Reproduction through development

We can now return to the problem of how multicellular organisms, such as humans, mice and oak trees, reproduce. Like unicellular organisms, their reproduction is based on cell division but instead of making many separate individuals, the cells stay together to gradually build up a multicellular individual. We can summarise the whole process with a life cycle. In the case of humans, for example, parents each contribute a cell: a sperm cell from the father and an egg cell from the mother. These two cells fuse together to form a fertilised egg. This slowly develops in the womb, first dividing to give two cells, then four, and so on. After many more rounds of division, the fertilised egg has grown into an embryo and eventually a new adult is formed, comprising many billions of cells. All of this depends on more and more cell divisions. Continuing with the life cycle: if the adult is female, some cells from her ovaries will divide in a special way to produce egg cells. If male, cell divisions in the testicles will give sperm cells. Finally, the sperm and egg cells are brought together and fuse to produce the fertilised egg for the next generation, closing the cycle. The life cycle involves alternation between a single-cell phase, the fertilised egg, and a complex multicellular adult phase. The two are linked by numerous cell divisions and an occasional sexual fusion.

A similar cycle underlies the reproduction of a flowering plant. When a grain of pollen from one flower lands on the female part of another flower, a sperm cell from the pollen fuses with an egg cell in the mother. The fertilised egg then divides repeatedly, doubling the number of cells each time until a tiny plant embryo forms. The process temporarily stops at this point and the plant releases the embryo with a protective hard outer coat, in the form of a seed. In the right conditions, as when the seed is planted in the ground, cell divisions resume in the embryo so that it eventually grows into a new plant, with flowers which produce more egg cells and pollen grains. As with humans, single-cell phases alternate with multicellular phases. One difference between flowering plants and humans is that many flowers are hermaphrodites, producing both male and female organs; this sometimes allows an individual to fertilise itself if pollen lands on female organs from the same plant. (There are also some plants, like willows and stinging nettles, that are more like us in having separate sexes.) Some multicellular organisms, such as aphids and dandelions, can side-step the requirement for sexual fusion during the life cycle and are able to develop from unfertilised eggs, although in these species there is still alternation of single-cell and multicellular phases.

To understand the mechanism by which multicellular organisms reproduce, it is not enough to know how a cell can grow and divide; we also need to know how a single cell, the fertilised egg, can give rise to a multitude of different types of cells in the complex arrangements that form the mature individual. The human body contains many organs and tissues, each made of various types of cells—nerve cells, blood cells, hair cells, etc.—each arranged in a precise way. These different cell types can be distinguished by characteristics such as size, shape, structure and behaviour. By behaviour I mean some of the more dynamic properties of cells like whether they move, grow, divide or even die. In a similar way, the various parts of a plant are made up of many different cell types with different properties, although unlike those of animals, plant cells do not usually move relative to each other because they are fixed in position by their cell walls.

The problem of development is to understand how the complex pattern and arrangement of different cell types that make up a mature organism can arise from a single cell in a consistent way each generation. This problem applies to multicellular organisms, and not to unicellular organisms that reproduce by simple cell division. Throughout this book I shall use the term development in this sense: to refer to the process whereby a single cell gives rise to a complex multicellular organism.

To see more dearly what the process of development involves, I will need to introduce three types of molecule that play a fundamental role in it: proteins, DNA and RNA.

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