Spiral structures of meiotic chromosomes in plants

Helical chromonema coiling is conserved in eukaryotes

Efficient chromatin condensation is required to transport chromosomes during mitosis and meiosis, forming daughter cells. While it is well accepted that these processes follow fundamental rules, there has been a controversial debate for more than 140 years on whether the higher-order chromatin organization in chromosomes is evolutionarily conserved. Here, we summarize historical and recent investigations based on classical and modern methods. In particular, classical light microscopy observations based on living, fixed, and treated chromosomes covering a wide range of plant and animal species, and even in single-cell eukaryotes suggest that the chromatids of large chromosomes are formed by a coiled chromatin thread, named the chromonema. More recently, these findings were confirmed by electron and super-resolution microscopy, oligo-FISH, molecular interaction data, and polymer simulation. Altogether, we describe common and divergent features of coiled chromonemata in different species. We hypothesize that chromonema coiling in large chromosomes is a fundamental feature established early during the evolution of eukaryotes to handle increasing genome sizes.

Mitotic chromosomes

Our understanding of the structure and function of mitotic chromosomes has come a long way since these iconic objects were first recognized more than 140 years ago, though many details remain to be elucidated. In this chapter, we start with the early history of chromosome studies and then describe the path that led to our current understanding of the formation and structure of mitotic chromosomes. We also discuss some of the remaining questions. It is now well established that each mitotic chromatid consists of a central organizing region containing a so-called "chromosome scaffold" from which loops of DNA project radially. Only a few key non-histone proteins and protein complexes are required to form the chromosome: topoisomerase IIα, cohesin, condensin I and condensin II, and the chromokinesin KIF4A. These proteins are concentrated along the axis of the chromatid. Condensins I and II are primarily responsible for shaping the chromosome and the scaffold, and they produce the loops of DNA by an ATP-dependent process known as loop extrusion. Modelling of Hi-C data suggests that condensin II adopts a spiral staircase arrangement with an extruded loop extending out from each step in a roughly helical pattern. Condensin I then forms loops nested within these larger condensin II loops, thereby giving rise to the final compaction of the mitotic chromosome in a process that requires Topo IIα.

Irene Manton, Erwin Schrödinger and the Puzzle of Chromosome Structure

Erwin Schrödinger's 1944 publication What is Life? is a classic of twentieth century science writing. In his book, Schrödinger discussed the chromosome fibre as the seat of heredity and variation thanks to a hypothetical aperiodic structure - a suggestion that famously spurred on a generation of scientists in their pursuit of the gene as a physico-chemical entity. While historical attention has been given to physicists who were inspired by the book, little has been written about its biologist readers. This paper examines the case of the English evolutionary botanist and cytologist Irène Manton, who took an interest in What is Life? for its relevance to her own research in chromosome structure as a clue to plant phylogeny. Drawing on recently discovered correspondence between Manton and Schrödinger, the paper reconstructs Manton 's path to the book (including the role of the chemist-philosopher Michael Polanyi) and her response to it by way of throwing new light on a pivotal moment in the history of the debate on chromosome structure.

MEIOTIC CHROMOSOME ORGANIZATION AND SEGREGATION IN PLANTS

During meiosis, homologous chromosomes are brought together to be recombined and segregated into separate haploid gametes. This requires two cell divisions, an elaborate prophase with five substages, and specialized mechanisms that regulate the association of sister chromatids. This review focuses on plant chromosomes and chromosome-associated structures, such as recombination nodules and kinetochores, that ensure accurate meiotic chromosome segregation.

Centromere structure and function in neoplasia

The mammalian centromere plays an essential role in maintenance of diploidy in the cell. It is therefore imperative that we understand the structure and function of the mammalian centromere in order to plan strategy to control the incidence of aneuploidy and resultant malformations of the nonneoplastic as well as neoplastic tissues. Even though considerable information is available about the structure and some functional aspects of centromeres of lower eukaryotes such as yeast, the structure of the mammalian centromere is still a matter of conjecture limited to an understanding of the base composition of the alphoid sequences putatively located in the centromeric DNA of higher apes. We do, however, have a better understanding of the structure and role of the kinetochore. In all eukaryotes analyzed so far, the centromeres in a given genome separate in a sequential manner dependent upon the time of replication of pericentric and centromeric DNA. Some chromosomes, generally found in neoplastic cells, that carry more than one centromere show premature separation of the accessory centromeres. These centromeres and the associated pericentric regions replicate their DNA in an earlier part of the S phase than those that show kinetochore activity; both, however, carry DNA of the same composition. The active centromeres in these chromosomes show kinetochore protein binding as detected by antikinetochore antibody; the inactive centromeres are usually devoid of these proteins. The double minutes in neoplastic cells also lack kinetochore proteins, perhaps due to a lack of any centromere. Some dicentric and multicentric chromosomes in cancer cells and transformed cell lines do not display premature centromere separation. In these chromosomes, all centromeric sites show kinetochore proteins and all centromeric regions replicate their DNA simultaneously. These chromosomes also exhibited meiotic-like behavior of some centromeres and show postanaphase separation of some centromeres, resulting in bridges. These bridges, upon breakage and rejoining of sister chromatids, generate new multicentric chromosomes. The resulting chromosomes also exhibit formation of compound kinetochores. Some of these phenomena are novel descriptions of the centromere behavior in cancer cells. This review also discusses the role of aberrant centromere separation in human biology, providing correlates between errors of centromere separation and neoplasia.