JUMPING

GENESSahar Zaidi14- MBT -14

CONTENTS• Introduction•History•General Characteristics•Mechanism•Types of transposons•Transposition•Transposable elements in bacteria and humans•Mutagenic effects•Applications•Conclusion

INTRODUCTIONJUMPING GENES:- are moderately repetitive DNA sequences interspersed at multiple sites through out the genome that can move or jump from one location in the genome to another.

These sequences are found in all organisms.

45% of human genome. 90% of maize genome.

HISTORY

Barbara Mc Clintock discovered transposableelements in 1948 in maize for which she wasawarded noble prize.

GENERAL CHARACTERISTICS•These elements encode enzymes that can insert their

sequence into new sites.

•Terminal inverted repeats are generally present at the ends.

•They generate direct repeats at the point of insertion.

GENERAL MECHANISM

TYPES OF TRANSPOSONS1. Retrotransposons2. DNA transposons.

RETROTRANSPOSONS Also called as CLASS 1 elements.

Transpose through RNA intermediate, are transcribed from transposable element and are then copied back by reverse transcriptase.

Retrotransposons are more common in eukaryotes.

DNA TRANSPOSONSAre CLASS 2 elements.

These transpose to new sites directly as DNA.

These are more common in prokaryotes.

TRANSPOSITIONThe process by which these sequences are copied and inserted is called as transposition.

On the basis of mechanism involved.1. Replicative transposition.2. Non replicative transposition.

REPLICATIVE TRANSPOSITION.In this type of transposition, a new copy of transposable element is introduced at a new site, old copy remains behind at original site.

NON REPLICATIVE TRANSPOSITIONIn this type of transposition, the transposable element excises fromold site and inserts at a new site.

TRANSPOSABLE ELEMENTS IN BACTERIA

1.Insertion sequences (IS). 2.Composite transposons. 3.Non composite transposons.

They generally use non-replicative transposition.

INSERTION SEQUENCES•They are the smallest and simplest transposable elements as they carry only the genetic information that is needed to sponsor their own transposition.

•They are denoted by IS followed by a number.

COMPOSITE TRANSPOSONS•Transposons that carry genes (like antibiotic resistance) in addition to those coding for transposition.

•They have a central region flanked by an IS element at each end.

•A composite transposon may transpose as a unit , but an active IS element at either end may also transpose independently.

•These are named Tn followed by a number.

NON COMPOSITE TRANSPOSONS•These transposable elements lack IS and are bracketed by short inverted repeats.•Carry genetic information for transposase and resolvase plus a gene that encodes for enzyme lactamase that provides resistance to ampicillin.

TRANSPOSONS IN HUMANS98% of transposable elements in humans are retrotransposons.

1. LINEs (long interspersed nuclear elements)2. SINEs (short interspersed nuclear elements)

LINES Represents 17% of human genome.

LINEs are retrotranspositionally active elements.

L1 LINE family are most commonly in human genome.

They are approximately 6kb in length. The consensus sequence contains 2 large reading frames• ORF1 encodes an RNA binding protein.• ORF2 encodes protein having both endonuclease and reverse transcriptase activity.

SINESThese represent 11% of human genome

•SINES are dependent on LINES for retrotransposition.

•These are 100 to 400bp long.

•Most common SINES in human genome are Alu elements

•Alu are 300bp long and 1 million copies per cell are present.

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MUTAGENIC EFFECTS• TEs are mutagens, they can damage the genome of their host cell in different ways:-• a transposon or a retroposon that inserts itself into a functional gene will most likely disable that gene;• after a DNA transposon leaves a gene, the resulting gap will probably not be repaired correctly;• multiple copies of the same sequence, such as Alu sequences can hinder precise chromosomal pairing during mitosis and meiosis , resulting in unequal crossovers one of the main reasons for chromosome duplication.• Diseases that are often caused by TEs include haemophilia, severe combined immunodeficiency , predisposition to cancer, and Duchenne muscular dystrophy LINE1 (L1) TEs that land on the human Factor VIII caused haemophilia and insertion of L1 into APC gene caused colon cancer and this confirms that the TEs play an important role for disease development.

USES OF TRANSPOSONS Transposable elements are nature`s tool of genetic engineering.•Transformation vectors for transferring genes.•Also drug resistance genes encoded by many transposons are useful in the development of plasmid as cloning vehicles.• Insertional mutagenesis. The Sleeping Beauty transposon system is a synthetic DNA transposon designed to introduce precisely defined DNA sequences into the chromosomes of vertebrate animals for the purposes of introducing new traits and to discover new genes and their functions.

CONCLUSION•Early view of mobile elements as junk DNA or completely selfish molecular parasites appears to be premature. Rather, they probably have made profound contributions to the evolution of higher organisms by serving as the sites of recombination, leading to the evolution of novel genes and new controls on gene expressions.

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