Cells can also take a break from the grind of the cell cycle, in a state called G0 or senescence (note that some cells are permanently in G0). The cell cycle is generally described as consisting of four main phases: G1, S phase, G2 and mitosis (or meiosis). Just like your day has a routine from day to night, cells have routines of their own. Ĭell division occurs as a part of the “cell cycle”. Oskar Hertwig described the fusion of egg and sperm in the transparent sea urchin egg in 1876. Walther Flemming in his 1882 work “Cell substance, nucleus and cell division.” This refers to the outcome of meiosis, where the genetic information in each new cell is halved. Mitosis is the Greek word for thread, after the thread-like chromosomes that can be seen under the microscope in dye-stained cells during cell division. Another example is Klinefelter syndrome, where XY males have an additional X chromosome. One example is Down’s syndrome, caused by trisomy 21. ![]() This can trigger miscarriage, but is occasionally tolerated. Uncontrolled mitosis occurs in cancer, where either genes that stop cell division ( tumour suppressors) are switched off, or genes that encourage cell division ( oncogenes) are overactive.Įrrors in meiosis can lead to the wrong number of chromosomes ending up in germ cells, this is called aneuploidy. What is an example of a disease caused by an error in this process? ![]() Other examples are 1-2 days in male fruit flies and ~ 24 days in human males. The process lasts 6 hours in yeast but can last more than 40 years in human females, due to a developmental hold at prophase I, until ovulation. Meiosis has various timescales in different organisms, which can be affected by several factors including temperature and environment of the organism, and the amount of nuclear DNA. The process can take over 10 hours for mammalian cells in culture, budding yeast can take ~80 minutes to complete a cell cycle, whilst bacteria can divide every 20 minutes. However, there are exceptions buddingyeast will form haploid spores under nutritional deprivation. Archaeaand bacteria do not do this, so it might be tempting to think that unicellularorganisms do not sexually reproduce. Only organisms which perform sexual reproduction. Mitosis is performed by unicellular and multicellular eukaryotes.Bacteria have their own version of mitosis called “binary fission”.This is distinct from meiosis as bacteria typically have one circular chromosome,which is not contained within a nucleus, like eukaryotic chromosomes. Two diploid cells with identical genetic information.įour haploid cells with different genetic information. Various steps in meiosis create opportunity for genetic diversity in the daughter cells. To create gametes with only one copy of the organism’s genetic information, in preparation for sexual reproduction. In a multicellular organism, the purpose can be to grow during development, or to repair or regenerate In a unicellular organism, the purpose of mitosis is to proliferate asa species. Meiosis: Overview and commonly asked questions ![]() We will also explore the research into these processes and how cell division might go awry to cause disease states such as cancer and Down’s Syndrome. This article will explore the characteristics of both kinds of cell division, shining a light on how they are similar and in which aspects they are crucially distinct. In the process of replicating themselves, cells have another choice: do they want to make an identical copy and be left with two cells? Or do they want to make four “half-copies”, in preparation for sexual reproduction, where their genetic content will be made whole again by the process of fertilisation? This choice is the choice between mitosis and meiosis. ![]() Other cells, such as neurons, will never divide again once they are terminally differentiated they are post-mitotic. In humans, tissues such as the skin and blood contain cells that are actively dividing, whilst other tissues such as fat contain cells that expand (good if you need energy for winter, bad if you are trying to fit into some expensive jeans). In order for organisms to grow, cells have two options: they must either replicate themselves to create more cells, or the cells themselves must expand in volume.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |