Epigenetics – beyond genes
Glossary
base (in DNA). Any one of four nitrogen-containing bases (adenine, thymine, guanine and cytosine). The sequence of the bases in DNA determines the sequence of amino acids in all proteins found in living things.
base pairs. Two bases held together by weak chemical bonds. The double helix shape of DNA is dependent on its two strands being held together by the bonds between the base pairs. In DNA, the bases that pair are adenine with thymine and guanine with cytosine.
chromosome. A long DNA molecule that contains the genes of the organism. Chromosomes are visible in cells during cell division.
DNA (deoxyribonucleic acid). The nucleic acid forming the genetic material of all organisms, with the exception of some viruses which have RNA. DNA is present in the nucleus and other organelles such as mitochondria and chloroplasts.
DNA repair genes. Encode proteins that correct mistakes in DNA caused by incorrect copying during replication and environmental factors such as by-products of metabolism, exposure to ultraviolet light or mutagens. The DNA repair process must operate constantly to correct any damage to the DNA as soon as it occurs. For more information about the role of DNA repair genes in cancer see Genetics of cancer (Learner.org, USA).
enzyme. A protein that acts as a catalyst. Every chemical reaction in living organisms is facilitated by an enzyme.
epigenetics. Is the study of heritable changes in gene activity that occur without a change in the sequence of the genetic material. Epigenetics literally means ‘in addition to genetics’.
gene. The basic unit of inheritance. A gene is a segment of DNA that specifies the structure of a protein or an RNA molecule.
genetic conditions. Those conditions or diseases that result from abnormalities in chromosomes or DNA, and are inherited.
genome. The total genetic material of an individual or species.
histones. Proteins found associated with DNA in eukaryotic cells that play a role in gene regulation. The DNA winds around the histone protein to form chromatin. For more information about the role of histones see The nucleus (Kimball’s Biology Pages, USA).
messenger RNA. RNA molecule that is transcribed from DNA and is used to direct the synthesis of a protein.
meiosis. A division of the nucleus that involves the separation of pairs of chromosomes into different cells. Meiosis takes place in the reproductive organs of sexually reproducing organisms. Meiosis involves two nuclear divisions, both of which may take place before division of the cell itself is complete. The eventual result is four cells, each with half the number of chromosomes present in the original cell. Crossing over of chromosomes during meiosis creates new combinations of genes in the progeny that were not present in either adult. For more information see How cells divide: Mitosis versus meiosis (Public Broadcasting Service, USA).
mutation. A change in the DNA sequence of a gene that may be harmful or beneficial. It is the only process that actually leads to new forms of a gene, and it is the ultimate source of all variation.
oncogenes. Mutated forms of genes which produce protein products that normally enhance cell division or inhibit normal cell death. For more information see Genetics of cancer (Learner.org, USA).
promoter. The DNA sequence adjacent to the coding sequence of a gene, which interacts with inducers or repressors and RNA polymerase to determine whether that gene is active or not.
protein. A large molecule composed of a linear sequence of amino acids. This linear sequence is a protein's primary structure. Short sequences within the protein molecule can interact to form regular folds (eg, alpha helix and beta pleated sheet) called the secondary structure. Further folding from interaction between sites in the secondary structure forms the tertiary structure of the protein.
Proteins are essential to the structure and function of cells. They account for more than 50 per cent of the dry weight of most cells, and are involved in most cell processes. Examples of proteins include enzymes, collagen in tendons and ligaments and some hormones. For more information see Protein structure and diversity (Molecular Biology Notebook, Rothamsted Research, UK).
RNA (ribonucleic acid). A nucleic acid similar to DNA. There are a number of types of RNA, the major ones being messenger RNA, transfer RNA and ribosomal RNA. RNA can serve as a messenger between DNA and proteins, as a structural molecule, as an enzyme and as regulators of gene expression. In some viruses RNA is the genetic material. For more information see Introduction to RNA and its functions (University of Newfoundland, Canada).
tumour suppressor gene. Genes that encode proteins that normally repress cell division or enhance cell death. For more information see Genetics of cancer (Learner.org, USA).
Posted September 2006.