Transposable elements in Maize And Drosophila

Transposable elements. types, Transposable elements in eukaryotes- drosophila and maize

1. Transposable Elements in Drosophila and Maize Presented By- Subhradeep Sarkar M.SC in Applied Genetics

2. Contents Introduction Transposition. Types of transposition. Transposable elements in eukaryotes Early experiments on transposons. Discovery ac & ds elements P – elements in drosophila. The genetic and evolutionary significance of transposable elements

3. Introduction Transposable elements are 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.

4. Transposition

5. Transposition Transposition is the 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.

6. Mechanisms of transposition Although each 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

7. Replicative A new copy of the transposable element is introduced at a new site while the old copy remains behind at the original site. T he number of copies of the transposable element increases.

8. Non-replicative The transposable element 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.

9. Retrotransposition In the third 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.

10. TRANSPOSABLE ELEMENTS IN EUKARYOTES

11. Transposable elements in eukaryotes 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).

12. Class 1 Class 1 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.

13. Discovery Of transposons

14. Early Experiment The early 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

15. Ac and Ds elements in maize Transposable elements were first identified in maize (corn), after 50 years by B arbara 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.

16. McClintock’s D iscovery of Transposons 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 ( A c ), also was present .

17. A Ds element, 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 C t nor the c allele confers pigment.

18. P- Elements in Drosophila

19. 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

20. Cross Between M & P Strains of Drosophila

21. Mechanism of Repression of P element

22. The Genetic and Evolutionary 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.

23. 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 /