Arabidopsis seedling

Understanding RNAs

Exploring the many roles of RNA

Ribonucleic acid (RNA) is similar to deoxyribonucleic acid (DNA). We usually think of RNAs as being single-stranded, although they may start out as double-stranded molecules like DNA before being split up by enzymes. RNA sequences are complementary to – rather than identical to – sequences in the DNA. The bases in an RNA sequence can bind to complementary RNA or DNA sequences. This seems to make RNA ideal for lots of the detailed regulation of gene activity.

As scientists have come to understand the many roles of RNA, they have started to give more specific names to the different types. There are miRNAs, siRNAs and piRNAs. There are A-RNAs and Y-RNAs. And there are some types of RNA that have only been found in non-human species so far, such as trans-acting siRNAs (ta-siRNAs), a subdivision of siRNAs that was discovered in the plant Arabidopsis and has since been recognised in rice and corn.

miRNAs (microRNAs) and siRNAs (short interfering RNAs) are probably the best understood, although there remains a lot to learn. They are both formed from short pieces of RNA, around 20 bases in length, and have regulatory roles.

Although miRNAs and siRNAs were at first viewed as distinct classes of RNAs, the lines between the two have blurred as time has gone on and their roles now seem to overlap. Both are involved in silencing processes that inhibit the expression of certain genes.

miRNAs are made by genes that specifically encode miRNAs. They regulate other genes by inhibiting the translation of messenger RNAs that are destined to make proteins. They were discovered in 1993, in worms, where they regulate genes involved in the timing of developmental processes.

siRNAs are often regarded as having a protective role, which includes defending the genome against invading genetic materials from viruses. siRNA-based therapies are a rapidly expanding area of medicine: the molecules can be used to switch off genes involved in cancer, for instance. However, getting enough of them into cells is difficult. Scientists are working on novel delivery approaches, such as nanoparticle carriers.

Lead image:

An Arabidopsis thaliana seedling

Blue Ridge Kitties/Flickr CC BY NC ND


About this resource

This resource was first published in ‘Genes, Genomes and Health’ in January 2010 and reviewed and updated in December 2014.

Genetics and genomics
Genes, Genomes and Health
Education levels:
16–19, Continuing professional development