This page appears in the following eBook. Aa Aa Aa. Walther Flemming's drawing of chromosomes. What happens during mitosis? Figure 1: During prophase, the chromosomes in a cell's nucleus condense to the point that they can be viewed using a light microscope. Prophase is the first phase of mitosis. During this phase, the chromosomes inside the cell's nucleus condense and form tight structures.
In fact, the chromosomes become so dense that they appear as curvy, dark lines when viewed under a microscope Figure 1. Because each chromosome was duplicated during S phase, it now consists of two identical copies called sister chromatids that are attached at a common center point called the centromere. Figure 2: The mitotic spindle white begins to form outside the cell's nucleus. Important changes also take place outside of the nucleus during prophase.
In particular, two structures called centrosomes move to opposite sides of the cell during this phase and begin building the mitotic spindle. The mitotic spindle plays a critical role during the later phases of mitosis as it orchestrates the movement of sister chromatids to opposite poles of the cell Figure 2.
After prophase is complete, the cell enters prometaphase. During prometaphase, the nuclear membrane disintegrates and the mitotic spindle gains access to the chromosomes. During this phase, a protein structure called the kinetochore is associated with the centromere on each sister chromatid. Stringlike structures called microtubules grow out from the spindle and connect to the sister chromatids at their kinetochores; one microtubule from one side of the spindle attaches to one sister chromatid in each chromosome, and one microtubule from the other side of the spindle attaches to the other sister chromatid Figure 3a.
Figure 3: a Metaphase and b Anaphase. In metaphase a , the microtubules of the spindle white have attached and the chromosomes have lined up on the metaphase plate. During anaphase b , the sister chromatids are pulled apart and move toward opposite poles of the cell. The nuclear membrane forms again and the cell body splits into two cytokinesis.
The various events of the cell cycle are tightly regulated. If errors occur at any one stage, the cell can stop cell division from progressing. Such regulatory mechanisms are known as cell cycle checkpoints, according to Cooper. There are three checkpoints within the G1, G2 and M phases. Damaged DNA stops cell cycle progression in the G1 phase, ensuring that an aberrant cell will not be replicated.
The G2 checkpoint responds to incorrectly duplicated, or damaged DNA. It prevents cells from moving into the M phase until the DNA is replicated correctly, or until the damage is repaired. The M phase checkpoint can halt the cell cycle in metaphase. It ensures that all the sister chromatids are properly hooked up to the mitotic spindle and that sister chromatids move towards opposite ends of the cell.
That's usually a good thing. Sometimes, abnormal cells manage not only to survive, but also to proliferate. Most often, these cells are implicated in cancer. And that's going to start pushing the cell cycle forward when it shouldn't be going forward," Hoyt said. The relationship between the cell cycle and cancer has led to the development of a class of cancer drugs that specifically target cancer cells during mitosis.
For example, microtubule poisons stop mitosis by targeting microtubules , the main component of the mitotic spindle. Damaging these thin, hollow, microscopic protein filaments ultimately prevents sister chromatids from being pulled apart. Examples of microtubule poisons are the medications paclitaxel Taxol and vinca alkaloids , which are used to treat a range of cancers, including certain ovarian and breast cancers.
However, microtubule poisons are not without their limitations. Organisms that reproduce sexually have two copies of each chromosome, one from their father and one from their mother.
A special form of cell division needed to produce sex cells - for example, sperm and eggs with only one copy of each chromosome. Main menu About this Site Table of Contents.
Growth, Development, and Reproduction. NGSS Performance Expectations: MS-LS Develop and use a model to describe why asexual reproduction results in offspring with identical genetic information and sexual reproduction results in offspring with genetic variation. The content and activities in this topic will work towards building an understanding of how aquatic plants and algae grow, develop, and reproduce.
Chromosomes are duplicated. Meiosis begins in a fashion similar to mitosis with chromosome replication. Matched sets of chromosomes pair together. Genes are swapped between matched chromosomes. The process of crossing over, or recombination, exchanges genetic information between chromosomes in a cell. The resulting chromosomes are brand new, unique combinations of genetic information. First division separates one of each chromosome pair.
The parent cell divides in half as in mitosis, producing two cells with a complete amount of DNA although they are not identical because of crossing over. Second division separates each chromosome, leaving one copy of each chromosome per cell. The two new cells divide a second time to produce four new gametes.
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