TRANSPOSONS

  • Transposons are jumping genes
  •  DNA sequence that can change its position with in the genome transposons are DNA   segments that are mobile
  •  Certain genetic elements were moving from one site to an entirely different site in the   chromosome.
  • These controlling elements were later on called as transposable elements.
  •  Transposons are found to encode a special protein named as transposase which catalysis the process of transposition.


Classes of Transposable Elements

• Transposable elements can be divided into two major classes based on method of                    transposition.
              Transposons (class 1)     Retrotransposons (class 2)



TRANSPOSONS (CLASS 1)

• DNA fragments transpose directly from DNA segment to DNA segment:
• Producing a DNA copy that transposes (replicative transposition)
• Or, cut/paste into a new locus (conservative transposition).
• Encode a transposase for transposition.
• Can carry additional genes.
• Found in eukaryotes and prokaryotes.


Retrotransposons (class 2)

• Retrotransposons, which use a retro-transcriptase encoded in their genome for         retro-transposition
• Retrotransposons were discovered in the fruit fly(Drosophila). The                             prototype Drosophila transposon IS called copia because it is present in the             genome in copious quantity.
• Similar transposable elements in yeast are called Ty, for “transposon yeast.” These transposable elements, about 6 kb in length.



 Retrotransposons divided into ;

LTR-containing retrotransposons
non-LTR retrotransposons



LTR-containing retrotransposons

Includes the retrotransposons with LTRs, which replicate in a manner very similar to retroviruses, except that they do not pass from cell to cell in virus particles. Not surprisingly, these are called LTR-containing retrotransposons.



Non-LTR retrotransposons

The second group includes the retrotransposons that lack LTRs (the non-LTR retrotransposons)



 Ty1

  • Ty1 encodes a reverse transcriptase. The tyb gene in Ty codes for a protein with an amino acid sequence closely resembling that of the reverse transcriptase encoded in the pol genes of retroviruses.
  • Ty Elements and their Genomes The Ty contain long terminal repeats (LTR) at both extremities of the element.
  • Use reverse transposase to make RNA intermediate for transposition.
  • Encode an integrase and reverse transcriptase for transposition. Their transposition is accomplished through a process which involves the synthesis of DNA by reverse transcription (i.e. RNA DNA) by using elements RNA as the      template (Fig. 4). This type of transposition involves an RNA    intermediate, the transposable DNA is transcribed to produce  an RNA molecule.
  • Commonly, two genes are found in LTR retrotransposons, representing gag and pol genes in retrovirus. The gag gene encodes structural proteins pol locus encodes a polyprotein with protease (PR or prot), integrase (IN or int), reverse transcriptase (RT), and RNase H (RH) catalytic domains. Similarly virus-like particles (VLPs), which constitute the transposition-competent structures, have been shown for the    yeast retroelements, Ty1 and Ty3.
  • This RNA is then used as a template for producing a complementary DNA by the activity of enzyme reverse transcriptase. This single stranded DNA copy so formed, is then made double stranded and then inserted into the target    DNA site. The transposable elements which require reverse transcriptase tor their movement are called retro transposons.
  • In yeast there are Ty1, Ty2, Ty3, Ty4, Ty5


 Non-LTR Retrotransposons

  • Retrotransposons that lack LTRs are much more abundant than those with LTRs, at least in mammals,
  • The most abundant of all are the long interspersed elements (LINEs), one of which is (L1) function is encoding endonuclease and a reverse transcriptase.
  • LINEs and LINE-like elements are retrotransposons that lack LTRs. 
  • These elements encode an endonuclease that nicks the target DNA. Then the element takes advantage of the new DNA 3’-end to prime reverse transcription of element RNA.