Transposons

Barbara McClintock, 1948

Term used in genetics. Transposons are coding DNA sequences that can change their location within the DNA (so-called 'jumping genes'). Transposons occur in all organisms. They can contain one or more genes. The locations at which transposons are integrated into the genome are usually random. They can cause mutations and alter the expression of other genes. A large proportion of natural mutations are caused by transposons. They are therefore also elements that increase the diversity of variants in nature.

Transposons are divided into:

• DNA transposons (class II transposons)
• and
• RNA transposons (class I transposons or retroelements).

In both cases, the enzymes required for jumping are encoded by the sequence placed on the transposon itself. The transposon thus brings its own tools with it and is therefore often regarded as a kind of "integrated virus", as the procedure is similar to that of retroviruses. DNA transposons are characterized by special, repetitive, non-coding sequences before and after the jumping gene, which serve as a recognition site for the transposase (so-called inverted repeats or IRs).

Transposons use breaks in the DNA, for example, although not every break is suitable - it must have short overhanging pieces of single-stranded DNA. Some transposons replicate themselves and then jump into these breaks.

In conservative transposition, a complete (i.e. double-stranded) DNA sequence is cut out and then reinserted somewhere else.

The transposon is often surrounded by non-coding sequences from the host DNA, so-called direct repeats (DRs). They are created by the fact that when the gap is opened, the transposase does not cut exactly 'opposite' on both DNA strands, but slightly offset. The resulting gap is therefore not identical on both DNA strands. After insertion of the transposon, the remaining gaps are filled again by normal DNA synthesis (insertion or addition of the missing bases).

Other transposons (class I transposons), which are known as retrotransposons or LTR retrotransposons, take the RNA route. They often have adjacent repeat sequences (long terminal repeats, LTRs). In this "replicative transposition", a DNA segment is copied into an mRNA transcript, translated into the so-called cDNA (complementary DNA) by means of a reverse transcriptase (enzyme that builds the strand complementary to the mRNA) and incorporated back into the DNA strand. The missing counter strand is then added by a DNA polymerase.r

Retrotransposons are transcribed like any other DNA. First, an RNA transcript is produced. However, the RNA does not then go into the cytoplasm for translation, but is converted back into double-stranded DNA by a special enzyme called reverse transcriptase (reverse transcription). The DNA copy is now inserted into a break site.