1.3 Making sense of the sequences

In the previous section you learned that millions of DNA fragments can be sequenced. But how do scientists make sense of all this information?

What comes next is a bit like assembling a puzzle. Powerful computer programs compare the DNA fragments and look for regions with a matching sequence of letters: When two sequences have the same letters at their ends, the program assumes they come from the same original piece of DNA and joins them together. By repeating this process many times, these fragments are combined into longer (assembled) sequences.

Figure 4 The process of assembling DNA sequences from DNA fragments (image taken from Pavlopoulos et al 2013).

The computers compare the assembled sequences to known genes in a database that contains known DNA sequences from many different organisms, and identify the organism(s) they come from (taxonomic assignment). The biological function of a gene can also be predicted. For example, scientists can predict which energy source a group of microorganism uses by looking at which type of metabolic genes are found in the sample.

Sometimes, not all sequences in a sample can be identified, meaning that they don’t have a matching sequence in the databases. The more scientists sequence genomes and organisms from the environment, the bigger the databases become and so the capacity to identify and study genes with metagenomics expands.

You can imagine that metagenomic studies generate a lot of data, making their storage and sharing very important. Data are often stored in online repositories such as the NCBI Sequence Read Archive (SRA).

Figure 5 Metagenomics workflow.

You will now try to make sense of some metagenomic data in a short activity.