Types of Sequences Present in Nucleotide Sequence Databases, cDNA Sequences, Genomic DNA Sequences, Organellar DNA Sequences, cDNA Molecules

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	Types of Sequences Present in Nucleotide Sequence Databases The databases on DNA sequences contain a variety of sequence types, viz., (i) cDNA sequences, (ii) genomic DNA sequences, (iii) expressed sequence tag (EST) sequences, (iv) genome sequence tag (GST) sequences ,(v) organellar DNA sequences (mtDNA and cpDNA), and
(vi) sequences of RNAs. EST sequences are obtained by sequencing only a part of the cDNA molecules produced using mRNA. These sequences are dubbed as tags because they can be used as probes for the isolation of the concerned genes from the genomic DNA. This approach was used by J. Craig Venter and his group for obtaining the sequence of expressed portion of human genome. The EST technique generated enormous sequence data that permitted the construction of a preliminary transcript map of the human genome. GSTs were developed for identifying the genes of Plasmodium falciparum. It was observed the enzyme mungbean nuclease (Mnase) cuts P.falciparum genomic DNA between genes. GSTs are developed by sequencing the DNA on either side of the points of cuts generated by Mnase.