Chapter 16 The images on this CD have been lifted directly, without change or modification, from...

Chapter 16

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Gene therapy, a method of inserting genetic material into a human to treat disease For example, defects in the protective immune system often result from defective genes. (1) Stem cells,

formative cells, are removed from the bone marrow of the afflicted infant. (2) The normal gene is carried into the defective stem cells by a benign virus, here a retrovirus, which has the ability to (3) indirectly insert the normal gene into the stem cell DNA. (4) The repaired stem cell, genetically engineered, is returned to the patient where it produces new immune system cells that now function normally, restoring the immune system to its role in thwarting infections.


Retrovirus The RNA genome of a retrovirus inserts a DNA copy of its viral genome into the

chromosome of the host. There, the viral DNA makes many more copies of the retrovirus RNA, which leave the host cell to repeat the cycle.


Genetic screening—fetus in uterus a) Amniocentesis. Cells sloughed from the embryo into the amniotic fluid can be collected

by syringe. b) Chorionic villi sampling (CVS). In even earlier embryos, the separate but intertwined blood vessels of mother and fetus can be collected through an inserted suction tube guided by ultrasound images. By both methods, cells from the fetus or its placenta can be analyzed for possible genetic defects to inform parents.


Influenza epidemic Over 21 million people died of the influenza epidemic of 1918-1919. Members of the Red

Cross often worked around the clock to aid infected people.


Viruses

Viruses attack bacteria, plants, and animals. Viruses are simple parasites consisting of a genome (usually DNA, sometimes RNA) held in a protective capsid. Shown is a T4 virus.


Lytic and lysogenic cycles of a virus In the lytic cycle, a virus infects, propagates, and bursts out. In the lysogenic cycle, a virus

infects, remains in residence, and propagates.


The HIV infection cycle 1) Surface molecules (gp 120 glycoproteins) on the HIV attach to surface molecules (CD4) on the white

blood cell, permitting the HIV to dock and insert its RNA into the cell. 2) In cooperation with the co-opted host cell replication machinery, HIV RNA produces, with the aid of a promoter molecule (reverse transcriptase), a DNA copy of itself to which the deceived cell produces a complementary copy, yielding a double-stranded DNA copy of the HIV. 3) This double-stranded viral DNA synthesizes copies of HIV. 4) After successful replication, the new HIV copies exit the cell, traveling in macrophages or in the blood to infect other white blood cells.


Diversity of HIV The genetic diversity of HIV is low before the immune system responds, then rises, and

then falls again after the immune system breaks down.


Ebola River Ebola fever was first reported from the area of the Ebola River in the Democratic Republic

of Congo (Zaire) in Africa.


Malaria The causative agent of malaria is Plasmodium carried from host to host by a species of

mosquito. When the carrier mosquito bites a human, the sporozoite stage of the parasite is inadvertently delivered to the blood of the host, carried in turn to the liver, and then eventually to the red blood cells. When another mosquito drinks of the host’s blood, it picks up and carries away the reproductive stages of the parasites (4, 5).


FIGURE 16.3 Somatic and Germ-Line Genetic Therapy Traits pass in the germ line from generation to generation. Each fertilized egg (solid dots)

grows the somatic body of an adult, setting aside gametes to produce the next generation, the germ line. (a) Somatic gene therapy. Changing the genome in the somatic cells of one individual changes the adult, but does not genetically propagate into future generations. (b) Germ-line therapy. Changing the gametes directly results in changes to adults, and these changes are also passed from generation to generation.


FIGURE 16.4 The Black Death In one of its outbreaks, the bubonic plague (“Black Death”) spread throughout Europe in

the fourteenth century, moving generally from east to west, and south to north. For five years it terrorized Europe, where one in four people died.


FIGURE 16.5 Epidemics (a) Diseases spread in epidemics as a result of three interacting factors—an agent that

infects a susceptible host promoted by favorable environmental circumstances. (b) Plagues, fierce epidemics, include a pathogen (agent), infecting an organism (host), where environment includes crowding, unsanitary conditions, and/or poor public health practices that promote spread of the disease.


FIGURE 16.9 Trichinosis (a) Domestic omnivores, such as pigs, act as hosts that pick up the parasite in

contaminated food. Digestion frees the juvenile parasites (cysts), which then penetrate the intestinal wall and mature to adults. Adult parasites produce juveniles, which migrate to muscles of the host where they roll up into cysts. (b) Humans (and carnivores such as bear and cougar) eat the meat, which contains the cysts. If the meat is not properly cooked, the juvenile parasites survive, are freed by digestion, and infect the human.


FIGURE 16.11 Edward Jenner This painting shows Jenner in the 1790s inoculating an infant with the less harmful cowpox,

which thereby confers protection from the related, but much more severe, smallpox virus.

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