Transposones
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This document discusses transposons, which are DNA segments that can move within a genome. Transposons carry genes and can generate DNA rearrangements that impact cell survival and evolution. They encode transposase proteins that catalyze the transposition process. There are different types of transposons based on their mechanism of movement, including cut-and-paste transposons, replicative transposons, and retrotransposons. Examples like Tn3 and bacteriophage Mu are provided. Transposons can cause mutations and have played a significant role in genome alteration and evolution over time.
Transposones
- 2. INTRODUCTION Transposons are DNA segments found in the genomes of living organism . Transposons are capable of moving from place to another place within a genome. It does not utilize an independent form of element like phage or plasmid but directly hops from one site to another.
- 3. They carry genes such as resistance to anti-bacterial, virulence and symbiotic factors and catabolism of xenobiotics. Transposons generate a variety of DNA rearrangements that have profound consequences for cell survival and evolution. Transposons are diverse in structure, highly regulated by a multitude of mechanism.
- 4. They are found to encode a special protein named transposase which catalyses the process of transposition. Transposons are particular to different groups of organism. Transposons have a major impact on changing the genetic composition of organism.
- 5. DEFINITION OF TRANSPOSONS “ A DNA sequence that is able to move or insert itself at a new location in the genome.” Transposons are small, mobile DNA sequences that move around chromosomes with no regard for homology and insertion of the transposons may produce deletions, inversions, chromosomal fusions and even more complicated rearrangement.
- 6. VARIOUS NAMES OF TRANSPOSONS Transposons have been variously called – Jumping genes Mobile elements Cassettes Insertion sequence Transposable elements
- 7. DISCOVERY OF JUMPING GENES Transposable elements were discovered by Barbara Mcclintock in 1965, through an analysis of genetic instability in maize. The instability involved the breakage of the chromosome and was found to occur at sites where transposable
- 8. CHARACTERISTICS OF TRANSPOSABLE ELEMENT They are DNA sequence that code for enzymes which bring about the insertion of an identical copy of themselves into a new DNA site. Transposition events involve both recombination and replication processes which frequently generate two daughter copies of the original transposable elements. one copy remains at the parent site while others appears at the target site.
- 9. The insertion of transposable elements invariably disrupts the integrity of their target genes. Transposable elements carry signals for the initiation of RNA synthesis, they sometimes activate previously dormant genes. A transposon is not a replicon, thus it cannot replicate apart from the host chromosome the way that plasmid and phage can. No homology exists between the transposons and the target site for its insertion.
- 10. TYPES OF TRANSPOSONS On the basis of their transposition mechanism, Transposons may be categorized into following types: 1 • Cut and paste • transposons 2 • replicative • transposons 3 • retro • elements
- 11. CUT AND PASTE TRANSPOSONS They transpose by cutting of the transposable sequence from one position in the genome and its insertion to another position within the genome. The cut and paste transposition involves two transposase subunits. Each transposase subunits binds to the specific sequences at the two ends of transposons. These subunit of transposase protein then come together and lead to the excision of transposon.
- 12. The excised ‘transposon-transposase complex’ then gets integrated to the target recipient site. In this manner, the transposons is cut from one site by a mechanism mediated by transposase protein. Examples of cut and paste type of transposons are IS-elements, P-elements, in maize, hobo- elements in drosophila etc.
- 15. REPLICATIVE TRANSPOSONS They transpose by a mechanism which involves replication of transposable sequence and this copy of DNA, so formed, is inserted into the target site while the donor site remains unchanged. In this type of transposition, there is a gain of one copy of transposon and both the donor and the recipient DNA molecule are having one-one transposable sequence each, after transposition. Example- Tn3-elements.
- 17. RETRO ELEMENTS Their transposition is accomplished through a process which involves the synthesis of DNA by reverse transcription by using elements RNA as the template. This type of transposition involves an RNA intermediate, the transposable DNA is transcribed to produce an RNA molecule. This RNA is then used as a template for producing a complementary DNA by the activity of enzyme reverse transcriptase.
- 18. The single stranded DNA copy so formed, is then made double stranded and then inserted into the target DNA site. The transposable elements which requires reverse transcriptase for their movement are called retro transposons. The retro elements may be viral or non-viral. Out of these two, non-viral retro elements are important and may further be classified as :
- 19. Retrovirus like elements carry long terminal repeats(LTR). Examples are copia, gypsy elements in drosophila. In retroposon LTR are absent. Examples are LINEs and SINEs in human. RETROVIRUS LIKE ELEMENTS RETROPOSONS
- 21. TRANSPOSABLE ELEMENTS IN PROKARYOTES Bacterial transposable elements are of following types :- Insertion sequences or IS elements Prokaryotic transposon element
- 22. INSERTION SEQUENCE OR IS ELEMENTS They are the transposable sequence which can insert at different sites in the bacterial chromosome. IS elements contains ITRs(inverted terminal repeats), these were first observed in E.coli. IS-elements are relatively short usually not exceeding 2500 bp. The IRTs present at the ends of IS-elements are an important feature which enables their mobility.
- 23. The IRTs present in the IS-elements of E.coli usually range between 18-40 bp. The term ‘Inverted Terminal Repeat’(ITR) implies that the sequence at 5 end of one strand is identical to the sequence at 5’ end of the other strand but they run in inverse opposite direction . In E.coli chromosome, a number of copies of several IS-element like IS1, IS2, IS3, IS4 and IS5 are present.
- 25. PROKARYOTIC TRANSPOSON ELEMENT These are also called composite transposons and are shown by the symbol ‘Tn’. It is made up of two IS elements, one present at each end of a DNA sequence, which contains genes whose functions are not related to the transposition process. These transposons have been found to have inverted repeats at the ends. the length of these inverted repeats ranges from a few nucleotides to about 1500 bp.
- 26. It can be said that these are the large transposons which are formed by capturing of an immobile DNA sequence within two insertion sequence thus enabling it to move. Examples of such transposons include the members of Tn series like, Tn1, Tn5, Tn9, Tn10, etc.
- 28. EXAMPLES OF TRANSPOSONS Tn3 transposons of E.coli Tn3 has 4957 bp and contains three genes such as tnp A, tnp R and bla,coding respectively for the following proteins : 1) Transposase – having 1015 amino acids and required for transposition. 2) Repressor(resolvase) – containing 185 amino acids which regulates the transposase. 3) Beta-lactamase – enzyme which confers resistance to the antibiotic ampicillin. On both sides of the Tn3 is an inverted repeat of about 38 bp.
- 30. BACTERIOPHAGE MU • The bacteriophage Mu is a temperate bacteriophage having typical phage properties and could be regarded as a giant transposon. • Phage Mu was thought to behave in a unique manner since it does not multiply in a lytic way. • It always starts its life cycle by inserting itself into the E.coli chromosome at random locations.
- 31. • Thus, phage Mu was originally known for its mutagenic properties. • During later part of phage life cycle, upon receipt of an appropriate signal, the Mu prophage somehow generates its transposition, but also activates the gene encoding for structural proteins which packages its DNA. • Mu phage is unique also in transposing much often than other transposons, providing nearly a hundred new copies of Mu during its hour-
- 33. SIGNIFICANCE OF TRANSPOSABLE ELEMENT Transposons may change the structural and functional characteristics of genome by changing their position in the genome. Transposable element cause mutation by insertion , deletion, etc. It makes positive contribution in evolution as they have tremendous impact on the alteration of genetic organisation of organisms.
- 34. They are useful as cloning vectors also, in gene cloning. For example, P-elements are frequently used as vector for introducing transgenes into drosophila. Transposons may also be used as genetic marker while mapping the genomes. Transposon-mediated gene tagging is done for searching and isolation of a particular gene.
- 35. REFERENCE PRESCOTT.HARLEY.KLEIN;SIXTH EDITION INTERNATIONAL EDITION 2005;MICROBIOLOGY;PAGE NO-291-295. VERMA P.S.AGARWAL V.K; 2004; CELL BIOLOGY,GENETICS,MOLECULAR BIOLOGY,EVOLUTION AND ECOLOGY;PAGE NO-254-260. WWW.BIOLOGYDISCUSSION.COM
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