n to microscopes, the ability to view adjacent objects as distinct structures.

resource partitioning  The division of resources such that a few dominant species exploit most of the available resources while other species divide the remainder; helps explain why a few species are abundant in a community while others are represented by only a few individuals.

respiration 1) breathing as part of gas exchange; or 2) cellular metabolism.

respiratory surface  A thin, moist, epithelial surface that oxygen can cross to move into the body and carbon dioxide can cross to move out of the body.

respiratory system  One of eleven major body organ systems in animals; moves oxygen from the external environment into the internal environment and removes carbon dioxide from the body. PICTURE

resting potential  The difference in electrical charge across the plasma membrane of a neuron.

restriction enzymes  A series of enzymes that attach to DNA molecules at speciÞc nucleotide sequences and cut both strands of DNA at those sites. A bacterial enzyme that cuts DNA at a specific recognition sequence. This is a bacterial defense against viral DNA and plasmid DNA and is now used as an important tool in biotechnology.

restriction fragment length polymorphism (RFLP)  A heritable difference in DNA fragment length and fragment number; passed from generation to generation in a codominant way.

retina  The inner, light-sensitive layer of the eye; includes the rods and cones.

retroviruses  Viruses that contain a single strand of RNA as their genetic Material and reproduce by copying the RNA into a complementary DNA strand using the enzyme reverse transcriptase. The single-stranded DNA is then copied, and the resulting double-stranded DNA is inserted into a chromosome of the host cell. PICTURE 1 | PICTURE 2 | PICTURE 3 | PICTURE 4

reverse transcriptase  An enzyme used in the replication of retroviruses; aids in copying the retrovirus's RNA into a complementary strand of DNA once inside the host cell. PICTURE 1 | PICTURE 2 | PICTURE 3

reverse transcription Process of transcribing a single-stranded DNA from a single-stranded RNA (the reverse of transcription); used by retroviruses as well as in biotechnology.

rheumatoid arthritis  A crippling form of arthritis that begins with inþammation and thickening of the synovial membrane, followed by bone degeneration and disÞ