In contrast, bacteria, plasmids, mitochondria, and chloroplasts have circular DNA.
In chromatin, the DNA molecule makes a double turn (DNA superhelix) at regular intervals on a core of eight histone units (nucleosomes). In eukaryotes, chromatin is formed by the association of DNA to basic proteins, predominantly histones. DNA strands are also complementary, with adenine pairing with thymine and guanine with cytosine. The double helix forms two grooves, one wider (major groove) than the other (minor groove). The DNA helix coils clockwise, both strands are antiparallel, one running in the 5′ to 3′ and the other in the 3′ to 5′ direction. DNA has a double helix structure with two polynucleotide strands wrapped around the same axis. DNA contains the bases A, G, T, and C, always with A/T and G/C being equal to 1. Individuals of the same species have equal amount of DNA in each cell (6 pg in humans) female and male gametes contain half of that amount. DNA, together with proteins, forms the chromatin of the cell nucleus and is also present in mitochondria. The last nucleotide has the C3′ nonesterified and it is considered the 3′ end of the molecule.
STORE AND TRANSMIT GENETIC INFORMATION FREE
One extreme of the chain has the 5′ of pentose free and is considered the start of the chain. Nucleotides form chains, by binding through ester links established by phosphate between the OH group in C5′ of the pentose of one nucleotide and the OH of C3′ of the pentose from another nucleotide. The aldopentose is d-ribose in ribonucleic acid (RNA) or d-2-deoxyribose in deoxyribonucleic acid (DNA). The base can be a pyrimidine: thymine (T), cytosine (C), and uracil (U) or a purine: adenine (A) and guanine (G). They are formed by the polymerization of units called nucleotides, which consist of a nitrogenous base, an aldopentose, and phosphoric acid. Nucleic acids (NA) contain the genetic information and play a key role in protein biosynthesis. Antonio Blanco, Gustavo Blanco, in Medical Biochemistry, 2017 Abstract
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