Transposable Elements inDrosophila and Maize

Presented By- Subhradeep Sarkar M.SC in Applied Genetics

CONTENTSIntroductionTransposition.Types of transposition.Transposable elements in eukaryotes Early experiments on transposons.Discovery ac & ds elementsP – elements in drosophila.The genetic and evolutionary significance of transposable elements

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.

Transposition

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.

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

Replicative A new copy 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.

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.

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.

TRANSPOSABLE ELEMENTS IN EUKARYOTES

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).

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.

DISCOVERY OF TRANSPOSONS

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

AC AND DS ELEMENTS 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.

McClintock’s Discovery 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 (Ac), also was present.

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 Ct nor the c allele confers pigment.

P- Elements in Drosophila

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

Cross Between M & P Strains of Drosophila

Mechanism of Repression of P element

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.

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/