An intron is any nucleotide sequence within a gene that is removed by RNA splicing during maturation of the final RNA product. The word intron is derived from the term intragenic region, i.e. a region inside a gene. The term intron refers to both the DNA sequence within a gene and the corresponding sequence in RNA transcripts. Sequences that are joined together in the final mature RNA after RNA splicing are exons. Introns are found in the genes of most organisms and many viruses, and can be located in a wide range of genes, including those that generate proteins, ribosomal RNA (rRNA), and transfer RNA (tRNA). When proteins are generated from intron-containing genes, RNA splicing takes place as part of the RNA processing pathway that follows transcription and precedes translation.
Retroposons are repetitive DNA fragments which are inserted into chromosomes after they had been reverse transcribed from any RNA molecule.
RNA splicing, in molecular biology, is a form of RNA processing in which a newly made precursor messenger RNA (pre-mRNA) transcript is transformed into a mature messenger RNA (mRNA). Splicing occurs only in eukaryotic introns. During splicing, introns are removed and exons are joined together. For nuclear-encoded genes, splicing takes place within the nucleus either during or immediately after transcription. For those eukaryotic genes that contain introns, splicing is usually required in order to create an mRNA molecule that can be translated into protein. For many eukaryotic introns, splicing is carried out in a series of reactions which are catalyzed by the spliceosome, a complex of small nuclear ribonucleo proteins (snRNPs). Self-splicing introns, or ribozymes capable of catalyzing their own excision from their parent RNA molecule, also exist.
Retrotransposons are genetic elements that can amplify themselves in a genome and are ubiquitous components of the DNA of many eukaryotic organisms. These DNA sequences use a "copy-and-paste" mechanism, whereby they are first transcribed into RNA, then converted back into identical DNA sequences using reverse transcription, and these sequences are then inserted into the genome at target sites.
The Pseudoviridae are a family of viruses, including the following genera:
Lentivirus is a genus of retroviruses that cause chronic and deadly diseases characterized by long incubation periods, in the human and other mammalian species. The best known lentivirus is the Human Immunodeficiency Virus (HIV), which causes AIDS. Lentiviruses are also hosted in apes, cows, goats, horses, cats, and sheep. Recently, lentiviruses have been found in monkeys, lemurs, Malayan flying lemur, rabbits, and ferrets. Lentiviruses and their hosts have worldwide distribution. Lentiviruses can integrate a significant amount of viral cDNA into the DNA of the host cell and can efficiently infect nondividing cells, so they are one of the most efficient methods of gene delivery. Lentiviruses can become endogenous (ERV), integrating their genome into the host germline genome, so that the virus is henceforth inherited by the host's descendants.
Exon shuffling is a molecular mechanism for the formation of new genes. It is a process through which two or more exons from different genes can be brought together ectopically, or the same exon can be duplicated, to create a new exon-intron structure. There are different mechanisms through which exon shuffling occurs: transposon mediated exon shuffling, crossover during sexual recombination of parental genomes and illegitimate recombination.
Long terminal repeats (LTRs) are identical sequences of DNA that repeat hundreds or thousands of times found at either end of retrotransposons or proviral DNA formed by reverse transcription of retroviral RNA. They are used by viruses to insert their genetic material into the host genomes.
Group I introns are large self-splicing ribozymes. They catalyze their own excision from mRNA, tRNA and rRNA precursors in a wide range of organisms. The core secondary structure consists of nine paired regions (P1-P9). These fold to essentially two domains – the P4-P6 domain and the P3-P9 domain. The secondary structure mark-up for this family represents only this conserved core. Group I introns often have long open reading frames inserted in loop regions.
A retron is a distinct DNA sequence found in the genome of many bacteria species that codes for reverse transcriptase and a unique single-stranded DNA/RNA hybrid called multicopy single-stranded DNA (msDNA). Retron msr RNA is the non-coding RNA produced by retron elements and is the immediate precursor to the synthesis of msDNA. The retron msr RNA folds into a characteristic secondary structure that contains a conserved guanosine residue at the end of a stem loop. Synthesis of DNA by the retron-encoded reverse transcriptase (RT) results in a DNA/RNA chimera which is composed of small single-stranded DNA linked to small single-stranded RNA. The RNA strand is joined to the 5' end of the DNA chain via a 2'-5' phosphodiester linkage that occurs from the 2' position of the conserved internal guanosine residue.
U24 is a member of the C/D class of snoRNA which contain the C (UGAUGA) and D (CUGA) box motifs. C/D box snoRNAs have been shown to act as methylation guides for a number of RNA targets. U24 is encoded within an intron of the gene for ribosomal protein L7a in mammals, chicken and Fugu. The U76/SNORD76 snoRNA is found in an intron of the uRNA host gene (UHG) growth arrest specific 5 (GAS5) transcript gene. snoRNAs Z20 and U76 snoRNAs show clear similarity to U24.
snoRNA U36 is a non-coding RNA (ncRNA) molecule which functions in the biogenesis (modification) of other small nuclear RNAs (snRNAs). This type of modifying RNA is located in the nucleolus of the eukaryotic cell which is a major site of snRNA biogenesis. It is known as a small nucleolar RNA (snoRNA) and also often referred to as a guide RNA.
Genomic deoxyribonucleic acid is chromosomal DNA, in contrast to extra-chromosomal DNAs like plasmids. It is also then abbreviated as gDNA. Most organisms have the same genomic DNA in every cell; however, only certain genes are active in each cell to allow for cell function and differentiation within the body.
60S ribosomal protein L7a is a protein that in humans is encoded by the RPL7A gene.
40S ribosomal protein S8 is a protein that in humans is encoded by the RPS8 gene.
LTR retrotransposons are class I transposable element characterized by the presence of Long Terminal Repeats (LTRs) directly flanking an internal coding region. As retrotransposons, they mobilize through reverse-transcription of their mRNA and integration of the newly created cDNA into another location. Their mechanism of retrotransposition is shared with retroviruses, with the difference that most LTR-retrotransposons do not form infectious particles that leave the cells and therefore only replicate inside their genome of origin.
Numerous key discoveries in biology have emerged from studies of RNA, including seminal work in the fields of biochemistry, genetics, microbiology, molecular biology, molecular evolution and structural biology. As of 2010, 30 scientists have been awarded Nobel Prizes for experimental work that includes studies of RNA. Specific discoveries of high biological significance are discussed in this article.
Maturase K (matK) is a plant plastidial gene, although higher plants have it moved into the nuclear genome. The protein it encodes is an organelle intron maturase, a protein that splices Group II introns. It is essential for in vivo splicing of Group II introns. Amongst other maturases, this protein retains only a well conserved domain X and remnants of a reverse transcriptase domain.
