Facultative dividers enter the G 0 phase but retain the capacity to re-enter the cell cycle when suitably stimulated. Stem cells in some tissues progress continually through the cell cycle to accommodate tissue growth or cell turnover, whereas terminally differentiated cells leave the cell cycle after the M phase and enter a state of continuous differentiated function designated as G 0 phase. The interval between the end of the S phase and the beginning of the M phase, the second gap or G 2 phase, is relatively short and is the period in which cells prepare for mitotic division. During the G 1 phase, cells differentiate and perform their specialised functions as part of the whole tissue. Between the end of the M phase and the beginning of the S phase is the first gap or G 1 phase this is usually much longer than the other phases of the cell cycle. Thus interphase may be divided into three separate phases. However, there is a discrete period during interphase when nuclear DNA is replicated this phase, described as the synthesis or S phase, is completed some time before the onset of mitosis. Historically, only two phases of the cell cycle were recognised: a relatively short mitotic phase ( M phase) and a non-dividing phase ( interphase), which usually occupies most of the life cycle of the cell. In these circumstances, cell death occurs by a mechanism known as apoptosis (see Fig. Cell division is tightly controlled to meet the needs of the organism uncontrolled cell division is one of the features of cancer.Ĭell division and differentiation are balanced by cell death, both during the development and growth of the immature organism and in the mature adult. Between these extremes are cells such as liver cells that do not normally divide but retain the capacity to undergo mitosis should the need arise ( facultative dividers). In contrast, the cells of certain other tissues, such as the stem cells of gut and skin, undergo continuous cycles of mitotic division throughout the lifespan of the organism, replacing cells lost during normal wear and tear. In the fully developed organism, the terminally differentiated cells of some tissues, such as the neurones of the nervous system, lose the ability to undergo mitosis. All body cells divide by mitosis except for male and female germ cells, which divide by meiosis to produce gametes (see Fig. The interval between mitotic divisions is known as the cell cycle. Most tissues, however, retain a population of relatively undifferentiated cells ( stem cells) that are able to divide and replace the differentiated cell population as required.
Prophase cell cycle skin#
Some of these daughter cells progressively specialise and eventually produce the terminally differentiated cells of mature tissues, such as muscle or skin cells. The fertilised egg ( zygote) divides by a process known as mitosis to produce two genetically identical daughter cells, each of which divides to produce two more daughter cells and so on.
![prophase cell cycle prophase cell cycle](https://www.mypathologyreport.ca/wp-content/uploads/2020/11/stages-of-mitosis-3000x1930.jpg)
![prophase cell cycle prophase cell cycle](https://pressbooks.bccampus.ca/conceptsofbiologygunness/wp-content/uploads/sites/34/2016/05/Figure_06_02_02-1024x8002.jpg)
![prophase cell cycle prophase cell cycle](https://i.pinimg.com/originals/93/6c/1e/936c1e5d2ca5dac04d367c3be7a5f9b2.png)
The development of a single fertilised egg cell to form a complex multicellular organism involves cellular replication, growth and progressive specialisation ( differentiation) for a variety of functions.