CLÁUSULAS Y CONDICIONES DE LA OFERTA

You should be able to:

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■ describe the structure of chromosomes ■

■ describe the cell cycle – the cycle of events by which body cells grow to a certain size and then divide into two

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■ explain how a nucleus divides into two genetically identical nuclei by mitosis

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■ prepare and observe a root tip squash in order to see stages of mitosis with a light microscope ■

■ explain the significance of mitosis ■

■ explain the significance of telomeres ■

■ explain the significance of stem cells ■

■ outline how uncontrolled cell division can lead to cancer

Is it useful to prolong human life? The forerunners of

modern chemists, the alchemists, thought so (Figure

5.1). They had two main aims: first, the ability to transform ‘base’ metals, such as lead, into the ‘noble metals’ (gold and silver) and second, to discover the elixir of life, which would confer eternal youth.

By the early 20th century, scientists had relegated these aims to impossible dreams. Now, however, we are once again challenging the idea that the process of ageing is inevitable.

Why do organisms grow old and die? Interest in the process of ageing was rekindled with the discovery of telomeres in 1978. These are protective sequences of nucleotides found at the ends of chromosomes, which become shorter every time a cell divides. A gradual degeneration of the organism occurs, resulting in ageing.

Some cells are able to replenish their telomeres using the enzyme telomerase. It is thought that cancer cells can do this and so remain immortal. It may therefore be possible to prevent the ageing of normal cells by keeping the enzyme telomerase active.

If the ageing process could be slowed or prevented, this would raise some important moral and ethical issues. Should the treatment be universally available? If not, who should benefit? What if you could live for 600 years? Should you be entitled to so many years of healthy life before the drug was withdrawn? If so, would this create a black market for the drug?

Why grow old?

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Figure 5.1 A 19th century oil painting showing an alchemist at work.

All living organisms grow and reproduce. Since living organisms are made of cells, this means that cells must be able to grow and reproduce. Cells reproduce by dividing and passing on copies of their genes to ‘daughter’ cells. The process must be very precisely controlled so that no vital genetic information is lost. In Chapter 6, we discuss how DNA can copy itself accurately. In this chapter, we consider how whole cells can do the same.

In Chapter 1, we saw that one of the most conspicuous structures in eukaryotic cells is the nucleus. Its importance has been obvious ever since it was realised that the nucleus always divides before a cell divides. Each of the two daughter cells therefore contains its own nucleus. This is important because the nucleus controls the cell’s activities. It does this through the genetic material, DNA, which is able to act as a set of instructions, or code, for life (Chapter 6).

So, nuclear division combined with cell division allows cells, and therefore whole organisms, to reproduce themselves. It also allows multicellular organisms to grow. The cells in your body, for example, are all genetically identical (apart from the gametes – reproductive cells); they were all derived from one cell, the zygote, which was the cell formed when two gametes from your parents fused.

Chromosomes

Just before a eukaryotic cell divides, a number of thread- like structures called chromosomes gradually become visible in the nucleus. They are easily seen, because they stain intensely with particular stains. They were originally termed chromosomes because ‘chromo’ means coloured and ‘somes’ means bodies.

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The number of chromosomes is characteristic of the species. For example, in human cells there are 46 chromosomes, and in fruit fly cells there are only eight.

Figure 5.2 is a photograph of a set of chromosomes in the nucleus of a human cell.

functioning of the organism. The fact that the two DNA molecules in sister chromatids, and hence their genes, are identical is the key to precise nuclear division. When cells divide, one chromatid goes into one daughter cell and one goes into the other daughter cell, making the daughter cells genetically identical.

So much information is stored in DNA that it needs to be a very long molecule. Although only 2 nm wide, the total length of DNA in the 46 chromosomes of an adult human cell is about 1.8 metres. This has to be packed into a nucleus which is only 6 μm in diameter. This is the equivalent of trying to get an 18 km length of string into a ball which is only 6 cm in diameter! In order to prevent the DNA getting tangled up into knots, a precise scaffolding made of protein molecules is used. The DNA is wound around the outside of these protein molecules. The combination of DNA and proteins is called chromatin. Chromosomes are made of chromatin. Chemically speaking, most of the proteins are basic (the opposite of acidic) and are of a type known as histones. Because they are basic, they can interact easily with DNA, which is acidic.

The precise details of chromatin structure are complex and you do not need to remember them, but they provide you with useful background knowledge. The solution to the packing problem is controlled coiling of the DNA. Coils can themselves be coiled to form ‘supercoils’; these may then be looped, coiled or folded in precise ways which are still not fully understood. We do, however, understand the basic unit of structure. This is called a nucleosome (Figure 5.4). (Although you do not need to know about nucleosomes, this will help you to understand how DNA forms chromosomes.) The nucleosome is cylindrical in Figure 5.2 Photograph of a set of chromosomes in a human

male, just before cell division. Each chromosome is composed of two chromatids held at the centromere. Note the different sizes of the chromosomes and the different positions of the centromeres.

Figure 5.3 Simplified diagram of the structure of a chromosome. Genes for different characteristics – in reality each chromosome is typically made up of several thousand genes. Centromere – holds the two chromatids together. There are no genes in this region.

telomeres

telomeres

Two identical chromatids make one chromosome. Each chromatid contains one DNA molecule.