These chromosomal loci show the ability to be transposed from one location to another, within and among the chromosomes of the compliment. Breaks on either side of the loci followed by insertion of the broken transposon into a new locus are known as transposition. Hence, a transposable element gets inserted into a chromosome, and also exists or relocates itself. A group of transposable elements includes insertion sequences, transposons, retrotransposons, bacteriophages, and P elements. The inverted repeats flank the region of transposable elements or the transposons. These mobile genetic elements are also known as jumping genes. These elements also belong to the class of junk DNA.
Transposable elements in prokaryotes:
· Insertion sequences:
It consists of only those genes which require mobilization and get inserted in a new location in a chromosome. The IS elements are present in the bacterial chromosomes and the plasmids. The studies related to bacterial IS elements and effect of the expression of the genes controlling the galactose metabolism revealed an insertion of 800 base pair DNA segment in the gene. It is known as the insertion sequence-1 involving the transposition. The IS elements get integrated into the chromosome randomly. Hence, such changes involve very high chances of the disruption of the coding sequences or the regulatory sequences. The crossing over between the segments having the IS elements lead to deletions or inversions. The transposition process requires an enzyme known as a transposase. This enzyme recognizes the IS elements and helps in initiating the transposition.
The process of insertion occurs at a target site. It leads to the duplication of the target site leading to the formation of the direct target repeats. Now, the IS element exactly places itself between the two inverted repeats flanked by direct target-site duplications. The gaps get filled by DNA polymerase and ligase enzymes.
· Transposon:
It is a complex structure consisting of the genes required for the insertion and mobilization of the DNA into the chromosome. The two types of transposons include composite and the non-composite transposons. The composite transposons have a complex structure. The central region consists of genes. This region gets flanked by IS elements on both the sides. The non-composite transposons include drug resistance genes. A composite transposon (for example Tn10) consists of a central region carrying a gene flanked by IS elements having inverted repeats. It also gets flanked by direct sites. The example of non-composite transposon includes Tn3. It consists of genes for three main enzymes in the central part. The bla gene encodes the beta-lactamase enzyme. The tnpA gene encodes transposase enzyme. The TnpB encodes resolvase enzyme.
Image 1: IS elements
Eukaryotic transposable elements:
Plants include several families of transposons. The transposons consist of autonomous and non-autonomous elements. The autonomous elements transpose by themselves since they lack the gene for transposition. The derivative of the autonomous element having loss of function of one or few genes is known as the nonautonomous element. The insertion of the autonomous element into the host gene makes the mutant allele unstable. Some of the corn kernels show the presence of spots. These spots arise due to the pigments produced by the cells having a transposable genetic element.
The corn consists of a pigment known as anthocyanin. It gives the corn kernel purple color. Mutation in the gene responsible for the production of anthocyanin leads to an unpigmented kernel. The corn kernel having a wild-type C gene gives a purple color. The recessive c gene gives colorless kernels. These mutations block the production of anthocyanin. The revertants of mutations give purple spots on the kernels. The colorless mutation arises due to the mobile controlling element. These mobile controlling elements are known as Ds depicting the dissociation. They get inserted into the C gene (wild-type). One more mobile controlling element, known as Ac is known as an activator. It helps in the transposition of Ds into the gene.
The Ac-Ds transposable elements in corn included studies carried out by Barbara McClintock. The cut and paste transposition of the Ac element occurs during the chromosome replication. Upon replication of the chromosomal region containing Ac site, each progeny chromatid gets a copy of Ac. The transposition of the Ac element to a replicated site on the chromosome results in the empty donor site on one chromatid and Ac element on the other chromatid. The Ac elements do not increase in number if they get inserted into an already replicated site. Consider another case in which the Ac transposition occurs in an unreplicated chromosome site. First, one of the chromatids gets an empty donor site. The other chromatid consists of the Ac element. In this case, the transposing element gets inserted into the nearby recipient site ready to be replicated. The result of replication involves the presence of the Ac element on both the chromatids. Hence, this type of transposition leads to an increase in the number of Ac elements. The transposition of the Ds elements occurs in the same way.
Image 2: Cointegration model
Ty elements in yeast:
These elements include long terminal repeats. The yeast Ty elements consist of a length of 5.9 kb. The long terminal repeats are known as directly repeated terminal sequences. The denotion of long terminal repeats involves a delta sign (δ). Each delta consists of a promoter region and sequences recognized by transposing enzymes. The Ty elements encode a single mRNA having promoter elements at the delta region. Open reading frames in the mRNA transcript encode different proteins. They include TyA and TyB open reading frames. The Ty elements transpose by making an RNA molecule of the integrated DNA sequence. Hence, it creates a new Ty element by the process of reverse transcription. The yeast Ty elements follow the same mechanism conducted by retroviruses. They replicate via double-stranded DNA.
Drosophila transposons:
The mobile genetic elements in Drosophila constitute about 15% of the total genome content. The example of Drosophila transposons includes P element. These elements have terminal inverted repeats. The autonomous P elements are the longest ones. The nonautonomous elements include the shortest ones. The autonomous P elements encode for transposase enzyme. This enzyme helps in the transposition of the P elements. The P elements serve as vectors for transferring the genes.
The mobile genetic elements in Drosophila constitute about 15% of the total genome content. The example of Drosophila transposons includes P element. These elements have terminal inverted repeats. The autonomous P elements are the longest ones. The nonautonomous elements include the shortest ones. The autonomous P elements encode for transposase enzyme. This enzyme helps in the transposition of the P elements. The P elements serve as vectors for transferring the genes.
LINEs and SINEs:
The human retrotransposons consist of LINES and SINES. Long interspersed sequences (LINEs) and short interspersed sequences (SINEs) occur in the moderately repetitive class of sequences. LINEs include autonomous elements that encode enzymes for the retrotransposition. The nonautonomous elements or SINEs also require those enzymes. The example of LINEs includes an L1 element with a size of 6500 base pair. The example of SINEs includes Alu family.
References:
[1] Biotechnology-3: Including Molecular Biology Biophysics, S. Mahesh
[2] Transposable element - Wikipedia
[1] Biotechnology-3: Including Molecular Biology Biophysics, S. Mahesh
[2] Transposable element - Wikipedia
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