Read the following text, paying particular attention to the highlighted words.
Cells are the basic units of life. They are the true miracle of evolution. Miracle in the figurative sense, since although we do not know how cells evolved, quite plausible scenarios have been proposed. Miraculous, none the less, in the sense that they are so remarkable. Most remarkable, and, in a way, a definition of life, is their ability to reproduce themselves. They are able to take in chemicals and convert them into usable energy and to synthesize all the components of the cell during growth that eventually leads to cell
Animals are made up of specialized cells, such as blood cells, cartilage cells, fat cells, muscle cells, nerve cells - humans have about 350 different cell types while lower animals, like hydra, only 10 to 20. Cells carry out an amazing range of specialised functions, such as carrying oxygen, transmitting messages, contracting, secreting chemicals, synthesizing molecules, and multiplying. The cells in the embryo are initially much less specialized and differ from each other in more subtle ways. All have certain basic characters and in order to understand their role in development it is helpful to be aware of four cell activities, three cell structures, and two main kinds of molecule.
The four cell activities are cell multiplication, cell movement, change in character, and cell signalling. These are mainly self-explanatory. Cells multiply by dividing and this usually requires cell growth, the cells doubling in size before dividing in two. Cells can also change shape, exert forces, and move from one place in the embryo to another. They can also change character: during development cells change from having rather unspecialised characters to mature cells with very specific functions. Cells in different parts of the embryo can be thought of as developing along quite different pathways, diverging more and more in character. Finally, cells give out and receive signals from neighbouring cells.
Cells cannot normally be seen without a microscope, being about one-thousandth of a millimetre in diameter. However, some cells, like the large eggs of frogs, are easily visible, and the human egg is just visible to the naked eye. The three key cell structures are the cell membrane, the cytoplasm, and the nucleus. Surrounding the cell is a very thin cell membrane which controls the entry and exit of molecules, and maintains the integrity of the cell. On the surface of the membrane are special receptors for signals from other cells, as well as molecules that enable cells to adhere to one another. Confined by the cell membrane the cytoplasm contains all the machinery for production of energy and cell growth; and there are also structures in the cytoplasm which can generate forces to change the shape of the cell, resulting in cell movement. Embedded within the cytoplasm is the cell nucleus surrounded by its own special membrane. Within the nucleus are the chromosomes of the cell, which contain the genes.
The life of the cell is dependent on the chemical reactions among the many million constituent molecules. Two key classes of molecules are nucleic acids and proteins which will be described much more fully in Chapter 5 and can be largely ignored for the present. Genes are made of the nucleic acid DNA, and they exert their effect by determining which proteins are made in the cell. Proteins are fundamental to the life of the cell because they are essential for all the key chemical reactions as well as providing the main framework of all the structures in the cell. Almost all the chemical reactions in the cell such as the provision of energy or the synthesis of key molecules will only take place in the presence of special proteins, known as enzymes, which allow the reactions to occur. Proteins are also the major structural molecules in the cell, providing, for example, the forces for cell movement, receptors at the cell surface, and the adhesive links between cells. Proteins also give different cells their special character. For example, the protein haemoglobin carries oxygen in the red blood cells, and insulin, another protein, is made in particular cells in the pancreas.
The wide variety of proteins in the cells are all coded for by the genes in the nucleus. While proteins themselves are synthesized in the cytoplasm, whether or not a protein is made is dependent on whether or not the gene that contains the information for that protein is active (Chapter 5). The only function of genes is to determine which proteins are made, thereby determining which chemical reactions take place and which structures will be present in the cell. In this way, genes control cell behaviour.
Cell behaviour can thus provide the crucial link between genes and animal structure and form. If we can understand how cells behave during development so as to make arms and brains we can then begin to ask how genes control the behaviour of the cells and so establish the link. Cells thus provide the key to understanding development because their behaviour brings about embryonic development and is controlled by gene activity. In very general terms there are, in development, three kinds of genes - those that control spatial organization, those that control change in form, and those that control cell differentiation.
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