10.3 Gene expression

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For the rest of this chapter, we consider how expression of a plant’s genome:

  • is controlled during development,
  • changes in response to external environmental signals, and
  • can be altered by human intervention through molecular biotechnology (Section 10.4).

Genes and genomes

Only a small fraction of the nuclear genome encodes functional genes (Table 10.1). Much of the remainder consists mostly of highly repeated DNA sequences, often referred to as ‘redundant’ or ‘junk’ DNA. Some of these sequences have structural roles in the centromere (spindle attachment) and telomere (chromosome end) regions. Other repeated sequences may have yet to be defined regulatory functions. We therefore do not have a complete understanding of genome organisation at the supramolecular level. Repeated sequences may have a role linking DNA to the nuclear membrane and nuclear skeleton. In some plants, unusually high DNA levels are a result of evolutionary pathways involving discrete hybridisation where whole genomes have been brought together, as in wheat. One of the smallest flowering plant genomes is in the model plant Arabidopsis thaliana, mainly due to a very low proportion of non-coding DNA.

Plant cells have plastid and mitochondrial genomes in addition to nuclear DNA. The compact genome of chloro-plasts is largely coding DNA, whereas plant mitochondrial DNA has a substantial non-coding component (Table 10.1). We are unclear about the role of this extra plant mitochondrial DNA, because it is lacking in other classes of organism which have small mitochondrial genomes.


Table 10.1