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3.7 Selecting the sex of a child

Once a pregnancy has been established, many couples are anxious to know the sex of their unborn baby. The reasons for this are many, ranging from the prosaic (will the baby be able to use its brother's or sister's old clothes) to the deeply religious (as described for Hindus in Section 2). In many communities there is so much social pressure on mothers to produce the ‘right’ sex (usually male) that infanticide of the ‘wrong’ sex is widely practised. Because this is illegal in most soc
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3.5 A new life

There is a common belief that life begins at the moment of conception, i.e. when a sperm fuses with an egg. This is a step forward from past years, when life was alleged to start at the time of ‘quickening’, i.e. when a woman could feel her fetus moving inside her. However, both these opinions suffer from an underlying falsehood: that life ‘begins’ at all. Life is a continuum; gametes are produced by living parents, and fuse to produce new living individuals, but unfused gametes are n
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3.1 How fertilization happens

Now that we have dealt with the basic biology, we can resume and give more detail to our story, and return to where we left it: fully mature, strongly swimming sperm have been deposited in the vagina, and will begin their race to the newly ovulated egg.


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2 Summary of Section 4

  1. Gametes are special cells because they contain only one set of chromosomes instead of the more usual two sets.

  2. The chromosome number is halved by meiosis.

  3. The crossing over and random assortment of chromosomes in meiosis produces a unique set of genes in every gamete, and thus in every individual (except for identical twins, who are derived from the same conceptus).

  4. Sperm production involves many rounds of
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4.7 Factors affecting fertilization

It is useful at this stage to summarize the main factors involved in a successful fertilization. First and foremost, fertile gametes must be made. This depends fundamentally on the health of the prospective parents. If they are diseased or undernourished, or have been exposed to high levels of radiation, then not only will they not produce healthy gametes, but they will probably not want to engage in the kinds of activity that might bring their gametes together.

DNA replication and prot
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4.6 Hormonal control of egg production

As you can see from the preceeding section, hormones play a crucial role in the maturation of the oocyte. Figure 3 showed you how levels of oestogen and progestogen vary throughout the menstrual cycle, and suggested that hormone balance is important for a woman's fertility, but you can now see how subtle the control really is. Cells have to develop sensitivity to hormones at the times when the hormones are likely to be present, otherwise the entire operation will fail.

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4.3 Gamete production in men

A sexually mature man is producing sperm all the time at a rate of around 300–600 per gram of testis per second. This provides the 500 million or so which are released at each ejaculation. But the formation of an individual sperm takes about nine weeks (64 days). Sperm are produced in the testes, and production is most efficient at a temperature several degrees lower than the normal body temperature of 371°C. For this reason the testes (plural of testis) are suspended outside the body cavi
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3.6 Summary of Section 3

  1. Many people wish to limit the number of their offspring, and so resort to contraceptive measures.

  2. Chemical contraceptives interrupt the production of gametes, or prevent implantation.

  3. Mechanical or barrier contraceptives prevent egg and sperm from meeting and, in the case of IUDs, prevent implantation.

  4. Surgical methods of contraception involve physical alteration of the reproductive tract so as to prevent e
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References

Sheldon, P. (2005) Earth’s Physical Resources: An Introduction (Book 1 of S278 Earth’s Physical Resources: Origin, Use and Environmental Impact), The Open University, Milton Keynes
Roy, A. (1999) The Cost of Living, Random House Inc, New York

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3.4.4 Sediment filling

The lifetime of reservoirs can vary greatly. Many reservoirs have lasted for over a hundred years, but some may be useful for only a much shorter period—fifty years or so — not because of the general deterioration of the dam as it gets older, but because sediment accumulates in the reservoir. Rivers carry large amounts of mud, silt and sand in suspension, particularly during floods, and when a river enters a reservoir it slows down and the sediment carried in suspension is deposited on th
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Acknowledgements

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Figure 1 Copyr
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7 Aquifers

A layer of rock that is sufficiently porous to store water, and permeable enough to allow water to flow through it, is called an aquifer. Consolidated porous and permeable rocks, for example, sandstone and limestone, can form important and extensive aquifers (e.g. Figure 15). Unconsolidated sands and gravels may also be good
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2 Infiltration

Precipitation that reaches the ground either runs off at the surface, or sinks into it. Infiltration is the movement of water through the ground surface into the soil and on downwards. The rate at which infiltration can take place depends, among other things, on the permeability of the soil or rock. Permeability is a measure of the ease with which water can move through a substance: the greater the permeability, the easier the infiltration.We shall deal with permeability of rock
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1 Wind energy

Wind energy was the fastest growing power source at the start of the 21st century, yet wind-driven mills and pumps, and nautical sails for transport were, along with waterwheels, the first mechanical devices to power industrial production. The advantages of harnessing wind energy are obvious; it is free, clean and widely available (but see later). Although a favoured source of ‘green’ energy, the increasing deployment of wind turbines where they are most efficient, on hilltops and coasts,
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2.1 Introduction

This section reminds you of the numerous specialised intracellular compartments of the eukaryotic cell, with how molecules are moved rapidly and specifically between them in eukaryotic cells.

  • What are the principal membrane-bound compartments of the cell and the trafficking pathways that connect them?

  • Early and late end
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1.5 Summary

  1. The cytoskeleton is formed of microtubules, microfilaments and intermediate filaments. Microtubules are formed by polymerisation of tubulin and microfilaments by polymerisation of actin. Assembly and disassembly are faster at the plus end of the filament. Both microtubules and microfilaments can display treadmilling and dynamic instability, in appropriate conditions.

  2. Actin is an ATPase, and actin-ATP is less readily dissociated from the ends o
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3.6.2 The JAK–STAT pathway

Another important protein kinase pathway is the JAK–STAT pathway. Cytokines (Section 2.2), are frequently used for signalling between cells of the immune system. Cytokine-induced signal transduction cascades are often direct pathways to the nucleus for switching on sets of genes. Janus kinases (JAKs, named after the two-faced Roman god) are a particular
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3.6.1 The MAP kinase pathway

The MAP kinase pathway is so called because the last component of the pathway was originally identified as a kinase activity in EGF-stimulated cells – hence the name ‘mitogen-activated protein kinase’ (MAP kinase), as it stimulates cell growth and proliferation.

  • What is the mechanism of activation of the EGF receptor?

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3.4 Second messengers

In the previous section, we have discussed the principles of second messengers (Section 1.5) and, in particular, those produced by PLC (IP3 and DAG) and PI3 kinase (PI(3,4)P2 and PI(3,4,5)P3). We shall now consider the roles and mechanisms of action of the other chief mediators, which are Ca2+ ions, cAMP and cGMP
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