Barbara McClintock

CHROMOSOME ORGANIZATION AND GENIC EXPRESSION

Presented at the Cold Spring Harbor Symposium in 1951, this paper focused on regulation and development rather than transposition, and strongly reflected McClintocks newfound interest in theory.

Controlling elements and the gene.

This paper presented at the 1956 Cold Spring Harbor Symposium provides a brief overview of McClintocks thoughts on controlling elements, including suppressor-mutator, or Spm.

The relation of a particular chromosomal element to the development of the nucleoli in Zea mays

Summary1.The nucleolus is organized in the telophase through the activity of a distinct deep-staining body having a definite position in one chromosome (the satellited chromosome) of the monoploid complement. Correlated with the number of satellited chromosomes present, the telophases of somatic tissue of haploids show one nucleolus, diploids, two nucleoli and triploids, three nucleoli. That the nucleolus develops through the activity of this body (refered to as the nucleolar-organizing body or element) was obtained from a reciprocal translocation which broke this body into two parts. Both interchanged chromosomes possessed a section. Nucleoli developed from each of these two segments. Thus, plants homozygous for the interchange developed four nucleoli in their somatic telophases; plants heterozygous for the interchange developed three nucleoli in their somatic telophases. Similarly, the telophase nucleoli resulting from the first division within the monoploid microspore of normal diploids show only one nucleolus, whereas, those of plants homozygous for the interchange are characterized by the development of two nucleoli.2.The functional capacity to develop a nucleolus is not the same for both segments of the severed nucleolar-organizing body. This is evident when the two interchanged chromosomes are present in the same nucleus. The segment of the nucleolar-organizing body possessed by one interchanged chromosome produced a large nucleolus, whereas, the segment of the nucleolar-organizing body possessed by the other interchanged chromosome produced a small nucleolus. When this latter chromosome, with the nucleolar-organizing element of slower rate of functional capacity is present without the former (i. e. without a competing nucleolarorganizing element) it produces, in contrast, a large nucleolus.3.The activity of the nucleolar-organizing element is hindered by certain genomic deficiencies. When this occurs, many small nucleolarlike bodies are produced and remain associated with the other chromosomes of the complement. These small nucleoli appear to develop from a swelling and later collection into droplets of the matrix substance of the chromosome.

The association of non-homologous parts of chromosomes in the mid-prophase of meiosis in Zea mays

Summary1.Association of chromosomes at pachytene in Zea mays is 2-by-2 whether or not the parts associated are homologous.2.Evidence for non-homologous association of parts of chromosomes has been obtained from monoploids, diploids, monosomics, trisomics, deficiencies, inversions, translocations, ring-shaped chromosomes, “asynaptic” plants and so-called “B-type” chromosomes.3.In many cases, the non-homologous association at pachytene appears to be as intimate as homologous association.4.Non-homologous association, present at pachytene, rarely continues into diakinesis.5.Translocations probably result from the association of non-homologous parts of chromosomes.

Some Parallels Between Gene Control Systems in Maize and in Bacteria

In this article, McClintock attempted to link the regulation of controlling elements to observations made by Francois Jacob, Jacques Monod, and Andre Lwoff on bacterial operons in 1960. In part, McClintock was trying to establish her preeminence in the study of gene regulation, but she also wanted to demonstrate that regulation was widespread among organisms. Many of the parallels she observed were not universally accepted and McClintock backed off most of her claims by 1968.

A Method for Making Aceto-Carmin Smears Permanent

Anthers are collected and placed in a solution of 1 part acetic acid to 3 parts of absolute alcohol. The contents of the anther are squeezed out on a slide in a drop of Bellings iron-aceto-carmin solution and a cover glass placed over the drop. Care should be taken to remove all anther walls and flower parts. Heat the slide over an alcohol flame for a second, repeating 4 or 5 times. Place the slide in a petri dish filled with a 10% solution of acetic acid. When the cover glass has risen away from the slide gently remove the cover glass and place in a Coplin jar containing equal parts of alcohol and acetic acid. Likewise, place the slide in this solution. Run both cover and slide thru the following solutions: 1 part acetic acid to 3 parts absolute alcohol, 1 part acetic acid to 9 parts absolute alcohol, absolute alcohol and finally equal parts of absolute alcohol and xylol. Recombine the cover and slide in xylol-balsam directly from this solution.

SPONTANEOUS ALTERATIONS IN CHROMOSOME SIZE AND FORM IN ZEA MAYS

Spontaneous aberrations in maize leading to changes in size and form of the chromosomes have not been investigated from the point of view of determining, systematically, the frequency and positions of breakages and reunions of broken ends of the chromosomes of the complement, as has been done in Tradescantia (Giles, 1940), in Allium (Nichols, 1941) and in other forms (Darlington and Upcott, 1941). Nevertheless, through studies of various problems not directed toward this goal, much has been learned of the process underlying the origin of changes in size and form of the chromosomes of maize which are not conditioned by the usual methods of inducing aberrations, such as X-radiation, ultraviolet radiation, high temperatures and aging.

Genetic Systems Regulating Gene Expression during Development

Publisher Summary This chapter provides an overview of the genetic systems regulating gene expression during development. Regulatory systems composed of two or more interacting elements are known. One of the elements is always at the locus of the structural gene whose expression is controlled by the system. The other element or elements of the system are located elsewhere in the chromosome complement. Each system of controlling elements is essentially autonomous in that the functioning of the elements of one systemdoes not interfere with the functioning of those of another system. Evidence that the Spm system can preset a gene locus at one stage in development to give a particular pattern of gene expression at a later stage was obtained from studies of several different gene loci that came under the control of the system.The review of modes of regulation of gene action by the Spm system illustrates the breadth of accomplishment of a system composed of only two genetic elements. Such systems provide a means of inducing extensive plasticity of expression of the genome. The presence of controlling elements may also be suspected merely on the basis of viewing plants in their native habitats. During development, all components of the genome must undergo sequential stages of programming, which must continue until the final stage of differentiation of a cell. The controlling elements associated with the genes may be the components that are concerned with all these varied expressions of gene regulation.