Assignment topic: TransposonsSubject : Molecular biology

LCWU Lahore

Transposable Elements

DEFINITION• DNA elements capable of moving

("transposing") about the genome. The movement of genetic information from one chromosomal location, the donor site, to another, the target site.

Discovery

• Discovered by Barbara McClintock.• Largely from cytogenetic studies in maize, but

since found in most organisms.• She liked to call them "controlling elements“

because they affected gene expression.• She got Nobel Prize in 1983 in Physiology and

Medicine

Genetic Transposition

• The ability of genes to change position on chromosomes, a process in which a transposable element is removed from one site and inserted into a second site in the DNA.

Mechanism of transposons

• Excises the IS element in the donor DNA• Makes staggered cuts in a short sequence in the

target DNA• Ligates the three prime termini of the IS element

to the five prime ends of the cut donor DNA• Host cell DNA polymerase fills in the single

stranded gaps, generating the short direct repeats that flank IS elements and DNA ligase joins the free ends

Characteristics • Comprise 45% of human chromosomal DNA “middle

repetitive DNA” contribute to spontaneous mutation, • genetic rearrangements, • and horizontal transfer of genetic material.• Aid speciation and genomic change.• In bacteria transposons are often associated with

antibiotic resistance genes.• Cells must depress transposition to insure genetic

stability.

Classification of transposition

• Conservative transposition:• The transposable element will only be found

at the target site, with no change in copy number.

• Also known as “cut-and-paste” transposition.

Duplicative transposition:

• Will result in two copies of the element, one at the donor site and one at the target site.

• copy-and-paste” transposition

Types of transposable elements:

Class I transposable elements (retro transposons).Class II transposable elements (DNA transposons).

Retro transposons:

• Retro transposons also called transposons via RNA intermediates are genetic elements that can amplify themselves in a genome and are ubiquitous components of the DNA of many eukaryotic organisms.

DNA transposons: • DNA fragments transpose directly from DNA segment to DNA

segment, Producing a DNA copy that transposes, cut or paste into a new locus.

• Insertion sequences:• Also known as an IS, an insertion sequence element, or an IS

element is a short DNA sequence that acts as a simple transposable element.

• Size:• 700 to 2,500 bp.• Content:• Contain information that required for transposition.

Example:

Some maize phenotypes caused by transposable elements excising in somatic tissues.

BACTERIAL TRANSPOSONS:

• In bacteria, transposons can jump from chromosomal DNA to plasmid DNA and back.

• Transposons in bacteria usually carry an additional gene for function other than transposition often for antibiotic resistance.

Example

Control of Transposons:

• Auto regulation: • Transcriptional silencing:

Transposons causing diseases:

• Hemophilia A and B• Severe combined immunodeficiency• Porphyria• Cancer• Duchenne muscular dystrophy.

Biological Significance of Transposons:

• To identifying the mutant allele.• To study the chemical mutagenesis methods.• To study gene expression• They provide a means for genomic change and

variation, particularly in response to stress.