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Chapter 8. How Cells Reproduce Part 2. Mitosis: Prophase. A cells chromosomes remain at least partially uncondensed during interphase to allow for transcription and DNA replication.
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Chapter 8 How Cells Reproduce Part 2
Mitosis: Prophase • A cells chromosomes remain at least partially uncondensed during interphase to allow for transcription and DNA replication. • However, in preparation for nuclear division, the chromosomes condense into tightly coiled packages that are more easily transported. • Once the chromosomes have condensed enough that they are visible under the light microscope, we say that the cell is in PROPHASE.
Mitosis: Prophase • In animal cells, a region near the nucleus that organizes microtubules while they are forming is called a centrosome. • The centrosome also contains two barrel-shaped structures called centrioles. • The centrosome is duplicated just before prophase begins. One centrosome (along with its two centrioles) moves to opposite ends (or poles) of the cell during prophase. • Microtubules then begin to grow from each centrosome. • These microtubules will form the spindle- a dynamic network of microtubules that attaches to and moves chromosomes during mitosis.
Mitosis: Prophase • At the end of prophase, the nuclear envelope breaks down and microtubules enter the region where the nucleus had previously been. • Some microtubules stop growing when they reach the middle of the cell, while others continue to grow until they reach a chromosome and attach to it. • At some point, different microtubules from opposite poles of the cell will be attached to each sister chromatid to ensure that each sister chromatid will be pulled to opposite ends of the cell later during mitosis.
Mitosis: Metaphase • The microtubules from opposite poles that are attached to each sister chromatid begin a “tug-of-war” in which the lengthen and shorten (by adding tubulin subunits), pushing and pulling the chromosomes with them. • Eventually, all of the microtubules reach the same length. • At this point, the chromosomes are aligned in the middle (or equator) of the cell along what is called the metaphase plate.
Mitosis: Anaphase • During anaphase, the microtubules that are attached to the sister chromatids of each chromosome begin to shorten. • As this shortening occurs, the two sister chromatid of each chromosome are pulled apart so that each moves to opposite poles of the cell. • Each DNA molecule (sister chromatid) has now become a separate chromosome.
Mitosis: Telophase • Telophase begins when the two clusters of chromosomes reach opposite poles of the cell. • Each cluster of chromosomes contains the same number as the parent cell had. Thus the chromosome number of each new cell that will arise from each pole will be diploid (2n). • A new nucleus forms around each cluster of chromosomes and the chromosomes begin to partially uncondense again. • The end result is two diploid nuclei.
Cytokinesis • After mitosis is over, the cell usually divides its cytoplasm so that two separate diploid daughter cells are formed. • Animal cells typically pinch in two. • This happens when a contractile ring composed of actin and myosin protein filaments that is part of the cell cortex (a layer of cytoskeletal proteins just under the plasma membrane) begins to contract. • As the contractile ring contracts, it drags the plasma membrane inward, forming an indentation that can be seen with a microscope. This indentation is called a cleavage furrow. • As the cleavage furrow deepens, the cytoplasm, and the cell, are pinched in two.
Cytokinesis • Plant cells go through cytokinesis differently than animal cells due to the presence of the cell wall composed of cellulose, which is a rigid structure that can’t be pinched in two. • By the end of anaphase in a plant cell, a set of short microtubules has formed on either side of the cell’s equator. • These microtubules guide vesicles from Golgi bodies and the cell surface to the equator of the cell. • At the cell’s equator, these vesicles carrying their cell wall building components fuse together, forming a disk-shaped structure called the cell plate. • The cell plate grows outward until it reaches the plasma membrane, where it attaches and, in doing so, partitions the cytoplasm into two approximately parts. • In time, the cell plate develops into cell wall and joins with the original cell wall of the parent cell, producing two daughter cells with their own plasma membranes and cell walls.
