Last revised by Francesco Sciacca on 14 May 2023

DNA (deoxyribonucleic acid) is a nucleic acid that encodes the genetic information (genome) necessary for RNA (ribonucleic acid) transcription (transcriptome) and protein synthesis (proteome1. It is contained in the nucleus of eukaryotic cells in the form of chromatin or chromosomes 7,8.

DNA is made up of two double-stranded polynucleotide chains. Each nucleotide consists of a pentose, 2-deoxy-D-ribose, to which the nitrogenous base and a phosphate group are covalently linked (N-glycosidic and phosphoester respectively). The nucleotides, in turn, are held together by "bridging" chemical bonds between the pentose molecules (phosphodiesteric chemical bond) 2,3.

The two polynucleotide chains, on the other hand, are joined by hydrogen bonds (H-bonds); they are established between the complementary heterocyclic base pairs: two H-bonds between adenine (A) and thymine (T) and three H-bonds between guanine (G) and cytosine (C) 4.

The primary structure is the sequence of heterocyclic bases along the polynucleotide chain that characterize the DNA 2.

The secondary structure equates to the right-handed double helix proposed in 1953 by James D Watson and Francis H Crick (B-form) for which they won the Nobel Prize 3. Each complete turn of the double helix takes ten pairs of nucleotides 6.

It is the branch of genetics that studies all the heritable modifications that vary the gene expression (phenotype) while not altering the DNA sequence 13-15.

DNA methylation is the process whereby specific regions of DNA can be altered by the addition of methyl groups (-CH3). This is an epigenetic mechanism used by the human cell to modulate gene expression and can be assessed using DNA methylation profiling

Circulating tumor DNA (ctDNA) are fragments of DNA released into the blood by cancer cells. The possibility of sequencing and monitoring it is the basis of recent diagnostic techniques known as "liquid biopsy" 9.

Polymerase chain reaction (PCR) is a molecular biology technique to multiply copies of DNA and quantify potential genomes present in a given sample preparation 10. The technique was developed by K B Mullis, Nobel Prize in Chemistry in 1993. In the specific case of RNA sequences, these must first be reverse-transcribed into DNA (RT-PCR) 11.

Some nitrogenous bases constituting the nucleic acids, if radiolabeled (for example 5 [125I] -2'-deoxyuridine) can be used, in vitro or in vivo, to estimate the extent of cell proliferation in oncology 12.

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