elements:The elements in this group of transposonsare larger than the IS elements and usuallycontain genes that are not necessary fortransposition. The transposition of Tn3 occurs in two stages. First, the transposase mediates the fusion of two circular molecules, one carrying Tn3 and other not. The resulting structure is called a Cointegrate. During this process, the transposon is replicated, and one copy is inserted at each junction in the cointegrate are oriented in the same direction. In the second stage tnpR encoded resolvase generates two molecules, each with a copy of the transposon.
Transposable elements in fungiThe TEs found in three orders of Fungi, Ascomycota, Basidomycota, andZygomycota. However, most were identified in Ascomycota species.Fungus TEs are divided into two main classes by their mode oftransposition and structural organization.1. Class I elements or retroelements, transpose by a “copy-and-paste” mechanism by the reverse transcription of an RNA intermediate.This class is sub divide into LTR Retrotransposons, which are flanked bylong terminal repeats sharing an overall organization similar toretrovimses, and non-LTR retroelements, which have structural featuresof long and short interspersed nuclear elements( LINEs and SINEs,RESPECTIVELY.) 2. Class II TEs, also called DNA transposons, are flanked by twoterminal inverted repeats ( TIRs) and transpose directly through a DNAform by a “cut-and-paste” mechanism.
Transposable elements in eukaryotes:In eukaryotes TE can be divided into 2 groupsOne group is structurally similar to TE found inbacteria.Other is retrotransposon, they use RNAintermediates.These include the Ty elements in yeast, copiaelements in Drosophila, Alu sequences in humans.
Transposons in maizeThe bacterial transposons were discoveredin 1940s by Barbara McClintock whoworked with maize. She found that theywere responsible for a variety of types ofgene mutation, usually Insertion Deletion Translocation
Ty elements in yeast⃗Ty elements are a retrotransposon found in yeast.⃗More than 30 copies of Ty elements present⃗At the end direct repeats called delta sequencesof 334-bp long present.⃗These are analogous to long terminal repeatsfound in retroviruses
Transposons in DrosophilaP elements are class II transposonsfound in drosophila’s they do littleharm because expression of theirtransposase gene is usually repressed.However, when male flies with pelements mate female flies lackingthem, the transposase becomes activein the germ line producing so manymutations that their offspring aresterile. P elements seem to have firstappeared I drosophila melanogasterabout 50 years a
Elements in HumansAbout 45% of human genome consists of sequence derivedfrom TE.Common TE in human genome is Alu transposed through anRNA intermediate.Alu belongs to repetitive sequences are collectively called asSINE’s constitute 11% of human genome.It also has LINE’s usually about 6000bp constitute 21% ofhuman genome.These two are identified as cause of mutations in 20 cases ofgenetic diseases
Effects caused by TransposonsI. Transposons are mutagens. They can cause mutations in several ways.II. A transposons inserts itself into a functional gene, it will probably damage it. Insertion into exons, introns, and even into DNA flanking the genes can destroys or alter the genes activity.III. Mutations responsible for some human geneticdiseased, including, a. Hemophilia A, Hemophilia B. b. X-linked severe combined immunodeficiency c. Porphyria d. Cancer , etc,.
Uses of Transposons.As cloning vehiclesTransformation vectors for transferring genes betweenorganisms.Also drug resistance genes encoded by manytransposons are useful in the development of plasmidsas cloning vehicles.Transposons mutagenesis:Use of transposons to increase rate of mutation due toinsertional inactivation
Conclusion:Transposons are present in the genomes of allorganisms, where they can constitute a huge fraction ofthe total DNA sequence. They are a major cause ofmutations and genome rearrangement.The ability of transposable elements to insert and togenerate deletions and inversions accounts for much ofthe macromolecular rearrangement.They cause mutation which is used in the production ofdifferent colour of grapes, corn and other fruits.As a result they are used in the genetic studies.
Referenceso Benjamin lewin.Genes.1997.Newyork. Printed in USA.o Daniel L.Hartl.Elizabeth.w.Jones., Genetics-Analysis of genesand genomes. 6th edition.2005.USAo Benjamin A.pierce.Genetics: A conceptual approach.thirdedition.Newyork 2008.o Monroe W.Stickberger.Genetics.third edition. 1985. USA.Websites: www.en.wikipedia.org/wiki www.ncbi.nim.nih.gov www.pnos.org www.biomedcentral.com