The Nature of TransposonsChapter 11pp 300-315

Outline• Nature of Transposons

• Transposons – Prokaryotic– Eukaryotic: Dr. McClintock’s research

• Retrotransposons vs Retroviruses

Transposons

• Are mobile DNA sometimes referred to as “jumping genes”.

Why are transposons in our genome?

• Cellular function hypothesis– Antibody diversity of VDJ

• Genetic variation hypothesis

• Self-DNA hypothesis

Common features associated with transposons

1. Direct repeats

2. Terminal Inverted repeats

CommonFeatures

Are:

Mechanisms of Transposition

• Movement through DNA intermediates

• Movement through RNA intermediates

• Movement through DNA intermediates

DNA TranspositionC

ut and Paste

Replicative Non-Replicative

Copy and Paste

Non-Replicative Movement by Transposition

• Requires transposase

• 1) Transposase cuts transposon out of donor DNA (blunt cut)

• 2) Transposase makes a staggered cut at the target sequence

• 3) Transposase ligates transposon into the target

• 4) Gaps are filled in with DNA pol I and ligase

Staggered cut

Direct repeats are generated

Staggered cut

Direct repeats are generated

Examples of: Transposable Elements in Bacteria

1. Simple Bacterial Transposon

INSERTION SEQUENCE

2. COMPOSITE TRANSPOSONIS + any DNA segment

Dr. Barbara McClintock1902-1992

Nobel Prize, 1983Physiology or Medicine

“I don’t understand a word that she said, but if she says it is so, it must

be so!”

Alfred Sturtevant

The Ds element often results in chromosomal breakage

Drosophilia melanogaster&

DNA transposition

“The P Element”

The P element Codes for both

• Transposase

• Repressor of transposition

Offspring sterile

Offspring norm

al

Why the difference?

Female P- and male P+ Female P+ and male P-

Hybrid dysgenesis Non-hybrid dysgenesis

Remember the P element Codes for

• A Transposase

• AND

• A Repressor of transposition

Mite

Science. 1991 Sep 6;253(5024):1125-8Possible horizontal transfer of Drosophila genes by the

mite Proctolaelaps regalis.

Houck MA, Clark JB, Peterson KR, Kidwell MG.

D. melanogaster

D. willistoni

The P element took a ride on a

That moved it onto

~50 yrs ago

Retrotransposons

mRNADNA

Examples of retrotransposons elements

• Ty

• Copia

• Alu

Yeast

Drosophila

Humans

General MechanismOf Retrotransposons

Note: retrotransposons are not found in prokaryotes

Types of Retrotransposons

• Ty

• Copia

Yeast

Drosophila

Humans

Viral

Non-Viral• Alu

Non-viral Retrotransposons

Humans

Non-Viral

LINES SINES

Alu Example: Line 1 and Line 2

ALU

• 300 bp

• Endonuclease cutting site: site5' AG/CT 3‘

• Retrotransposon

• 1 million in genome

• Linked to variety of disorders

Alu has inserted in NF-1 gene

Neurofibromatosis: autosomal dominantGrowth of nodules mainly affecting nerves in the skin

Viral retrotransposons look a little like retroviruses—but are NOT the same !

Now we must compare a Viral RNA with a Viral Retrotransposon

VIRAL RNA

Now a viral retrotransposon

Here’s the Copia element of Drosophila

Compare to the retrovirus

Similar gag and pol sequences

Here’s the Ty element, again notice similarities to retroviruses

Delta sequences function like LTR’s in viruses

Summary Table

Viral and non-viralTy and Copia viral retrotransposons

Alu non viral retrotransposons

A topic of scientific discussion is what is a “prion”?

Blunt cut from donor

HUMANIMMUNODEFICIENCYVIRUS

Human Helper T Lymphocyte