- Demonstrate an understanding of the cell cycle and the process and importance of mitosis.
- Perform laboratory investigations to study the processes of mitosis.
- Organize data that illustrate the number of chromosomes in various cells.
The Cell CycleEdit
Cells reproduce through a continuous sequence of growth and division known as the cell cycle. In the growth stage, called interphase, the cell makes new molecules increasing the volume and mass of the cell.
During the first part of interphase, called gap 1 (G1), the cell carries out metabolic activities to prepare for the synthesis phase (S phase) during which DNA is replicated. Once a cell has completed G1 phase it can progress to S phase or enter a rest phase. Cells that enter the rest phase still function but do not progress through the cell cycle.
After S phase the cell enters gap 2 phase (G2). In this phase the cell prepares to divide.
- G1 Phase: Rapid growth and metabolic activity
- S Phase: DNA synthesis and replication
- G2 Phase: Centrioles replicate, cell prepares for division
The timing of the cell cycle and the lengths of the phases depend on the type of cell and its environment.
What is the Function of Mitosis?Edit
The process of mitosis allows organisms to grow, repair, and maintain their functions. Mitosis can form new cells to repair damaged tissue and in some cases regrow entire body parts. It is also important in replacing dead or improperly functioning cells.
Mitosis occurs in all somatic (body) cells. New cells produced for growth or repair are identical to the previously existing cells. Chromosomes within the nucleus hold the genetic information needed to maintain the cell and produce new copies of the cell. Each chromosome is made up of two sister chromatids which are held together by a centromere. Sister chromatids are genetic copies of one another.
Mitosis must maintain the same number of chromosomes from cell to cell. During cell division the original parent cell divides to produce two daughter cells. Mitosis ensures the daughter cells have the same number of chromosomes and genetic code as the parent cell. Before mitosis a parent somatic cell in a human has 46 chromosomes and each daughter cell has the same number.
The Phases of MitosisEdit
Although the process of mitosis is continuous, for convenience they are divided into four stages.
During prophase, chromatin, which is made of DNA and proteins, condenses and thickens to form visible duplicated chromosomes. At this stage, each chromosome, having been replicated during S phase, is X-shaped. Each half of the X is one copy of the original chromosome (chromatid), but together are considered one chromosome since they are still connected.
The nuclear membrane and nucleolus disappear. Centrioles made of microtubules move to opposite poles of the cell. Spindle fibres, also made of microtubules, start to form between the two centrioles.
During metaphase the spindle fibres attach to the centromeres of the chromosomes. The chromatids are guided by the spindle fibres to the middle of the cell, also called the cell's equator. Each centriole pair attaches one spindle fibre to each chromosome on opposite chromatids. The chromosomes are arranged in a single-file line in the middle of the cell.
During anaphase the mircotubules in the spindle fibres shorten, pulling the chromatids apart and splitting the centromeres. Chromatids are pulled to opposite poles of the cell by the spindle fibres.
Telophase begins when the chromatids have reached the two opposite poles in the cell. The chromatids are now single non-replicated chromosomes and begin to unwind.
The separation of the cytoplasm and formation of two new daughter cells is called cytokinesis. The spindle fibres are no longer needed so they break down and disappear. The nucleolus reappears and a nuclear membrane forms around each new set of chromosomes located at opposite poles of the cell. The cytoplasm divides between the two halves and new cell membranes are formed. In plant cells a cell wall is also formed and separates the two newly formed nuclei. After cytokinesis two daughter cells have been formed.
Errors in MitosisEdit
A serious error can occur if the mitotic process is disrupted by mutations. Mutations can be caused by various mutagens, such as toxic compounds, radiation, or viruses. Mutations cause a permanent change in the DNA molecule. For example certain chemical compounds in cigarette smoke can alter the structure of chromosomes in parent cells and this mutation is passed on to daughter cells.
Certain genes work like switches to regulate the rate of mitosis. If these genes are altered by a mutation, the rate of mitosis will also be affected. For example, once a cell has completed its cell cycle, certain genes are activated which proteins that stop the process of mitosis. When the genes are deactivated mitosis can continue. A mutation could permanently deactivate these genes and the cell could begin to divide uncontrollably. The rate of mitosis can also be regulated through genes that start mitosis. A mutation causing these genes to be permanently activated would also result in uncontrollable cell division. Genes that can be activated by a mutation are known as oncogenes.
Retinoblastoma is a retinal cancer that is either inherited or results from a mutation to both copies of the retinoblastoma gene. This mutation results in the formation of a tumour on the retina of the eye. Wilms tumour is a cancer that can develop as a result of a mutation to the Wilms tumour gene in a kidney cell. Breast cancer is another example of a mutation to both breast cancer 1 (BRCA1) genes.