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In meiotic prophase, homologous chromosomes pair and recombine, which ensures correct segregation of the homologs at the metaphase to anaphase transition of the first meiotic division. As a first step, early in meiotic prophase, DNA double-strand breaks (DSBs) are introduced by SPO11 at hundreds of sites throughout the genome. In mammals, formation and repair of these breaks is essential for the pairing process. The sex chromosomes, X and Y, are largely heterologous and pair only in the small pseudoautosomal homologous regions. The large heterologous regions show delayed repair of DSBs and asynapsis, which triggers meiotic sex chromosome inactivation (MSCI), leading to the formation of the transcriptionally silenced XY body. Initiation of XY body formation is known to require phosphorylation of H2AX, which is also associated with damage-induced DSB repair in somatic cells. MSCI is considered as a specialized form of a general meiotic silencing mechanism that also silences autosomal unsynapsed chromatin during male and female meiotic prophase: meiotic silencing of unsynapsed chromatin (MSUC). Whereas the XY pair remains always largely unsynapsed and inactivated, autosomal nonhomologous regions frequently show heterologous synapsis and escape from inactivation. Hence, MSUC is less efficient than MSCI. However, if MSUC occurs, MSCI is incomplete, indicating that the two mechanisms are functionally interacting. Herein, we briefly review how X and Y evolved from a pair of autosomes, in relation to X-chromosomal dosage compensation in females and MSCI in males. Next, we describe current ideas about initiation and maintenance of MSCI and MSUC. We also discuss MSCI in birds, a species with female heterogamety. Insight in the mechanism of meiotic silencing is highly relevant for our understanding of male infertility associated with translocations, particularly when these occur between an autosome and the X. Furthermore, we anticipate that dysregulation of MSCI may impact on early embryonic development. [ABSTRACT FROM AUTHOR]

A million million spermatozoa, All of them alive: Out of their cataclysm but one poor Noah Dare hope to survive. And among that billion minus one Might there have chanced to be Shakespeare, another Newton, a new Donne, But the One was Me. Although biopolymers are more sophisticated than their simpler cousins, such as polythene and rubber, this alone does not give them an obvious reason for existing. Nucleic-acid codes for proteins, some of which contribute to the formation of further nucleic acid, but if these products were free to float away through the surrounding water, no advantage would be gained by the original molecule. The essential extra factor was discovered over 300 years ago. In 1663, using one of the earliest microscopes, Robert Hooke observed that cork has a structure reminiscent of a honeycomb, with pores separated by diaphragms so as to produce a collection of little boxes, which he called cells. Their universality was not appreciated until 1824, when Henri Dutrochet concluded that entire animals and plants are aggregates of cells, arranged according to some definite plan. He also suggested that growth is caused by an increase in either cell size or cell number, or both. The idea of the cell was further elevated in 1838, when Matthias Schleiden proposed that it is the basic structural unit in all organisms. [ABSTRACT FROM AUTHOR]

This section presents definitions of Canadian English terms starting with letter "Y." A yak refers to a long-haired humped Tibetan ox. A Y chromosome is a sex chromosome that occurs only in male cells. The term "yeast" refers to a greyish-yellow fungous substance obtained from fermenting malt liquors and used as a fermenting agent.

The article focuses on the study of Y chromosome of white campion (Silene latifolia). The Y chromosome contains key genes controlling the sexual dimorphism. In male plants, a filamentous structure replaces the pistil, while in female plants the stamens degenerate early in flower development. In the absence of stamen promoting function located on the Y chromosome, the male developmental arrest in the female flower results from the lack of parietal initials and the subsequent degeneration of sporogenous cell initials during anther differentiation. Thus, the female plant (XX constitution) contains the genetic information necessary to initiate anther development up to the early sporogenous stage. Subsequently, anther development is arrested because the genes required to proceed beyond that stage are not expressed or are missing. flower. In the presence of gynoecium suppression function (GSF), located on the Y chromosome, the female developmental arrest in the male flower results from a block in carpel initiation. The male plant (XY constitution) contains all the genetic information to produce a functional hermaphroditic flower, while the GSF causes a sudden arrest of cell proliferation in whorl 4 of male flowers at the time of partitioning between whorls 3 and 4, at the flower meristem centre.

A definition of the term "pseudoautosomal" is presented. It refers to sex-chromosome linked genes that are inherited as if they were autosomes. As the consequence of the difference between the human X- and Y-chromosomes in respect to their genes, the males have only one allele of an X- or a Y-linked gene.