TransposonsImage courtesy of Lauren Solomon, Broad Communications The reddish streaks on these corn grains are caused by transposons. Submitted to: Zaahir Salam Dr. S.Kannan
Transposable Elements (Transposons) “JUMPING GENES” Discovered by Barbara McClintock, largely from cytogenetic studies in maize, but since found in most organisms.1. Nobelprize.org (1983 Nobel Prize in Physiology and Medicine)2. profiles.nlm.nih.gov/LL/Corn (maize) varieties Barbara McClintock 1902-1992
We now know that only 1.1% to 1.4% of our DNA actually encodes proteins. Snapshot of the human genome. The chart shows the proportions of our genome made up of various types of sequences. More than 50% of our genome consists of short, repeated sequences, the vast majority of which—about 45% of our genome in all—come from transposons.
What is a Transposon• Segments of DNA that move from one genomic location to other.• The simplest transposable elements are Insertion Sequences(IS).• IS is a short sequence of DNA carrying only the genes needed for transposition and bounded at both ends by sequences of nucleotides in reverse orientation called Inverted repeats.
Between the IR’s there is the gene for transposase, which recognizes the end ofIS because the Enzyme is not specific for a particular sequence of genometransposable elements appear to move to random destinationsSome also contain Antibiotic Genes or Toxin Genes. These are called CompositeTransposon.
When a transposon inserts at a target site, the target sequence is duplicated so that short, direct-sequence repeats flank the transposon’s terminal inverted repeats.
Transposable Genetic Elements Move from One Location to Another• It is the third general type of recombination system: recombination that allows the movement of transposable elements, or transposons.• These segments of DNA, found in virtually all cells, move, or“jump,” from one place on a chromosome (the donor site) to another on the same or a different chromosome (the target site).• DNA sequence homology is not usually required for this movement, called transposition; the new location is determined more or less randomly.• Transposons are perhaps the simplest of molecular parasites, adapted to replicate passively within the chromosomes of host cells.
• There are two distinct types: – Class II transposons. These consist of DNA that moves directly from place to place. – Class I transposons. These are retrotransposons that first transcribe the DNA into RNA and then use reverse transcriptase to make a DNA copy of the RNA to insert in a new location.
Bacterial Transposition• Most have short repeated sequences at each end that serve as binding sites for the transposase.• When transposition occurs, a short sequence at the target site (5 to 10 bp) is duplicated to form an additional short repeated sequence that flanks each end of the inserted transposon.• These duplicated segments result from the cutting mechanism used to insert a transposon into the DNA at a new location. Duplication of the DNA sequence at a target site when a transposon is inserted. The sequences that are duplicated following transposon insertion are shown in red. These sequences are generally only a few base pairs long, so their size relative to that of a typical transposon is greatly exaggerated in this drawing.
• There are two general pathways for transposition in bacteria. – Direct transposition – Replicative transposition• In direct (or simple) transposition cuts on each side of the transposon excise it, and the transposon moves to a new location. This leaves a double-strand break in the donor DNA that must be repaired.• At the target site, a staggered cut is made the transposon is inserted into the break, and DNA replication fills in the gaps to duplicate the target site sequence.• In replicative transposition the entire transposon is replicated, leaving a copy behind at the donor location.• A cointegrate is an intermediate in this process, consisting of the donor region covalently linked to DNA at the target site. Two complete copies of the transposon are present in the cointegrate, both having the same relative orientation in the DNA.
Eukaryotic Transposition• The difference is however, the mechanism of transposition seems to involve an RNA intermediate.• Eukaryotic DNA transposons from sources as diverse as yeast and fruit flies have a structure very similar to that of retroviruses; these are sometimes called retrotransposons. Retrotransposons encode an enzyme homologous to the retroviral reverse transcriptase, and their coding regions are flanked by LTR sequences.• They transpose from one position to another in the cellular genome by means of an RNA intermediate, using reverse transcriptase to make a DNA copy of the RNA, followed by integration of the DNA at a new site. Eukaryotic transposons. The Ty element of the yeast Saccharomyces and the copia element of the fruit fly Drosophila serve as examples of eukaryotic transposons, which often have a structure similar to retroviruses but lack the env gene. The Delta sequences of the Ty element are functionally equivalent to retroviral LTRs. In the copia element, INT and RT are homologous to the integrase and reverse transcriptase segments, respectively, of the pol gene.
So Are Transposons Good or Bad?• In the process of inserting into the genome, transposons can – interrupt the normal coding of DNA, – creating gene mutations with a variety of effects. – They may turn nearby genes off, preventing their ability to create protein, or they may turn them on, increasing the amount of protein made.• There is evidence that transposons aren’t just “selfish genes” intent on replicating themselves or genomic “junk” that provides no benefit to the host. They may play a creative role in building new functional parts of the genome .• Recent research has shown that transposons may help plants respond and adapt to environmental stress by regulating other genes.• In bacteria, transposons often carry genes that impart resistance to antibiotic substances, helping the bacteria survive.