Thursday, April 13, 2017
Wednesday, April 5, 2017
The Cell Cycle
Cell cycle- (http://www.medicinenet.com/script/main/mobileart.asp?articlekey=7107)
A cell is the basic unit of life.
How do we make new cells?
A cell is a microscopic structure containing nuclear and cytoplasmic material enclosed by a semipermeable membrane and, in plants, a cell wall; the basic structural unit of all organisms. We have prokaryotic cells and eukaryotic cells.
A prokaryotic cell possesses a simple structure with no nuclear envelope and usually a single circular chromosome. A eukaryotic cell possesses a more complex structure, with a nucleus and multiple linear chromosomes consisting of DNA complexed to histone proteins.
Cell reproduction requires a copy of the genetic material, separation of the copy, and cell division. In a prokaryotic cell, the single chromosome replicates, each copy moves toward opposite sides of the cell, and the cell divides. In eukaryotic cells, reproduction requires mitosis and meiosis to ensure that a complete set of genetic information is transferred to each new cell. In eukaryotic cells, chromosomes are typically found in homologous pairs.
Functional chromosomes consist of a centromeres, telomeres, and multiple origins of replication. The centromere is the point at which the kinetochore assembles and to which microtubules attach. Telomeres are the stable ends of chromosomes. After a chromosome is copied, the two copies remain attached to the centromere, forming sister chromatids.
The cell cycle consists of the stages through which a eukaryotic cell passes between cell divisions. It consists of: G-subzero, Interphase, G-subone, A-phase, G-subtwo, M-phase, Prophase, Prometaphase, Metaphase, Anaphase, Telophase, and Cytokinesis.
"The cell cycle governs the entire life of a cell, including growth, division and death. There are defined phases of the cell cycle, during which the cell grows, duplicates its DNA and eventually divides into two new cells (http://sciencing.com/cell-cycle-20206.html)."
At the end of the cell cycle the chromosomes line up in the center of the cell. Sister chromatids separate and become independent chromosomes, which then migrate to opposite ends of the cell. The nuclear membrane reforms around chromosomes at each end of the cell, and the cytoplasm divides.
Mitosis results in the production of two genetically identical cells. Progression through the cell cycle is controlled by interactions between cyclins and cyclin-dependent kinases.
Sexual reproduction produces genetically variable progeny and allows for accelerated evolution. It includes meiosis, in which haploid sex cells are produced, and fertilization, the fusion of sex cells.
The major events of meiosis include: Meiosis one, Prophase one, Metaphase one, Anaphase one, Telophase one, Cytokinesis, Meiosis two, Prophase two, Metaphase two, Anaphase two, Telophase two, and Cytokinesis.
Genetic variation in meiosis is produced by crossing over and by the random distribution of maternal and paternal chromosomes. The result of meiosis is the production of four haploid cells that are genetically variable.
"Genetic variation can refer to differences between individuals or to differences between populations. Mutation is the ultimate source of genetic variation, but mechanisms such as sexual reproduction and genetic drift contribute to it as well (https://geneed.nlm.nih.gov/topic_subtopic.php?tid=48&sid=50)."
Grasping mitosis and meiosis requires more than simply memorizing the sequence events that take place in each stage, although these events are important. The key is to understand how genetic information is apportioned in the course of cell reproduction through a dynamic interplay of DNA synthesis, chromosome movement, and cell division. These processes bring about the transmission of genetic information and are the basis of similarities and differences between parents and progeny.
Subscribe to:
Posts (Atom)