Cell Division

Cells divide for three main reasons: growth and development, replace worn-out or injured cells, and reproduction of offspring. Cell division is part of the cell cycle divided into five distinct phases. The diploid state of the cell is the normal chromosomal number in species. During sexual reproduction, the cell's chromosome number is reduced to a haploid state to ensure constancy in chromosome number and thus continuation of the species. The process of cell division is controlled by regulatory proteins. Mitosis occurs in all body cells and is divided into four phases. Meiosis, which occurs in only the germ cells involved in reproduction, divides the chromosomes in two rounds termed meiosis I and meiosis II (reduction division). The human lifecycle starts with gametogenesis, the process that forms gametes which then combine to form a zygote. The zygote quickly becomes an embryo and develops rapidly into a foetus. This chapter explores cell division.

Spermatogenesis: The Commitment to Meiosis

Mammalian spermatogenesis requires a stem cell pool, a period of amplification of cell numbers, the completion of reduction division to haploid cells (meiosis), and the morphological transformation of the haploid cells into spermatozoa (spermiogenesis). The net result of these processes is the production of massive numbers of spermatozoa over the reproductive lifetime of the animal. One study that utilized homogenization-resistant spermatids as the standard determined that human daily sperm production (dsp) was at 45 million per day per testis (60). For each human that means ∼1,000 sperm are produced per second. A key to this level of gamete production is the organization and architecture of the mammalian testes that results in continuous sperm production. The seemingly complex repetitious relationship of cells termed the “cycle of the seminiferous epithelium” is driven by the continuous commitment of undifferentiated spermatogonia to meiosis and the period of time required to form spermatozoa. This commitment termed the A to A1 transition requires the action of retinoic acid (RA) on the undifferentiated spermatogonia or prospermatogonia. In stages VII to IX of the cycle of the seminiferous epithelium, Sertoli cells and germ cells are influenced by pulses of RA. These pulses of RA move along the seminiferous tubules coincident with the spermatogenic wave, presumably undergoing constant synthesis and degradation. The RA pulse then serves as a trigger to commit undifferentiated progenitor cells to the rigidly timed pathway into meiosis and spermatid differentiation.

Investigation of Peer Discussions on Genetic Concepts

This study is an investigation on how students express their understanding of genetic concepts and their relations during peer discussions. Participants in this study were non-major students from a Swedish upper secondary school. Special attention was paid to how the groups treated the domain- specific vocabulary, how they expressed their understanding of reduction division and how they connected concepts from different biological organization levels. These subject areas have been reported as difficult for students in earlier studies. The results show discussions concerning the three subject areas and in the discussions the students help each other to make the meaning of the genetic concepts clear. The analysis is based on socio-cultural perspectives with focus on how the participants treated the genetic content from the previously presented subject areas in their discussions.

Automictic parthenogenesis in the parasitoid Venturia canescens (Hymenoptera: Ichneumonidae) revisited

Both arrhenotokous and thelytokous reproduction are known to occur in the parasitoid wasp Venturia canescens. The cytological mechanism of thelytoky was previously reported to involve the formation of a restitution metaphase after the reduction division, but the exact nature of the subsequent divisions, whether reductional or equational, remained unclear. We reinvestigated the cytological mechanisms in a thelytokous strain collected in France. Our observations confirm previous results, but an equational and not a reduction division was observed after restitution. This type of reproduction can be classified as central fusion automictic parthenogenesis. In two arrhenotokous strains the normal pattern of oogenesis and syngamy of Hymenoptera was observed. In addition, we used PCR amplification to show that thelytoky in V. canescens is not caused by Wolbachia bacteria. The results are discussed in relation to maintenance of heterozygosity and female sex.Key words: automictic parthenogenesis, central fusion, genetic variation, restitution, Venturia canescens, Wolbachia bacteria.

Cryptospores: The Origin and Early Evolution of the Terrestrial Flora

The Cryptogamic, or spore-producing, plants of today are composed of three nonvascular, bryophyte groups (mosses, liverworts, and hornworts) and several vascular groups (ferns, club mosses, and horsetails). All of these plants produce abundant spores, which serve as propagules for dispersing and, to some extent, preserving plants through periods of ecological stress. Plant spores are typically formed as the end products of meiosis (reduction division) from a dividing sporocyte, or spore mother cell (smc). Because of this, they typically occur in groups of four, with each individual spore bearing a characteristic trilete, or Y-shaped mark on its common contact surface. Spore walls, composed of an inert, heterogeneous polymer called sporopollenin, are extremely resistant to the chemical vicissitudes of the terrestrial environment. This property of typical plant spores ultimately allows them to be quite abundant in fine grained clastic rocks. Although fossilized spores represent only a small part of the once-living plant, in many cases, they represent an important component of the plant fossil record, especially when the preservation of macroscopic tissues is lacking.