Вариант 3

The environment as a factor in cancer was first suggested as early as 1775. By 1910 the newly discovered viruses were proposed as the cause. Two other late-nineteenth-century discoveries—X rays and radioactivity—were eventually found to cause cancer as well.

Despite the essential correctness of the view that cancer can result from exposure to chemicals in the environment, from radiation exposure (whether from radium or from the Sun), or from infection by specific viral agents, the precise mechanisms at work were far from clear. Furthermore, some kinds of cancer seemed to "run in families," while other cancers were apparently affected by mental stress. Finally, in 1967, K.D. Zang and H. Singer identified a specific change in a chromosome found in a tumor.

All of these theories began to make sense when biologists discovered how DNA and various proteins directed by DNA control cell growth. It was first determined that malfunctions of certain specific genes resulted in uncontrolled growth of cancer. Later it was demonstrated that these genes are the ones that direct normal growth.

In some cases, the gene is damaged by something in the environment, such as a chemical or radiation. A virus can also damage a gene while directing the DNA in the cell to make copies of the virus.

Вариант 3

Since 1988 hundreds of lives have been saved by the three ounces of blood contained in a typical placenta and umbilical cord. That blood is now known to be a rich source of so-called hematopoietic stem cells, the precursors of every-thing in the blood from infection-fighting white blood cells to the red blood cells that carry oxygen to the platelets that facilitate blood clotting after an injury.

The stem cells from a single placenta are sufficient to rebuild the blood and immune system of a child with leukemia, whose own white blood cells are abnormally dividing and must be killed by chemotherapy. In the past, physicians had to seek a living donor to provide such children with transplants of bone marrow, Which also contains stem cells that pro-duce blood and immune cells. Unfortunately, many people have died during the long search for a donor with a matching tissue type or from complications if the donated marrow did not match well. Cord blood, which can be stored, is more likely to provide a suitable match and less likely to cause complications, because its stem cells are immunologically different from and more tolerant than those in adult bone marrow.

The benefits of umbilical cord blood transplantation have been demonstrated most conclusively in leukemia, but the process has other uses.

Вариант 4

It is not the intact heart only that beats. A mere portion will beat if properly perfused. It was found, in this way, that different parts of the heart will beat at different rates. That part beating most rapidly, however, forces its rate upon the remainder of an intact heart, for each rise and full of electric potential moves out along the heart muscle from that most rapidly beating portion and the rest of the heart must follow, having no opportunity to set up potential fluctuations at its own rate. The most rapidly beating part of the heart is therefore referred to as а pacemaker.

In the two-chambered heart of the fish, the pacemaker lies in the sinus venosus (“hollow of the vein" L). This is a widening at the end of the vein leading into the auricle. The beat begins there and progresses down the auricle and ventricle.

The sinus venosus persists in the embryos of birds and mammals but is gone by time of birth. It fades into the right auricle and its remnants can still be made out there as a bundle of special cells. Because those cells represent the fusion of the sinus venosus and the auricle, they are referred to as the sinoauricular node, or, in abbreviated form, the S-A node. It is the S-A node that is the pacemaker of the human heart.