In each cell only some genes are switched on.

So how are cells able to be selective about which proteins they make? It all comes back to genes. Genes can be turned on or off like light switches. When a gene is in the 'on' position a protein can be made. But when the gene is in the 'off' position, no protein is made. Cells are different from one another because they have different combinations of genetic switches turned on. This genetic switching is what enables a single fertilised cell to develop into a complex human being.

When a male sperm fuses with a female egg, the fertilised egg formed is the first cell of a new life. It contains a mixture of genes from the mother and the father. All the genetic information necessary for the subsequent development of the embryo is contained in the DNA of that single cell. The single cell divides again and again, eventually forming billions of cells, each with a set of genes identical to those in the fertilised egg. While this is happening, a precise and ordered sequence of genetic switching takes place.

As the cells divide and grow, specific combinations of genes are turned on or off. The combination of genes turned on in one cell affects which genes are turned on in neighbouring cells, so that as the embryo continues to grow, cells become specialised and organised into the different tissues and organs of the body.

Nine months after conception we emerge into the world with all our bits and pieces just where they should be. Skin on the outside, muscle, bones and organs on the inside; mouth at the head end and feet on the ends of our legs.

Tiny genetic differences make us each individual.

A parent and child share 99.95 per cent of the information in their genes. But this doesn't seem so special when you consider that we share 99.90 per cent of our genetic information with everyone else on earth. We also share 98.5 per cent with chimpanzees and 40 -50 per cent with bananas and cabbages! So our genes connect us not only with our immediate ancestors, our parents, grandparents and so on, but also with all our evolutionary relatives.

Human beings, chimps, insects, oak trees, seaweed and bacteria, to name but a few, are all very closely related, and our genes play a crucial part in the similarities and differences we see between ourselves and all other living things.

(http://www.bookweb.org)

Answer the questions on the text.

1. What are genes? How are they inherited?

2. What substance are genes made of?

3. Define the main function of DNA.

4. What is DNA composed of?

5. What are chromosomes?

6. Why do the genes the chromosomes carry come in pairs?

7. How are sex cells called scientifically? How many chromosomes do they contain?

8. How many base pairs does a typical gene consist of?

9. Describe the process of cells development after egg-fertilization.

10. Do genes connect people only with immediate ancestors? Provide figures by means of your answer illustration.

Summarize the information from the text in three or four paragraphs using the worlds below. (Make up a plan of your summary first thing.)

Tiny strings of chemicals, coded information, to inherit, a complete copy of genes in the nucleus of a cell, DNA, a long twisting ladder, helix, to control heredity, chemical unit, a base, adenine, thymine, guanine, cytosine, a linked pair of aminoacids, a sequence, to manufacture proteins, to read the genetic code, thread-like structures, chromosomes, skin cells, muscle cells, gametes, sperm egg, fertilization, dead genes, virus infections, automatically copied, to pass down from generation to generation, to trace family relationships, to produce proteins, human genome, height, build, eye and hair colour, strengths, weaknesses, susceptibility to disease, genetic switching, the subsequent development of the embryo, tissues and organs, genetic information, immediate ancestors, evolutionary relatives, similarities and differences.

RENDERING

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