The terminus of a DNA helix has been called its Achilles' heel. Thus to prevent possible incomplete replication and instability of the termini of linear DNA, eukaryotic chromosomes end in characteristic repetitive DNA sequences within specialized structures called telomeres. In immortal cells, loss of telomeric DNA due to degradation or incomplete replication is apparently balanced by telomere elongation, which may involve de novo synthesis of additional repeats by novel DNA polymerase called telomerase. Such a polymerase has been recently detected in HeLa cells. It has been proposed that the finite doubling capacity of normal mammalian cells is due to a loss of telomeric DNA and eventual deletion of essential sequences. In yeast, the est1 mutation causes gradual loss of telomeric DNA and eventual cell death mimicking senescence in higher eukaryotic cells. Here, we show that the amount and length of telomeric DNA in human fibroblasts does in fact decrease as a function of serial passage during ageing in vitro and possibly in vivo. It is not known whether this loss of DNA has a causal role in senescence.

Telomeres shorten during ageing of human fibroblasts.

We report an improved draft nucleotide sequence of the 2.3-gigabase genome of maize, an important crop plant and model for biological research. Over 32,000 genes were predicted, of which 99.8% were placed on reference chromosomes. Nearly 85% of the genome is composed of hundreds of families of transposable elements, dispersed nonuniformly across the genome. These were responsible for the capture and amplification of numerous gene fragments and affect the composition, sizes, and positions of centromeres. We also report on the correlation of methylation-poor regions with Mu transposon insertions and recombination, and copy number variants with insertions and/or deletions, as well as how uneven gene losses between duplicated regions were involved in returning an ancient allotetraploid to a genetically diploid state. These analyses inform and set the stage for further investigations to improve our understanding of the domestication and agricultural improvements of maize.

/pdf/the-b73-maize-genome-complexity-diversity-and-dynamics-2hdsrsw8xl.pdf

The B73 Maize Genome: Complexity, Diversity, and Dynamics

Identification of a specific telomere terminal transferase activity in tetrahymena extracts

It is concluded from a review of the literature that plant cell culture itself generates genetic variability (somaclonal variation). Extensive examples are discussed of such variation in culture subclones and in regenerated plants (somaclones). A number of possible mechanisms for the origin of this phenomenon are considered. It is argued that this variation already is proving to be of significance for plant improvement. In particular the phenomenon may be employed to enhance the exchange required in sexual hybrids for the introgression of desirable alien genes into a crop species. It may also be used to generate variants of a commercial cultivar in high frequency without hybridizing to other genotypes.

Somaclonal variation — a novel source of variability from cell cultures for plant improvement

https://delangelab.rockefeller.edu/pubs/50_Griffith_Cell_99.pdf

Mammalian Telomeres End in a Large Duplex Loop

An attempt is made to outline several experiments conducted by the author that revealed how a genome may react to conditions for which it is unprepared, but to which it responds in a discernible, but initially unforeseen manner. One of the experiments involves the effect of X-rays on chromosomes of maize. 35 references, 3 figures.

https://science.sciencemag.org/content/sci/226/4676/792.full.pdf

The significance of responses of the genome to challenge

This article constitutes part of McClintock's larger body of work on ring chromosomes in which she found that the chromatid
 breakage-fusion-bridge (BFB) cycle occurred only in the germ cells and endosperm, while the chromosome BFB continued in the
 sporophyte, which meant that she could selectively produce persistent variegation in plant tissues.

https://www.genetics.org/content/genetics/26/2/234.full.pdf

The Stability of Broken Ends of Chromosomes in Zea Mays.

In the course of an experiment designed to reveal the genic composition of the short arm of chromosome 9, a phenomenon of rare occurrence (or recognition) in maize began to appear with remarkably high frequencies in the cultures. The terms mutable genes, unstable genes, variegation, mosaicism, mutable loci or “position-effect” have been applied to this phenomenon. Its occurrence in a wide variety of organisms has been recognized. The most extensive investigations of this phenomenon have been undertaken in Drosophila melanogaster.1 In this organism, the conditions associated with the origin of genic instability have been well defined. The part played by the heterochromatic materials of the chromosomes, in inducing and controlling the type of variegation and its time and frequency of occurrence, has been established. It has not been generally recognized that the instability of genic expression in other organisms may be essentially the same as that occurring in Drosophila.

https://www.pnas.org/content/pnas/36/6/344.full.pdf

The origin and behavior of mutable loci in maize

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

Chromosome organization and genic expression.

In a recent brief review (McClintock, 1956), a description was given of types of elements carried in the maize chromosomes that serve to control gene action and to induce, at the site of the gene, heritable modifications affecting this action. These elements were initially discovered because they do not remain at one position in the chromosome complement. They can appear at new locations and disappear from previously determined locations. The presence of one such element at or near the locus of a known gene may affect the action of this gene. In so doing, it need not alter the action potentials of the genic substances at the locus. Therefore, these elements were called controlling elements. It was also shown that controlling elements fall into groups, the members of each operating as an integrated system in the control of gene action.

Barbara McClintock

Papers

The significance of responses of the genome to challenge

The Stability of Broken Ends of Chromosomes in Zea Mays.

The origin and behavior of mutable loci in maize

Chromosome organization and genic expression.

Controlling elements and the gene.