Download cell cycle - user web page

CELL CYCLE
Margaret Modupe Kajotoni
March 26, 2007
The cell is the basic unit of life. All organisms are made up of at least one cell. Most
cells are very small and invisible without using a microscope.There are two main types or of
cells: prokaryotic cells and eukaryotic cells. All cells are surrounded by a plasma membrane,
which is made of a double layer (a bilayer) of phospholipids. Within this membrane, is the
cytoplasm which is composed of the fluid and organelles of the cell. The contents of a cell
are called the protoplasm. Cells are small compartments that hold all of the biological equipment necessary to keep an organism alive and successful on Earth. The nucleus is a membrane
bound organelle that contains the genetic information in the form of chromatin, highly folded
ribbon-like complexes of DNA (deoxyribonucleic) acid and a class of proteins called histones.
DNA (Deoxyribonucleic) acid is a nucleic acid that contains the genetic instructions for the
development and functioning of living organisms. Ribosomes are special organelles that are
directly involved in protein synthesis. They are made of RNA (ribonucleic acid).
The series of events involving the growth, replication, and division of a eukaryotic cell is known
as the cell cycle. The cell cycle consists of two major phases: M phase and the interphase.
During the interphase peroid, the cell is constantly synthesising RNA, producing protein and
growing in size.The interphase is divided into stages: Gap 0 (G0), Gap 1 (G1), S (synthesis)
phase, Gap 2 (G2).Cells that have temporarily or reversibly stopped dividing are said to have
entered the G0 phase. Some cell types in mature organisms, such as parenchymal cells of the
kidney, enter the G0 phase and can only be induced to begin dividing again under very specific
circumstances. Cells increase in size in Gap 1(G1), produce RNA and synthesise protein. An
important cell cycle control mechanism activated during this period (G1 Checkpoint) ensures
that everything is ready for DNA synthesis. DNA replication occurs during the S (synthesis)
phase to produce two similar daughter cells. During the gap between DNA synthesis and mitosis, the cell will continue to grow and produce new proteins. At the end of this gap is another
control checkpoint (G2 Checkpoint) to determine if the cell can now proceed to enter M phase
and divide.
M phase is when nuclear chromosomes divides to produce two identical daughter cells and
cytoplasm(cytokinesis) physically divides.The process of mitosis is designed to insure that exact copies of the DNA in chromosomes are passed on to daughter cells. Mitosis is further
divided into 4 phases: prophase, prometaphase, metaphase, anaphase, and telophase. During
1
the prophase, the chromatin, diffuse in interphase, condenses into chromosomes. Each chromosome has duplicated and now consists of two sister chromatids.At the end of prophase, the
nuclear envelope breaks down into vesicles. During the metaphase, the chromosomes align at
the equitorial plate and are held in place by microtubules attached to the mitotic spindle and to
part of the centromere.At the anaphase, the centromeres divides and sister chromatids separate
and move toward the corresponding poles. Daughter chromosomes arrive at the poles and the
microtubules disappear during the telophase. The condensed chromatin expands and the nuclear envelope reappears. The cytoplasm divides, the cell membrane pinches inward producing
two daughter cells (phase: Cytokinesis).The final step of cell division in which the membrane
cleaves and there are two separate cells. The original cell has stopped dividing.The cell cycle
is as shown in figure 1 and figure 2.
Figure 1: The cell cycle
The importance of cell division in any organism cannot be overemphazid. However, when
it is not regulated, it leads to a disease called cancer.The molecular events that regulates the
cell cycle occurs in a sequential fashion and it is impossible to ”reverse” the cycle. However,
the passage of a cell through the cell cycle is controlled by proteins in the cytoplasm.There are
two key classes of regulatory molecules that determine a cell’s progress through the cell cycle:
cyclins and cyclin-dependent kinases. Leland H. Hartwell, R. Timothy Hunt, and Paul M.
Nurse won the 2001 Nobel Prize in Physiology or Medicine for their discovery of these central
molecules in the regulation of the cell cycle.
References
http : //www.cellsalive.com/cellc ycle.html
http : //en.wikipedia.org/wiki/Cellc ycle
http : //users.rcn.com/jkimball.ma.ultranet/BiologyP ages/C/CellCycle.html
http : //www.cancerquest.org/index.cf m?page = 58
2
Figure 2: Mitosis
3