Key Concepts of Section 5.1

Key Concepts of Section 5.1

Structure of Nucleic Acids

  • 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), all of which function in protein synthesis (see Figure 5-1).

  • All DNAs and most RNAs are long, unbranched polymers of nucleotides. A nucleotide consists of a phosphorylated pentose linked to an organic base, either a purine or a pyrimidine.

  • 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.

  • Adjacent nucleotides in a polynucleotide are linked by phosphodiester bonds. The entire strand has a chemical directionality with 5′ and 3′ ends (see Figure 5-2).

  • Natural DNA (B DNA) contains two complementary antiparallel 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 (see Figure 5-3). Base pairing between the strands and hydrophobic interactions between adjacent base pairs stacked perpendicular to the helix axis stabilize this native structure.

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    The bases in nucleic acids can interact via hydrogen bonds. The standard Watson-Crick base pairs are G·C, A·T (in DNA), and G·C, A·U (in RNA). Base pairing stabilizes the native three-dimensional structures of DNA and RNA.

  • 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 (Tm) of DNA increases with the percentage of G·C base pairs. Under suitable conditions, separated complementary nucleic acid strands will renature.

  • Circular DNA molecules can be twisted on themselves, forming supercoils (see Figure 5-8). Enzymes called topoisomerases can relieve torsional stress and remove supercoils from circular DNA molecules. Long linear DNA can also experience torsional stress because long loops are fixed in place within chromosomes.

  • Cellular RNAs are single-stranded polynucleotides, some of which form well-defined secondary and tertiary structures (see Figure 5-9). Some RNAs, called ribozymes, have catalytic activity.