Mitosis

Mitosis is a special kind of cell division used by eukaryote cells. You will probably find a series of diagrams in your biology textbook to show the stages of mitosis. They are:

• prophase;

• metaphase;

• anaphase;

• telophase.

Mitosis is really a division of the nucleus into two identical nuclei. It is followed by cytokinesis which is the division of the cytoplasm into two.

Just imagine for a moment. You started your life as a single cell as a fertilized egg or a zygote. By mitosis, you divided into two cells, both of your cells divided again (4) and again (8) ... really boring but this is what you did. During mitosis, one cell divides into two genetically identical cells. Before mitosis starts you cannot see any chromosomes in the nucleus, however, all the DNA in the nucleus is replicated exactly. This stage is not a part of mitosis: it is called interphase (between divisions).

Prophase

During prophase the chromosomes coil up. They become shorter and fatter. The nuclear envelope seems to disappear. If you squash the cell and look at it under a microscope it is possible to see chromosomes and count them. You would see 46 chromosomes in a normal human cell. This stage may take over one hour.

Metaphase

This is a very short stage in mitosis. The chromosomes move and line themselves up at the equator of the cell. It should be possible with a good microscope to see that each chromosome has divided into two daughter chromatids. These are genetically identical to each other. This stage only takes about 15 minutes.

Anaphase

During anaphase the chromatids separate (are pulled apart) and move to the poles (opposite ends of the cell). You may be able to see spindle fibres which help to pull the chromatids apart. Although this stage only takes about ten minutes, it is the most interesting stage because it shows that the cell division is genetically exact.

Telophase

Now there are 46 daughter chromatids at each end of the cell. Each one uncoils: it gets longer and thinner. It is no longer possible to see and count individual chromosomes. This stage takes much longer but it is not so interesting.

After mitosis has taken place (the division of the nucleus) the cell divides into two cells. This process is called cytokinesis. New cell membrane is formed dividing the cell into two. Meiosis is another kind of cell division. This is the one which makes sperms and eggs.

Task 9. Match words and phrases with their Russian equivalents.

1) nucleus (N)	a) оплодотворённая яйцеклетка
2) fertilized egg	b) разматывать, раскручивать
3) coil up (V)	c) ядро
4) uncoil (V)	d) сплющить, раздавить
5) nuclear envelope	e) полюс
6) squash (V)	f) свертываться кольцом или спиралью
7) pole (N)	g) ядерная оболочка
8) spindle fibres	h) веретенообразные волокна

Task 10. Choose the correct answer.

1. What is the smallest unit of life that can still replicate?

a) Proteins c) Cells

b) Quarks d) Organelles

2. How many different types of cells has the average human body?

a) 20 c) 400

b) 200 d) 1000

3. What are the parts of cells called?

a) Cellular parts c) Organs

b) Organelles d) Celluoplasts

4. What part of a plant cell is involved in photosynthesis?

a) Mitochondria c) Smooth Reticulum

b) Nucleus d) Chloroplast

5. What is mitosis?

a) Cell death c) Cell division

b) Cellular respiration d) Cellular communication

6. Who was the first person to use the term "cell?"

a) Aristotle c) Robert Hooke

b) Captain Hooke d) James Watson

7. During which phase do the chromatids separate and move to the poles?

a) Metaphase c) Telophase

b) Anaphase d) Prophase

Task 11. Challenge question.

The four most common elements in the human body and in all living things are oxygen, carbon, hydrogen and nitrogen. What is the most common molecule in the body?