TRANSPOSABLE ELEMENTS IN

PROKARYOTESSUBMITTED BY:- REETA

YADAVCLASS:- M.SC(P) BOTANY

ROLL NO. 01

IntroductionMechanisms of transpositionBacterial transposons IS elementsComposite transposonsNon-composite transposonsMedical significance of bacterial transposons

ConclusionReferences

CONTENTS

Transposable elements are also known as “TRANSPOSONS” , “JUMPING GENES” , “MOBILE GENETIC ELEMENTS”.

A DNA sequence that can change its position within the genome, sometimes creating or reversing mutations and altering the cells genome size.

Term was given by “Hedges and Jacob(1947). Barbara McClintock’s discovery of these

jumping genes, through an analysis of genetic instability in Maize, earn her noble prize in 1983.

INTRODUCTION

1) Conservative transposition:-

2) Replicative transposition:-

Mechanisms of transposition

3)Retrotransposition

IS elementsComposite transposonsTn3 elements

Bacterial transposons

simplest bacterial transposons (small DNA fragment).

first detected in certain lac(-) gene mutations of E. coli (it reverses the wild type phenotype).

compactly organized (~2500 bp) and contain only genes whose products are involved in transposition.

Inverted terminal repeats are found at the ends. Some IS elements encode transposase, an

enzyme.

IS elements (Insertion Elements)

The IS50 Element

These are Cut & Paste transposons.

Two different way to cut DNA by

restriction enzymes:

-blunt ends -over hanging

ends -(sticky ends)

Insertion of an IS Element Causes Target Site Duplication

bacterial chromosome & plasmids may contain IS elements.

Conjugative R plasmids have spread multiple drug resistance in bacterial populations.

Formation of Conjugative R Plasmid by Recombination of IS Elements

bacterial cut-and-paste transposons

--denoted by the symbol Tn.

--are created when two IS elements insert near each other.

Have two IS elements flanking a region that contains one or more genes for antibiotic resistance.

Composite transposons

REGULATION OF TN5

don’t have IS elements at each of their ends. larger than the IS elements contain genes that are not required for

transposition

NON-COMPOSITE TRANSPOSONS(TN3 ELEMENTS)

Tn3 is a replicative transposon that transposes by temporarily fusing DNA molecules into a cointegrate

when the cointegrate is resolved, each of the constituent DNA molecules emerges with a copy of Tn3

Transposition of Tn3 elements

Many bacterial transposons carry genes for antibiotic resistance.

It is relatively simple matter for these genes to move from one DNA molecule to another- for instance, from a chromosome to a plasmid. This genetic flux has a profound medical significance because many of the DNA molecules that acquire resistance genes can be passed on to other cells.

This process has occurred in several species pathogenic to humans, including strains of Staphylococcus, Neisseria, Sbigella & Salmonella.

Many bacterial infections causing diseases such as dysentery, tuberculosis, & gonorrhea are difficult to treat.

Medical significance of bacterial transposons

The spread of multiple drug resistamnce in bacterial populations has been accelerated by evolution of conjugative R plasmids that carry the resistance genes .

These plasmids have two components- one called the resistance transfer factor , or RTF, contains the genes for conjugative transfer between cells, the other, called the R-determent, contains the genes for antibiotic resistance.

These can be passed from one species to another, even between quite dissimilar cell types-for e.g between a coccus and a bacillus.

Thus, once multiple drug resistance has evolved in a part of the microbial kingdom, it can spread to other parts with relative ease.

Contd.

Snustad, D.Peter & Simmons J.Michael.2003. Principles Of Genetics(2nd edition) John Wiley & Sons. Newyork. page no. 441-446.

Weaver , F Robert & Hedrick W.Philip .1997. Genetics(3rd edition). Wm.C.Publishers. page no. 354-365

Web links:-

http://www.nature.com/scitable/topicpage/transposons-the-jumping-genes-518

http://www.ndsu.edu/pubweb/~mcclean/plsc431/transelem/trans5.htm

http://bio.classes.ucsc.edu/bio105l/EXERCISES/F%20TRANSFER/Tn.pdf

References