The Gene: An Intimate History - Siddhartha Mukherjee Page 0,236

indomitable Sarah Chalfant, who first saw this book through the pinhole of a two-paragraph proposal, you gave Gene shape, form, clarity, gravity, and urgency. Thank you.

Glossary

Allele: A variant or alternative form of a gene. Alleles are usually created by mutations, and can be responsible for phenotypic variations. A gene can have multiple alleles.

Central dogma, or Central theory: The theory that biological information in most organisms moves from genes in DNA to messenger RNA to proteins. This theory has been modified several times. Retroviruses contain enzymes that can be used to build DNA from an RNA template.

Chromatin: The material from which chromosomes are composed. Chromatin takes its name from chroma (“color”), since it was initially found by staining cells with dyes. Chromatin may consist of DNA, RNA, and proteins.

Chromosome: A structure within a cell comprised of DNA and proteins that stores genetic information.

DNA: Deoxyribonucleic acid, a chemical that carries genetic information in all cellular organisms. It is usually present in the cell as two paired, complementary strands. Each strand is a chemical chain made up of four chemical units—abbreviated A, C, T, and G. Genes are carried in the form of a genetic “code” in the strand and the sequence is converted (transcribed) into RNA and then translated into proteins.

Enzyme: A protein that accelerates a biochemical reaction.

Epigenetics: The study of phenotypic variations that are not caused by changes in the primary DNA sequence (i.e., A, C, T, G) but by chemical alterations of DNA (e.g., methylation) or changes in packaging of DNA via DNA-binding proteins (e.g., histones). Some of these alterations are heritable.

Gene: A unit of inheritance, normally comprised of a stretch of DNA that codes for a protein or for an RNA chain (in special cases, genes might be carried in RNA form).

Genome: The full complement of all genetic information within the organism. A genome includes protein-encoding genes, genes that do not encode proteins, the regulatory regions of genes, and sequences of DNA with yet-unknown functions.

Genotype: An organism’s collection of genetic information that determines its physical, chemical, biological, and intellectual characteristics (see “phenotype”).

Mutation: An alteration in the chemical structure of DNA. Mutations can be silent—i.e., the change might not affect any function of the organism—or can result in a change in the function or structure of an organism.

Nucleus: A membrane-enclosed cellular structure or organelle that is found in animal and plant cells, but not in bacterial cells. Chromosomes (and genes) in animal cells are contained in the nucleus. In animal cells, most genes are nuclear, although some genes are also found in mitochondria.

Organelle: A specialized subunit within a cell that is typically dedicated to a specific function. Individual organelles are usually separately enclosed within their own membranes. Mitochondria are organelles dedicated to the production of energy.

Penetrance: The proportion of organisms that carry a particular variant of a gene that also expresses the associated trait or phenotype. In medical genetics, penetrance refers to the proportion of individuals carrying a genotype that manifest the symptoms of an illness.

Phenotype: The set of an individual’s biological, physical, and intellectual traits, such as skin color or eye color. Phenotypes can also include complex traits, such as temperament or personality. Phenotypes are determined by genes, epigenetic alterations, environments, and random chance.

Protein: A chemical comprised, at its core, of a chain of amino acids that is created when a gene is translated. Proteins carry out the bulk of cellular functions, including relaying signals, providing structural support, and accelerating biochemical reactions. Genes usually “work” by providing the blueprint for proteins. Proteins can be modified chemically by the addition of small chemicals such as phosphates or sugars or lipids.

Reverse transcription: The process by which an enzyme (reverse transcriptase) uses a chain of RNA as a template to build a chain of DNA. Reverse transcriptase is found in retroviruses.

Ribosome: A cellular structure composed of protein and RNA that is responsible for the decoding of messenger RNA into proteins.

RNA: Ribonucleic acid, a chemical that performs several functions in the cells, including acting as an “intermediate” message for a gene to be translated into a protein. RNA is composed of a chain of bases—A, C, G, and U—strung together along a sugar-phosphate backbone. Typically, RNA is found as a single strand in a cell (unlike DNA, which is always double-stranded), although double-stranded RNA can be formed under special conditions. Some organisms, such as retroviruses, use RNA to carry their genetic information.

Traits, dominant and recessive: A physical or biological feature of an organism. Traits are typically encoded by genes. Many genes might encode