Amino Acid
Small molecule consisting of a central carbon atom surrounded by an amino group, an acid group, a hydrogen, and a variable "R" group. Amino acids are the monomers for proteins. There are 20 different amino acids found in living organisms on Earth.

Chromosome
The cellular structure which carries DNA. Chromosomes are made up of DNA and proteins. Different species of organisms can have different numbers of chromosomes. There are 23 human chromosomes (therefore a human has 46 chromosomes because we get one copy of each from our mother and one from our father).

Cloning
Making a genetically identical copy. It is possible to clone a single gene inside a bacterium. Or an entire organism can be cloned, meaning its full genome is copied and a new individual with the same genetic composition born.

Codon
A sequence of three nucleotides in DNA or mRNA that represents a specific amino acid. Codons are recognized during translation by tRNA molecules, which have an anticodon (a three-nucleotide sequence complementary to the codon) which binds to the codon. The specific amino acid encoded by the codon is bound to the tRNA with the anticodon complementary to the codon, and this amino acid is added to the protein sequence when the codon and anticodon bind.

DNA
Deoxyribonucleic acid, the molecule which contains the genetic information of all living organisms on earth. DNA is made up of four different nucleotide monomers, known as A, T, C, and G; and the pattern in which these nucleotides are arranged is the code that tells cells how to build the proteins that make up an organism. The shape of a DNA molecule is a double-stranded helix, with the two strands of nucleotides running antiparallel to each other.

DNA Polymerase
The main enzyme in DNA replication. It is a large protein that "sits" on an open unwound DNA strand and uses that strand as a template to create a new DNA strand complementary to the first. It only works in the 5' to 3' direction, so it replicates the first strand of a DNA helix directly, and must replicate the second in small segments as the helix unwinds. The segments are then joined together by another enzyme, and the end result is that one double helix of DNA becomes two, each identical to the first.

Gamete
The "sex cell" of an organism. Male gametes are sperm and femal gametes are eggs, or ova. Gametes are produced by meiosis and therefore contain only half of the genetic information of the rest of the body's cells. During sexual reproduction, two gametes combine to make a complete new organism.

Gap 1
Also abbreviated "G1," this is the first and longest stage of the cell cycle, ocurring immediately after cell division. The cell performs all of the normal metabolic functions for which it is specialized. A cell may stay in Gap 1 permanently and never divide again; in this case the cell is said to be in "Gap 0."

Gap 2
Also abbreviated "G2," this is the third stage of the cell cycle, ocurring after the Synthesis phase. In this phase, all the DNA in the nucleus has already been replicated and the cell prepares to divide in two.

Gene
The functional unit of the genome. A gene contains the information necessary to make a single protein. The human genome contains about 30,000 genes.

Gene Therapy
A medical technique for correcting some genetic disorders. When the cause of a disease can be traced to a defective gene, it's sometimes possible to combat the disease by replacing the defective gene with a normal copy. There are different methods of introducing the functional gene into the body, depending on the individual and the disease.

Genome
The entire collection of an organism's DNA.

Karyotype
A chart of all of an individual's chromosomes. Cells can be removed from a person and their cycles halted at mitosis, when the chromosomes are easiest to see. The cells are then broken up and the chromosomes identified based on size and shape. Karyotyping can be used to screen for genetic problems caused by incorrect numbers of chromosomes or defective chromosomes.

Meiosis
A special kind of cell division that occurs only in gametes. Meiosis involves two consecutive rounds of cell division, and its end result is gamete cells with only half of the genetic information of the rest of the body's cells. It also involves the independent assortment of chromosomes inherited from both parents into the new gametes, plus the crossing over or recombination of genetic information between chromosomes. Because of independent assortment and recombination, no two gametes are identical.

Mitosis
The stage of the cell cycle in which cell division occurs. The DNA in the nucleus is pulled into two identical new nuclei, each of which will become the nucleus of a new cell.

Nucleotide
The monomer of nucleic acids. Nucleotides consist of a phosphate group attached to a 5-carbon sugar (ribose in RNA, deoxyribose in DNA), which in turn is attached to a nitrogenous base. There are five different nitrogenous bases: adenine (abbreviated "A"), thymine (T), cytosine (C), guanine (G), and uracil (U). A, T, C, and G are found in DNA; A, U, C, and G are found in RNA.

Protein
Class of large biological molecules that provide structure for cells and organisms and catalyze chemical reactions within them. Proteins are made of long chains of amino acids, folded into shapes specific for their particular function.

Ribosome
Cellular structures that make proteins. A ribosome is made of protein and ribosomal RNA (rRNA). Ribosomes have a large and small subunits, between which an mRNA strand slides in order for tRNA to "read" it and bring in the correct amino acids to make a protein.

RNA
Ribonucleic acid. It is similar in structure to DNA, except that it is single-stranded, contains the sugar ribose instead of deoxyribose, and uses the base uracil instead of thymine. There are three types of RNA in cells:

• mRNA, short for "messenger RNA," travels to ribosomes where it provides the code, transcribed from the DNA, to make a protein. About 5% of the RNA in cells is mRNA.
• tRNA, short for "transfer RNA," is a t-shaped strand of RNA with a series of three nucleotides called an anticodon at one end and an amino acid specific to the anticodon at the other. The anticodon pairs with its complementary three-nucleotide sequence on an mRNA strand at a ribsome. The amino acid is then released from the tRNA. This happens over and over, and the released amino acids link into a growing protein chain. About 5% of the RNA in cells is tRNA.
• rRNA, short for "ribosomal RNA," is a component of ribosomes (proteins are the other component). About 90% of the RNA in cells is rRNA.

RNA Polymerase
The main enzyme involved in transcription. It is a large protein that "sits" on an open unwound DNA strand and inserts RNA nucleotides, using the DNA strand as a template (A pairs with U in transcription). This creates a new strand of mRNA.

Synthesis
Also abbreviated "S," this is the second stage of the cell cycle, in which the DNA in the nucleus replicates.

Transformation
The process in which genetic material is transmitted from one organism to another, and properties of the donor organism thereby conferred upon the recipient. The recipient organism is said to have been transformed.

Transgenic
Having DNA that originally came from a different species.

X-ray Crystallography
A technique for determining the shapes of molecules. First the molecule is crystallized. Then x-rays are aimed through the crystal. The regularly spaced atoms of the crystal then diffract the x-rays into an orderly pattern. Next the x-rays hit a sheet of photographic film and leave exposed spots in the pattern in which they were deflected. By analyzing the pattern on the film, a scientist can determine the shape of the original molecule.