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Home >> Genetics Dictionary >> Exon, Expressed sequence tag
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Exon
The protein-coding DNA sequence of a gene. See also: intron.
2. A transcribed region of a gene that is present in mature messenger RNA. The region(s) of the gene coding for proteins.
3. The segments of a gene that are transcribed and included in the final processed transcript.
4. The nucleotide sequences of some genes consist of parts that code for amino acids, with other parts that do not code for amino acids interspersed among them. The coding parts, which are translated, are called exons; the interspersed noncoding parts are called introns.
5. A portion of the split gene that is included in the transcript of a gene and survives processing of the RNA in the cell nucleus to become part of a spliced messenger of a structural
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6. In split genes, a portion that is included in the ribonucleic acid (RNA) transctipt of a gene and survives processing of the RNA in the cell nucleus to become part of a spliced messenger RNA (mRNA) or structural RNA in the cell cytoplasm. Split genes are those in which regions that are represented in mature mRNAs or structural RNAs (exons) are separated by regions that are transcribed along with exons in the primary RNA products of genes, but are removed from within the primary RNA molecule during RNA processing steps (introns).
Exons comprise three distinct regions of a protein-coding gene. The first is not portion that is not translated into protein, but contains the signal for the beginning of RNA synthesis, and sequences that direct the mRNA to ribosomes for protein synthesis. The second is a set of exons containing information that is translated into the amino acid sequence of a protein
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The third region of a gene that becomes part of an mRNA is an untranslated end portion that contains signals for transcription termination and for the addition of a polyadenylate tract at the end of a transcript.
The evolutionary origin of the exon-intron organization of eukaryotic genes has not been clarified. However, it has been determined from comparisons of structural-functional domains in proteins with the exon domains in corresponding genes that there is often a correlation of the two. For example, the gene for one of the human cell-surface proteins that are important in the recognition of self and nonself by the immune system, the HLA-B7 gene, has nine exons.
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Exon 1 encodes the 5’ (beginning) untransalated portion of the mRNA that is presumably responsible for steps in the initiation of protein synthesis from the mRNA. Exon 2 encodes the site of translation initiation and the sequence of amino acids (the signal peptide) that designates the protein being made as one that will either be a component of membrane or pass through a membrane to be secreted. Exons 3, 4, and 5 encode the three domains of the HLA-B7 protein that are modified by the addition of carbohydrates and are exposed to the cell exterior. Exon 6 encodes the transmembrane portion of the portion, being composed of codons for a series of amino acids that are able to interact with the lipids of the cell membrane. Exons 7 and 8 encode the cytoplasmic (“anchor”) portion of the HLA-
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Map of a eukaryotic gene, primary RNA transcript, and messenger RNA for a protein. Exonsn the unmarked segnebts of the map, are separated by intron sequences, which are spliced out during synthesis of messenger RNA. The appearance of the 5’ cap, a modified guanine nucleotide, precedes or accompanies any splicing activity. The 3’ end, always a series pf adenine nucleotides, is added at the same time. The start codon, AUG, specifies Methionine. The stop codon, which signals the term ination of protein-chain translation, is a ribonculeotide triplet.
| 1. Gene |
2 & 3. Intron Sequence |
4. Primary RNA Transcript |
| 5 & 6. Intron Sequence |
7. Poly A |
8.& 14. Gppp |
| 9. AAAAAAA 3' |
10. 5' Cap |
11. Messenger RNA |
| 12. AUG |
13. Introns excised during RNA processing |
15. AAAAAAA |
| 16. Start codon |
17. Stop codon |
18. Translation |
19. H2N |
20. Protein |
21. COOH |
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B7 protein, and exon 9 contains polyadenylation signals. The mechanism by which the exons are joined in RNA copies of genes is called RNA splicing, and it is part of the maturation of mRNAs and some transfer and ribosomal RNAs (tRNAs and rRNAs) from primary transcripts of genes (see illus.). Three different RNA splicing processes have been identified. One involves mRNA precursors in nuclei, and specific sequences at exon-intron junctions that are recognized by certain nuclear ribonucleoprotein partivles that factilitate the cleavage and ligation of RNA. Another applies to nuclear precursors of tRNA, where splice sites are determined by structural features of the folded RNA molecule. The third form of splicing was discovered in studies of protozon rRNA synthesis, and has also been shown to be a part of the maturation of both rRNA and mRNA in yeast mitochondria; it is an autocatalytic process that requires neither an enzyme nor added energy such as from adenosine triphosphate. In bacteria, it has been observed that certain mRNAs in bacteriophage-infected cells result from the same sort of self-splicing that has been described for protozoan rRNA
Exonuclease
An enzyme that cleaves nucleotides sequentially from free ends of a linear nucleic acid substrate.
2. An enzyme that digests DNA, beginning at the ends of the strands
3. An enzyme that cleaves a nucleic acid molecule only at its ends.
Expressed gene
See: gene expression
Expressed sequence tag (EST)
A short strand of DNA that is a part of a cDNA molecule and can act as identifier of a gene. Used in locating and mapping genes.
Expressivity
In relation to. Autosomal inheritance, a term referring to the extent to which a genetic condition is expressed. If three is variable expressivity, the trait may vary in expression from mild to severe but it is never completely unexpressed, i.e., an individual will have some symptoms but they can vary from mild to severe.
2. The range of phenotypes expressed by a given genotype under any give set of environmental conditions, or over a range of environmental conditions.
Extinction
The disappearance of a species or a population.
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