Life Science Encyclopedia

This is a glossary of terms commonly used in the study of genetics and related disciplines in biology. It is intended as introductory material for novices; for more specific and technical detail, please see the article corresponding to each term. Introductory articles in the field include:

• DNA
• Gene
• Genetics
• Genome
• Mendelian inheritance

A

• Adenine: One of the four nucleotide bases in DNA or RNA; pairs with thymine in DNA or uracil in RNA.
• Allele: one of multiple alternative forms of a single gene, each of which is a viable DNA sequence occupying a given position, or locus on a chromosome. For example, in humans, one allele of the eye-color gene produces blue eyes and another allele of the eye-color gene produces brown eyes.

B

• Base pair: A pair of nucleotide bases on complementary DNA or RNA strands organized in a double helix.

C

• Chromosome: a molecular "package" for carrying DNA in cells, organized as two double-helical DNA molecules that encode many genes. Some simple organisms have only one chromosome made of circular DNA, while most eukaryotes have multiple chromosomes made of linear DNA.
• Cytosine: One of the four nucleotide bases in DNA or RNA; pairs with guanine.

D

• DNA: A polymeric molecule made of deoxyribonucleotides, hence then name deoxyribonucleic acid. Most often has the form of a "double helix", which consists of two paired DNA molecules and resembles a ladder that has been twisted. The "rungs" of the ladder are made of base pairs, or nucleotides with complementary hydrogen bonding patterns.

G

• Gene: A set of segments of nucleic acid that contains the information necessary to produce a functional RNA product in a controlled manner. In living organisms, it is a unit of heredity and typically encoded in DNA. A particular gene can have multiple different forms, or alleles, which are defined by different sequences of DNA.

• Gene expression: The process in which the information encoded in a gene is converted into a form useful for the cell. The first step is transcription, which produces a messenger RNA molecule complementary to the DNA molecule on which a gene is encoded. For protein-coding genes, the second step is translation, in which the messenger RNA is read by the ribosome to produce a protein.

• Gene pool: the sum of all the alleles shared by members of a single population.

• Genetics: the field of biology that studies genes and their inheritance.

• Genotype: The complement of alleles present in a particular individual's genome that give rise to the individual's phenotype.

• Genome: The entire complement of genetic material in the chromosome set of an organism, virus or organelle.

• Guanine: One of the four nucleotide bases in DNA or RNA; pairs with cytosine.

L

• Locus: a location on a chromosome where a particular gene resides.

M

• Mutation: is a change in the base pair sequence of DNA or RNA.

N

• Nucleotide: one of the four monomers that make up a DNA molecule.

P

• Phenotype: The observable physical or behavioral traits of an organism, largely determined by the organism's genotype.
• Protein: A linear polymeric molecule made of amino acids linked by peptide bonds. Proteins carry out the majority of chemical reactions that occur inside the cell.

T

• Thymine: One of the four nucleotide bases in DNA; pairs with adenine. In RNA, thymine is replaced with uracil.

• Transcription: The first step in gene expression, in which a messenger RNA molecule complementary to particular gene encoded in DNA is synthesized by enzymes called RNA polymerases. To produce a functional protein, transcription is followed by translation.

• Translation: The second step in gene expression, in which a messenger RNA molecule is read by the ribosome to produce a functional protein. Translation is always preceded by transcription.

U

• Uracil: One of the four nucleotide bases in RNA; pairs with adenine. In DNA, uracil is replaced with thymine.