transposable element

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TRANSPOSONS Also called as transposable elements/jumping genes / mobile DNA These are DNA sequences that move from one location in a chromosome to another location within the same chromosome or into another chromosome Discovered by Barbara McClintock largely from cytogenetic studies in maize, but now found in many organisms. Discovered by Barbara McClintock, largely from cytogenetic studies in maize, but since found in most organisms

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Studies of the genetic basis of this variation led to the discovery of TRANSPOSABLE ELEMENTS 1983 Nobel prize in physiology and medicine. Ac-Ds system in maize(1950)

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Autonomous element : Ability of transposition and encodes active transposase. Non autonomous element : don't have the ability to transpose, they can transpose only when an autonomous member of same family is present in same nucleus.

TYPES 1.cut and paste transposons.:

TYPES 1.cut and paste transposons. One group propagates itself by transposition as DNA copies e.g.: insertion sequence , composite transposons. Transposition is accomplished by excising an element from its position in a chr. and inserting in another position . Excision & insertion events are catalyzed by enzyme transposase. ( encoded by element itself)


2.REPLICATIVE TRANSPOSONS Replication of transposable element’s DNA takes place. Transposase mediate interaction between element and potential insertion site. During interaction element is replicated and one copy inserted to new site.


3.RETROTRANSPOSONS Insertion of copies of an element that were synthesized from the element’s RNA. Enzyme reverse trascriptase uses element’s RNA as template to synthesize DNA molecules. These DNA molecules get inserted to new site. E.g.: retro viruses


MECHANISM OF MOVEMENT Replicative transposition : element is duplicated during reaction Enzymes: Transposase -acts at the end of original transposon Resolvase : acts on duplicated copies

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Non replicative transposition : Transposing element moves from one site to another and is conserved. Requires only transposases . Conservative transposition: Another type of non replicative event in which excision from donor site and insertion to target site. Nucleotide bond is conserved.


TRANSPOSABLE ELEMENT IN HUMAN 4 Basic transposable element 1.LINE(Long interspersed nuclear element) 2.SINE(short interspersed nuclear element) 3.Retrovrus like elements 4.Cut and paste transposons. These account for 44% of all human DNA


SIGNIFICANCE Major cause of mutation and chromosome rearrangement Some transposon confer selective advantage on the organism that carry them, others are genetic parasites. It can be called as nature’s tool for genetic engineering The first transposon to be molecularly isolated was from the plant snapdragon


INSERTION SEQUENCE The simplest bacterial transposons are insertion sequences or IS elements. It is called so because it can insert at many different sites in bacterial chromosomes and plasmids. They are compactly organized and contains genes whose products are involved in promoting or regulating transposition. IS elements are demarcated by short identical or nearly identical sequences at its ends. These terminal sequences are always in inverted orientation w.r.t each other, so called as INVERTED TERMINAL REPEATS (90-40 nucleotide pairs) Short DNA of about 800-1400 bp ,have ability to transpose. Eg.E.coli IS elements These are transposons that code for enzymes needed for transposition flanked by short inverted terminal repeats. Integration of IS element within a gene prevents the fn. of the gene & causes mutation. IS elements promotes illegitimate recombination.

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IS elements when inserted into chromosome , creates duplication of part of DNA sequence at the site of insertion . Duplications are located on either side of the elements. These short directly repeated sequence are called TARGET SITE DUPLICATION (2-13 nucleotide pairs)

Events during insertion :

Events during insertion 1)Staggered breaks in target DNA 2)Transposon joins to the protruding single stranded ends. 3)Remaining gaps are then filled generating the repeat of target DNA at the site of insertion


CHAPERONES These are proteins that mediate correct assembly by causing a target protein to acquire one possible conformation instead of others. They prevent the formation of incorrect structures by interacting with unfolded proteins to prevent them from folding incorrectly. Chaperonin system consists of oligomeric assembly which forms a structure into which unfolded proteins are inserted. Heat shock protein 70 (Hsp 70) & some associated proteins form major class of chaperones that act on target proteins. Chaperonins which are molecular machines use chemical energy in the form of ATP to promote protein folding in all cells.

Cellular Stresses Promote Misfolding and Aggregation of Proteins:

Cellular Stresses Promote Misfolding and Aggregation of Proteins High temperatures Heavy metals Alcohol Anoxia Osmotic stress Energy starvation Reducing agents

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Group 1 Found in bacteria, organelles of endosymbiotic origin GRoEL - Double ring GRoES -Single ring Group 2 Found in eukaryotic cytosol TRiC-Ring complex


ROLE AFTER SYNTHESIS when the protein after synthesis exits the ribosome to enter the cytosol it appears as unfolded. Spontaneous folding then occurs due to interaction b/w different surfaces. Chaperones influence the folding process by controlling the accessibility of the reactive surfaces.

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DURING DENATURATION During denaturation new regions are exposed and become able to interact. These interactions are recognized by chaperones Chaperones help in renaturation or lead to its removal by degradation


MECHANISM OF ACTION Chaperonins undergo conformational changes during folding reaction due to enzymatic hydrolysis of ATP. These conformational changes allow chaperonins to bind an unfolded/misfolded protein It encapsulates the protein within one of its cavities formed by the two rings & release the protein back into solution. Upon release the protein will either be folded or requires further rounds of folding

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As the ribosome moves along the chain of mRNA,a chain of amino acid is built up to produce new protein molecule. The chain is protected against unwanted interactions with other cytoplasmic molecules by chaperonins until it has successfully completed its folding. Chaperonins assist the folding of nascent polypeptide into their functional state. Cells respond to abrupt rise in temperatue by inducing synthesis of HSPs