GENETIC ENGINEERING: Redrawing the Blueprint of Life - 1. The World Inside the Cell
Figure 1. Almost all animal and plant cells have a cell membrane, a filling of cytoplasm, and a nucleus. Many other cell parts are found in the cytoplasm.
Figure 2. These particular cells, in which the nuclei can be seen as dark dots, have been taken from a person's spinal cord.
Figure 3. DNA is shaped like a ladder that has been twisted around. The rungs are made of pairs of chemical bases. C always goes with G and A always goes with T.
Figure 4. How the DNA code helps to make a protein.
Figure 5. From the whole person to the gene.
Figure 6. Fertilization happens when an egg cell and sperm unite Note: Egg and sperm not shown to scale.
Elephants, oak trees, ants, and human beings may look very different from one another, but like all living things on Earth they contain the same fundamental working parts. These working parts are called CELLS (see Figure 1).
An adult's body contains about 10 trillion cells that, individually, are too small to be seen without a microscope.
Each cell has properties that make it ideally suited to a particular task. A nerve cell, for instance, which carries messages to and from the brain, is long and thin, like a fine wire. A muscle cell, on the other hand, can change shape and is very elastic. At first sight, nerve cells look nothing like muscle cells. However, their basic structure is the same (see Figure 2).
All animal and plant cells have three important parts in common. They are surrounded by a clear, flexible covering, called a CELL MEMBRANE, inside which is a jelly-like filling, or CYTOPLASM. Within the cytoplasm is the most important part of all – a small, dark speck known as the NUCLEUS.
Inside the Nucleus
The nucleus directs the making of essential substances, called PROTEINS, on which all life depends. Chemical messages sent out by the nucleus inform the rest of the cell how to put together the required proteins. These proteins then enable the cell to process food into energy, to grow and divide, and to carry our repairs. The instructions needed to make proteins are stored within a special, complex chemical found inside the nucleus, called DNA (see Figure 3).
DNA is shaped like a twisted rope ladder. The rungs of the adder are made up of four chemical bases. They are called adenine (A), thymine (T), guanine (G), and cytosine (C, each of which has a different shape, like a piece from a jigsaw puzzle. A and T are shaped so that they fit together exactly. C and G also form a perfectly matched pair. But any other combination, such as A and G, will not lock together. The rungs of the DNA rope ladder, then, are made of A-T and C-G pairs.
In Morse code, letters are represented by a series of dots and dashes. Since letters make up words and words make up sentences, Morse code provides a way of representing any amount of information with just two symbols. The A-T and C-G pairs are like chemical dots and dashes. They enable long, coded instructions to be stored in a simple way inside a length of DNA.
The Workings of the Cell
By following the instructions stored in its DNA, a cell is able to manufacture a great variety of chemicals. Thousands of different proteins have to be produced constantly inside your body to help you stay alive and healthy. Proteins make up most of your muscles. They help you digest your food. Even your fingernails and hair are built up from a tough kind of protein called keratin.
Like DNA, proteins are highly complex substances. They consist of long chains of smaller chemical units called AMINO ACIDS. Only 20 different amino acids occur in nature. But just as all the words in the English language are made from only 26 letters, many thousands of different proteins can result from different combinations of the basic amino set.
It is the order of amino acids that gives a protein its special properties, making it, for example, a flower, root, muscle, or skin protein. The instructions stored in the DNA chemical code are used to put the amino acid units into the correct order to make every kind of protein found in cells.
DNA in Action
When a protein needs to be made, a section of the DNA spiral unravels and pulls apart. One side of the unwound DNA acts as the pattern for a particular protein. This protein is assembled, in a long chain, from amino acid "links."
Each amino acid is represented in the DNA code by its own special group of three base units. For example, ACC is the code for one particular amino acid, AAG is the code for another, and so on. Looking along an unwound length of DNA, we could read off its base units in groups of three. By following along, we could read off the sequence of amino acids, and therefore the protein, it specifies. A section of DNA that has the complete code for a single protein is called a gene (see Figure 4).
Genes determine the type of proteins our bodies make. Genes, therefore, control a huge variety of factors that help make us unique individuals. Genes play a part in determining everything from the color of your hair and eyes to the size of your feet. And since nobody (unless you have an identical twin) has exactly the same set of genes as you have, nobody looks exactly like you, either.
Genes are not found as separate bits of DNA inside the nuclei of our cells. Instead, they are strung out like beads on long strands of DNA known as CHROMOSOMES. Nearly all of the cells in your body contain 46 chromosomes, arranged in 23 pairs. Each chromosome has thousands of genes strung out along it. If all the DNA making up the chromosomes inside one of your cells were unraveled and joined end to end, the DNA would stretch out almost two yards. All the DNA from all your cells, joined end to end, would reach from the Earth to the Sun and back about 250 times!
Each cell in your body contains the complete set of DNA needed to make a perfect copy of yourself (see Figure 5). If all the information in this DNA were printed out as instructions in English, it would fill a set of encyclopedias with about a million pages. But not every cell in your body uses every instruction on the DNA in its nucleus. It is one of the most remarkable facts of nature that each cell "reads" only those parts of the DNA code needed to manufacture certain proteins.
Most of our cells contain 46 chromosomes. But there are two types of cells in human beings that have only half this number. These are the egg cells in females and the sperm cells in males. When fertilization takes place, a sperm joins with an egg, and the 23 chromosomes from each combine to make a new set of 46 (see Figure 5).
Of the 46 chromosomes in your normal body cells, then 23 have been inherited from your mother and 23 from your father. This means that all the genes on your chromosomes come in two versions, one set inherited from each of your parents.
In the case of some genes, only one of the two versions of the gene is ever used. This is the DOMINANT gene. The other member of the pair is said to be RECESSIVE. In the case of the other genes, the instructions of two corresponding genes may be combined. The overall effect of the two types of genes is that you are, in some ways, like one parent; in other ways, like the other parent; and you have some features that combine traits from both your parents.
Most people have genes that work correctly throughout their lives and cause no serious health problems. But not everyone is so lucky. Genes that are abnormal can give rise to a variety of inherited illnesses, or GENETIC DISEASES.
|Solving the Mystery of DNA|