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amino acids — 20 molecules that contain nitrogen and together form the building blocks of proteins.

base pairs — a set of two bonded nucleotides on opposite strands of DNA.

chromosome — a rod — shaped structure of tightly coiled DNA found in the cell nucleus of plants and animals.

cloning — the process used to make genetically identical copies of an organism.

cytoplasm — substance within the cell but outside the nucleus in which various cell parts are suspended.

DNA — deoxyribonucleic acid, the genetic material of cells that carries their genetic code.

gamete — an organism’s sex cell, which contains half the total number of chromosomes.

gene — a section of a chromosome made up of DNA, the fundamental unit of heredity.

gene markers — landmarks for a particular gene, either detectable traits that are inherited along with the gene or distinctive segments of DNA.

gene mapping — the process of determining the positions of genes on a chromosome and the distance between them.

gene therapy — the treatment of disease by replacing or changing non — functioning genes.

genetics — the scientific study of heredity.

genome — all the genetic material in the chromosomes of a particular organism.

genome map — a chart that shows the arrangement of the genes or other DNA markers on the chromosomes.

genotype — the inherited genetic code of an organism for a specific trait.

helix — a winding shape, similar to a spiral; the DNA molecule has a double — helix shape, which is two helixes twisted around each other.

human genome — the collection of genes needed to produce a human being.

Human Genome Project — the international research effort to identify and map all the genes in the human body.

inherit — to receive a trait from an ancestor.

meiosis — the division of the genetic information in reproductive cells, so that they have only half the normal number of chromosomes.

molecule — a combination of atoms, and also the basic building — block of DNA and RNA. Each molecule has its own shape and attaches only to certain other molecules to form the DNA helix.

mutation — a change in the genetic code that can occur because of environmental reasons or over long periods of time as part of evolution.

nucleotide — a unit of DNA or RNA, consisting of one chemical base plus a phosphate molecule and a sugar molecule.

nucleus — the center of a cell, where all of the DNA, packaged in chromosomes, is contained.

phenotype — an organism’s inherited physical appearance.

polymerase chain reaction — a genetic engineering technique used to reproduce segments of DNA millions of times; it is used in forensics and chemical and biological experiments.

protein — a chain of amino acids; examples include hormones, enzymes, and antibodies.

ribosome — an organelle that produces protein in cells.

RNA — ribonucleic acid, similar to DNA, except that it has ribose instead of deoxyribose sugar, and uracil instead of thymine as a nitrogen base.

sex chromosomes — chromosomes that determine an organism’s sex. Human females have two X chromosomes; males have one X chromosome and one Y chromosome.

somatic cells — all body cells, except for the reproductive cells.

trait — an organism’s physical feature, determined by a gene.