Worlds of David Darling > Children's Encyclopedia of Science > Genetic Engineering > 1. The World Inside the Cell


GENETIC ENGINEERING:
Redrawing the Blueprint of Life


a book in the Beyond 2000 series by David Darling



Genetic Engineering book cover Contents
Introduction
1. The World Inside the Cell
2. Inheriting Disease
3. Gene Therapy:
     Hopes and Fears

4. Designer Genes
5. Genetic Information:
     Ownership and Privacy

Glossary





1. The World Inside the Cell



basic parts of a cell
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.
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.

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.

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.


spinal cord cells
These particular cells, in which the nuclei can be seen as dark dots, have been taken from a person's spinal cord

Inside the Nucleus

DNA
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.
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.

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

how the DNA code helps to make a protein
How the DNA code helps to make a protein
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.

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. 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.


From the whole person to the gene
From the whole person to the gene

Inheriting Genes

fertilization
Fertilization happens when an egg cell and sperm unite
Note: Egg and sperm not shown to scale

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.

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

James Watson (left) and Francis Crick (right) next to the first model they made of the structure of DNA in 1953
James Watson (left) and Francis Crick (right) next to the first model they made of the structure of DNA in 1953
Working like detectives, scientists gradually uncovered enough clues to be able to solve the mystery of DNA's structure. By the early part of the twentieth century, it was known that DNA has three components: a sugar, an acid, and four different bases (A, T, G, and C). In 1949, the Austro-American biochemist Erwin Chargaff discovered that in any sample of DNA there is always an equal amount of the bases A and T and of the bases G and C. In the early 1950s experiments using X-ray beams, by Rosalind Franklin at the University of London, showed that DNA is a long, thin molecule coiled in a spiral, or helix. Franklin's role in determining the makeup of DNA was far more important than is sometimes recognized. Finally, in 1953, James Watson and Francis Crick of the Cavendish Laboratory in Cambridge, England, put all of the scientific evidence together and worked out DNA's precise structure. From Chargaff's results, Watson and Crick deduced that base A was probably always paired with base T and that base G was always paired with C. This pattern could only happen, they realized, if DNA was composed of two strands twisted together to form a "double helix." The bases form the rungs, and the sugar and the acid make up the sides.


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