Molecular biology: power sequencing

Abstract

Advances in DNA-sequencing technology provide unprecedented insight into the entire collection of four genomes' transcribed sequences; they herald a new era in the study of gene regulation and genome function.

Figure 1. mRNA-Seq

In this technique, which was used for analysis of transcriptomes of five organisms^–, the isolated mRNA is analysed by one of three procedures. a, In the first procedure, mRNAs are randomly sheared, linker molecules are attached to their ends, and they are then converted to DNA. b, Alternatively, after shearing, the mRNA fragments are converted to DNA, followed by the attachment of linker molecules. c, In a third procedure, mRNAs are first copied into DNA sequences, which are then sheared and attached to linkers. In all three cases, the resulting DNA are analysed by next-generation sequencing technology and the data are compared with the reference genome for that particular organism using bioinformatics, to determine the genomic regions from which the sequences were derived.

Figure 2. Determining mRNA expression levels using mRNA-Seq

a, For a gene expressed highly, several sequence reads (orange) map to each of its exons and some reads (pink) to the two exons spanning an intron. No or few reads map to introns, suggesting that intron removal in this case is very efficient. b, But when a gene is expressed at low levels, fewer sequence reads map to the exons or to two exons spanning an intron, whereas some reads map to the introns (brown). That almost equal number of reads map to regions within the intron and to those spanning it suggests that splicing of these gene transcripts is inefficient.