Transposable elements: mechanisms of transposition, genome evolution, and epigenetic silencing

Intuition Beginner

Most of your DNA does not code for proteins. Nearly half of the human genome is made of transposable elements — DNA sequences that can copy or move themselves to new locations. They are sometimes called "jumping genes."

Barbara McClintock discovered transposable elements in the 1940s while studying colour patterns in Indian corn (maize). She found that certain DNA segments moved between chromosomes, turning genes on and off and producing the spotted kernel patterns. Her work was initially met with scepticism because it violated the assumption that genes had fixed positions.

Transposable elements fall into two broad categories. Retrotransposons (Class I) copy themselves through an RNA intermediate: the element is transcribed into RNA, then reverse-transcribed back into DNA and inserted elsewhere. The original copy stays put, so the element increases in copy number. DNA transposons (Class II) move directly as DNA, excising themselves from one spot and reinserting elsewhere.

Visual Beginner

Think of the genome as a book. Most pages contain instructions (genes). Transposable elements are like self-copying sticky notes scattered throughout the book. Retrotransposons photocopy themselves and paste the copy on a new page — the original stays. DNA transposons physically peel off and stick themselves somewhere else.

Worked example Beginner

The most common transposable element in humans is LINE-1 (L1), a retrotransposon that makes up about 17% of the genome. A full-length LINE-1 is about 6,000 base pairs long and carries instructions for two proteins: one that binds RNA, and one with reverse transcriptase activity that copies RNA back into DNA.

The mobilisation process works as follows:

1. LINE-1 DNA is transcribed into RNA.
2. The RNA is translated into the two proteins.
3. The RNA and proteins form a complex that re-enters the nucleus.
4. The reverse transcriptase copies the RNA into DNA and inserts it into a new genomic location.

Most LINE-1 copies are truncated or mutated and cannot jump. Only about 100 copies in each human genome are competent to mobilise, and even fewer are active in any given cell.

Check your understanding Beginner

Formal definition Intermediate+

Transposable elements (TEs) are DNA sequences capable of changing their genomic position through mechanisms classified into two major classes based on the nature of the transposition intermediate.

Class I: Retrotransposons

Retrotransposons transpose via an RNA intermediate that is reverse-transcribed into DNA before integration. This copy-and-paste mechanism increases genomic copy number. Three major subclasses exist in mammals:

LINE-1 (Long Interspersed Nuclear Element 1): Approximately 6 kb full-length autonomous element encoding two open reading frames. ORF1p is an RNA-binding protein with chaperone activity. ORF2p encodes an endonuclease (EN) domain and a reverse transcriptase (RT) domain. The endonuclease nicks genomic DNA at a loose consensus site (5'-TTTT/A-3', where / marks the nick), exposing a 3'-OH that primes reverse transcription of the LINE-1 RNA directly at the integration site — a process called target-primed reverse transcription (TPRT).

TPRT proceeds as follows:

1. ORF2p endonuclease nicks one strand of target DNA at a T-rich site.
2. The 3'-OH at the nick serves as a primer. LINE-1 RNA anneals via its poly(A) tail to the T-rich overhang.
3. ORF2p reverse transcriptase extends from the 3'-OH, synthesising cDNA using the LINE-1 RNA as template.
4. Second-strand cleavage and cDNA synthesis complete the insertion.
5. Host DNA repair machinery fills gaps and seals nicks.

The result is a new LINE-1 copy flanked by short target site duplications (TSDs) of 7-20 bp, generated by the staggered nature of the initial cleavage on opposite strands.

SINEs (Short Interspersed Nuclear Elements): Non-autonomous retrotransposons of 100-400 bp that parasitise LINE-1 reverse transcriptase for mobilisation. The most abundant SINE in humans is the Alu element (~280 bp, derived from 7SL RNA). Alu elements have no coding capacity — they consist of two similar monomers derived from 7SL RNA separated by an A-rich linker, followed by a poly(A) tail. With over one million copies, Alu elements account for approximately 11% of the human genome. Alu RNA is transcribed by RNA polymerase III and hijacks the LINE-1 ORF2p protein in trans for its own retrotransposition.

SVA elements: Composite non-autonomous retrotransposons (~2 kb) unique to hominid lineae. SVA elements contain SINE-R (derived from an endogenous retrovirus), VNTR (variable number tandem repeat), and Alu-like components. They are the youngest retrotransposon family in humans and remain actively mobilising.

LTR retrotransposons and endogenous retroviruses (ERVs): Elements flanked by long terminal repeats (300-1000 bp) that contain promoter, enhancer, and polyadenylation signals. Full-length ERVs (~7-10 kb) encode gag, pol (reverse transcriptase, integrase, protease), and sometimes env genes. Most human ERV copies are ancient fossils with disabling mutations; the HERV-K (HML-2) family is the most recently active. ERVs replicate via an extrachromosomal DNA intermediate: the RNA transcript is reverse-transcribed into double-stranded DNA in a virus-like particle, and the integrase inserts the cDNA into the genome.

Class II: DNA transposons

DNA transposons move directly as DNA, typically through a cut-and-paste mechanism catalysed by a self-encoded transposase enzyme. The transposase binds to short inverted repeat sequences (terminal inverted repeats, TIRs) flanking the element, excises it, and inserts it into a new target site.

Cut-and-paste transposition:

1. Transposase binds the TIRs at both ends of the element, forming a synaptic complex (transpososome).
2. The element is excised from the donor site by double-strand cleavage at both ends.
3. The excised transposase-element complex captures a new target DNA site.
4. Transposase catalyses staggered single-strand cuts at the target, inserts the element, and host repair machinery fills in the gaps.
5. The filled-in gaps become the target site duplication (TSD).

Because the element is excised from the donor, cut-and-paste transposition does not inherently increase copy number. Copy number increases only if the donor site is replicated before excision (replicative transposition) or if the gap at the donor is repaired using the sister chromatid as a template.

Helitrons (rolling-circle transposons): A distinct subclass that replicates via a rolling-circle mechanism, encoding a RepHel protein with helicase and replication initiator activity. Helitrons capture and duplicate host gene fragments, creating chimeric transcripts. They lack TIRs and do not generate TSDs. First discovered in plants and nematodes, they are major contributors to gene duplication in C. elegans and maize.

Autonomous vs. non-autonomous elements: An autonomous element encodes all proteins needed for its own mobilisation. A non-autonomous element retains the cis-acting sequences (TIRs, poly(A) tail) recognised by the transposition machinery but lacks functional coding capacity — it mobilises in trans using proteins encoded by autonomous elements. Miniature inverted-repeat transposable elements (MITEs) are short (50-500 bp) non-autonomous DNA transposons with TIRs but no transposase gene; they are among the most abundant TE types in plant genomes.

Insertion site preferences and target site duplications

Different TE families show distinct insertion site preferences. LINE-1 prefers T-rich sequences (consistent with the endonuclease nicking at TTTT/A) and inserts into AT-rich regions. Alu elements prefer A+T-rich regions but also target genes. DNA transposon insertion is generally less sequence-specific but may show structural preferences (e.g., TA dinucleotide target sites for Tc1/mariner elements, generating 2 bp TSDs).

Target site duplications arise from the staggered cleavage at the insertion site. When the transposase (or endonuclease for LINE-1) makes single-strand cuts offset by a few base pairs on the two strands, the resulting single-stranded overhangs are filled in by host polymerases, duplicating the sequence at the insertion junction. TSD length is a family-level diagnostic: LINE-1 produces 7-20 bp TSDs, Alu produces 7-21 bp TSDs, Tc1/mariner produces 2 bp TSDs, and hAT elements produce 8 bp TSDs.

Key mechanism Intermediate+

Target-primed reverse transcription and the LINE-1 retrotransposition cycle.

LINE-1 is the only currently active autonomous retrotransposon in humans and the driver of nearly all ongoing retrotransposition. Its mobilisation mechanism — target-primed reverse transcription (TPRT) — is mechanistically distinct from retroviral integration and represents the predominant mode of non-LTR retrotransposition in vertebrates.

The full cycle begins with transcription of a full-length LINE-1 from its internal promoter in the 5' untranslated region (UTR). The bicistronic mRNA is exported to the cytoplasm, where ORF1p and ORF2p are translated. Translation of ORF2p requires a poorly characterised ribosomal frameshift signal at the ORF1/ORF2 junction, ensuring that ORF2p (the enzymatic machinery) is produced at roughly one copy per 10-20 ORF1p molecules. This stoichiometric imbalance is functionally important: excess ORF1p coats the LINE-1 RNA, forming an RNP particle that protects the RNA from degradation.

The RNP re-enters the nucleus during mitosis when the nuclear envelope breaks down. Integration proceeds by TPRT:

1. Endonucleolytic nicking. The ORF2p EN domain nicks one strand of target DNA at a loose consensus sequence. The preferred site is 5'-TTTT/AA-3' (nick on the bottom strand). The nick exposes a 3'-OH with a T-rich overhang.

2. Annealing. The poly(A) tail of the LINE-1 RNA base-pairs with the T-rich overhang at the nick site, positioning the RNA template adjacent to the 3'-OH primer.

3. First-strand cDNA synthesis. The ORF2p RT domain extends from the 3'-OH, using the LINE-1 RNA as template. Reverse transcription proceeds toward the 5' end of the RNA, synthesising the first cDNA strand. In most cases, reverse transcription is incomplete, terminating before reaching the 5' end of the template — this produces the 5' truncated insertions that constitute the majority of genomic LINE-1 copies.

4. Second-strand cleavage and synthesis. The second strand of target DNA is cleaved at a position offset from the first nick, and second-strand DNA synthesis completes the insertion. Host DNA repair enzymes process the remaining nicks and gaps.

The hallmark of successful TPRT is a new LINE-1 copy flanked by target site duplications of 7-20 bp, with variable 5' truncation and occasional 3' transduction (if the polyadenylation signal is bypassed and downstream genomic sequence is included in the RNA transcript). 3' transductions can carry flanking genomic DNA to new locations, creating a mechanism for exon shuffling.

LINE-1 also drives the mobilisation of non-autonomous elements (Alu, SVA) and cellular mRNAs. Cellular mRNAs that are reverse-transcribed and inserted by the LINE-1 machinery become processed pseudogenes (retropseudogenes) — they lack introns, possess a poly(A) tract, and are flanked by TSDs, but are typically non-functional due to the absence of promoter sequences.

Exercises Intermediate+

Epigenetic silencing, exaptation, and the transposon-driven genome Master

DNA methylation and heterochromatin silencing of transposable elements

The host genome deploys multiple epigenetic defence layers against transposable element activity. The primary silencing mechanism in mammals is DNA methylation at CpG dinucleotides. Nearly all CpG sites within transposable elements are methylated in somatic tissues, maintaining transcriptional repression. The de novo DNA methyltransferases DNMT3A and DNMT3B, along with the maintenance methyltransferase DNMT1, establish and propagate TE methylation across cell divisions.

The connection between DNA methylation and TEs runs deep. The CpG dinucleotide, the substrate for vertebrate DNA methylation, is the target of most genomic methylation precisely because TEs are the predominant CpG-containing sequences. The methylation machinery may have evolved originally as a TE defence system that was later co-opted for gene regulation.

In mouse germ cells, a second wave of demethylation and remethylation during primordial germ cell development resets the epigenetic landscape. During this vulnerable window (embryonic days 10.5-13.5), DNA methylation is globally erased, including at TEs. The cell compensates by deploying the piRNA pathway (see below) to maintain TE silencing independently of DNA methylation during this period. Failure of either system leads to TE reactivation, DNA damage, and germ cell apoptosis.

H3K9me3 (trimethylation of histone H3 lysine 9) marks heterochromatin and is enriched at TE loci. The SETDB1 (ESET) histone methyltransferase deposits H3K9me3 at ERV elements in embryonic stem cells, maintaining their silencing. The KRAB-ZFP/KAP1 system (see below) recruits SETDB1 to specific TE families through sequence-specific DNA binding.

The piRNA pathway: germ cell defence against transposons

The Piwi-interacting RNA (piRNA) pathway is the primary small-RNA-based transposon silencing system in animal germ cells. piRNAs are 24-31 nucleotide RNAs that associate with Piwi-family Argonaute proteins. The pathway operates through two arms: primary processing and ping-pong amplification.

Primary piRNA biogenesis. piRNA clusters are specialised heterochromatic loci that serve as a genetic memory of past transposon invasions. They contain a diverse archive of TE fragments in both sense and antisense orientations. In Drosophila, the Rhino-Deadlock-Cutoff (RDC) complex promotes single-stranded transcription of dual-strand piRNA clusters, bypassing normal transcriptional termination. The long precursor transcripts are exported to the perinuclear nuage (in germ cells) or Yb bodies (in somatic follicle cells) and processed into individual piRNAs by the endonuclease Zucchini (Zuc)/PLD6.

The ping-pong amplification cycle (described in Exercise 6) creates a positive-feedback loop that amplifies piRNAs targeting the most active TEs. The ping-pong signature — 1U bias in Aub-bound piRNAs and 10A bias in Ago3-bound piRNAs — is the diagnostic hallmark of an active cycle.

Transcriptional silencing. Nuclear Piwi proteins (Piwi in Drosophila, MIWI2 in mouse) are loaded with antisense piRNAs and enter the nucleus, where they guide heterochromatin formation at complementary TE loci. The mechanism involves recruitment of histone methyltransferases (Eggs/dSETDB1 in Drosophila, establishing H3K9me3) and the Panoramix (Panx)/Panoramix (Silencio) complex, which recruits additional chromatin effectors. The result is stable transcriptional silencing of TE loci that persists even after the initiating piRNAs decay.

In mice, MIWI2 (nuclear) and MILI (cytoplasmic) are the two essential Piwi proteins. MIWI2 loads with piRNAs in the cytoplasm, enters the nucleus, and guides de novo DNA methylation at TE loci — directly connecting the piRNA pathway to the DNA methylation silencing machinery. MILI performs the ping-pong cleavage of TE transcripts in the cytoplasm. Mice lacking MIWI2 or MILI show TE reactivation, meiotic arrest, and male sterility.

KRAB-ZFP expansion and TE domestication

Vertebrate genomes have evolved a large family of KRAB-zinc finger proteins (KRAB-ZFPs) that silence specific TE families through sequence-specific DNA binding. KRAB-ZFPs are the largest family of transcription factors in mammals: humans have approximately 350 KRAB-ZFP genes, and mice have approximately 600. Each KRAB-ZFP has an N-terminal KRAB domain (which recruits the KAP1/TRIM28 co-repressor) and a C-terminal array of C2H2 zinc fingers that determine DNA-binding specificity.

The silencing mechanism operates as follows:

1. A KRAB-ZFP binds a specific TE sequence via its zinc finger array.
2. The KRAB domain recruits KAP1 (TRIM28).
3. KAP1 serves as a scaffold for the SETDB1 histone methyltransferase (depositing H3K9me3), the HP1 heterochromatin protein, and the NuRD histone deacetylase complex.
4. H3K9me3 and HP1 binding establish a repressive heterochromatin domain over the TE.

The evolutionary dynamic between KRAB-ZFPs and TEs is an arms race. When a new TE family invades a genome, it spreads unchecked until a KRAB-ZFP evolves targeting specificity for its sequence. The KRAB-ZFP gene family expands through tandem duplication and diverges through positive selection on the zinc finger residues that contact DNA, generating new specificities. Each TE invasion selects for new KRAB-ZFP genes, which explains the rapid expansion of the family in mammalian lineages.

After a TE family accumulates disabling mutations and becomes inactive, its cognate KRAB-ZFP persists as a "fossil record" of the past invasion. Some of these KRAB-ZFPs are later co-opted to regulate host genes that happen to contain related sequence motifs — a form of TE domestication.

Exaptation: transposable elements as sources of regulatory innovation

TEs are not purely genomic parasites. Over evolutionary time, host genomes have recruited TE-derived sequences for essential cellular functions — a process termed exaptation (Gould and Vrba, 1982).

Syncytin and placental evolution. The syncytin genes (syncytin-1, syncytin-2 in humans) are derived from the env (envelope) gene of endogenous retroviruses. The retroviral envelope protein mediates membrane fusion between virus and host cell during infection. In mammals, syncytin mediates fusion of trophoblast cells to form the syncytiotrophoblast — the multinucleated layer of the placenta that mediates nutrient and gas exchange between mother and foetus. Without thisERV-derived gene, placental mammals could not exist in their current form. Syncytin was independently co-opted from different ERV families in primates, rodents, lagomorphs, and carnivorans — at least six independent exaptation events producing the same functional outcome.

TE-derived regulatory elements. TEs carry their own promoters, enhancers, insulators, and polyadenylation signals. When inserted near host genes, these regulatory sequences can be co-opted to control gene expression. Genome-wide analyses reveal that a substantial fraction of human enhancers, especially those involved in immune response and early development, are derived from TE sequences:

• ERV LTRs serve as tissue-specific enhancers and alternative promoters.
• Alu elements contain binding sites for transcription factors and contribute to hormone-responsive gene regulation.
• SINE elements have been co-opted as insulator elements and boundary elements in chromatin organisation.

The MER41 family of ERV insertions in the human genome contains binding sites for STAT1 (a key interferon-responsive transcription factor) and has been co-opted as interferon-inducible enhancers regulating immune genes. This represents direct domestication of an ancient ERV invasion for innate immune defence.

TE-derived coding sequences. TEs have contributed novel exons and protein domains to host genes through a process called exonisation. Alu elements inserted into introns are alternatively spliced into mature mRNA, creating new protein isoforms. The Rag1/Rag2 recombinase — essential for V(D)J recombination in adaptive immunity — is derived from a Transib superfamily DNA transposon, providing one of the most consequential exaptations in vertebrate evolution. The RAG1/2 transposase was domesticated approximately 500 million years ago and now performs the programmed DNA rearrangement that generates antibody and T-cell receptor diversity.

Genome size, the C-value paradox, and transposon accumulation

The C-value paradox is the lack of correlation between genome size and organismal complexity. The human genome (3.2 Gb) is 200 times larger than the C. elegans genome (100 Mb) but has only about 5 times as many protein-coding genes (~20,000 vs ~4,000). The primary explanation for genome size variation across eukaryotes is transposable element accumulation.

Plant genomes illustrate the extremes. Arabidopsis thaliana has a compact 135 Mb genome with relatively few TEs. Bread wheat (Triticum aestivum) has a 17 Gb genome — over 5 times larger than human — of which approximately 85% consists of TEs, primarily LTR retrotransposons (Copia and Gypsy families) that have undergone massive proliferation in the wheat lineage. The onion (Allium cepa) genome is 16 Gb, vastly larger than needed for its gene complement.

The equilibrium genome size reflects a balance between TE proliferation and removal. TEs add sequence through retrotransposition and DNA transposition. DNA is removed through large deletions (unequal crossing over, illegitimate recombination). In species with large effective population sizes, purifying selection removes TE insertions efficiently because the fitness cost of each insertion (however small) is visible to selection. In species with small effective population sizes (like humans, with a long-term $N_e$ of approximately 10,000), purifying selection is less efficient, and slightly deleterious TE insertions can drift to fixation — the mutational hazard hypothesis (Lynch and Conery, 2003).

The rate of LTR retrotransposon removal can be estimated from the ratio of solo LTRs (single LTR circles remaining after recombination between flanking LTRs of a full-length element) to intact elements. In rice, the solo LTR ratio is approximately 10:1, indicating rapid removal. In wheat, the ratio is much lower, consistent with slower removal and net TE accumulation.

LINE-1 reverse transcriptase and retropseudogenes

The LINE-1 reverse transcriptase does not exclusively act on LINE-1 RNA. It promiscuously reverse-transcribes any polyadenylated RNA that is present in the same RNP complex, including cellular mRNAs and non-coding RNAs. This trans-mobilisation produces processed pseudogenes (retropseudogenes) when cellular mRNAs are reverse-transcribed and inserted into the genome.

Processed pseudogenes bear the hallmarks of LINE-1-mediated insertion: loss of introns (because the mRNA template is already spliced), a 3' poly(A) tract, flanking target site duplications, and 5' truncation. The human genome contains approximately 10,000-15,000 processed pseudogenes. Most are non-functional because they lack promoter sequences and regulatory elements. However, some processed pseudogenes retain open reading frames and are expressed, and a small number have acquired new functions (e.g., the snRNP pseudogene-derived small nuclear RNAs).

LINE-1 reverse transcriptase also generates interspersed segmental duplications through a process called 3' transduction. When the LINE-1 polyadenylation signal is bypassed during transcription, downstream genomic sequence is included in the RNA. This downstream sequence is co-reverse-transcribed and inserted at the new site along with the LINE-1 element, creating a duplicate of the flanking region at a distant genomic location. 3' transductions average 50-200 bp but can exceed 1 kb, contributing to genome structural variation.

Somatic retrotransposition and mosaicism

While most TE insertions occur in the germline and are inherited in Mendelian fashion, LINE-1 retrotransposition also occurs in somatic cells, generating genomic mosaicism — the presence of different LINE-1 insertion patterns in different cells of the same individual.

Somatic retrotransposition was first demonstrated in cancers, where whole-genome sequencing of tumours revealed LINE-1 insertions absent from matched normal tissue. In some cancers (oesophageal, colorectal, hepatocellular), LINE-1 insertions are abundant and clonal, indicating they occurred early in tumour evolution. These insertions can disrupt tumour suppressors (APC, PTEN) or activate oncogenes, directly contributing to carcinogenesis.

Somatic LINE-1 activity also occurs in normal tissues. Neuronal genomes show LINE-1 insertions absent from other tissues, indicating retrotransposition in neural progenitor cells. Single-cell sequencing has estimated that each neuron in the human brain contains approximately 0.5-5 unique LINE-1 insertions. The functional significance of neuronal mosaicism is debated: it may contribute to neuronal diversity and individuality, or it may be a purely stochastic process with pathological consequences when it strikes critical genes.

The endonuclease-independent pathway provides an alternative route for LINE-1 insertion in somatic cells. In cells with high levels of DNA damage (where nicks and breaks are abundant), LINE-1 RNA can be reverse-transcribed and inserted at pre-existing DNA breaks without requiring ORF2p endonuclease activity. This pathway is enhanced by ionizing radiation and chemotherapy and may contribute to the increased LINE-1 activity observed in cancer cells.

CRISPR evolutionary origin

The CRISPR-Cas adaptive immune system in bacteria and archaea is derived from a domesticated Cas1 integrase that shares evolutionary ancestry with transposases. The CRISPR array — the locus where bacterial cells store sequence memories of past phage infections — is assembled by the Cas1-Cas2 integrase complex, which captures short fragments of invading phage DNA and inserts them as spacers between repeat sequences in the CRISPR array. This integration mechanism is mechanistically analogous to transposon insertion: staggered cuts at the integration site generate short duplications (again, target site duplications), and the inserted sequence is oriented in a specific direction.

The evolutionary trajectory from transposon to immune system involved several steps: (a) domestication of a transposase as the Cas1 integrase; (b) acquisition of a CRISPR RNA biogenesis pathway (tracrRNA, RNase III); (c) coupling of spacer acquisition to interference (Cas9 cleavage of complementary phage DNA). The bridge between transposition and adaptive immunity is the Cas1 integrase — a transposase descendant that now performs programmed, regulated DNA integration for a defensive function, mirroring the exaptation of the RAG1/2 transposase for V(D)J recombination in vertebrates.

Synthesis. Transposable elements are simultaneously genomic parasites, evolutionary innovators, and major determinants of genome architecture. The host genome maintains a multi-layered defence (DNA methylation, H3K9me3 heterochromatin, piRNA pathway, KRAB-ZFP targeting) that suppresses TE activity in the germline and somatic tissues. When these defences falter — during epigenetic reprogramming in early development, in ageing cells with declining methylation, or in cancer cells with disrupted heterochromatin — TEs reactivate and generate insertional mutagenesis. Yet the same elements have been repeatedly domesticated for host functions: syncytins from ERVs for placental development, RAG1/2 from a DNA transposon for adaptive immunity, Cas1 from a transposase for CRISPR immunity, and innumerable TE-derived enhancers and promoters for gene regulation. The genome is not a clean blueprint but a palimpsest written over by billions of years of TE activity, with each insertion event simultaneously a potential threat and a source of evolutionary novelty. The bridge to 17.06.04 pending runs through the epigenetic silencing machinery: DNA methylation, H3K9 methylation, and heterochromatin formation are the mechanisms the cell uses to keep TEs silent, and understanding TE silencing is prerequisite for understanding how these epigenetic marks are established and maintained genome-wide.

Connections Master

• Mutation and repair 17.06.01. Transposable element insertions are a form of mutation — they disrupt coding sequences, alter splicing, and create insertional mutagenesis. TE excision (for DNA transposons) generates double-strand breaks that require repair by NHEJ or HR 17.06.02 pending. The host repair machinery processes the gaps at insertion sites, generating target site duplications.

• DNA repair pathways 17.06.02 pending. The staggered cuts made by transposases and LINE-1 endonuclease are processed by host DNA repair enzymes. NHEJ repairs the gaps at insertion sites. The DNA damage response (ATM/ATR signalling) is activated by TE-induced DSBs. LINE-1 endonuclease-independent insertion exploits pre-existing DNA damage as integration sites.

• DNA replication 17.05.01. Retrotransposon copy number increases when the donor site is replicated before excision. LINE-1 transcription is upregulated during S phase. The TPRT mechanism uses a 3'-OH primer structurally analogous to a replication primer.

• Transcription and gene expression 17.05.02. TEs carry their own promoters (LINE-1 5' UTR, LTR promoters, Pol III promoters in SINEs) that can drive ectopic gene expression when inserted near host genes. TE silencing by DNA methylation and heterochromatin directly regulates transcription.

• Cell cycle and cancer 17.08.01. Somatic LINE-1 retrotransposition is documented in many cancers. TE insertions can disrupt tumour suppressors or activate oncogenes. Cancer cells often show global DNA hypomethylation (reactivating TEs) combined with local hypermethylation of tumour suppressor promoters.

• Immunology 17.10.01. The RAG1/RAG2 recombinase, essential for V(D)J recombination in lymphocyte development, is derived from a Transib DNA transposon. The CRISPR-Cas system in prokaryotes is derived from a domesticated transposase. Both are examples of TE exaptation for immune defence.

• Evolutionary genetics 19.02.05. TE insertions are a source of genetic variation that feeds into population-genetic processes. The balance between TE proliferation (increasing copy number) and purifying selection (removing deleterious insertions) determines equilibrium TE load. Species with small effective population sizes accumulate more TEs due to less efficient selection (the mutational hazard hypothesis).

• Epigenetic regulation 17.06.04 pending. DNA methylation, H3K9me3, and heterochromatin formation are the primary mechanisms for silencing TEs. Understanding TE silencing provides the motivation for the epigenetic mechanisms covered in 17.06.04. The piRNA pathway connects small RNA biology to DNA methylation and histone modification.

Historical notes Master

Barbara McClintock discovered transposable elements in maize (Zea mays) through a decade of meticulous cytogenetic work published in a series of papers from 1948 to 1953. Her 1950 PNAS paper, "The origin and behavior of mutable loci in maize," reported that certain genetic loci could change position on chromosomes, producing the variegated kernel colour patterns she called "dissociation" (Ds) and "activator" (Ac). She showed that Ac was an autonomous element that could mobilise itself, while Ds was non-autonomous and required Ac for transposition.

McClintock's findings challenged the prevailing view that genes had fixed chromosomal positions. The initial reception was sceptical: many geneticists found the concept of mobile genetic elements incompatible with the linear gene map being built from Drosophila crosses. The discovery was not widely accepted until the 1960s-1970s, when mobile elements were found in bacteria (IS elements, discovered by Starlinger and Saedler in 1972), Drosophila (P elements, discovered by Kidwell in 1977), and yeast (Ty elements, discovered by Cameron and Davis in 1974). McClintock received the Nobel Prize in Physiology or Medicine in 1983, over thirty years after her discovery.

The discovery of retrotransposons followed from the characterisation of reverse transcriptase (Temin and Baltimore, 1970 Nobel Prize). The yeast Ty elements were shown to transpose through an RNA intermediate by Boeke, Garfinkel, and Fink in 1985 — the first demonstration of retrotransposition. The LINE-1 element was characterised as a human retrotransposon by Kazazian, Moran, and colleagues in the 1990s, who developed the cultured-cell retrotransposition assay that definitively demonstrated LINE-1 mobility.

The piRNA pathway was discovered in 2006 by four independent groups (Aravin, Brennecke, Gunawardane, and Vagin) who identified a new class of small RNAs (24-31 nt) associated with Piwi proteins in mouse and Drosophila germ cells. The ping-pong amplification cycle was described by Brennecke et al. (2007) and Gunawardane et al. (2007), based on the signature 10-nt complementarity between sense and antisense piRNAs.

The concept of exaptation was introduced by Gould and Vrba in 1982 to describe characters that evolved for one function but were later co-opted for another. The discovery that syncytin-1 (Mi et al., 2000; Blond et al., 2000) was derived from an ERV envelope protein and was essential for placental syncytiotrophoblast formation provided the most dramatic example of TE exaptation in mammals. The identification of RAG1/2 as a domesticated transposon was proposed by Thompson in 1995 and supported by structural studies showing the RAG1/2 catalytic domain is homologous to transposase/integrase active sites.

The C-value paradox was named by Thomas in 1971, though the discrepancy between genome size and complexity had been known since the 1950s from DNA quantification by spectrophotometry. Lynch and Conery (2003) proposed the mutational hazard hypothesis, explaining genome size variation through the population-genetic efficiency of selection against slightly deleterious TE insertions.

Somatic LINE-1 retrotransposition in the human brain was demonstrated by Coufal et al. (2009) and quantified by single-cell sequencing in studies by Evrony, Lee, and colleagues (2012-2015), revealing that individual neurons carry unique LINE-1 insertion profiles.

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