Expressed sequence tag

An expressed sequence tag (EST) is a short sub-sequence of a cDNA (complementary DNA) sequence. ESTs are used to identify genes and are instrumental in gene discovery and gene sequence determination. They are generated by sequencing either one or both ends of an expressed gene.

Generation

ESTs are typically generated by isolating mRNA from a tissue of interest, converting it into cDNA using the enzyme reverse transcriptase, and then sequencing the cDNA. The resulting sequences are then compared to known sequences in databases to identify new genes or to study gene expression patterns.

Applications

ESTs have several applications in molecular biology and genomics:

• **Gene Discovery**: ESTs can be used to discover new genes by comparing the sequences to known genes in databases.
• **Gene Expression**: By analyzing the abundance of different ESTs in a library, researchers can infer the expression levels of genes in different tissues or under different conditions.
• **Genome Annotation**: ESTs help in annotating genomes by providing evidence for the existence of genes and their exon-intron structure.
• **Marker Development**: ESTs can be used to develop genetic markers for genetic mapping and quantitative trait loci (QTL) analysis.

Databases

Several databases store EST sequences, including:

• GenBank
• dbEST
• UniGene

Limitations

While ESTs are valuable tools, they have limitations:

• **Short Length**: ESTs are typically short sequences, which can make it difficult to assemble full-length genes.
• **Redundancy**: EST libraries often contain redundant sequences, which can complicate data analysis.
• **Error Rate**: The process of generating ESTs can introduce errors, which need to be accounted for in analyses.

See also

• Complementary DNA
• Gene expression
• Genomics
• Molecular biology
• Reverse transcriptase

Related Pages

• Gene
• Genetic marker
• Genetic mapping
• Quantitative trait loci
• Genome annotation