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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.