Minggu, 28 Oktober 2012

Cell Cycle

The cell cycle, or cell-division cycle, is the series of events of Cell division and duplication (replication).

Cell division
is the process by which a parent cell divides into two or more daughter cells. Cell division is usually a small segment of a larger cell
cycle. This type of cell division in eukaryotes is known as mitosis.

In cells without a nucleus (prokaryotic), the cell cycle occurs via a process termed binary fission or a mitosis ( pembelahan biner)
. In cells with a nucleus (eukaryotes), the cell cycle can be divided in two periods:
1. interphase
2. the Mitosis (M) phase

The Interphase
Typically, this phase accounts for 90% of the cell cycle. It is a time of high metabolic activity. The cell grows by producing proteins and organelles,
and chromosomes are replicated.
The Interphase consists of three phases:
1. G1 phase ( gap1)
2. S phase (synthesis),
3. G2 phase ( gap 2)
The first phase within interphase , from the end of the previous M phase until the beginning of DNA synthesis is called G1 (G indicating gap) . It is
also called the growth phase
The S phase starts when DNA synthesis (or replication).
At the beginning of the phase, each chromosome is single. At the end, after DNA replication, each chromosome consists of two sister chromatids. ; when
it is complete, all of the chromosomes have been replicated
The cell then enters the G2 phase, which lasts until the cell enters mitosis.
G2 phase is a period of metabolic activity and growth. During this phase the cell makes final preparations for division.
is the process where one cell splits off from its sister cell. It usually occurs after cell division. The Cell Cycle is the sequence of growth, DNA
replication, growth and cell division that all cells go through. Beginning after cytokinesis, the daughter cells are quite small and low on ATP. They
acquire ATP and increase in size during the G1 phase of Interphase. Most cells are observed in Interphase, the longest part of the cell cycle. After
acquiring sufficient size and ATP, the cells then undergo DNA Synthesis (replication of the original DNA molecules, making identical copies, one "new
molecule" eventually destined for each new cell) which occurs during the S phase. Since the formation of new DNA is an energy draining process, the
cell undergoes a second growth and energy acquisition stage, the G2 phase. The energy acquired during G2 is used in cell division (in this case
Regulation of the cell cycle is accomplished in several ways. Some cells divide rapidly (beans, for example take 19 hours for the complete cycle; red
blood cells must divide at a rate of 2.5 million per second). Others, such as nerve cells, lose their capability to divide once they reach maturity.
Some cells, such as liver cells, retain but do not normally utilize their capacity for division. Liver cells will divide if part of the liver is
removed. The division continues until the liver reaches its former size.
Cancer cells are those which undergo a series of rapid divisions such that the daughter cells divide before they have reached "functional maturity".
Environmental factors such as changes in temperature and pH, and declining nutrient levels lead to declining cell division rates. When cells stop
dividing, they stop usually at a point late in the G1 phase, the R point (for restriction).

The cell division process that produces new cells for growth, repair, and the general replacement of older cells is called mitosis. In this process, a
somatic cell divides into two complete new cells that are identical to the original one. Human somatic cells go through the 6 phases of mitosis in 1/2
to 1 1/2 hours, depending on the kind of tissue being duplicated.

Six phases of mitosis
Some human somatic cells are frequently replaced by new ones and other cells are rarely duplicated. Hair, skin, fingernails, taste buds, and the
stomach's protective lining are replaced constantly and at a rapid rate throughout our lives. In contrast, brain and nerve cells in the central nervous
system are rarely produced after we are a few months old. Subsequently, if they are destroyed later, the loss is usually permanent, as in the case of
paraplegics. Liver cells usually do not reproduce after an individual has finished growing and are not replaced except when there is an injury. Red
blood cells are also somewhat of an exception. While they are being constantly produced in our bone marrow, the specialized cells from which they come
do not have nuclei nor do the red blood cells themselves.

Before mitosis begins, cells are in the interphase stage, which is the rest period between mitotic divisions. At this time, the chromosomes are
not identifiable but are an undifferentiated loose mass of stretched out DNA molecules within the nucleus. Chromosome replication and other cell
processes are occurring in preparation for mitosis. In addition, cells carry out normal metabolic activities at this time. Most mammal somatic cells,
including those of humans, spend at least 90% of their time in interphase.

At the beginning of the first mitotic stage, prophase, the thread-like doubled chromosomes contract and become visible. The two centrioles move
to opposite sides of the nucleus. At the same time, the nuclear membrane begins to break down.

During metaphase, the nuclear membrane disappears and the chromosomes become aligned half way between the centrioles. The centromere of each
doubled chromosome becomes attached by thread-like spindle fibers to the centrioles which are at polar opposite sides of the cell.

During the following anaphase stage, the chromatids of each chromosome separate at their centromeres and then migrate to the opposite poles of
the cell.

During telophase, two distinct nuclear membranes develop encompassing the two identical sets of chromosomes. At the same time, the cytoplasm
divides between the two new cells, and the cell membrane begins to pinch off the cell contents into two daughter cells.

Finally, the new identical cells separate and once again enter interphase in preparation for a later mitotic division. While the chromosomes are
no longer visible, they will be replicated just before mitosis begins.


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