Overview

Eukarya, like bacteria, regulate the amounts of rRNA, tRNA, and mRNA by controlling rates of synthesis and degradation. On the synthesis side, control is exerted in transcription and splicing. Degradation is controlled by RNases, and is influenced by RNA processing.

The rate-limiting binding step of transcription is complicated both by the inaccessibility of DNA in chromatin and by large genome size. Transcription goes hand in hand with the disassembly of chromatin, and a large genome requires a large number of regulatory DNA sequences and transcription factors. Chromatin disassembly is effected by the chemical modification of histones and by the actions of chromatin remodeling complexes.

RNA Degradation

Stable versus Unstable RNAs

Like bacteria, eukarya have much more stable RNA than unstable RNA. The cytoplasmic RNA of a human cell is ≈80% rRNA, ≈15% tRNA and other small RNAs, and ≈1% to 5% mRNA. In growing eukaryal cells, 50% to 70% of transcription is from class I genes (big rRNAs), 20% to 40% of transcription is from mRNA genes, and the remaining 10% is from class III genes (tRNA and other small RNAs). The rarity of mRNA compared with the rate of transcription by RNA polymerase II reflects the short half-life of eukaryal mRNAs.

mRNA Degradation

Capping and polyadenylation increase RNA's resistance to degradation. These modifications are significant in mRNA, because virtually all mRNAs are capped, and most are polyadenylated.