Some Helpful Terms
Cells: The “subunit” of all living things. ALL living things are made of cells
Cells can reproduce by dividing. In multi-cellular organisms, different cells are specialized for different
functions. All cells contain genetic material like DNA.
Tissue: Many cells of the same type. (E.g: Many muscle cells, nervous cells, connective cells (like blood or
bone), or epithelial cells.
Cell Membrane: The semi-permeable “skin” of a cell through which materials diffuse in and out of the
cell.
Cell Nucleus: The “organelle” in many cells where the DNA is held and stored.
Atom: The smallest subunit of all matter. Both living and non-living things are made of atoms. There are
approximately seven trillion atoms in one cell. (If a cell is the size of an ocean liner, than an atom is the
size of a BB pellet.)
Molecule: Two or more atoms bonded together by a chemical bond (the sharing or trading of electrons)
Chain molecule: A large molecule made up of many smaller molecules (which are each made up of several
atom). Helpful analogy: chain molecules are like necklaces and their subunits, or building blocks, are like
beads.
Organic Molecule: The types of chain molecules, which make up living things.
Carbohydrate: The type of organic molecule that animals use to make energy. It is composed of many
sugar molecules strung together. Plants make carbohydrates to store the sugar they make during
photosynthesis. Animals get carbohydrates when they eat plants and plant products – especially plant
organs that are used specifically to store carbohydrates, like tubers.
Lipid: The type of organic molecule that makes up semi-permeable cell membranes, that is used to store
energy, or that is used to make steroid hormones.
Protein: The type of organic molecule that makes up enzymes, the transport channels in cell membranes,
and non steroid hormones. Proteins are made up of many amino acids strung together. Proteins are also
structurally important (E.g. Collagen.)
DNA: The type of organic molecule (a nucleic acid) that stores the code, or instructions, for how to
assemble proteins. It also is the hereditary material that allows one generation to pass on its traits to the
next generation. DNA is double stranded and is in the shape of a double helix (a spiral staircase). The
“railings” of the staircase are composed of sugar and phosphate molecules. The “stairs” of the staircase are
made up of nitrogenous bases. The building block of DNA is a “nucleotide,” which is made up of one
sugar, one phosphate, and one nitrogenous base all chemically bonded together.
DNA Code: The order of nitrogenous bases in the DNA provides the instructions for the order of amino
acids in a protein molecule. There are 20 different kinds of amino acids. Some amino acids are attracted
while others repel each other. As a result, the order of amino acids and the attractions and repulsion
between them determine the shape of the protein which then determines the function.
RNA: A type of nucleic acid which has various functions – it acts as the messenger between the DNA code
and the assemblage of protein molecules n a part of the cell known as the ribosome.
Gene: A segment of DNA. Each gene codes for a single protein.
Chromosome: A bundle of DNA. Each chromosome contains approximately 1000 genes. Humans have 23
pairs of chromosomes in each of our body cells.
Homologous Chromosomes: Each pair of chromosomes contains one chromosome from the mother’s egg
and one from the father’s sperm. Each chromosome in the homologous pair contains genes for the same
traits although those genes may not code for the same versions of those traits.
Allele: One version of a gene for a given trait. (Eg. If the gene codes for eye color, one allele may be
“brown,” while another is “blue.”)
Dominant: When one allele is expressed (transcribed and translated) more than the allele on the other
homologous chromosome within the homologous pair. A person needs only one copy of a dominant allele
for it to show in their appearance.
Recessive: When one allele is expressed (transcribed and translated) LESS than the allele on the other
homologous chromosome within the homologous pair. A person needs only one copy of a dominant allele
for it to show in their appearance.
Gametes: The general word for sex cell - a sperm or an egg. A gamete contains 23 chromosomes
(haploid), NOT 23 pairs of chromosomes (diploid).
Fertilization: When a sperm and egg fuse, the resulting fertilized egg, or zygote, contains 23 pairs of
chromosomes.
Zygote: A single-celled embryo (a fertilized egg.)
Somatic (body) Cell: In a human, somatic cells, unlike gametes, are diploid and contain 23 PAIRS of
chromosomes.
Blastocyst: An embryo made up of many up of 70-100 cells.
Embryonic Stem Cells: Cells that have not yet become “specialized” (ie. on track to becoming on type of
cell (nervous, epithelial, connective, or muscle). Therefore, they have the potential to become various types
of cells if the correct genes are expressed. In the four-cell stage of an embryo, the stem cells are
“totipotent,” and each cell has the potential to give rise to an entirely new organism.