![]() These can be categorized in two types: LINEs (long interspersed nuclear elements) and SINEs (short interspersed nuclear elements). LINES and SINES: There are non-LTR retrotransposons that essentially lack the long terminal repeat of the HERV. In humans, these HERVs comprise a substantial 8% of the total genome. Both the HERV and its MaLR counterpart in diverse mammal genomes contain specialized LTR sequences that flank its internal coding regions. ![]() HERVs: In humans, there is a specialized variant of a mammalian apparent LTR-retrotransposon (MaLR) called a human endogenous retrovirus (HERV). I will briefly discuss each group in turn. Retrotransposons come in a variety of distinct categories and are classified based on the presence or absence of specialized long terminal repeats (LTR) in the flanking boundaries of the sequence. This allows the active retrotransposons to retain their original location and structure in the genome while accumulating copies of themselves elsewhere. Instead, they actually proliferate by being copied into an RNA intermediate in a “copy and paste” type of mechanism. RNA transposons, also known as retrotransposons or retroelements, are not cut and pasted like DNA TEs. However, in some plant, insect, and yeast genomes, they can occupy 20 to 80% of the total DNA sequence. For the most part, DNA transposons only constitute about 3% of the genome of mammals and are not actively transposing. 4,5Īt the flanking ends of a DNA transposon are specialized sequences called inverted repeats. These transpose by a “cut and paste” mechanism in which the transposon is excised from a region and moved to another via the aid of a transposase enzyme encoded by the transposon itself or another one. The TEs that McClintock discovered are in a TE class known as DNA transposons. 3 It was found that the Ac element was a small TE that encoded a single transposase enzyme (facilitating transposition) and that the Ds element was a deleted derivative of Ac. In 1983, researcher Nina Federoff isolated the Ac and Ds TEs and mapped the DNA sequences. Remarkably, McClintock’s work in identifying TEs in the 1940s and 1950s through radiation, controlled matings, and light microscopy of stained chromosomes was vindicated as the golden age of molecular biology took off decades later. She noticed it could switch back and forth between an active and inactive form. McClintock located another notable TE she called suppressor-mutator ( Spm). The Ac element could also initiate its own transposition. However, she also discovered that activity for Ds required another TE she termed activator, or Ac. One of the first TEs McClintock discovered was associated with a chromosome breakage event she called Ds for dissociation locus. Amazingly, all of this research was done before researchers had access to modern DNA sequencing technology. ![]() ![]() McClintock also found that the TEs themselves interacted with each other as activating factors. Thus, the coloration patterns in the corn kernels were found to be caused by the interplay between a TE and a pigment gene. In other words, she observed that the TEs could move to new locations and modify the expression of genes where they inserted. She noticed that some of the variegation patterns involved chromosome breaks that she documented by microscope photography of stained cell nuclei.įrom 1944 to 1947, McClintock demonstrated that the observed chromosomal breakpoints were the result of genetic activity by what she called controlling elements (now called TEs). 2 In addition to the predicted breakage and deletion patterns that produced abnormal ring-shaped chromosomal structures, McClintock obtained a large number of viable plants with unusual patterns of kernel color variegation. Image credit:Smithsonian Institution Archives public domainĪs an offshoot of McClintock’s work with irradiated corn plant cells, she also noted that breakpoints in chromosomes were not random. A goal for the future would be to determine the extent of knowledge the cell has of itself and how it utilizes this knowledge in a “thoughtful” manner when challenged. The sensing devices and the signals that initiate these adjustments are beyond our present ability to fathom. There must be numerous homeostatic adjustments required of cells. Regarding this discovery, McClintock said in her 1983 Nobel Prize speech: Thirty-five years after McClintock’s first report of transposable elements, she was awarded the Nobel Prize. In fact, she astutely noticed that the chromosomal response to the damaging X-rays was not random but comprised an innate surveillance and repair system with untold complexity. She observed that mutagenic X-rays did not produce isolated random changes in the DNA of corn plants. Geneticist Barbara McClintock made an important discovery in the 1930s that eventually led to her later work in transposable elements. Transposable Elements: A Brief History of Their Discovery
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