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Sunday, January 15, 2017

A comparison of the two principal nucleic acids: RNA (left) and DNA (right)

A comparison of the two principal nucleic acids: RNA (left) and DNA (right), showing the helices and nucleobases each employs.
  • Deoxyribonucleic acid (DNA), the genetic material, carries information to specify the amino acid sequences of proteins. It is transcribed into several types of ribonucleic acid (RNA) including messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA), which function in protein synthesis.
  •  Both DNA and RNA are long, unbranched polymers of nucleotides. Each nucleotide consists of a heterocyclic base linked via a five-carbon sugar (deoxyribose or ribose) to a phosphate group
  •  DNA and RNA each contain four different bases. The purines adenine (A) and guanine (G) and the pyrimidine cytosine (C) are present in both DNA and RNA. The pyrimidine thymine (T) present in DNA is replaced by the pyrimidine uracil (U) in RNA.
  •  The bases in nucleic acids can interact via hydrogen bonds. The standard Watson-Crick base pairs are G·C, A·T (in DNA), and A·U (in RNA). Base pairing stabilizes the native three-dimensional structures of DNA and RNA.
  •  Adjacent nucleotides in a polynucleotide are linked by phosphodiester bonds. The entire strand has a chemical directionality: the 5′ end with a free hydroxyl or phosphate group on the 5′ carbon of the sugar, and the 3′ end with a free hydroxyl group on the 3′ carbon of the sugar . Polynucleotide sequences are always written in the 5′ → 3′ direction (left to right).
  •  Natural DNA (B DNA) contains two complementary polynucleotide strands wound together into a regular right-handed double helix with the bases on the inside and the two sugar-phosphate backbones on the outside. Base pairing (A·T and G·C) and hydrophobic interactions between adjacent bases in the same strand stabilize this native structure.
  •  Binding of protein to DNA can deform its helical structure, causing local bending or unwinding of the DNA molecule.
  •  Heat causes the DNA strands to separate (denature). The melting temperature of DNA increases with the percentage of G·C base pairs. Under suitable conditions, separated complementary nucleic acid strands will renature.
  •  Local unwinding of the DNA helix induces stress, which is relieved by twisting of the molecule on itself, forming supercoils. This process is regulated by topoisomerases, which can add or remove supercoils.
  •  Natural RNAs are single-stranded polynucleotides that form well-defined secondary and tertiary structures. Some RNAs, called ribozymes, have catalytic activity.