4 Day Break Research Paper

Matthew Xu
Mr. Angus
A.P. Biology Period 9
17 February 2016
4 Day Break Essay
1). In the majority of species, a small yet motile sperm fertilizes a much larger but also immobile egg to create a zygote. This zygote must then undergo cleavage. In humans, cleavage occurs about 24 hours after the forming of a zygote. Cleavage is a succession of mitotic cell divisions without cell growth between division cycles. There is close to no cell growth because during cleavage, the embryonic cells undergo the S and M phases of the cell cycle but practically avoid the G1 and G2 phases. Thus, the embryo does not increase in cytoplasmic material or enlarge but merely partitions its zygote into smaller blastomeres. The first five to seven divisions produce a
Cell differentiation is the process by which cells become specialized in structure and function. Without cell differentiation, cell division would only produce a large ball of identical cells. Cell differentiation is not random, it organizes tissues and organs in a particular arrangement in the process of morphogenesis. The specific genes expressed in any particular cell of a developing organism determine that cell’s path. This is to say that by regulating the genes expressed by certain cells, differentiation can occur. There are multiple ways in which the cell controls which genes are being expressed. One way differentiation is controlled is in the egg’s cytoplasmic material. Before and after fertilization, the egg and the future zygote has an unequal distribution of cytoplasmic material. This uneven distribution will then in the future have a profound impact on the expression of genes in the future embryo. Another method of differentiation is the environment around a particular cell. Importantly, signals from other embryonic cells nearby have the greatest influence. Signals from neighboring cells cause induction in a target cell. These neighboring cells convey these signals with cell-surface receptors to send the cell down a specific developmental path by causing changes in its gene expression. These early differentiations have a large impact on the determination of the cells in the future. Homeotic genes control pattern formation in the late embryo, larva and adult.
A stem cell is a relatively unspecialized cell that can both reproduce itself indefinitely and under appropriate conditions, differentiate into specialized cells of one or more types. In this way, stem cells can replenish its own population and generate cells that travel down specific pathways. Embryonic stem cells are cells that are isolated from early embryos at the blastula stage. These embryonic stem cells reproduce indefinitely and can be made to differentiate into a variety of specialized cells. On the other hand, adult stem cells are stem cells that are present in the adult body and are used to replace non-reproducing specialized cells as needed. Unlike embryonic stem cells, adult stem cells are not able to become all cell types. However, adult stem cells are able to create multiple types. Most adult animals have only a tiny number of stem cells. Induced pluripotent stem cells are specialized adult cells that have been transformed into embryonic stem cells through the introduction of extra cloned copies of stem cell master regulatory genes with retroviruses. Induced pluripotent stem cells have been proven to be able to do anything an embryonic stem cell can do. A totipotent cell is a cell that can give rise to all parts of an embryo as well as an adult. A pluripotent cell is a cell that give rise to many parts of an organism, but not all parts. Finally, a multipotent cell is a cell that can develop into more than one cell type but is by far more limited than