Volume 1

Gene expression / Benjamin Lewin.

  • Benjamin Lewin
Date:
1974-1977
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Gene expression / Benjamin Lewin. Source: Wellcome Collection.

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    of strand association, forces other than hydrogen bonding—such as hydro phobic interactions from base stacking—contribute to the stability of the duplex structure (reviewed by Marmur, Rownd and Schildkraut, 1963; Lewin, 1967, 1973). The phosphodiester backbone of the helix is highly negatively charged—one charge per phosphate group—and in physiological conditions this charge is neutralized by interactions with positive groups. In bacteria, the positive charges may be provided by cations. In the chromosomes of higher organisms, basic proteins are associated with DNA; these histone proteins possess sequences of amino acids whose positive charges interact electrostatically with the negatively charged phosphate groups of the double helix (see chapter three of volume two). Because the two strands of DNA are joined only by hydrogen bonds and hydrophobic interactions, they can unwind without breaking any covalent bonds; the single strands which result can direct the formation of replicas by their ability to hydrogen bond specifically to their complementary bases, adenine to thymine and guanine with cytosine. The specificity of complemen tary base pairing accounts not only for the reproduction of DNA but also for its expression. RNA also possesses adenine, guanine and cytosine, but has uracil, which lacks the C3-methyl group of thymine, as its fourth base; the sugar groups of the nucleotides of RNA possess a hydroxy] group at the 2' position of the ribose ring where DNA has hydrogen. Although RNA is generally encountered as a single strand polynucleotide chain, its bases have the same specificity of hydrogen bonding as DNA and recognition between complementary sequences to give duplex regions can occur between RNA and DNA and within RNA molecules, where it is important in the structure of bacteriophage RNAs (see chapter 3), ribosomal RNAs (see chapter 4) and transfer RNAs (see chapter 5). Short nucleotide sequences of transfer RNA molecules also hydrogen bond to complementary sequences in messenger RNAs (see chapter 2). Colinearity of Gene and Protein The concept that genes act by specifying proteins implies that the segment of DNA comprising a gene must in some way be responsible for the sequence of amino acids which constitute the protein for which it codes. (It seems likely that the secondary and tertiary structures of a protein are determined by its primary structure.) The simplest and most obvious way for a length of nucleic acid to specify a protein is for them to be colinear; the sequence of nucleotides along the gene should correspond to the sequence of amino acids making up the protein. This is equivalent to saying that the sequential structure of the chromosome extends to within genes as well as between them and leads to the picture of chromosomes as structures which contain long molecules of DNA, successive regions coding for different proteins.
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