3.2: DNA

What is DNA?

Deoxyribonucleic acid (DNA) is a molecule that carries most of the genetic instructions used in the development, functioning and reproduction of all known living organisms and many viruses.

DNA is a nucleic acid; alongside proteins and carbohydrates, nucleic acids are one of the three major macromolecules essential for all known forms of life. DNA stores biological information and is involved in the expression of traits in all living organisms.

The Path to Discovery

In the 1950s, Francis Crick and James Watson worked together to determine the structure of DNA at the University of Cambridge, England. At the time, other scientists like Rosalind Franklin, Linus Pauling and Maurice Wilkins were also actively exploring this field. Pauling had discovered the secondary structure of proteins using X-ray crystallography.

Cloning

Reproductive cloning is a method used to make a clone or an identical copy of an entire multicellular organism. In cloning both the original organism and the clone have identical DNA. Identical twins are, in one sense, clones of each other; they have identical DNA, having developed from the same fertilized egg.

Cloning became an issue in scientific ethics when a sheep became the first mammal cloned from an adult cell in 1996. Since then several animals such as horses, bulls, and goats have been successfully cloned, although these individuals often exhibit facial, limb, and cardiac abnormalities.

Figure \(\PageIndex{1}\): Modern understanding of DNA structure and function has led to cloning: Dolly the sheep was the first large mammal to be cloned.

There have been attempts at producing cloned human embryos as sources of embryonic stem cells, sometimes referred to as ‘cloning for therapeutic purposes’. Therapeutic cloning produces stem cells to attempt to remedy detrimental diseases or defects (unlike reproductive cloning, which aims to reproduce an organism). Still, therapeutic cloning efforts have met with resistance because of bioethical considerations.

CRISPR

CRISPR (Clustered, Regularly-Interspaced Short Palindromic Repeats) allows scientists to edit genomes, far better than older techniques for gene splicing and editing. The CRISPR technique has enormous potential application, including altering the germline of humans, animals and other organisms, and modifying the genes of food crops. Ethical concerns have surfaced about this biotechnology and the prospect of editing the human germline and making so-called ‘designer babies’.

The monomeric building blocks of DNA are deoxyribomononucleotides (usually referred to as just nucleotides), and DNA is formed from linear chains, or polymers, of these nucleotides. The components of the nucleotide used in DNA synthesis are a nitrogenous base, a deoxyribose, and a phosphate group. The nucleotide is named depending on which nitrogenous base is present. The nitrogenous base can be a purine such as adenine (A) and guanine (G), characterized by double-ring structures, or a pyrimidine such as cytosine (C) and thymine (T), characterized by single-ring structures. In polynucleotides (the linear polymers of nucleotides) the nucleotides are connected to each other by covalent bonds known as phosphodiester bonds or phosphodiester linkages.

The two polynucleotide strands are anti-parallel in nature. That is, they run in opposite directions. The sugars and phosphates of the nucleotides form the backbone of the structure, whereas the pairs of nitrogenous bases are pointed towards the interior of the molecule. The twisting of the two strands around each other results in the formation of uniformly-spaced major and minor grooves bordered by the sugar-phosphate backbones of the two strands.

Contributors and Attributions

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