Transposons had been broken. She used self-pollinated plants

Transposons A series of genetic elements can occasionally move, or transpose, from one position on a chromosome to another position on the same chromosome or on a different chromosome. They somehow alter the DNA genetic identity as well as genome size. These are the genes which keep on moving their position with time. Most of the Eukaryotic genome is made up of transposons. Transposons make up of almost 45% human genome, 37.5% of mouse genome and 42% of the dog. So these elements are considered to possess the most mass of DNA. Transposons are very important for the researchers for changing the DNA within a living organism. In oxytricha transposons played a very vital role. A bacterial DNA transposon is shown in figure. There are 2 main classes of TEs considered by scientists: class I may be called retrotransposons used copy and paste technique for transposing DNA and reverse transcriptase is also used in this technique, while in class II or DNA transposons, in this class intermediate DNA is transferred by using cut and paste method. Discovery Before 1930s, it was widely considered that genes are fixed in their specific positions and can’t be moved. But in 1943-44 Barbara McClintock researched on maize plant and find out transporting elements. She was researching in Cold Spring Harbor Laboratory in New York on maize plant whom chromosome had been broken. She used self-pollinated plants means they can be pollinated by their own anther. After few generations of maize, she found that some of leaves show different phenotype from the rest of them. They contained identical albino patches on them. She suggested that after few cell divisions some cell lost DNA while these lost DNA can be captured by other cells. But after comparing chromosomes of the current generation to them she came to know that some genes had transferred it position on these chromosomes. So, this led to a major breakthrough as it was widely considered that chromosomes are fixed in position and could not shift its position. She also suggested that genes couldn’t only transpose but also on and off according to environmental conditions. She wrote an article in genetics in 1953 entitled “Induction of Instability at Selected Loci  schematic diagram shows the nucleotide sequence of terminal inverted repeats and flanking direct repeats on a transposable element. The transposable element is depicted as a horizontal, double-stranded segment of DNA. Elongated, vertical, rectangles represent individual nucleotides on each DNA strand. The rectangles are grey, except for 11 nucleotide pairs near each of the DNA segment’s opposite ends. These rectangles are colored and labeled with the letters A, T, G, or C. The color of the rectangle represents the chemical identity of the nitrogenous base and is blue (guanine), red (thymine), green (adenine), or orange (cytosine). The six innermost nucleotide pairs at both ends are enclosed in a bracket and labeled as terminal inverted repeats. The five outermost nucleotide pairs at both ends are enclosed in a separate bracket and labeled as flanking direct repeats.in Maize”. But her work wasn’t getting any attraction and rejected but later accepted and she was given noble prize in 1983 for her work in genetics. Classification TEs have been classified in two classes on the basis of its mechanisms. Class I is using copy and paste method while class II is using cut and paste method. Different classes of transposable elements are found in the genomes of different eukaryotic organisms (Figure 1).  Figure 1: The relative amount of retrotransposons and DNA transposons in diverse eukaryotic genomes Class I Class I TEs are copied in two stages: first, they are transcribed from DNA to RNA, and the RNA produced is then reverse transcribed to DNA. This copied DNA is then inserted back into the genome at a new position. The reverse transcription step is catalyzed by a reverse transcriptase, which is often encoded by the TE itself. The characteristics of retrotransposons are similar to retroviruses, such as HIV. Retrotransposons are commonly grouped into three main orders: . TEs with long terminal repeats (LTRs), which encode reverse transcriptase, similar to retroviruses . Long interspersed nuclear elements (LINEs, LINE-1s, or L1s), which encode reverse transcriptase but lack LTRs, and are transcribed by RNA polymerase II . Short interspersed nuclear elements do not encode reverse transcriptase and are transcribed by RNA polymerase III Class II (DNA transposons) The cut-and-paste transposition mechanism of class II TEs does not involve an RNA intermediate. The transpositions are catalyzed by several transposase enzymes. Some transposases non-specifically bind to any target site in DNA, whereas others bind to specific target sequences. The transposase makes a staggered cut at the target site producing sticky ends, cuts out the DNA transposon and ligates it into the target site. A DNA polymerase fills in the resulting gaps A schematic diagram shows the nucleotide sequence of terminal inverted repeats and flanking direct repeats on a transposable element. The transposable element is depicted as a horizontal, double-stranded segment of DNA. Elongated, vertical, rectangles represent individual nucleotides on each DNA strand. The rectangles are grey, except for 11 nucleotide pairs near each of the DNA segment’s opposite ends. These rectangles are colored and labeled with the letters A, T, G, or C. The color of the rectangle represents the chemical identity of the nitrogenous base and is blue (guanine), red (thymine), green (adenine), or orange (cytosine). The six innermost nucleotide pairs at both ends are enclosed in a bracket and labeled as terminal inverted repeats. The five outermost nucleotide pairs at both ends are enclosed in a separate bracket and labeled as flanking direct repeats.http://www.nature.com/scitable/content/ne0000/ne0000/ne0000/ne0000/57623/Classes_large_2.jpgfrom the sticky ends and DNA ligase closes the sugar-phosphate backbone. This results in target site duplication and the insertion sites of DNA transposons may be identified by short direct repeats (a staggered cut in the target DNA filled by DNA polymerase) followed by inverted repeats (which are important for the TE excision by transposase). The structure of a DNA transposon is shown in figure. Cut-and-paste TEs may be duplicated if their transposition takes place during S phase of the cell cycle, when a donor site has already been replicated but a target site has not yet been replicated. Such duplications at the target site can result in gene duplication, which plays an important role in genomic evolution. Not all DNA transposons transpose through the cut-and-paste mechanism. In some cases, a replicative transposition is observed in which a transposon replicates itself to a new target site. Class II TEs comprise less than 2% of the human genome, making the rest Class I. Autonomous and non-autonomous Transposition can be classified as either “autonomous” or “non-autonomous” in both Class I and Class II TEs. Autonomous TEs can move by themselves, whereas non-autonomous TEs require the presence of another TE to move. This is often because dependent TEs lack transposase (for Class II) or reverse transcriptase (for Class I). Activator element (Ac) is an example of an autonomous TE, and dissociation elements (Ds) is an example of a non-autonomous TE. Without Ac, Ds are not able to transpose.  Figure 2: Classes of mobile elements. Examples • The first TEs were discovered in maize (Zea mays) by Barbara McClintock in 1948, for which she was later awarded a Nobel Prize. She noticed chromosomal insertions, deletions, and translocations caused by these elements. These changes in the genome could, for example,