THE BLUEPRINT OF LIFE

Problem. What controls heredity ? What makes you grow up to look like you and not someone else? The answer to this question is the key to life.

Background. For centuries men have wondered why men look like men. Some people thought that the blood of a mother and father mixed and the characteristics carried in the blood of the parents were passed on to the children.This theory was proven false in 1902, when Walter Sutton discovered the chromosomes. The chromosomes are tiny threadlike structures in the nucleus of a cell that contain the heredity of the cell.

Each human cell has forty-six chromosomes, which are split into forty-six identical pairs whenever the cells divide. Each of the daughter cells receives one-half of the chromosomes. But scientists did not know why or how the chromosomes split into two identical groups.

Explanation. The key discovery in the study of heredity was made by molecular biologists who discovered the chemical make-up of the chromosomes. They found that the chromosomes are made of a certain nucleic acid, called deoxyribonucleic acid. Biologists usually refer to this by its abbreviation, DNA.

In 1953 Doctors Francis Crick and James Watson perfected a three-dimensional model that explained all the characteristics of DNA. The model showed that the molecule was shaped like a spiral staircase. The sides of the stair treads were made of two chemicals and the "treads" were made of four other chemicals.

Many experiments proved that it is the arrangement of these four kinds of "treads" that determines a person’s characteristics.

The cells of the body contain about three feet of DNA, if the molecules are laid end-to-end. There are three or four encyclopedias of information in these DNA molecules. They use another nucleic acid, RNA (ribonucleic acid), to translate this information into the proteins that make up new tissue.

Further research. For an excellent discussion of DNA, read The Cell, one of the books in the Life Science Library.

The Respiratory System

Respiration. You have learned that cell respiration takes place when the cells take in oxygen and give off carbon dioxide. This process maintains the life of the cell by providing oxygen for the burning of food in the cell.

Body respiration, or breathing, takes place in the lungs. When you breathe, you draw air into your lungs by moving a large sheet of tissue called the diaphragm, and by the action of the rib muscles. Look at the illustration to see how the diaphragm causes the lungs to expand and contract.

When air is drawn into your lungs, it passes through several parts of the respiratory system. It is first drawn through the nose and mouth. There it is cooled or warmed by the moist tissues that line the passages. Foreign bodies are filtered out by tiny hairs in the nose.

The air passes through the nasal passages and the throat and is drawn into the trachea, or wind-pipe, and the bronchial tubes. These tubes lead to air sacs called alveoli. In the alveoli, the transfer of gases into the blood takes place.

Oxygen in blood. The alveoli are surrounded by a dense concentration of capillaries that carry the blood from the veins. This blood has a high concentration of carbon dioxide and a low concentration of oxygen. Because the walls between the capillaries and the air sacs are so thin, both carbon dioxide and oxygen can pass through them.

Carbon dioxide passes out of the blood, and oxygen passes into it. The oxygen is picked up by a compound called hemoglobin. Hemoglobin gives the blood its red color.

After the blood obtains oxygen in the alveoli, it is pumped to the left side of the heart, then through the arteries to the capillaries, and then to the cells of the body, where cell respiration takes place, and the process begins again.