Genetic organization : a comprehensive treatise / edited by Ernst W. Caspari, Arnold W. Ravin.

Date:: 1969-

Catalogue details

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

Credit: Genetic organization : a comprehensive treatise / edited by Ernst W. Caspari, Arnold W. Ravin. Source: Wellcome Collection.

29/552 page 11

I. GENETICS IN HISTORICAL PERSPECTIVE 11 current investigation which will be dealt with by other authors in this treatise. The chief alterations in views of the transmission system required by discoveries in genetics after about 1945 concerned the concept of the gene as the fundamental unit of organization of the genetic material. Today genes appear to be complex linear sequences of a hundred or niore separately mutable sites along the polynucleotide chain of DNA. Each site probably consists of a single pair of nucleotides. It is this pair of small molecules, or the bonds which hold them together, that consti¬ tute the elementary structures. Micromaps of genes, attained by classic methods of recombination, reveal an order at a level below that of the gene as a unit of function, and research chiefly with microorganisms has revealed that genes are also elements in an organization of higher order. They participate as structural genes, that is, genes specifying protein structure, and along with regulator and contiguous operator genes are organized into regulatory circuits. The activity of the structural genes is modulated and controlled by the regulatory circuit. Regulation, where best understood as in bacteria, usually takes place by selective repression. The regulatory substance from the regulator gene passes through the cytoplasm where it may be activated or inhibited en route to the struc¬ tural genes in the chromosome. Finally, the manner in which a structural gene specifies the structure of the protein for which it is responsible is embodied in a set of rules known as the genetic code. The chief feature of this is that the nucleotide pattern of one of the strands in a segment of DNA acts as a template upon which is formed a complementary strand of ribonucleic acid in which и (uracil) takes the place of the thymine of DNA. The DNA order is thus transcribed into a corresponding RNA order. This RNA copy acts as a template upon which the amino acids from the pool in the cell will be assembled in the sequence specified by the order of bases (nucleotides) in the RNA. The message in the RNA is in a four-letter code, the letters representing the bases A (adenine), U( uracil), G (gua¬ nine), and С (суtosine). This is to be decoded or translated into the twenty-letter language of the amino acids found commonly in proteins. The code consists of triplets or codons of three successive bases such as AGU upon which a specific amino acid (e.g., methionine) is attached by a specific enzyme. The message transcribed from the DNA order of nu¬ cleotides is translated, by means of the code, into a sequence of amino acids assembled in polypeptide chains to form a specific protein. Experimental studies with microorganisms, especially bacteria and viruses, have provided material for much of the new body of principles

digitised image 1

digitised image 2

digitised image 3