VIII. Answer the following questions. Use all information given before.

1. What enables a multicellular organism to grow and replace worn out or damaged cells?

2. What does cell division start with?

3. What two forms of nuclear division do you know?

4. What are chromosomes?

5. What do chromosomes consist of?

6. What is known as homologous pairs?

7. What is the difference between diploid and haploid?

8. What are three main stages of the cell cycle?

9. How does the duration of the cell cycle vary?

IX. Match the sentence halves. Make complete sentences:

1.	Cell division starts	A.	the individual’s mother and the other from the father.
2.	Chromosomesare the structures that provide	B.	and produces two identical nuclei.
3.	Chromosomes consist of	C.	occurring at some point in the life cycle of organisms that reproduce sexually.
4.	One chromosome in each pair comes from	D.	DNA, the genetic material of the cell, wrapped in protein.
5.	In mitosis, the nucleus divides once	E.	according to conditions such as temperature and the type of cell.
6.	The cell cycle is the sequence of events	F.	with the division of the nucleus.
7.	The duration of the cell cycle varies	G.	that occurs between one cell division and the next.
8.	Meiosis is the basis of sexual reproduction,	H.	would be doubled in each generation.
9.	Without meiosis in the life cycle, the number of chromosomes of a sexually reproducing species	I.	continuity between one generation of cells and the next.

X. Read and translate the short text without any dictionary:

Fact of life: Some laboratory-grown mammalian cells appear to obey an internal “biological clock” that allows them to divide by mitosis a maximum number of times. For example, a fibroblast (connective tissue cell) taken from a fetus divides on average about 50 times; the same type of cell taken from an adult divides only 14 to 19 times.

 

XI. Food for thought: Although meiosisoccurs at some stage in the life cycle of sexually reproducing plants, their gametes are usually formed by mitosis. Suggest reasons for this.

Text 2.3. Microscopes

■ Essential targets:

By the end of this text you should be able to:

● describe the main features of a light microscope and an electron microscope

● distinguish between the terms magnification and resolving power

● give the approximate size of different biological structures using an appropriate unit of measurement.

Pre-reading

■ Discuss these questions with your partner.
Then find the answers in the text.

1.Who invented a microscope?

2. What types of microscopes are used today?

 

■ Read the given text and make your essential assignments:

A microscope is used to produce a magnified image of an object or specimen. Anton van Leeuwenhoek (1632-1723) was the first to invent a microscope powerful enough to explore the world of microbes. His discoveries stimulated an explosion of interest in the scientific use of microscopes. Since the 18^th century many new types have been invented, of which the most commonly used today are the compound light microscopeand the electron microscope.