Cell division results in genetically identical daughter cells
- The continuity of life is based on the reproduction of cells or cell division.
- The cell division process is an integral part of the cell cycle, the life of a cell from the time it is first formed from a dividing parent cell until its own division into two cells.
Cellular Organization of the Genetic Material
A cell’s endowment of DNA, its genetic information is called its genome.
Before the cell can divide to form genetically identical daughter cells, all of the DNA must be copied and then two copies separated so that each daughter cell ends up with a complete genome.
The replication and distribution of DNA is manageable because the DNA molecules are packaged into chromosomes.
The nuclei of a human somatic cell (all body cells except the reproductive cells) each contain 46 chromosomes made up of two sets of 23, one set inherited from each parent.
Reproductive cells or gametes-sperm and eggs-have half as many chromosomes as somatic cells, or only one set of 23 chromosomes.
Eukaryotic chromosomes are made of chromatin, a complex of DNA and associated protein molecules.
Distribution of Chromosomes During Eukaryotic Cell Division
After DNA duplication, the chromosomes condense: Each chromatin fiber becomes densely coiled and folded, making the chromosomes much shorter and thick.
Each duplicated chromosome has two sister chromatids. The two chromatids, each containing an identical DNA molecule, are initially attached along their lengths by adhesive protein complexes called cohesins. This attachment is known as the sister chromatid cohesion.
The duplicated chromosome has a narrow waist at the centromere, a specialized region where the two chromatids are most closely attached.
Later in the cell division process, the two sister chromatids of each duplicated chromosome separate and move into two new nuclei, one forming at each end of the cell.
Mitosis, the division of the nucleus is usually founded immediately by cytokinesis, the division of the cytoplasm.
You produce gametes by a variation of cell division called meiosis, which yields nonidentical daughter cells that have only one set of chromosomes.
The mitotic phase alternates with interphase in the cell cycle
Phases of the Cell Cycle
The mitotic phase (M) phase, which includes both mitosis and cytokinesis, is usually the shortest part of the cell cycle.
Mitotic cell division alternates with a much longer stage called interphase, which often accounts for about 90% of the cell. It is during interphase that the cell grows and copies its chromosomes in preparation for cell division.
Interphase can be divided into subphases:
- G1 phase (“first gap”);
- S phase (“synthesis”);
- G2 phase (“second gap”)
Mitosis is conventionally broken down into five stages: prophase, prometaphase, metaphase, anaphase, and telophase.
The Mitotic Spindle
Many of the events in mitosis depend on the mitotic spindle, which begins to form in the cytoplasm during prophase. This structure consists of fibers made from microtubules and associated proteins.
In animal cells, the assembly of spindle microtubules starts at the centrosome, a subcellular region containing material that functions throughout the cell cycle to organize the cell’s microtubules.
An aster, a radial array of short microtubules. The spindle includes the centrosomes, the spindle microtubules, and the asters.
Each of the two sister chromatids of a replicated chromosome has a kinetochore, a structure of proteins associated with specific sections of chromosomal DNA at the centromere.
During prometaphase, the spindle microtubules attach to the kinetochores which then moves the chromosomes toward the pole from which those microtubules extend.
At metaphase, the centromeres of all the duplicated chromosome are on a plane midway between the spindle’s two poles. This plane is called the metaphase plate.
Cytokinesis occurs by a process known as cleavage. The first sign of cleavage is the appearance of a cleavage furrow.
The contractile ring of actin microfilaments act as drawstrings. The cleavage furrow deepens, until the parent cell is split in two, creating two daughter cells.
In plant cells, vesicles from the Golgi apparatus move along microtubules to the middle of the cell, where they coalesce, producing a cell plate.
The asexual reproduction of single-celled eukaryotes includes mitosis and occurs by a type of cell division called binary fission, meaning “division in half”.
Prokaryotes also reproduce by binary fission, but the prokaryotic process does not involve mitosis.
In E. coli, the process of cell division is initiated when the DNA of the bacterial chromosome called the origin of replication, producing two origins.
The origin replicates while the other origin moves to the opposite end of the cell. The cell elongates and replication finishes and a new cell wall is deposited, which in result creates tow daughter cells.
The Evolution of Mitosis
Since prokaryotes evolved before eukaryotes, mitosis may have evolved from binary fission.
Certain protists exhibit types of cell division that seem intermediate between binary fission and mitosis.
The eukaryotic cell cycle is regulated by a molecular control system
Evidence for Cytoplasmic Signals
- Hypothesis: The cell cycle is driven by specific signaling molecules present in the cytoplasm
- Evidence comes from an experiment where they induced cultured mammalian cells at different phases of the cell cycle to fuse.
The Cell Cycle Control System
The sequential events of the cell cycle are directed by a distinct cell cycle control system, a cyclically operating set of molecules in the cell that both triggers and coordinates key events in the cell cycle.
A checkpoint in the cell cycle is a control point where stop and go-ahead signals can regulate the cycle (using signal transduction pathways).
If a cell receives a go-ahead signal at the G1 checkpoint, it will usually complete the G1, S, G2, and M phases and divide.
If it does not receive a go-ahead signal at that point, it will exit the cycle, switching into a nondividing state called the G0 phase.
The Cell Cycle Clock: Cyclins and Cyclin Dependent Kinases
Rhythmic fluctuations in the abundance and activity if cell cycle control molecules pace the sequential events of the cell cycle. These regulatory molecules are mainly proteins of two types: protein kinases and cyclins.
Many of the kinases that drive the cell cycle are actually present at a constant concentration in the growing cell, but much of the time they are in inactive form.
To be active, such a kinase must be attached to a cyclin, a protein that gets its name from its cyclically fluctuating concentration in the cell.
Because of this requirement, these kinases are called cyclin-dependent kinases, or Cdks.
The activity of a Cdk fluctuates with changes in the concentration of its cyclin partner.
MPF (maturation-promoting factor) is a cyclin-Cdk complex that triggers a cell’s passage past the G2 checkpoint into the M phase
Stop and Go Signs: Internal and External signals at the Checkpoints
An example of an internal signal is that kinetochores not attached to spindle microtubules send a molecular signal that delays anaphase
A growth factor is a protein released by certain cells that stimulates other cells to divide.
Different cell types respond specifically to different growth factors or combinations of growth factors. For example, platelet-derived frpwth factor stimulates the division of a human.
The effect of an external physical factor on cell division is clearly seen in density-dependent inhibition, a phenomenon in which crowded cells stop dividing.
Most animal cells also exhibit anchorage dependence. To divide the must be attached to a substratum, such as the inside of a culture jar or the extracellular matrix of a tissue.
Loss of Cell Cycle Controls in Cancer Cells
Cancer cells do not heed the normal signals that regulate the cell cycle. They divide excessively and invade other tissues. In addition to their lack of density-dependent inhibition and anchorage dependence, cancer cells do not stop dividing when growth factors are depleted.
A logical hypothesis is that cancer cells do not need growth factors in their culture medium to grow and divide.
The problem of cancer begins when a single cell in a tissue undergoes transformation, the process that converts a normal cell to a cancer cell. The body’s immune system normally recognizes a transformed cell as an insurgent and destroys it.
If the abnormal cells remain at the original site, the lump is called a benign tumor.
In contrast, a malignant tumor becomes invasive enough to impair the function of one or more organs. These tumors can proliferate and spread to locations distant from their original site in a process called metastasis.
Cite this essay
The Cell Cycle. (2017, Jan 20). Retrieved from https://studymoose.com/the-cell-cycle-essay