7.2.2 Environment & Gene Expression

Environment and Gene Expression

• An organism’s internal or external environment can influence gene expression patterns
• The levels of regulatory proteins or transcription factors can be affected in response to environmental stimuli such as light, and chemicals including drugs and hormones
• For example, enzymes are activated in response to ultraviolet radiation and increase the expression and production of melanin, leading to skin pigmentation
• Temperature can also influence gene expression as demonstrated by organisms
  • The Himalayan rabbit (Oryctolagus cuniculus L.) possesses a gene for the development of pigmentation in its fur
    • The gene is inactive above 35°C but active between 15°C and 25°C
    • In the parts of the body that are cooler such as ears, feet and nose the gene becomes active making these areas black
  • Temperature also affects the expression of the sex chromosomes in Australia's bearded dragon lizards (Pogona barbata)
    • Lizards raised in hot temperatures were female in appearance and were capable of bearing offspring
    • Despite having the ZZ chromosomes usually found in male lizards
• There is much debate about whether a particular phenotype or behaviour can be attributed to inheritance or environment
• Environmental factors explain the differences observed between identical twins as they age
  • Identical twins have the same DNA but their individual genomes can come under different outside influences
  • They can change independently, leading to phenotypic differences such as height
  • Twin studies have shown that environmental factors have a greater influence on many disease states (e.g. cancer and rheumatoid arthritis) compared to genetic influence

DNA is wrapped around histone proteins which form a nucleosome. Nucleosomes coil tightly around each other to form the chromosome structure.

NOS: Looking for patterns, trends and discrepancies; there is mounting evidence that the environment can trigger heritable changes in epigenetic factors

• Epigenetics is the genetic control by factors other than an individual’s DNA sequence
• Epigenetics is a broad term that encompasses (m)any alterations
  • That induce switching-on and switching-off of genes, but
  • Without changing the actual genetic code
• Epigenetics involves heritable changes in gene function, without changes to the DNA sequence
• In eukaryotic cells, nuclear DNA is wrapped around proteins called histones to form chromatin
• Chromatin can be chemically modified in different ways to alter gene expression
  • Methylation of DNA (chemical addition of a -CH₃ group)
  • Histone modification via acetylation, methylation and phosphorylation of amino acid tails
• Such modifications are called epigenetic tags and collectively, all the epigenetic tags in an organism is called the epigenome
• The epigenome can undergo changes due to environmental factors
  • Smoking, stress, exercise and diet can cause epigenetic changes
  • Internal signalling from the body's own cells can also cause modifications to occur
• Epigenetic modification is independent, histone modification in one area is not linked to modification in another
• Like the genome, the epigenome is heritable
  • Mounting evidence demonstrates that modifications to the epigenome in one generation can be passed on to the next generation at cellular or whole organism level