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DNA (deoxyribonucleic acid): found inside every living cell, DNA is the building material of genes, the fundamental units of heredity. The structure of DNA is a double helix that resembles a ladder. Each rung on the ladder is made of two elements which both fit together perfectly and correspond to a base. There are four such bases: A (Adenine), C (Cytosine), G (Guanine), and T (Thymine). Base A on one side (or strand) of the ladder is always paired with base T; base C is always paired with base G.

Epigenetic: refers to all processes that act on DNA and modify the expression of genes (DNA chemical modification, DNA packaging, etc.).

Exon skipping: a technique which involves removing part of a gene that carries a mutation in order to restore the complete or partial function of the protein.

Genes: regions of DNA that encode proteins.

Genome: the entire DNA (the complete set of genes) present in the nucleus of each cell of an organism. Only 1.2% of the human genome, corresponding to 20,000 to 25,000 genes, code for proteins. The function of the remaining 98% of the genome's DNA remains largely unknown.

Messenger RNAs: found in every living cell, they carry the genetic information encoded in DNA from the cell nucleus to the cytoplasm, where the machinery for protein synthesis is located. The molecule is a nucleic acid very similar in structure to DNA but present only in single stranded form.

Proteins: macromolecules encoded by genes and present in all living organisms. They perform all the functions required for the survival of an organism: enzymatic, immune, structural, etc.

Restriction enzymes: proteins that cut DNA at specific sites. They are used to map DNA, and to cut the long DNA molecules into smaller fragments. The fragments can then be multiplied in bacteria
for further analysis. The introduction of these fragments, with or without modification, into the same organism or another one, results in what are known as genetically modified, or transgenic, organisms. These techniques were the basis of what has been termed the biological revolution of the 1960s.

Reverse transcriptase: an enzyme capable of synthesizing DNA from the complementary RNA molecule. This enzyme has facilitated genetic analysis and manipulation by allowing information to move from mRNA, hence protein, back to DNA.


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