Glossary

Chromosome
  Genomes are organized as DNA molecules called chromosomes. Bacterial cells contain a single chromosome, whereas the genomes of higher organisms (plants, animals) include various numbers of chromosomes, the same number for each species; each chromosome is generally present in two copies which are not identical; one copy is inherited from the mother and the other from the father. In humans there are 23 pairs of chromosomes including one pair of sex chromosomes which are different in men (X and Y) and the same in women (XX). There are thus 24 types of human chromosomes, which are numbered more or less according to size. During cell division, DNA molecules which are associated with proteins, condense and the chromosomes appear as rods visible in the microscope.

Cloning
  In molecular biology this word designates identical reproduction of a DNA fragment which is integrated into a bacterium and recopied by it as if it were its own DNA. After spreading over a Petri dish, each bacterial clone—the totality of cells originating from the division of a single bacterial cell—form colonies. This is the means used for isolating DNA fragments from a mixture, to conserve them and prepare them in large quantity. The word clone is also used to designate the fragment of DNA present which is reproduced infinitely in a bacterial clone.

Complementary DNA (cDNA)
  Molecule of DNA obtained by copying a molecule of messenger RNA (mRNA). In contrast to RNA, the molecule of cDNA can be cloned and amplified, and thus provides access to the sequence of the mRNA. Its name comes from the fact that one of the strands of DNA is complementary to the RNA strand.

DNA
  Deoxyribonucleic acid (DNA) is the molecule which contains the genetic information. This information is encoded by a chain, or sequence, of nucleotides, which are the basic subunits of DNA. A molecule of DNA is formed of two chains with complementary sequences, paired and twisted into a double helix. This structure was elucidated by James Watson and Francis Crick in 1953.