allele - one variant of a particular gene; for example, there are blue and brown alleles of the eye-color gene.

anaphase - phase of mitosis where sister chromatids separate and daughter chromosomes migrate to opposite poles of the cell.

autosome - any chromosome not involved in sex determination.

cell cycle - the progression of a parent cell from creation through growth and development through DNA synthesis to cellular division to produce to daughter cells.

cell membrane - the part of the cell which separates the external environment from the cytoplasm; cell membranes have hydrophilic external surfaces and hydrophobic interiors.

centrioles - structures in the cytoplasm from which the spindle apparatus forms.

centromere - constriction in an unreplicated chromosome which results in two regions or arms ; serves as an attachment site for sister chromatids and spindle fibers .

chromatid - one half of a replicated chromosome which is joined to the other half at the centromere.

chromosome replication - chromosome replication takes place during S phase of the cell cycle; the two strands of a DNA double helix separate and each serves as a template for synthesis of complementary daughter strand.

chromosomes - discrete structures which occur in eukaryotic cell nuclei, contain one or two DNA double helices (in their unreplicated and replicated forms respectively), and are associated with protein especially when condensed.

cytokinesis - division of the cytoplasm, usually at the end of nuclear division.

cytoplasm - gel-like substance in which all cellular components outside the nucleus are immersed.

cytoskeleton - protein lattice structure which gives shape to our cells.

daughter cell - one of two cells resulting from the division of a single cell.

daughter chromosomes - produced by the separation of identical sister chromatids in anaphase of mitosis.

diploid - a cell that contains two of each type of chromosome, such as a human somatic cell.

DNA double helix - the structure of our genetic material; human DNA consists of two complementary strands which twist around each other like two spiral staircases.

DNA synthesis - DNA synthesis takes place during S phase of the cell cycle; the two strands of a DNA double helix separate and each serves as a template for synthesis of complementary daughter strand.

extended chromosome - the relaxed DNA state found during interphase of the cell cycle.

G1 stage - first stage in interphase, during which cellular growth and development takes place; precedes DNA synthesis or the S stage .

G2 stage - third stage of interphase during which organelle replication and protein synthesis takes place; follows DNA synthesis or the S stage.

gametes - reproductive cells; sperm and egg cells in animals.

gene - section of a chromosome which codes for a protein or RNA product.

hereditary material - the information which is passed from one cellular generation to the next (encoded in DNA in humans).

homologous chromosome - chromosomes having the same size, shape, binding pattern, and array of genes; DNA coding for hair color in the allele from your mother is homologous to DNA coding for hair color in the corresponding allele from your father.

interphase - the portion of the cell cycle where the cell is not dividing; includes G1, S and G2 stages.

metaphase - stage of mitosis in which replicated chromosomes align at the center of the cell.

mitosis - process of cellular division in which the daughter cells are genetically and morphologically identical to themselves and to the mother cell.

nuclear membrane - similar in structure to the cell membrane, this structure separates the cytoplasm from the nucleus; contains large open pores for transfer of the macromolecules like RNA.

nucleus - organelle in the center of the cell which contains the chromosomes.

organelles - membrane bound structures found inside the cell.

prophase - stage of mitosis in which replicated chromosomes condense; nuclear membrane dissociates; centrioles migrate to the poles of the cell.

protein - polypeptide chain which serves a structural or enzymatic role or other role in the cell.

replicated chromosomes - chromosomes which have undergone DNA replication and contain two sister chromatids.

RNA product - DNA genes are copied into RNA products; messenger RNA molecules are then translated into proteins; other RNA products including transfer RNA and ribosomal RNA are not translated into protein.

S-phase - second stage of interphase between G1 and G2; period of DNA replication.

sister chromatids - two identical copies of a parent chromosome which are attached to one another at the centromere.

somatic cell - any cell in the body that is not a germ cell (reproductive cell).

spindle fibers - protein structures which move the chromosomes during cell division.

telophase - mitotic stage where nuclear membrane reforms and the spindle fibers disappear.

unreplicated chromosomes - chromosomes that have not undergone replication and contain just one DNA double helix.