Anti-Parallel: the opposing orientation of the two nucleotide chains in a DNA molecule. (Lewis, 169) One (DNA) "strand" runs in the '5-prime' to '3-prime' direction from top to bottom, while the other strand is oriented 3-prime to 5-prime from top to bottom. (Brooker, 218)

Base Pair: strong hydrogen bonded adenine-thymine and guanine-cytosine. Hold together the two strands of DNA. (Watson, 52) Two chemical bases bonded to one another forming a "rung of the DNA ladder." The DNA molecule consists of two strands that wind around each other like a twisted ladder. Each strand has a backbone made of alternating sugar (deoxyribose) and phosphate groups. Attached to each sugar is one of four bases--adenine (A), cytosine (C), guanine (G), or thymine (T). The two strands are held together by hydrogen bonds between the bases, with adenine forming a base pair with thymine, and cytosine forming a base pair with guanine. (NHGRI) Only specific combinations of the bases are possible, a fact which facilitates accurate DNA replication; when quantified, this term refers to the actual number of base pairs in a “sequence” of nucleotides. (NCI3) Also referred to as ‘nitrogenous bases.’

Chargaff’s Rules: an “adenine”  "base" in one strand forms two hydrogen bonds with a “thymine” base in the opposite strand. A “guanine” base forms three hydrogen bonds with a “cytosine” base. (Brooker, 218)

Complementary DNA (cDNA): DNA that is synthesized in the laboratory from a messenger RNA template. (HGPIA)

Double Helix: an alpha helix that resembles a gently twisted ladder. The rails of the ladder, which run in opposite directions, contain alternating units of “deoxyribose sugar” and “phosphate.” The nucleotides stack tightly on top of one another, forming the rungs of the "helical" ladder.  Each rung is composed of a pair of nucleotides held together by relatively weak hydrogen bonds. There are 10 base pairs per turn of the helix. Adenine always pairs with thymine, and cytosine always pairs with guanine. Thus the nucleotide ‘alphabet’ on one half of the DNA helix determines the alphabet of the other half. (DNA Science, 34) “Crick and I arrived at the double helix first precisely because most chemists at that time thought DNA too big a molecule to understand by chemical analysis.”  What got us most excited was the complementarity of the base sequences along the two chains. If you knew the sequence - the order of bases - along one chain, you automatically knew the sequence along the other. It was immediately apparent that this must be how the genetic messages of genes are coped so exactly when "chromosomes" duplicate prior to "cell division." The molecule would ‘unzip’ to form two separate strands. Each separate strand then could serve as the template for the synthesis of a new strand, one double helix becoming two. The double helix made sense chemically and it made sense biologically. (Watson, 52-53) In a DNA double helix, two DNA strands are twisted together to form a structure that resembles a spiral staircase. (Brooker, G-11) It is a writhing, spinning helix as a result of the forces of random "brownian motion." (Venter, 45)

Mitochondrial DNA (mtDNA): the small circular chromosome found inside mitochondria. The mitochondria are organelles found in cells that are the sites of energy production. The mitochondria, and thus mitochondrial DNA, are passed from mother to offspring. (NHGRI) Double-stranded DNA. In eukaryotes, the mitochondrial genome is circular and codes for ribosomal RNAs, transfer RNAs, and about 10 proteins. (MeSH) A mini-chromosome in a mitochondria carries 37 genes. 24 of 37 genes encode RNA molecules (22 encode tRNA and 2 encode rRNA) (The other) 13 genes function in cellular respiration (making ATP). Encoded genes act in the mitochondria and encode proteins that participate in protein synthesis and energy production. Does not “cross over.” About 1 in 200 people have a mutation,  however diseases are very rare. (Lewis, 98) Cannot repair itself, which accounts for its higher mutation rate. (Lewis, 228)

Plasmid: small, often circular DNA molecule found in bacteria and other cells. Plasmids are separate from the bacterial chromosome and replicate independently of it. They generally carry only a small number of genes, notably some associated with antibiotic resistance. Plasmids may be passed between different bacterial cells. (NHGRI) Double-strand, closed DNA molecules found in “cytoplasm” of a variety of bacterial species that generally confer some evolutionary advantage to the host cells. (UMLS) Distinct from the normal bacterial genome. Some plasmids are capable of integrating into the host genome. A number of artificially constructed plasmids are used as cloning vectors. (HGPIA)

Sugar-Phosphate Backbone: nucleotides join into long chains when chemical bonds form between the "deoxyribose sugars" and the "phosphates." This creates a continuous sugar-phosphate backbone. (Lewis, 169)

Phosphodiester Linkage: refers to a double linkage (two 'phosphodiester' bonds) that holds together adjacent nucleotides in DNA and RNA strands. (Brooker, G-28) Two adjacent sugars are connected via oxygens in a single phosphate to form “esters.” The energy for putting in “nucleotides” comes from breaking the phoshodiester bond of the entering nucleotide “triphosphate.” (Micklos, 40)