An introduction to DNA

Deoxyribonucleotide

DNA is a long chain polymer composed of four deoxynucleotides, namely:

Adenine deoxynucleotide (dAMP), thymidine deoxynucleotide (dTMP), cytosine deoxynucleotide (dCMP), guanine deoxynucleotide (dGMP).

Deoxyribonucleic acid (DNA) is a long polymer composed of repeated nucleotide sequences, about 22 to 24 angstroms (2.2 to 2.4 nm) wide, and each nucleotide unit is about 3.3 angstroms (0.33 nm) long. In the entire DNA polymer, there may be millions of linked nucleotides. For example, chromosome 1, the largest in a human cell, has 220 million base pairs. Normally in organisms, DNA is not a single molecule, but two molecules that pair together and are tightly bound together, coiled like vines into a double helix. Parts of each nucleotide molecule are linked together to form a long backbone; The other part, called the base, binds the two pairs of DNA to each other. A nucleotide is a nucleoside plus one or more phosphate groups, and a nucleoside is a base plus a sugar molecule.

The DNA backbone is composed of interacting phosphoric acid and carbohydrate groups. "The sugars that make up deoxyribonucleic acid are circular 2-deoxyribose, which is one of the five carbon sugars." The two oxygen atoms of the phosphate group are attached to the three and five carbon atoms of the five-carbon sugar, respectively, to form a phosphate diester bond. The asymmetric position of the covalent bonds on both sides makes each long DNA strand directional. The two strands of nucleotides in the double helix are arranged in opposite directions to each other, and this arrangement is called antiparallel. The two asymmetric ends of a DNA strand are called the 5' end and the 3' end. One of the major differences between deoxyribonucleic acid and RNA is the difference in the composition of the sugar molecule. DNA is 2-deoxyribose, while RNA is ribose.

Deoxyribose (a five-carbon sugar) is linked to a phosphate molecule by ester bonds to form a long chain skeleton, arranged on the outside, and the four bases arranged on the inside. Each sugar molecule is linked to one of four bases, a sequence of bases arranged along a long strand of DNA that makes up the genetic code that guides protein synthesis. The process of reading the code is called transcription, which transcribe a nucleic acid molecule called mRNA (messenger RNA) from a single strand of the DNA duplex. Most Rnas carry messages for protein synthesis, while others have specific functions, such as rRNA, snRNA, and siRNA.

In cells, DNA can combine with proteins to form chromosomes, and the whole set of chromosomes is collectively called the chromosome set. In humans, a normal human cell contains 46 chromosomes. Before cell division, chromosomes will first complete replication in interphase, which can be divided into G1 phase - pre-DNA synthesis phase, S phase -DNA synthesis phase, and G2- post-DNA synthesis phase. For eukaryotes, such as animals, plants and fungi, chromosomes mainly exist in the nucleus. In prokaryotes, such as bacteria, it is mainly present in the nucleoid in the cytoplasm. Chromatin proteins on chromosomes, such as histones, organize and compress DNA to help it interact with other proteins to regulate gene transcription.

DNA is a high polymer, and the DNA solution is a high polymer solution, which has a high viscosity and can be stained green by methyl green. DNA can absorb ultraviolet rays (260nm), and the content of DNA can be determined by using this property. When the nucleic acid is denatured, the absorbance increases, which is called hyperchromic effect. When the denatured nucleic acid is renatured again, the absorbance will return to the original level. "High temperature, organic solvents, acid-base reagents, urea, amides, etc. can cause denaturation of DNA molecules, that is, the hydrogen bond between the bases of the DNA double strand is broken, and the double helix structure is uncoiled - also known as DNA unwinding."

Molecular structure

DNA is a long chain of many deoxynucleotides linked to each other by 3 ', 5 '-phosphodiester bonds in a certain base order. Most DNA contains two such long strands, and some DNA is single strand, such as E. coli bacteriophage φX174, G4, M13, etc. DNA can be divided into circular DNA and strand DNA. In some types of DNA, 5-methylcytosines can replace cytosines to a limited extent, with wheat embryo DNA being particularly abundant in 5-methylcytosines. In some phages, 5-hydroxymethylcytosine is substituted for cytosine. In the late 1940s, E.Chargaff found that the base composition of DNA in different species was different, but the adenine number was equal to the thymine number (A=T), and the guanine number was equal to the cytosine number (G=C), so the sum of the purine number was equal to the pyrimidine number. Generally, the structure of DNA was described in several layers.

Primary structure

"It refers to the four basic building blocks of nucleic acids, deoxyribonucleotides (nucleotides), linear multimers linked to each other by 3', 5' -phosphodiester bonds, and the order of their basic units, deoxyribonucleotides."

Deoxyribonucleotide

"Each deoxyribonucleotide is composed of three parts: a nitrogenous base, a five-carbon sugar (deoxyribose), and a phosphate." The nitrogen-containing bases of nucleic acids can be divided into four groups: adenine (abbreviated as A), thymine (abbreviated as T), cytosine (abbreviated as C), and guanine (abbreviated as G). The four nitrogen-containing base compositions of DNA are species specific. That is, the proportions of the four nitrogen-containing bases are consistent among individuals of the same species, but vary among species. The ratios of the four nitrogen-containing bases in DNA have A curious regularity: A=T, C=G Chagaev's rule (complementary pairing of bases) in the DNA of every organism.