Deoxyribonucleic acid (DNA) is a molecule that has the biological instructions that make each species unique and it is present in the cell. DNA is passed from adult organisms to their offspring during reproduction.
DNA is a nucleic acid, one of the four major groups of biological macromolecules.
All nucleic acids are made up of nucleotides. In DNA, each nucleotide is made up of three parts: a 5-carbon sugar called deoxyribose, a phosphate group, and a nitrogenous base.
In the 1950s, a biochemist named Erwin Chargaff discovered that the amounts of the nitrogenous bases (A, T, C, and G) were not found in equal quantities. However, the amount of A is always equal to the amount of T, and the amount of C is always equal to the amount of G.
The discovery of the double helix structure of DNA was made thanks to a number of scientists in the 1950s.
DNA molecules have an antiparallel structure, that is, the two strands of the helix run in opposite directions of one another. Each strand has a 5′ end and a 3′ end.
Usually, Watson-Crick models are used to describe the structure of DNA.
Attached to each sugar ring is a nucleotide base, one of the four bases Adenine (A), Guanine (G), Cytosine (C), and Thymine (T). The first two (A, G) are examples of a purine which contains a six atom ring and five atom rings sharing two atoms. The second two (C, T) are examples of a pyrimidine which is composed of a single six atom ring. A base pair is one of the pairs A-T or C-G. It has been noticed that each base pair consists of a purine and a pyrimidine. The nucleotides in a base pair are complementary, which means their shape allows them to bond together with hydrogen bonds. The A-T pair forms two hydrogen bonds. The C-G pair forms three. The hydrogen bonding between complementary bases holds the two strands of DNA together. Hydrogen bonds are not chemical bonds. They can be easily disrupted. This allows the DNA strands to separate for transcription (copying DNA to RNA) and replication (copying DNA to DNA).
The phosphate group of one nucleotide covalently bonded with the sugar molecule of the next nucleotide, and so on, forming a long polymer of nucleotide monomers. The sugar–phosphate groups line up in a “backbone” for each single strand of DNA, and the nucleotide bases stick out from this backbone. The carbon atoms of the five-carbon sugar are numbered clockwise from the oxygen as 1′, 2′, 3′, 4′, and 5′ (1′ is read as “one prime”). The phosphate group is attached to the 5′ carbon of one nucleotide and the 3′ carbon of the next nucleotide.
Replication relies on complementary base pairing, that is the principle explained by Chargaff’s rules. However, three models are used to describe replication style in DNA.
- Semi-conservative replication
In this model, the two strands of DNA unwind from each other, and each acts as a template for synthesis of a new, complementary strand. As a result, two DNA molecules with one original strand and one new strand are formed.
- Conservative replication
In this model, DNA replication results in one molecule that consists of both original DNA strands (identical to the original DNA molecule) and another molecule that consists of two new strands (with exactly the same sequences as the original molecule).
- Dispersive replication
In the dispersive model, DNA replication results in two DNA molecules that are mixtures, or “hybrids,” of parental and daughter DNA. In this model, each individual strand is a patchwork of original and new DNA.