Transposons

Transposons ppt

1. BY: SANJAY SINGH BHOJ M.SC 2 nd SEMESTER,ZOOLOGY TRANSPOSABLE ELEMENTS

2. CONTENTS INTRODUCTION. DISCOVERY OF TRANSPOSABLE ELEMENTS IN MAIZE. CHARACTERISTIC OF TRANSPOSABLE ELEMENTS. MECHANISM OF TRANSPOSITION (A) DNA INTREMEDIATES (B) RNA INTERMEDIATES. TRANSPOSABLE ELEMENT IN PROKARYOTES & EUKARYOTES .

3. INTRODUCTION Transposable elements are mobile DNA sequences found in the genome of all organisms. Pieces of DNA or transposable elements can move to new positions within the same chromosome or even to a different chromosome. By which these sequences are copied & inserted into a new site in the genome is called transposition . They have been detected genetically in model organisms such as E.coli, maze, yeast , Drosophilia through the mutation that they produce when they inactivate genes into which they insert.

4. DISCOVERY OF TRANSPOSABLE ELEMENTS IN MAIZE The first person to suggest that genetic elements were capable of moving around the genome was Barbara McClintock. In the late 1940 ,McClintok found that certain mutations were very unstable appearing & disappearing from one generation to the next or even during the life time of an individual plant. She called this genetic rearrangement , Transposition & mobile genetic element Then, in the late 1960 several laboratories discovered that certain DNA sequence in bacteria moved on rare occasion from one place in the genome to another. These bacterial transposable element were called tranposons

5. Kernels of corn are typically uniform in color. The spots on this kernel result from a mutation in gene that codes for an enzyme involved in pigment production. Example of transposable element :

6. Characteristics of transposons . C T G A T G A T C A T C A T C T A C T A G T A G T A G A T G A T C A T C A C T A G T A G T A A T Staggered cut are made in the target DNA G A T G A T C A T C C T A T A C T A G T A G T A Staggered cut leave short, single stranded Pieces of DNA Transposable element (1) Staggered cut

7. G A T G A T C A T C A C T A G T A G T A C T A C T A G T A G T G A T C A T C A T Gape filled in by DNA polymerase. Replication of single Stranded DNA creates the Flanking direct repeats. FLANKING DIRECT REPEAT (2) Terminal inverted repeats At the end of many but not all transposable element are terminal inverted repeats, 9 to 40 bp in length that are complementary of one another. For example: 5’G A G C ----- G C T C3’ 3’C T C G ----- C G A G5’

8. Mechanism of transposition Several different mechanisms are required for transposition in both prokaryotic & eukaryotic cell. Some transpose through DNA , whereas other use RNA intermediates . (1) DNA intermediates (2) RNA intermediates Replicative or Nonreplicative Replicative (copy & paste) : a new copy of the transposable element is introduced at a new site while the old copy remains at the original site, the number of copies of the transposable element increases.

9. Nonreplicative (cut & paste) : the transposable element excises from the old site & insert at a new site. Although short sequence in the target DNA are replicated, generating flanking direct repeats. Cleavage requires a transposase enzyme .The joining of target DNA & transposable element is carried out by normal replication & repair enzyme .

10. Transposition through an RNA intermediate Transcription Reverse transcription Staggered cuts Old copy New copy Replication fills, gaps & create flanking direct repeats (b) (c) (d) (e) (a)

11. Transposable element in prokaryote The simplest type of transposable element in bacterial chromosome & plasmid is INSERTION SEQUENCE (IS). This type of element carries only the genetic information necessary for its movement. Insertion sequence(IS) are typically from 800 to 2000 bp in length & possess mainly two characters: (A) Terminal inverted repeat (B) Flanking direct repeat An inverted repeat, usually containing 50 bp, present at each end of an insertion sequence, between the inverted repeats is a region that encodes TRANSPOSAE , an enzyme required for transposition of the insertion sequence(IS).

12. IS Element General structure of bacterial Is element Any segment of DNA that becomes flanked by two copies of an insertion sequence is called a . composite transposon s

13. Noncomposite Transposons Some transposable element in bacteria lack insertion (IS) sequence & are referred to as noncomposite transposons. Noncomposite transposons are flanked by (IR) sequence, these sequence are shorter & do not encode the transposase enzyme that is necessary for transposition. Noncomposite tansposons encode their own transposase in addition to carrying bacterial genes. An example of noncomposite Transposons is Tn 3 is shown in figure.

14. Transposable element in eukaryotes Eukaryotic transposable elements falls into two classes: (1) Retrotansposons : Eukaryotic transposable element that transpose through RNA intermediate are called retrotransposons. Resembled a well characterized class of animal viruses called retrovirus, like presence of LTRs, gag & pol genes. Retrotransposons also generate direct repeats at the point of insertion. Retrotransposons includes: Ty elements in yeast, copia element in Drosophilia Alu sequences in humans.

15. (2) DNA transposons: Some mobile elements found in eukaryotes appear to transpose by mechanism similar to those in bacteria. Transposable Elements In Human Genome Almost half of the human genome is derived from transposable elements. Majority of these transposable elements are of two type of retrotransposons: (1) Long interspersed nuclear elements or LINEs : LINEs move by retrotransposition with the use of an element encoded reverse transcriptase. (2) Short interspersed nuclear elements or SINEs : SINEs can be best described as nonautonomous LINEs, because they have structural features of LINEs but do not encoded their own reverse transcriptase.

16. REFERENCES Griffith,A.J.F., An Introduction to Genetic Analysis. 5 th ed. Pierce,B.A., Genetics A Conceptual Approach. 4 th ed.

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