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Transposable elements in Maize And Drosophila
- 1. Transposable Elements in Drosophilaand Maize Presented By- Subhradeep Sarkar M.SC in Applied Genetics
- 2. CONTENTS Introduction Transposition. Types of transposition. Transposableelements in eukaryotes Early experiments on transposons. Discovery ac & ds elements P – elements in drosophila. The genetic and evolutionary significance of transposable elements
- 3. • Transposable elementsare mobile DNA sequences found in the genomes of all organisms. • A variety of colourful names have been applied to these genetic elements: controlling elements, jumping genes, mobile genes, mobile genetic elements, and transposons (Tn). • They have been detected genetically in model organisms such as E. coli, maize, yeast, and Drosophila through the mutations that they produce when they inactivate genes into which they insert.
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- 5. Transposition • Transposition isthe movement of a transposable element from one location to another. • several different mechanisms are there for transposition in both prokaryotic and eukaryotic cells. • Nevertheless, all types of transposition have several features in common: • 1.Staggered breaks are made in the target DNA. • 2.The transposable element is joined to single-stranded ends of the target DNA. • 3.DNA is replicated at the single strand gaps.
- 7. Mechanisms of transposition •Althougheach kind of transposable element has its own special characteristics, most can be classified into one of three categories based on how they transpose •1.Replicative •2.Conservative (non replicative) •3.Retrotransposition
- 8. Replicative A newcopy of the transposable element is introduced at a new site while the old copy remains behind at the original site. The number of copies of the transposable element increases.
- 9. Non-replicative The transposableelement excises from the old site and inserts at a new site also called cut-and-paste transposition. the number of copies of the transposable element remain same.
- 10. Retrotransposition In thethird category, transposition is accomplished through a process that involves the insertion of copies of an element that were synthesized from the element’s RNA. An enzyme called reverse transcriptase uses the element’s RNA as a template to synthesize DNA molecules, which are then inserted into new chromosomal sites.
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- 12. Transposable elements ineukaryotes • Eukaryotic transposable elements fall into one of two classes, • 1.Class 1 elements • 2.Class 2 elements • They are distinguished by whether the DNA element transposes through an RNA copy (class 1) or directly (class 2).
- 13. CLASS 1 • Class1 elements are also called RNA elements because the DNA element in the genome is transcribed into an RNA copy. • Insertions of class 1 elements into the genome are essentially permanent. That is, they cannot be excised from the donor site. Class 2 • Class 2 elements are called DNA elements • Class 2 elements can excise from the donor site, which means that, if insertion into a gene has created a mutation, excision of the element can lead to the reversion of the original mutation.
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- 17. Early Experiment • Theearly work of Rollins A. Emerson on the maize genes that caused variegated (multi- coloured) kernels. • Most corn kernels are either wholly pigmented or colourless (yellow), but Emerson noted that some yellow kernels had spots or streaks of colour. • He proposed that these kernels resulted from an unstable mutation Coloured Spotted Colourless
- 18. AC AND DSELEMENTS IN MAIZE • Transposable elements were first identified in maize (corn), after 50 years by Barbara McClintock. • McClintock published her conclusion in 1948. • The significance of McClintock’s early discoveries was finally recognized in 1983, when she was awarded the nobel prize in physiology or medicine.
- 19. McClintock’s Discovery ofTransposons in Corn • McClintock discovered that the cause of the unstable mutation was a gene that moved. • c/c = White kernels; C/C= Purple Kernels . • If reversion of c to C occurs in a cell, cell will produce purple pigment and a spot. • She concluded “c” allele results from an non- autonomous transposon called the “Ds” inserted into the “C” gene. Ds= Dissociation • But only if another gene, the activator (Ac), also was present.
- 20. A Dselement, transposing under the influence of a nearby Ac element, may insert into the C allele, destroying its ability to produce pigment. After the transposition of Ds into the C allele, this kernel will be colorless (white or yellow), because neither the Ct nor the c allele confers pigment.
- 22. P- Elements inDrosophila
- 23. P- Elements &Hybrid Dysgenesis in Drosophila • P elements are transposable elements in drosophila. It is transposed only in germ cells. • Hybrid Dysgenesis means assortment of aberrant traits( Chromosome break, sterility, mutation etc). • There are two strains of Drosophila, M and P strains
- 24. Cross Between M& P Strains of Drosophila
- 27. Mechanism of Repressionof P element
- 29. THE GENETIC ANDEVOLUTIONARY SIGNIFICANCE OF TRANSPOSABLE ELEMENTS • The evolutionary significance of transposable elements is unknown, but the hypotheses have been proposed to explain their common occurrence. • The cellular function hypothesis suggests that transposable elements provide some important function for the cell. • The genetic variation hypothesis proposes that transposable elements provide evolutionary flexibility by inducing mutations.
- 30. REFERENCE:- Cell Biology,Genetics, Molecular Biology, Evolution And Ecology ;First edition by P.S Verma & Agarwal; Page no-254 The cell- A molecular Approach,;Forth Edition by M.Cooper; Page no- 163 Genetics; Third edition by Monroe w. strickberger; page no- 543 https://www.youtube.com/watch?v=vSgUaLDa0n8 https://www.youtube.com/watch?v=bdnqpoeIs6A&t=319s https://www.ncbi.nlm.nih.gov/books/NBK21254/
Editor's Notes
- #20 Autonomous – not subjected to control from outside.