1. Why is epigenetics important?
Epigenetics is the study of changes in organisms caused by modification of the way in which genes are expressed rather than alteration of the genetic code in DNA itself. Epigenetic change occurs normally and naturally, and is influenced by external environmental factors which switch genes on or off. It is needed for normal development (the process by which a fertilised egg cell grows into an embryo and then into an adult body). Epigenetic markers are also associated with disease.
In plants, characteristics that are controlled epigenetically include:
• Flowering time which is a very important characteristic for crop production because: it is a critical component of the adaptation to different climatic conditions of cultivated crops and it is an important determinant of yield: in cereals, early flowering extends the time seeds can develop; contrarily, early flowering in leafy crops (such as cabbage and fodder grasses) reduces production levels and complicates harvesting since the flowers need to be discarded.
It can help avoid drought stresses: lack of water affects seed crops such as maize particularly just before and during flowering. Drought-tolerant maize varieties often flower early.
• Root length is important as it can significantly change productivity under conditions of drought.
• Hybrid vigor (the increased vitality of F1 hybrids) is also controlled epigenetically. Epigenetic control is also implicated in establishing barriers when different species are hybridised, an important tool in plant breeding.
• Epigenetics regulation is important for the success of plants that reproduce asexually (also known as clonal) as it allows for a fast adaptation to environmental stresses. Bananas, cassava and potato are important clonal crops.
2. What is the difference between genetic and epigenetic variation?
3. What is the molecular basis of epigenetics?
DNA molecules are not found loose in the cell’s nucleus: DNA is first wrapped around proteins called histones, and then wrapped again forming a fibre called chromatin which is further coiled, making up a chromosomes.
• The level of DNA packaging is however not uniform: tightly packed areas alternate with more loosely packed chromosomal regions.
• The level of packing influences the accessibility to the DNA of the molecules responsible for gene expression: loose areas are open and more actively
expressed while tightly coiled areas are closed and transcribed to a lesser extent which is also referred to as being ‘silenced’ (turned off).
• The addition of epigenetic tags (methylation) to the DNA and associated proteins promotes a more close configuration of the DNA, and hence the silencing of gene expression.
Figure 2. Chromosome structure
The degree of coiling along individual chromosomes is not uniform: areas of tightly packed chromatin (DNA together with associated proteins) occur alongside more loosely packed areas which usually contain higher numbers of genes and higher levels of gene expression. For this reason tightly packed areas are sometimes referred to as ‘silent’ as opposed to the ‘active’, more accessible chromatin fibres.