Course 9.1Lesson 2 of 3
Mitosis is the part of M phase in which the copied chromosomes are shared equally between two nuclei. It runs through prophase, prometaphase, metaphase, anaphase and telophase. This lesson follows the chromosomes, the spindle and the nuclear envelope through each stage.
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By the end of G2, a cell holds two copies of every chromosome. They are joined as sister chromatids. Mitosis shares them out so that each new nucleus gets one complete set. Because the daughter cells have the same number of chromosomes as the parent, mitosis is called equational division.
Mitosis is one smooth process. The stages are only names for what we see on a slide, and no line can be drawn between them. NCERT describes four stages. Many A-Level, IB and AP books add a fifth, prometaphase, between prophase and metaphase. Both are correct. NCERT includes the events of prometaphase in the end of prophase and the start of metaphase.
Prophase follows S and G2 phases. The chromatin condenses and the chromosomes become visible as compact rods. Each is made of two chromatids joined at the centromere. The centrosomes move to opposite poles, and the spindle starts to form from microtubules. By the end of prophase, the Golgi complex, the endoplasmic reticulum, the nucleolus and the nuclear envelope are no longer seen.
The nuclear envelope breaks into small vesicles. On each centromere, a disc shaped protein structure called the kinetochore forms on each sister chromatid. Spindle microtubules from the poles attach to the kinetochores, and they push and pull the chromosomes towards the middle of the cell.
The nuclear envelope is completely gone. The chromosomes lie on the equator of the spindle in a plane called the metaphase plate. One chromatid of each chromosome is joined by its kinetochore to fibres from one pole, and its sister is joined to fibres from the other pole. The chromosomes are most condensed now. This is the stage at which their shape is best studied. Cells in culture are often stopped at metaphase to count the chromosomes.
The centromere of every chromosome splits at the same moment. The sister chromatids separate. Each is now called a chromosome. The chromosomes move to opposite poles with the centromere in front and the arms trailing behind. The spindle fibres shorten, and the poles move apart. At the end of anaphase, each pole has a full set of chromosomes.
The chromosomes reach the poles. They decondense and lose their separate shapes, and the chromatin gathers in a mass. A nuclear envelope forms round each group. The nucleolus, the Golgi complex and the endoplasmic reticulum appear again. Telophase is prophase run backwards. Cytokinesis usually follows.
The count of 4n at anaphase and telophase surprises many students. The number of chromosomes is doubled only because the sister chromatids are now counted as chromosomes. The whole cell still holds 4C of DNA until it divides.
You should see that the chromosomes double in number at anaphase, and that the DNA molecules do not change from prophase until the cell divides.
The program lists the chromosomes, the joined chromatids and the DNA at each stage for a cell with 2n = 8.
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
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