Sequence Databases

The NCBI Sequence Database

All published genome sequences are available over the internet, as it is a requirement of every scientific journal that any published DNA or RNA or protein sequence must be deposited in a public database. The main resources for storing and distributing sequence data are three large databases: the NCBI database (www.ncbi.nlm.nih.gov/), the European Molecular Biology Laboratory (EMBL) database (www.ebi.ac.uk/embl/, and the DNA Database of Japan (DDBJ) database (www.ddbj.nig.ac.jp/). These databases collect all publicly available DNA, RNA and protein sequence data and make it available for free. They exchange data nightly, so contain essentially the same data.

In this chapter we will discuss the NCBI database. Note however that it contains essentially the same data as in the EMBL/DDBJ databases.

Sequences in the NCBI Sequence Database (or EMBL/DDBJ) are identified by an accession number. This is a unique number that is only associated with one sequence. For example, the accession number NC_001477 is for the DEN-1 Dengue virus genome sequence. The accession number is what identifies the sequence. It is reported in scientific papers describing that sequence.

As well as the sequence itself, for each sequence the NCBI database (or EMBL/DDBJ databases) also stores some additional annotation data, such as the name of the species it comes from, references to publications describing that sequence, etc. Some of this annotation data was added by the person who sequenced a sequence and submitted it to the NCBI database, while some may have been added later by a human curator working for NCBI.

The NCBI database contains several sub-databases, the most important of which are:

• the NCBI Nucleotide database: contains DNA and RNA sequences
• the NCBI Protein database: contains protein sequences
• EST: contains ESTs (expressed sequence tags), which are short sequences derived from mRNAs
• the NCBI Genome database: contains DNA sequences for whole genomes
• PubMed: contains data on scientific publications

Classwork 2

Q1. What information about the rabies virus sequence (NCBI accession NC_001542) can you obtain from its annotations in the NCBI Sequence Database?

What does it say in the DEFINITION and ORGANISM fields of its NCBI record? Note: rabies virus is the virus responsible for rabies, which is classified by the WHO as a neglected tropical disease.

Q2. How many nucleotide sequences are there from the bacterium Chlamydia trachomatis in the NCBI Sequence Database?

Note: the bacterium Chlamydia trachomatis is responsible for causing trachoma, which is classified by the WHO as a neglected tropical disease.