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

Fact of life:

Robert Hooke described his observations of the cork cells: “I counted several lines of these pores, and found that there were usually about three-score of these small Cells placed end-ways in the eighteenth part of an inch in length, whence I concluded that there must be near eleven hundred of them, or somewhat more than a thousand in length of an inch and therefore in a square inch above a Million, or 1 166 400, and in a Cubick Inch, above twelve hundred million, or 1 259 712 000, a thing almost incredible, did not our Microscope assure us of it by ocular demonstration.

XI. Food for thought: Suggest why red blood cells appear to contradict the cell theory.

Text 2.2. Introduction To Cell Division

■ Essential targets:

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

● describe the main stages of the cell cycle

● distinguish between mitosis and meiosis.

Pre-reading

■ Working in pairs discuss these questions with your partner. Then scan the text to find the answers and compare them with your discussion.

1. Is cell division essential to life?

2. What is the basis of reproduction in every organism?

3. What provides continuity between one generation of cells and the next?

4. How manychromosomes does each human cell have?

■ Read the given text and make your essential assignments:

One of the most important concepts in biology is that cells arise only by the division of existing cells. Cell division is essential to all life. It enables a multicellular organism to grow and to replace worn out or damaged cells. It is also the basis of reproduction in every organism. Cell division starts with the division of the nucleus. There are two forms of nuclear division: mitosis andmeiosis.

Chromosomes: carrying information

Chromosomesare the structures that provide continuity between one generation of cells and the next. Their name comes from the Greek: chroma = coloured, soma = body, because of their affinity for certain stains used in microscopy. Chromosomes consist of DNA, the genetic material of the cell, wrapped in protein. They become visible in the nucleus where the more dispersed chromatin existed before. Whole chromosomes can be examined microscopically after breaking a dividing cell open and staining it with a suitable dye.

Chromosomes form homologous pairs

If the chromosomes are cut out they can be arranged into matching pairs according to their size and certain other features. These are called homologouspairs. Apart from the sex chromosomes, both chromosomes in a pair normally contain the same genes (for example, for eye or hair colour). However, these may be different forms of the gene (for example, one chromosome carries the form for green eyes, the other for brown eyes).

Human cells each have 46 chromosomes (23 pairs). Other species have different numbers, for example, chimpanzee cells each have 48 (24 pairs) and cabbage plant cells each have 18 (9pairs).

One chromosome in each pair comes from the individual’s mother and the other from the father.

- Cells that have the normal two sets of chromosomes are called diploid.

- Cells that give rise to gametes (eggs and sperm) have only one chromosome of each pair, so they have half the normal number of chromosomes. Such cells are calledhaploid.

- In humans, n =23, so normal diploid cells have 46 chromosomes and the haploid gametes have 23 chromosomes.

Mitosis: two identical daughter cells

In mitosis, the nucleus divides once and produces two identical nuclei. The new daughter cells are genetically identical to the parental cell (unless their DNA has been changed in some way, for example by a mutation). So mitosis doubles the number of cells without changing the genetic information. New cells for growth of a multicellular organism, asexual reproduction, and wound healing, for example, are produced by mitosis.

The cell cycle

The cell cycle is the sequence of events that occurs between one cell division and the next. It consists of three main stages:

1. During interphase,the cell grows, carries out its functions, and replicates its DNA. After the DNA is replicated, new protein becomes attached to it. The chromosome now consists of two strands called sister chromatids which contain identical genetic information.Sister chromatids are joined at some point along their length by a centromere. These become visible under the light microscope only during mitosis. Typically, interphase lasts for about 90 per cent of the cell cycle.

2. Nuclear division takes place during mitosis. The chromatids containing replicated DNA are separated from each other and are redistributed as chromosomes in the nuclei of the two new daughter cells.

3. In cell division(also called cytokinesis) the cytoplasm divides to form two daughter cells.

The duration of the cell cycle varies according to conditions such as temperature and the type of cell. The cell cycle of some plant cells (for example, stamen cells of Tradescantia) takes less than 30 minutes at 45°C, but more than two hours at 10°C. Cells in the growing root tip of an onion divide every 22 hours at 20°C. Some cells such as human nerve cells do not divide at all once they have become specialized.

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