Answer:

Expressed Sequence Tags (ESTs) are short sequences of DNA that are derived from the expressed genes of an organism. They represent a small portion of the transcribed genome and provide valuable information about the genes that are actively being transcribed and expressed as RNA.

Explanation:

ESTs are generated through a process called Expressed Sequence Tagging, which involves the following steps:

1. cDNA Library Construction: The first step in EST generation is the construction of a complementary DNA (cDNA) library. This is done by isolating and purifying the total RNA from a specific tissue or cell type. The RNA is then reverse transcribed into cDNA using reverse transcriptase enzyme.

2. Fragmentation: The cDNA is then fragmented into smaller pieces to create a library of DNA fragments. These fragments represent the expressed genes of the organism.

3. Cloning and Sequencing: The fragmented cDNA is then ligated into a vector, such as a plasmid, and transformed into a bacterial host for cloning. The bacterial colonies containing the cloned cDNA fragments are selected and sequenced using Sanger sequencing or next-generation sequencing technologies.

4. Sequence Analysis: The obtained EST sequences are then analyzed to identify the corresponding genes and their functions. This analysis involves comparing the EST sequences with known DNA sequences in databases to determine their similarity and potential function.

Genes expressed as RNA: The correct answer option 'A' states that ESTs refer to genes expressed as RNA. This is because ESTs are derived from the transcribed portions of the genome, representing the genes that are actively being expressed and transcribed into RNA molecules.

ESTs provide valuable information about the gene expression patterns, alternative splicing events, and potential protein products of the expressed genes. They are widely used in functional genomics, gene discovery, and comparative genomics studies. EST databases have been created to store and provide access to the vast amount of EST sequences generated from various organisms.

In summary, Expressed Sequence Tags (ESTs) are short sequences of DNA that represent the genes expressed as RNA in an organism. They are generated through a process called EST tagging and provide valuable information about gene expression and function.