14.8 Gene Expression

205

of DNA shape, or rigidity, by the modified histones? There may also be proteins other

than histones, likewise susceptible to modification, associated with nucleosomes. It

is appropriate to consider the nucleus as a highly dynamic object full of proteins

reacting with and diffusing to, from, and along the DNA.

RNA Interference

For many years, the rôles of RNA were thought to be confined to messenger RNA,

transfer RNA, and ribosomal RNA; remarkably, the very extensive activity of the so-

called “noncoding RNA” transcribed from intergenic regions and possibly introns in

regulating gene expression was unsuspected until recently. Currently, two classes of

this small (about two dozen nucleotides) RNA are recognized: microRNA (muμRNA

or miRNA) and small interfering RNA (siRNA). They appear to originate from their

own microgenes, or are formed from RNA hairpins (cf. Fig. 15.5) resulting from

mistranscribed DNA.

These small RNA molecules seem to be as abundant as mRNA, and their basic

function is to block transcription by binding to complementary DNA sequences, or to

block translation by binding to complementary RNA sequences (“RNA silencing”).

The varied applications of this function include plant defence against viruses. ⁵⁵

Long Non-coding RNAs

A large fraction of the genome of complex organisms is occupied by DNA sequences

specifying the so-called long non-coding RNAs (lncRNAs). These RNAs include

RNAp. Many are cell type-specific and regulate aspects of cell differentiation. The

variety of functions makes nomenclature and classification rather difficult; work is

ongoing. ⁵⁶

Promoter Sites and Transcription Factors

The affinity of RNAp to DNA is strongly dependent on the presence or absence of

other proteins on the DNA upstream of the sequence to be transcribed (cf. Fig. 14.3),

and associated with the RNAp. The principle of activation and repression by the

55 Voinnet (2001), Ding and Voinnet (2014).

56 Mattick et al. (2022).