The Cell Cycle
A cell spend's the majority of its life in interphase. Interphase prepares the cell for the mitotic phase. There are three stages of interphase. The G1 phase, the S phase, and the G2 phase. In the G1 phase, the cell grows in size, copies its organelles, and creates the molecular building blocks that it will need to use later. In the S phase, the cell synthesizes a complete copy of the DNA. In the G2 phase, the cell grows even more, makes proteins and organelles, and beings to reorganize its contents in preparation for mitosis.
In the prophase, the cell's chromosomes start to condense. Then, the mitotic spindle begins to form. The spindle is made from microtubules, and it will organize the chromosomes later. The nucleolus disappears, meaning that the nucleus is getting ready to break down. The nuclear envelope breaks down, and the chromosomes are released. Some of the chromosomes are captured by the spindles.
By this time, the spindle has captured all of the chromosomes and lined them up at the middle of the cell.The cell will then check that all the chromosomes are at the metaphase plate with their kinetochores correctly attached to microtubules. This is called the spindle checkpoint and helps ensure that the sister chromatids will split evenly between the two daughter cells when they separate in the next step. If a chromosome is not properly aligned or attached, the cell will halt division until the problem is fixed.
In anaphase, the sister chromatids separate from each other. The protein "glue" is broken down, and each is now its own chromosome. The chromosomes of each pair are pulled towards the opposite ends of the cell. The microtubules not attached to chromosomes elongate and push apart, separating the poles and making the cell longer. All of these processes are driven by motor proteins, molecular machines that can “walk” along microtubule tracks and carry a cargo. In mitosis, motor proteins carry chromosomes or other microtubules as they walk.
In telophase, the cell is nearly done dividing, and it starts to re-establish its normal structures as cytokinesis takes place. The mitotic spindle is broken down into its building blocks. Two new nuclei form, one for each set of chromosomes. Nuclear membranes and nucleoli reappear. The chromosomes begin to return to their “stringy” form. Cytokinesis, the division of the cytoplasm to form two new cells, overlaps with the final stages of mitosis. It may start in either anaphase or telophase, depending on the cell, and finishes shortly after telophase.
The telomeres are repetitive regions at the very ends of chromosomes. Telomeres act as caps that protect the internal regions of the chromosomes, and they're worn down a small amount in each round of DNA replication. Telomeres consist of hundreds or thousands of repeats of the same short DNA sequence, which varies between organisms but is 5'-TTAGGG-3' in humans and other mammals. The repeats that make up a telomere are eaten away slowly over many division cycles, providing a buffer that protects the internal chromosome regions bearing the genes. Telomere shortening has been connected to the aging of cells, and the progressive loss of telomeres may explain why cells can only divide a certain number of times.
Cancer begins with a mutation. A neoplasm forms, which is an accumulation of cells that lost control over how fast they grow and divide. A neoplasm that forms a lump in the body is called a tumor. The gene affected by the tumor is called an oncogene. Certain neoplasms become malignant. They get progressively worse, because they spread to other cells. The process by which malignant cells break free from their home tissue and invade other parts of the body is called metastasis. The disease cancer occurs when the abnormally dividing cells of a neoplasm disrupts body tissues, both physically and metabolically. Screenings can detect cancerous cells. Early detection can remove malignant neoplasms before metastasis occurs.
Biology: Concepts and Applications