Understandings |
Applications & Skills |
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Mitosis is the splitting of the nucleus into two identical daughter nuclei
Chromosomes condense by supercoiling during mitosis Cytokinesis occurs after mitosis and is different in plant and animal cells Interphase is a very active phase of the cell cycle with many processes occurring in the nucleus and cytoplasm. Cyclins are involved in the control of the cell cycle Mutagens, oncogenes, and metastasis are involved in the development of primary and secondary tumors |
App: The correlation between smoking and cancer incidence
Skill: Identify the phases of mitosis in electron micrographs Skill: Determination of a mitotic index from a micrograph |
Image courtesy of shmoop.com.
The cell cycle describes the behavior of cells as they grow and divide. In most cases, the cell produces two cells that are genetically identical to the original. Cell division as a whole used to be called mitosis before more advanced cellular research determined that there are a number of significant nuclear processes that take place before the cell actually undergoes mitosis (called interphase). Mitosis itself has four phases: prophase, metaphase, anaphase, and telophase (PMAT).
The cell cycle describes the behavior of cells as they grow and divide. In most cases, the cell produces two cells that are genetically identical to the original. Cell division as a whole used to be called mitosis before more advanced cellular research determined that there are a number of significant nuclear processes that take place before the cell actually undergoes mitosis (called interphase). Mitosis itself has four phases: prophase, metaphase, anaphase, and telophase (PMAT).
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Interphase. This is the longest phase in the cell cycle. It is comprised of three smaller phases: G1, S and G2. In G1, the major event is cell growth. At this point, the cell is as small as it will ever be. In S, the major event is the replication of DNA. In G2, the cell actually grows and prepares for mitosis. Organelles increase in number and DNA condenses from chromatin to chromosomes and microtubules form. Special proteins called cyclins control the cell's progress through the cell cycle. Cyclins (there are two; one for each checkpoint) bond to substances called CDKs (cyclin-dependent protein kineases) that allow them to act as enzymes. Cyclins were discovered by accident. The first cyclin triggers DNA replication at the G1 checkpoint. The second cyclin triggers mitosis at the G2 checkpoint.
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Prophase. During prophase (to the left), the nuclear envelope disappears and chromatin (loose DNA) coils to form chromosomes. Spindle fibers form and centrosomes move to the opposite ends of the cell.
Metaphase. Chromosomes move to the center of the cell--called the metaphase plate (seen below). The centromeres lie on the equator of the plate. Each half of the chromosome is called sister chromatids. The spindle fibers move the chromosomes and the centrosomes are at opposite ends of the cell. |
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Anaphase. This is the shortest phase (seen below). The sister chromatids split and the chromosomes move towards opposite sides of the cell because the microtubules are shortening. Centromeres move first. At the end of this phase, each side (or pole) of the cell has an identical set of chromosomes.
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Telophase. The chromosomes are now at the poles of the cell and the nuclear envelope re-forms (seen below). The chromosomes uncoil to become chromatin once more. Nucleoli reappears, spindle fibers disappear, and the cell is now ready to undergo cytokinesis.
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Cytokinesis. There is an inward pinching of the plasma membrane to form cleavage. In plant cells, due to the existence of the cell wall, a cell plate forms across the center of the cell. At the end of cytokinesis, there are two identical daughter cells. See below.
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Chromatin condenses to form chromosomes through a process called supercoiling. The DNA wraps around histones (alkaline proteins ) to produce nucleosomes. Nucleosomes coil to form solenoids. Solenoids coil to form looped domains, and looped domains coil once more to form a chromosome.
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