Epigenetics involves changes in gene expression that are independent of DNA sequence. Such changes are different from genetics, which involve the DNA sequence and genes themselves. Genes are the basic units of heredity (inheritance). They help determine what traits living things inherit from their parents. Genes are encoded within the structure of a molecule called DNA (deoxyribonucleic acid). Inside a living cell, DNA is arranged into threadlike structures called chromosomes. Each gene consists of a sequence of DNA found on a chromosome. Genes direct the formation, growth, and reproduction of cells and organisms through a process known as gene expression. In this process, the cell makes a protein according to the instructions carried by a gene.
Cellular and developmental processes are in part controlled through epigenetics. These processes are stable during cell growth, much like genetic processes. Epigenetic processes include the development of stem cells. A stem cell is a cell that can develop into any of the body’s different cell types (see Stem cell ). This ability is due, in part, to the reduced presence of epigenetic marks present in the cells, called a reduced epigenetic state. Scientists have also found that an organism’s environment can bring about epigenetic changes.
Epigenetic processes.
Several epigenetic processes can alter gene expression in organisms. One process is called DNA methylation. In this process, a group of atoms called a methyl group is attached to the DNA. The amount of DNA methylation helps determine whether a gene is switched on or off. Another epigenetic process involves histones. Histones are proteins that bind to DNA. They bend and coil the long DNA molecules to form chromosomes. Many chemicals can modify histones, thus altering gene expression. Yet another epigenetic process involves small molecules of RNA (ribonucleic acid). RNA molecules can interact with protein or DNA to alter and regulate the activity of certain genes.
The entire set of chemical instructions that control heredity in a living thing is called its genome. The complex interaction of all epigenetic processes makes up the epigenome. The epigenome has an essential role in regulating the activity of the genome.
Environmental epigenetics
is the study of environmental factors that can alter living things through epigenetic processes. Such factors include nutrition as well as exposure to environmental chemicals and stress. For example, even identical twins, who share the exact same genome, differ in some respects. Some of these differences are in part a result of epigenetic changes caused by the environment. For example, one twin may have been exposed to stress during a critical time in its development. Such stress may lead to epigenetic changes affecting the twin’s physiology. The study of environmental epigenetics has important applications in environmental science, nutrition, and toxicology (the study of environmental chemicals).
Scientists know that epigenetic processes can play a role in the development of diseases, including cancer and diabetes. Scientists have long suspected that early exposure to certain chemicals causes epigenetic changes that lead to disease later in life.
Epigenetics and inheritance.
Epigenetic changes can be passed, along with genes, to future generations through heredity involving the sperm or egg. For example, people who smoke cigarettes may alter their gene expression in ways that can be passed to their children. The inherited alterations could predispose the children to certain health problems, such as diabetes and cancer. Because some epigenetic changes are heritable, scientists believe epigenetics may play an important role in evolution, the development of living things over time.
History.
The English biologist Conrad Waddington coined the term epigenetics in the 1940’s. He used it to describe all developmental interactions between genes and the environment. However, scientists did not identify the first molecular epigenetic factor, DNA methylation, until the 1970’s. Today, researchers in epigenetics are working to better understand how epigenetic and genetic processes control the action of genes in organisms.
See also Cloning (Differences among clones) .
