Positive feedback amplifies the effects of cell activities.
- The hormone oxytocin, for example, increases uterine muscle contractions during labor and childbirth.
- The contraction of the uterine muscles causes the synthesis of even more oxytocin, which causes the contractions of the uterine muscles to grow.
- This positive feedback mechanism causes labor contractions to intensify, becoming increasingly powerful throughout birthing.
The longest phase of the cell cycle is the interphase.
- During interphase, the cell expands to the point where it may divide into two daughter cells.
- During interphase, the cell also duplicates its genetic material (DNA).
Interphase is comprised of the three successive stages as listed below.
- G1—During this stage, the cell expands and prepares for DNA replication, as well as the replication of some cellular organelles (such as centrioles).
- S—DNA replication occurs at the S (synthesis) stage. Each chromosome has one chromatid when the S stage begins. Following the completion of DNA replication, each chromosome has two identical chromatids linked together by a single centromere. The cell has double the quantity of DNA at the end of the S stage as it had at the end of G1.
- G2—During the G2 stage, the cell continues to develop and prepares components for mitosis, such as the proteins that will form the spindle fibers. Prophase, metaphase, anaphase, and telophase are the four phases of mitosis. The nuclear membrane breaks and the chromosomes separate during prophase.
The nuclear membrane melts during prophase, and the chromosomes condense and become visible.
- Spindle fibers begin to develop as well.
- The spindle fibers have entirely connected to the centromeres of each chromosome during metaphase.
- The chromosomes are then arranged in a single column along the cell's "equator."
- The metaphase plate is the core of the mitotic spindle.
- ^^Each chromosome separates at its centromere during anaphase when opposing spindle fibers shorten.^^
- The identical chromatids are drawn to opposing ends of the cell. Each chromatid now has its own centromere and is considered a distinct chromosome at this stage.
%%The cell contains twice as many chromosomes at the conclusion of anaphase as it had at the beginning of the cell cycle.%%
Two new nuclear membranes develop during telophase.
- The two nuclei now have the same number of chromosomes.
- ^^Cytokinesis is the division of the cytoplasm and all of its cellular components into two daughter cells.^^
^^Cytokinesis happens following mitosis.^^
- A cleavage furrow is generated during cytokinesis in animal cells, which divides the cytoplasm and its contents between the two new cells.
- Plant cells have a cell wall and perform cytokinesis in a unique way.
- A cell plate is formed within the dividing cell in plant cells, giving fresh cell wall material for each daughter cell.
Some cells may cease to divide either momentarily or permanently.
- Cells may cease dividing when they reach a mature, completely differentiated state or when environmental circumstances are unfavorable for ongoing division.
- The cell cycle must be properly regulated in order for cells in living creatures to grow, repair, and reproduce.
- During the cell cycle, checkpoints are used to regulate this process.
- Interactions between cyclins and cyclin-dependent kinases regulate several of these checkpoints.
Throughout the cell cycle, cyclin-dependent kinases are present at constant levels.
- ^^These kinases activate other molecules by adding phosphate groups to them.^^
- Cyclin-dependent kinases, on the other hand, are dormant until they bind to cyclin proteins.
Cyclin protein levels fluctuate during the cell cycle, peaking right before mitosis.
- Cells can enter G0 at any point in the cell cycle and can rejoin it if driven by the right chemical signals.
- Cells in tissues, for example, will cease dividing if they get overcrowded.
- Many types of somatic cells are also anchorage-dependent, which means they require attachment to a surface in order to divide.
- %%Cancer cells are not inhibited by density-dependent inhibition or anchorage dependency, and they can continue to grow and divide under settings that would cause properly functioning somatic cells to cease proliferating.%%
- Many genes are also involved in cell cycle control.
- Proto-oncogenes drive cell division at a certain rate, similar to how an accelerator drives a car.
- Proto-oncogenes are required for controlled and regulated cell proliferation.
When proto-oncogenes are altered, they can become oncogenes, which stimulate abnormally high rates of cell proliferation.
- An oncogene functions similarly to a tumor suppressor.
- Somatic body cells are all of the cells in an organism that are not engaged in sexual reproduction.
- Somatic cell division, like that of tissues, can be influenced by density-dependent inhibition.
- BRCA (sometimes known as the "breast cancer gene") is a tumor suppressor gene mutation.
As a person has a BRCA mutation, he or she has a significantly elevated chance of developing certain forms of cancer when compared to someone who does not have the gene.
- ==A person with the mutation in one allele would still have one functioning tumor suppressor allele, thus cancer would not be guaranteed, but it would be more likely than in a person who does not have the mutation and has two functional copies.==
An oncogene works in the same manner as a car's accelerator stuck in the down position causes it to travel too quickly.
- When cell division happens too rapidly and too often without respect for surrounding cells, these oncogenes can cause tumors to arise.
A single allele of a proto-oncogene mutation can cause a cell to develop out of control and create a tumor.
- Because a single allele mutation may cause a cell to develop out of control, proto-oncogenes are said to act dominantly.
- Tumor suppressor genes encode proteins that detect mutations in cells that may lead to the development of cancers.
- Tumor suppressor genes work similarly to the brakes on a car, stopping tumors from forming.
- A tumor may develop if both alleles of a tumor suppressor gene are mutated.
- Tumor suppressor genes are believed to work recessively because a cell cannot develop out of control unless both alleles of a tumor suppressor gene are nonfunctional.
Sometimes the life of a live thing is dependent on certain cells dying and not replicating.
- Apoptosis is the name given to this type of planned cell death.
- When a cell gets a cancer-causing mutation, apoptosis may be triggered.
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