Mitosis
What is Mitosis?
There are various structures within the cell, but many are too difficult to see. For example, within the nucleus lie the chromosomes. These are important for heredity and reproduction. When a cell splits and becomes two, certain processes occur within the nucleus first. One of these processes involves the splitting of the chromosomes. This process is called Mitosis and there are four distinct stages.
In the drawings below, you can see the chromosomes in the nucleus going through the process of mitosis, or division. If you have a microscope (400x) and a properly stained slide of the onion root tip (or allium root tip), you can see the phases in different cells, frozen in time. You will be looking at strands of DNA inside the cell!
The 4 Phases of Mitosis
First Phase: Prophase
The DNA molecules of the chromosomes condense. The outer boundary of the cell is the faint circle just inside the box.
Second Phase: Metaphase
The chromosomes line up in the center of the cell, separate and become a pair of identical chromosomes.
Third Phase: Anaphase
Each set of chromosomes moves toward the opposite end of the cell.
Fourth Phase: Telophase
Here, the spindle fibers disappear, the nuclear membrane appears and the cell divides into two daughter cells. Notice the indentation starting on the outer cell wall. Eventually these two indentations will touch and the cell will become two.
With your stained onion root tip slide, look down near the pointed end (tip). You should be able to identify all the stages of mitosis. At left is an actual picture of an onion root tip specimen. Can you identify the phase of the chromosomes in the center that look like a spider? (Answer below)
Summary:
When a cell divides, first the chromosomes split and become two. Each new one looks exactly like the original old one. Then the chromosomes move away from each other and become two similar nuclei. Finally, the cell splits in two, making two new identical cells, each with an identical nucleus.
Memory Aid:
An easy way to learn the names of the phases of mitosis in order is to remember the "word" PMAT. Visualize a floor mat with a big letter "P" inscribed on it.
Other interesting information about cells:
Many cells of the same kind are called tissue. Blood is a tissue. When many different kinds of cells work together, we get an organ, like a heart. Finally, when many organs work together, they form a complex living thing, like a flower, a flea, or a human. Think of all the different types of cells and organs working together to keep you alive!
Answer to question above: metaphase.
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