R

Radiation hybrid

A hybrid cell containing small fragments of irradiated human chromosomes. Maps of irradiation sites on chromosomes for the human, rat, mouse, and other genomes provide important markers, allowing the construction of very precise STS maps indispensable to studying multifactorial diseases.
See also: sequence tagged site

Rare-cutter enzyme

See: restriction-enzyme cutting site

Recessive gene

A gene which will be expressed only if there are 2 identical copies or, for a male, if one copy is present on the X chromosome.

Reciprocal translocation

When a pair of chromosomes exchange exactly the same length and area of DNA. Results in a shuffling of genes.

Recombinant clone

Clone containing recombinant DNA molecules.
See also: recombinant DNA technology

Recombinant DNA molecules

A combination of DNA molecules of different origin that are joined using recombinant DNA technologies.

Recombinant DNA technology

Procedure used to join together DNA segments in a cell-free system (an environment outside a cell or organism). Under appropriate conditions, a recombinant DNA molecule can enter a cell and replicate there, either autonomously or after it has become integrated into a cellular chromosome.

Recombination

The process by which progeny derive a combination of genes different from that of either parent. In higher organisms, this can occur by crossing over.
See also: crossing over, mutation

Regulatory region or sequence

A DNA base sequence that controls gene expression.

Repetitive DNA

Sequences of varying lengths that occur in multiple copies in the genome; it represents much of the human genome.

Reporter gene

See: marker

Resolution

Degree of molecular detail on a physical map of DNA, ranging from low to high.

Restriction enzyme, endonuclease

A protein that recognizes specific, short nucleotide sequences and cuts DNA at those sites. Bacteria contain over 400 such enzymes that recognize and cut more than 100 different DNA sequences.
See also: restriction enzyme cutting site

Restriction fragment length polymorphism (RFLP)

Variation between individuals in DNA fragment sizes cut by specific restriction enzymes; polymorphic sequences that result in RFLPs are used as markers on both physical maps and genetic linkage maps. RFLPs usually are caused by mutation at a cutting site.
See also: marker, polymorphism

Restriction-enzyme cutting site

A specific nucleotide sequence of DNA at which a particular restriction enzyme cuts the DNA. Some sites occur frequently in DNA (e.g., every several hundred base pairs); others much less frequently (rare-cutter; e.g., every 10,000 base pairs).

Retroviral infection

The presence of retroviral vectors, such as some viruses, which use their recombinant DNA to insert their genetic material into the chromosomes of the host's cells. The virus is then propogated by the host cell.

Reverse transcriptase

An enzyme used by retroviruses to form a complementary DNA sequence (cDNA) from their RNA. The resulting DNA is then inserted into the chromosome of the host cell.

Ribonucleotide

See: nucleotide

Ribose

The five-carbon sugar that serves as a component of RNA.
See also: ribonucleic acid, deoxyribose

Ribosomal RNA (rRNA)

A class of RNA found in the ribosomes of cells.

Ribosomes

Small cellular components composed of specialized ribosomal RNA and protein; site of protein synthesis.
See also: RNA

Risk communication

In genetics, a process in which a genetic counselor or other medical professional interprets genetic test results and advises patients of the consequences for them and their offspring.

RNA (Ribonucleic acid)

A chemical found in the nucleus and cytoplasm of cells; it plays an important role in protein synthesis and other chemical activities of the cell. The structure of RNA is similar to that of DNA. There are several classes of RNA molecules, including messenger RNA, transfer RNA, ribosomal RNA, and other small RNAs, each serving a different purpose.

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