EKU General Botany Lab Mitosis Review

Mitotic cell division begins by the DNA molecules coiling and thickening to begin to form recognizable chromosomes. This process usually takes a long time. Here is a nucleus that is not actively dividing. Nuclei like this are in interphase of the cell cycle. Notice that the nucleoli and the nuclear envelope (membrane) are clearly visible. These nuclei are in prophase of mitosis. The nucleoli are becoming indistinct and the chromosomes are coiling and shortening. This gives the nucleus a granular appearance.

Mitotic division continues When the chromosomes move to the middle of the cell. This phase of mitosis is called metaphase. Then, every chromosome divides lengthwise and the halves move in opposite directions. This stage is called anaphase.

Finally, the chromosomes reach the ends... of groups of microtubules called the spindle. When they have reached that point, we say that the cell is in telophase of mitosis. During telophase, if the rest of the cell is going to divide, a new cell wall forms, and the “daughter” nuclei gradually return to interphase. These (this slide and the previous two slides) are the main events of mitotic cell division.

Stages of the cell cycle with some details interphase prophase metaphase anaphase telophase

Interphase Is the part of a cell’s life when it is not dividing. DURING INTERPHASE, The cell “hums” with normal metabolic activity. DNA molecules are fully extended, making RNA molecules Nucleoli are plainly visible. At some point during interphase, the single DNA molecules each make an entire lengthwise copy of themselves. Each new DNA molecule remains attached to the DNA molecule that produced it. These are plant cells. Notice the cell walls.

Prophase is the stage where the nucleus is preparing to divide. A lot happens, but it all happens gradually. DURING PROPHASE, The DNA molecules and their associated protein molecules shorten, thicken, and become recognizable as double-strand chromosomes. The nucleolus (sometimes there’s more than one) becomes indistinct and disappears. The nuclear envelope (double membrane) disappears. A new organelle, the spindle, forms. The spindle is constructed from microtubules and microfibrils. The spindle is the structure that will actually divide the nucleus, and make sure each new nucleus receives exactly the right number of chromosomes. The chromosomes move to the center of the spindle and attach to spindle microtubules.

When the centromeres attach to the spindle, the cell is in the stage we call metaphase. Then, the centromeres of each chromosome divide. When that happens, anaphase, the next stage, has begun. Animal or plant?

Anaphase begins when the centromeres divide When the centromeres divide, it means each of the chromatids (= long halves of a chromosome) is now separate from its duplicate copy. The two chromatids of each chromosome move in opposite directions along the spindle. All of the other chromosomes are doing the same thing – one chromatid goes in one direction, and the other chromatid goes in the opposite direction. Eventually, all of the chromatids reach the ends of the spindle and stop moving. (The ends of the spindle are sometimes called the “poles” of the spindle.) When the chromosomes reach the end of the spindle, telophase has begun. (Notice the spindle fibers between the two groups of anaphase chromosomes, inside the red circle.)

Telophase is the final phase of the division of a nucleus Telophase is basically the reverse of prophase. The chromosomes become indistinct and gradually disappear, as the DNA molecules become extended once more. The nuclear envelope and nucleolus reappear. Normal cell metabolism begins to occur, and the “daughter cells” slip into interphase. Usually, cytokinesis (= division of the cytoplasm) occurs during telophase. In animals, microfibrils contract inside the cell membrane to pinch the cell in half. In plants, the endomembrane system secretes non-living material in a plane through the middle of a cell, dividing the cytoplasm approximately in half. During cytokinesis, other cell organelles (other than the nucleus) are not necessarily divided equally into the new daughter cells.

Here are some views of cytokinesis. In plants, the structure that forms the two new cells is called a phragmoplast, or cell plate. It looks like a line across the middle of the spindle. In animals, the structure that forms the two new cells is called a cleavage furrow.

Summary of mitosis Mitosis results -- 2 cells with same number of chromosomes as parent cell genetically identical to parent cell ~1/2 the volume of the parent cell Mitosis significance – It is not part of the sexual process It is how multicellular organisms grow Please note: these single-strand chromosomes will make copies of themselves during interphase.