Transposable elements (TEs), mobile genetic elements that can relocate within genomes, represent a significant portion of the human genome and are studied in Drosophila melanogaster for their mechanisms of mobilization. The document specifically highlights the copia retrotransposable elements, their structure, transposition process, and the implications of TEs in genetic research, including potential mutations. Future research opportunities continue to expand as more genomes are sequenced, aiding in understanding TEs' roles in genetic functions and disorders.

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MOLECULAR GENETICS | UNIT IV | Transposable elements in Drosophila | Copia
RNA TRANSPOSABLE ELEMENTS (COPIA) IN Drosophila
TRANSPOSABLE ELEMENTS
 Transposable elements (TEs) are mobile genetic elements that can mobilize
within host genomes.
 As TEs comprise more than 40% of the human genome and are linked to
numerous diseases, understanding their mechanisms of mobilization and
regulation is important.
 Drosophila melanogaster is an ideal model organism for the study of eukaryotic
TEs as its genome contains a diverse array of active TEs.
 Also referred to as “jumping genes,” TEs move, or transpose, to different locations
throughout the genomes in which they reside.
TYPES OF TRANSPOSABLE ELEMENTS
 The discovery of TEs in maize by Barbara McClintock in the 1940s, proposed that
these elements be classified into 2 major groups
1. Retro transposons (class I elements) – Example: Copia elements
2. DNA transposons (class II elements) – Example: P Element
NOTE: Within these groups, numerous families of TE (defined by sequence similarity) still many unclassified
RETRO TRANSPOSABLE ELEMENTS IN Drosophila
 Retro transposons, or RNA transposons, comprise more than 30% of the human
genome and are the most abundant class of TEs in the D. melanogaster genome.
 Retro transposon genes resemble those of retroviral genomes in both structure and
function.
 Retro transposons are primarily characterized by the presence of gag and pol
genes that may be overlapping and may also be encoded in a single fused ORF.
 The retro transposon pol gene encodes a poly protein, typically consisting of a
protease, an integrase, and a reverse transcriptase (RT) with an RNase H
domain and DNA polymerase activity.
 The RT is common to all autonomous retrotransposons, as it is required for
reverse transcription of the RNA intermediate to generate DNA copies of these TEs.
 The protease is involved in processing of precursor proteins, such as the Pol poly
protein. The integrase is required for insertion of cDNA into the host genome.
 Gag is the primary component of virus-like nucleo capsid particles, formed by
polymerization of Gag monomers, which provide a structural coat for components
involved in the reverse transcription event of retro transposon mobilization
Retro transposons include;
1. LTR (Long Terminal Repeat)
retro transposons
Example: Copia, Gypsy
2. Non-LTR (Non-Long Terminal
Repeat) retro transposons
 2a. Long Interspersed Nuclear
Elements (LINEs) and LINE-
like elements)
 2b. Short Interspersed Nuclear
Elements (SINEs)
3. Other TEs
Note: Some retro transposons contain a third gene encoding the retroviral envelope (env) protein necessary for
mobilization of retro elements outside of their host cells

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MOLECULAR GENETICS | UNIT IV | Transposable elements in Drosophila | Copia
COPIA TRANSPOSABLE ELEMENTS IN Drosophila
 Copia transposable element is one of the first retro transposon to be identified.
 In 1975, D. Hogness and his colleagues D. Finnegan, and G. Rubin, identified
genes in Drosophila that transcribe “copious” amount of RNA.
 It produces copious (abundant) amount of RNA, hence termed as COPIA.
STRUCTURE OF COPIA TRANSPOSABLE ELEMENTS
 Similar to any other Retro transposons, copia element consists of a central
coding region as basic structure and flanked by long terminal repeats (LTRs),
which are oriented in the same direction.
 Each LTR is bounded by Short-inverted repeats.
 The copia element consists of ~5 kb of DNA long with 0.3 kb direct repeats.
 Copia elements have only genes, which are homologous to gag and pol genes of
the retrovirus.
 The retro transposon pol gene encodes a poly protein, typically consisting of a
protease, an integrase, and a reverse transcriptase (RT) with an RNase H
domain and DNA polymerase activity.
 The RT is common to all autonomous retro transposons, as it is required for
reverse transcription of the RNA intermediate to generate DNA copies of these TEs.
 The protease is involved in processing of precursor proteins, such as the Pol poly
protein.
 The integrase is required for insertion of cDNA into the host genome.
 Gag is the primary component of virus-like nucleo capsid particles, formed by
polymerization of Gag monomers, which provide a structural coat for components
involved in the reverse transcription event of retro transposon mobilization.
TRANSPOSITION OF COPIA TRANSPOSABLE ELEMENTS IN Drosophila
 The transposition of Copia element involves reverse transcription of RNA.
 A segment of copia DNA is transcribed to produce ss RNA.
 Simultaneously, Copia DNA signals reverse transcriptase synthesis.
 Reverse transcriptase uses ssRNA as template to create the dsDNA – Copia.
 The newly synthesized DNA gets integrated into the genome using the flanking
sequences.
FUTURE ASPECTS:
 With the ever-growing number of sequenced genomes from different branches of the tree
of life, there are increasing TE research opportunities.
 Transposable elements have many potential applications in genetic research, including
insertional mutagenesis, gene mapping, gene cloning, gene transfer within and between
species, and identification of genes expressed in specific tissues at a particular time.
 Their insertion may cause some mutations, including one affecting eye colour and
segment formation in Drosophila. For example, the eye colour mutant white apricot (Wa),
an allele of the white (w) gene, contains a copia insertion element within the gene.
(Removing the transposable element sometimes restores the wild type allele).

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MOLECULAR GENETICS | UNIT IV | Transposable elements in Drosophila | Copia