What are transposons?
Transposons are the mobile segments of DNA which can jump or transpose from one place to another within the genome of a single cell. At least 50 % of the human genome consists of these transpososns. Transposons cannot exist independently as they do not have an origin of replication.
What is transposition?
Transposition or the process of jumping is carried with the help of an enzyme known as transposase. This enzyme helps to make two single-stranded breaks in the target DNA.
The mechanism of transposition can lead to phenotypically significant mutations.
Example 1 - Mice having almost identical DNA can exhibit a variation in their fur color.
Reason behind this could be that the gene coding for the fur color has been altered by the insertion of a transposon.
Example 2 - Corn grains come in different colors (yellow, white or purple).
The aleurone layer of the corn grains is responsible for pigmentation. Depending upon the position of transposons some cells express pigmentation and some do not. This leads to the phenotypic variation within the corn grains.
Some characteristics of transposons
Discovery of transposons
The jumping genes or transposons were discovered by Barbara McClintock and came into publication in the year 1948. She was also awarded a Nobel Prize in Medicine in the year 1983 for this discovery.
Her discovery was indeed a breakthrough in her time as DNA was considered to be stable and deterministic at that time. However these genes were later isolated from bacterium Escherichia coli in the late 1960's.
Classification of transposons
Transposons are broadly classified into 4 classes which are as follows:
Insertion sequences or IS elements
The diagram of IS is given below:
Transposons are the mobile segments of DNA which can jump or transpose from one place to another within the genome of a single cell. At least 50 % of the human genome consists of these transpososns. Transposons cannot exist independently as they do not have an origin of replication.
What is transposition?
Transposition or the process of jumping is carried with the help of an enzyme known as transposase. This enzyme helps to make two single-stranded breaks in the target DNA.
The mechanism of transposition can lead to phenotypically significant mutations.
Example 1 - Mice having almost identical DNA can exhibit a variation in their fur color.
Reason behind this could be that the gene coding for the fur color has been altered by the insertion of a transposon.
Example 2 - Corn grains come in different colors (yellow, white or purple).
The aleurone layer of the corn grains is responsible for pigmentation. Depending upon the position of transposons some cells express pigmentation and some do not. This leads to the phenotypic variation within the corn grains.
Some characteristics of transposons
- They are seen in almost all organisms but are relatively more abundant in eukaryotic organisms.
- Eukaryotic genome consists of a large quantities of repetitive sequences and transpsosons can be the source of this repetitive DNA.
- Transposons can stimulate structural, regulatory or both kinds of changes within the same organism.
- During transposition it can adopt either of the two mechanisms which are “copy and paste" or "cut and paste”.
Discovery of transposons
The jumping genes or transposons were discovered by Barbara McClintock and came into publication in the year 1948. She was also awarded a Nobel Prize in Medicine in the year 1983 for this discovery.
Her discovery was indeed a breakthrough in her time as DNA was considered to be stable and deterministic at that time. However these genes were later isolated from bacterium Escherichia coli in the late 1960's.
Classification of transposons
Transposons are broadly classified into 4 classes which are as follows:
Insertion sequences or IS elements
- ·They are the smallest bacterial transposons which do not carry host genes.
- ·They carry only those genes which are needed for their own transposition.
- ·They have inverted repeats at the termini and carry the gene for transposase enzyme.
- They contain the signals for the abrupt chain termination or termination of the process of transcription very frequently
The diagram of IS is given below:
Composite transposons or Class-I transposons or (retrotransposons)
The diagram of a composite transposon is given below:
- · An antibiotic resistance gene (or genes) is flanked by two IS elements
- · Some times a terminal IS element can act as an individual transposable element.
- · Most commonly occurring types of retrotransposons are:
- Long interspersed nuclear elements (LINEs) – They lack LTRs (long terminal repeats) and are transcribed by RNA polymerase II. They code for reverse transcriptase.
- Short interspersed nuclear elements (SINEs) – They are transcribed by RNA polymerase III and do not code for reverse transcriptase.
The diagram of a composite transposon is given below:
Noncomposite transposon or Class-II transposons (DNA transposons)
- · They are simple segments of DNA
- · They range in length from 1,000 to as many as 40,000 base pairs
- · They contain a transposase gene, Inverted repeats and several antibiotic resistance genes.
Mechanism of transposition
There are two basic methods of transposition
Conservative transposition ·
There are two basic methods of transposition
Conservative transposition ·
- This is seen with class II transposons which adopt a cut and paste mechanism.
- · Transposase enzyme is generally seen within these elements and it cuts the transposon from the donor DNA (blunt cuts).
- · Transposase also cleaves target sites (through a staggered cut).
- · The transposon gets ligated into the target through transposase.
- · DNA pol I and ligase help to fill the gaps by synthesizing new nucleotides.
- · During this process the donor DNA gets destroyed and the target DNA gets duplicated.
Note: Not all DNA transposons transpose through a cut-and-paste mechanism. In some cases a replicative transposition is observed in which transposon replicates itself to a new target site.
Replicative transposition
This mechanism is adopted by transposons belonging to class I and they adopt a copy and paste mechanism.
Replicative transposition
This mechanism is adopted by transposons belonging to class I and they adopt a copy and paste mechanism.
- · Requires transposase, resolvase and an IRS (internal resolution site) recognized by resolvase
- · These transposons generate a second copy which gets inserted somewhere else. The original copy is left behind within the donor.
- · In the fist step transposable DNA is copied into RNA.
- · RNA which jumps to a different location gets copied into DNA in the presence of reverse transcriptase.
- · This copied DNA gets inserted into a different location within the genome.
- · The Transposase generates the nicks and helps in the process of ligation.
- · Gaps during the process of ligation are filled by DNA pol I and ligase enzyme.
- · Resolvase catalyzes site specific recombination between the 2 IRSs
- · This mechanism is involved in the spread of transposable elements within higher organisms.
- · Leads to the insertion of repetitive sequences throughout the genome.
- · The donor is not destroyed as the transposon does not leave it rather two copies of transposons are produced.
Nomenclature of transposons :
Scientific applications of transposons
Assist in the transfer of bacterial resistance to antibiotics
A simple transposon generally contains only a copy of transposase enzyme and does not have any additional genes. Simple transposons act like parasites which are not beneficial to the host in any way.
Some transposons contain some additional genes apart from the gene for transposase enzyme. For example gene coding for antibiotic resistance – The genes for antibiotic resistance are generally carried by the transposable elements which later get incorporated into plasmids. Such plasmids can be used to transfer the genes of antibiotic resistance from one organism to another.
Transposons can lead to diseases
Transposons stimulate chromosomal rearrangements or gene mutations which might lead to diseases within human beings.
Generally transposons do not serve any specific function and hence are also termed as junk DNA. However in rare cases they may cause diseases within human beings.
Diseases generally associated with transposable elements are SCID, hemophilia A and B, Duchenne muscular dystrophy and porphyria.
The regulatory unit of a gene consists of promoter and operator sequences. If a transposon gets inserted near the promoter region, it may hyperactivate the process of gene transcription. As a result the genes get over activated which might also lead to a situation like cancer. Or in some other cases after the transposon is cut from the genome the cell may fail to repair the cut. This mutation may alter the gene regulation leading to cellular dysfunctioning.
Effect on embryo and tumor development
Within animal cells transposons may effect the development of embryo and tumor formation.
Effect on oncogenes
Oncogenes get activated due to random reshuffling of transposons especially when they get located adjacent to the oncogenes.
- Transposons are abbreviated as Tn followed by the numbers for identification. For example Tn1, Tn2, Tn3 etc.
- · Genes present on transposons are represented by their genotypic name for example if a transposon contains an ampicillin resistant gene it is denoted as Tn1(ampR).
- · Insertion sequences or IS elements were the first ones to be identified and they do not carry host genes. Hence they are labeled as IS1, IS2 etc.
- · When transposons are located within the gene: For example within the Lac gene, they are labeled with the help of gene name, allele number followed by two colons and the transposon present inside the gene ((like lacZ87::Tn3).
Scientific applications of transposons
Assist in the transfer of bacterial resistance to antibiotics
A simple transposon generally contains only a copy of transposase enzyme and does not have any additional genes. Simple transposons act like parasites which are not beneficial to the host in any way.
Some transposons contain some additional genes apart from the gene for transposase enzyme. For example gene coding for antibiotic resistance – The genes for antibiotic resistance are generally carried by the transposable elements which later get incorporated into plasmids. Such plasmids can be used to transfer the genes of antibiotic resistance from one organism to another.
Transposons can lead to diseases
Transposons stimulate chromosomal rearrangements or gene mutations which might lead to diseases within human beings.
Generally transposons do not serve any specific function and hence are also termed as junk DNA. However in rare cases they may cause diseases within human beings.
Diseases generally associated with transposable elements are SCID, hemophilia A and B, Duchenne muscular dystrophy and porphyria.
The regulatory unit of a gene consists of promoter and operator sequences. If a transposon gets inserted near the promoter region, it may hyperactivate the process of gene transcription. As a result the genes get over activated which might also lead to a situation like cancer. Or in some other cases after the transposon is cut from the genome the cell may fail to repair the cut. This mutation may alter the gene regulation leading to cellular dysfunctioning.
Effect on embryo and tumor development
Within animal cells transposons may effect the development of embryo and tumor formation.
Effect on oncogenes
Oncogenes get activated due to random reshuffling of transposons especially when they get located adjacent to the oncogenes.