Epigenetic tags act as a kind of cellular memory. A cell’s epigenetic profile — a collection of tags that tell genes whether to be on or off — is the sum of the signals it has received during its lifetime.

As a fertilized egg develops into a baby, dozens of signals received over days, weeks, and months cause incremental changes in gene expression patterns. Epigenetic tags record the cell’s experiences on the DNA, helping to stabilize gene expression. Each signal shuts down some genes and activates others as it nudges a cell toward its final fate. Different experiences cause the epigenetic profiles of each cell type to grow increasingly different over time. In the end, hundreds of cell types form, each with a distinct identity and a specialized function.

Even in differentiated cells, signals fine-tune cell functions through changes in gene expression. A flexible epigenome allows us to adjust to changes in the world around us, and to learn from our experiences.

After birth and as life continues, a wider variety of environmental factors start to play a role in shaping the epigenome. Social interactions, physical activity, diet and other inputs generate signals that travel from cell to cell throughout the body. As in early development, signals from within the body continue to be important for many processes, including physical growth and learning. Hormonal signals trigger big changes at puberty.

Even into old age, cells continue to listen for signals. Environmental signals trigger changes in the epigenome, allowing cells to respond dynamically to the outside world. Internal signals direct activities that are necessary for body maintenance, such as replenishing blood cells and skin, and repairing damaged tissues and organs. During these processes, just like during embryonic development, the cell’s experiences are transferred to the epigenome, where they shut down and activate specific sets of genes.

To learn more, visit An Example of Cell Communication: The Fight or Flight Response

1. SWITCH SPECIFIC GENES ON OR OFF

A gene regulatory protein attaches to a specific sequence of DNA on one or more genes. Once there, it acts like a switch, activating genes or shutting them down.

2. RECRUIT ENZYMES THAT ADD AND REMOVE EPIGENETIC TAGS

Gene regulatory proteins also recruit enzymes that add or remove epigenetic tags. Enzymes add epigenetic tags to the DNA, the histones, or both.

Epigenetic tags give the cell a way to “remember” long-term what its genes should be doing.

Using the original DNA strands as a template, methyl copying enzymes attach methyl tags to newly replicated DNA copies. One original DNA strand and one copy will be passed to each daughter cell.

Because identical twins develop from a single fertilized egg, they have the same genome. So any differences between twins are due to their environments, not genetics. Recent studies have shown that many environmentally induced differences are reflected in the epigenome

The insight we gain from studying twins helps us to better understand how nature and nurture work together. For well over a century, researchers have compared characteristics in twins in an effort to determine the extent to which certain traits are inherited, like eye color, and which traits are learned from the environment, such as language. Typically taking place in the field of Behavioral Genetics, classical twin studies have identified a number of behavioral traits and diseases that are likely to have a genetic component, and others that are more strongly influenced by the environment.

Depending on the study and the particular trait of interest, data is collected and compared from identical or fraternal twins who have been raised together or apart. Finding similarities and differences between these sets of twins is the start to determining the degree to which nature and environment play a role in the trait of interest.

Twin studies uncover genetic and environmental contributions