Deoxyribonucleic acid (DNA) is a type of nucleic acid with a double helix structure. It is a macromolecular biomaterial responsible for the inheritance and expression of genetic information in many organisms on the earth.
Constituents and double helix structure
DNA is a nucleic acid composed of deoxyribose (pentosaccharide), phosphoric acid, and base. There are four types of bases: adenine (A) and guanine (G), which are purine bases, and cytosine (C) and thymine (T), which are pyrimidine bases. A base bound to the 1'position of 2-deoxyribose is called a deoxynucleoside, and a phosphate bound to the 5'position of the deoxyribose of this nucleoside is called a deoxynucleotide.
Nucleotides are the smallest unit of nucleic acid, while DNA is a polymer of deoxynucleotides. The sugar used by nucleic acid as a constituent substance is called a constituent sugar, and the nucleic acid using ribose as a constituent sugar is called ribonucleic acid (RNA). Nucleotide molecules form phosphodiester bonds in the form of water that can be taken from the 3'-position OH group of sugar and the OH group of phosphoric acid, and these bonds continuously form a chain-like molecular structure. A polynucleotide (or polynucleotide chain) in which 100 or more nucleotides are linked is called a single-stranded structure of DNA. There is a concept of directional in DNA, and during replication, DNA polymerase synthesizes DNA in the direction of the 5'→ 3'end. Transcription of RNA also follows this direction.
In double-stranded DNA, two polynucleotide strands are oriented antiparallel and take a right-handed helical morphology (double helix structure). The two polynucleotide chains are linked via hydrogen bonds of complementary base (A / T, G / C) pairs. Base complementarity is the property that if one of the four types of bases A, T, G, and C is determined, the other type that is bound to it by a hydrogen bond is also determined. There are two hydrogen bonds between A / T and three between C / G, and the stability is different. Exceptionally, special sequences may have a left-handed helical structure, which is called Z-type DNA.
The significance of this complementary double-stranded structure is that one is left for preservation (sense strand) and the other is for transcription (antisense strand) that transfers genetic information to mRNA as much as necessary. .. In addition, since one of the double strands is inherited as it is, accurate DNA replication can be easily performed, which is crucial for transmitting genetic information. It also helps repair DNA damage (double helix for details).
The length of DNA varies. The unit of length is bp (base pair) for double-stranded and nt (nucleotide) for single-stranded.
Intracellular DNA has a circular shape such as prokaryotic and mitochondrial DNA, and a linear shape commonly found in eukaryotes. The number of spiral turns of DNA in nature is slightly smaller than the theoretical value (10.4 bases per rotation). There is no problem with linear DNA, but with circular DNA, the ring is twisted to eliminate the instability caused by this difference, which is called DNA supercoil (or negative supercoil).
The hydrogen bonds that connect the two polynucleotides are unstable and therefore separate in boiling water to form a single strand. However, when cooled slowly, the polynucleotide rebinds from complementarity and returns. The fact that DNA becomes a single strand in this way is called "DNA denaturation", and the restoration to its original state is called annealing. The temperature at which 50% denaturation occurs is marked Tm, and the higher the A / T pair, the lower. Tm is used when the concentration of monovalent cations is low, or when hydrogen bonds are easily released, urea or hol.