DNA is the blueprint that makes you, well you.  All cells have a nucleus (or I should say, all cells start with a nucleus... red blood cells spit out the nucleus before they get to work!).  Within this nucleus are chromosomes, which are made of proteins and DNA.  DNA contains "genes," which are instructions on how to build proteins.

What are chromosomes?

Chromosomes are groups of protein and DNA that are found in the nucleus of cells - 46 in most cells, 23 from your mother and 23 from your father.  Sperm and eggs are special cells which contain only 23 chromosomes.  A karyotype is a picture of all 46 chromosomes.  This picture can be used to determine if the chromosomes are normal in number and appearance.  For example, in Down syndrome there is an extra chromosome number 21.

What is DNA?

DNA is a group of molecules containing the genetic code of a living thing.  Human DNA contains 4 types of molecules: adenosine (A), cytosine (C), guanine (G), and thymine (T).  These molecules are lined up in pairs on a long spiral chain called a "double helix."  Imagine a ladder that is twisted like a spiral noodle.  DNA may be copied or converted to RNA.

What is RNA?

RNA is a "copy" of a portion of the genetic code that travels outside of the nucleus to be used for creating proteins.  RNA, similar to DNA, contains four types of molecules, but uses a molecule called uracil (U) instead of thymine.  These 4 types of molecules (sometimes called "bases") provide a code that describes how to build protein molecules.  Each group of 3 bases represents a "codon."  Each codon represents a specific amino acid or acts as a "start" or "stop" message.  Amino acids are attached together on a long chain to form a protein.  Once a protein chain is complete, it folds into a unique shape and is ready for duty! 

Photo 1 - Normal male karyotype. http://www.genome.gov/.  Used with permission.

Photo 2 - From DNA to a chromosome. http://www.genome.gov/.  Used with permission.