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minutes, doubles the number of DNA molecules. After 20 cycles, a single fragment produces more than one million (220) copies! In a few hours, 100 billion copies of the fragment can be manufactured. PCR, now fully automated, has revolutionized many aspects of science and medicine because it allows the investigation of ...
o one end of a single-stranded DNA of unknown sequence. Reaction products GT T T GT T T GT T T GT A A CA d d a Template Large fragments Small fragments 2. The primed DNA fragment is combined with DNA polymerase and free nucleotides and then is added to four tubes. Each tube contains a different, chainterminating dideox...
red hair or elevated cholesterol in your blood. As the human genome project charges toward completion, its researchers are excitedly assembling a huge database of SNPs. Research indicates that SNPs can be expected to occur at a frequency of about one per thousand nucleotides, scattered about randomly over the chromosom...
o cultured mammalian cells. These cells produce many copies of the recombinant virus, which has the outside coat of a herpes or hepatitis virus. When this recombinant virus is injected into a mouse or rabbit, the immune system of the infected animal produces antibodies directed against the coat of the recombinant virus...
ited States—in 1999, over half of the 72 million acres planted with soybeans in the United States were planted with seeds genetically modified to be herbicide resistant, with the result that less tillage has been needed, and as a consequence soil erosion has been greatly lessened. These benefits, while significant, hav...
he water has completely emptied before initiating the spin cycle, so the cell checks that everything needed is on hand before initiating cell division. Campbell reasoned: “Maybe the egg and the donated nucleus need to be at the same stage in the cell cycle.” This proved to be a key insight. In 1994 researcher Neil Firs...
ce a continuous supply of new islet cells, replacing those lost to immune attack. Because there would always be cells to make insulin, the diabetes would be cured. No one knew just what such a stem cell looked like, but the researchers knew they come from the epithelial cells that line the pancreas ducts. Surely some m...
ivation, the foods themselves are not altered in any detectable way, and no nutritional test could distinguish them from "organic" varieties. So there seems to be little if any health need for a GM label for genetically engineered foods. The right of the public to know what it is eating is a very different issue. There...
ntal tests of evolution, it is first necessary to identify a population in nature upon which strong selection might be operating (see above). Then, by manipulating the strength of the selection, an investigator can predict what outcome selection might produce, then look and see the actual effect on the population. 419 ...
ass the long-neck trait on to their offspring. (b) Darwin's theory: variation is inherited. FIGURE 20.2 How did giraffes evolve a long neck? in the genetic makeup of populations. Allele frequencies can also change as the result of repeated mutations from one allele to another and from migrants bringing alleles into a p...
nchanged from one generation to the next. This simple relationship has proved extraordinarily useful in assessing actual situations. Consider the recessive allele responsible for the serious human disease cystic fibrosis. This allele is present in North Americans of Caucasian descent at a frequency q of about 22 per 10...
ecreases in a population’s size are founder effects and bottlenecks. Founder Effects. Sometimes one or a few individuals disperse and become the founders of a new, isolated population at some distance from their place of origin. These pioneers are not likely to have all the alleles present in the source population. Thu...
esistant target site Insect cell membrane (a) Insect cells with resistance allele at pen gene: decreased uptake of the pesticide Target site Five factors can bring about a deviation from the proportions of homozygotes and heterozygotes predicted by the Hardy-Weinberg principle. Only selection regularly produces adaptiv...
ult, pollen—and the alleles it carries— can be blown for great distances, leading to levels of gene flow between mine sites and unpolluted areas high enough to counteract the effects of natural selection. In general, the extent to which gene flow can hinder the effects of natural selection should depend on the relative...
m Peak shifts Peak gets narrower 25 50 75 100 125 0 25 50 75 100 125 0 25 50 75 100 125 FIGURE 20.13 Three kinds of natural selection. The top panels show the populations before selection has occurred, with the forms that will be selected against shaded red and the forms that will be favored shaded blue. The bottom pan...
olism of gluconate, so that only 6-PGD mediated the utilization of this sole source of carbon, he obtained very different results: several alleles were markedly superior to others. Selection was clearly able to operate on these alleles, but only under certain conditions. The ability of selection to produce evolutionary...
history of life as revealed by the fossil record. Millions of years ago 440 Part VI Evolution Gaps in the Fossil Record This is not to say that the fossil record is complete. Given the low likelihood of fossil preservation and recovery, it is not surprising that there are gaps in the fossil record. Nonetheless, paleon...
ement with Darwin, most evolutionary biologists would agree that natural selection is the process responsible for most of the major evolutionary changes that have occurred through time. Although we cannot travel back through time, a variety of modern-day evidence confirms the power of natural selection as an agent of e...
cated that dark moths had a far better chance of surviving in these polluted woods, where the tree trunks were dark. In the relatively unpolluted Dorset woods, Kettlewell recovered 12.5% of the light moths but only 6% of 446 Part VI Evolution FIGURE 21.10 Tutt’s hypothesis explaining industrial melanism. These photogra...
the latter used as a rudder), an arched back (to increase the length of its stride), and great muscle mass. By contrast, the odd proportions of the ungainly basset hound resulted from selection for dogs that could enter narrow holes in pursuit of rabbits and other small game. In other cases, breeds have been developed ...
ose descendants will initially exhibit fairly high overall similarity in their DNA. However, as the descendants evolve independently, they will accumulate more and more differences in their DNA. Consequently, organisms that are more distantly related would be expected to accumulate a greater number of evolutionary diff...
es. 21.4 The theory of evolution has proven controversial. Darwin’s Critics In the century since he proposed it, Darwin's theory of evolution by natural selection has become nearly universally accepted by biologists, but has proven controversial among the general public. Darwin's critics raise seven principal objection...
n a quarter of the world’s fruit fly species are found on the Hawaiian Islands. Lake Victoria Cichlid Fishes. Isolation has led to extensive species formation among these small fishes. New Zealand Alpine Buttercups. Repeated glaciations have fostered waves of species formation in alpine plants. Diversity of Life throug...
sa. The valley oak, a graceful deciduous tree that can be as tall as 35 meters, occurs in the fertile soils of open grassland on gentle slopes and valley floors. In contrast, the scrub oak is an evergreen shrub, usually only 1 to 3 meters tall, which often forms the kind of dense scrub known as chaparral. The scrub oak...
on Species utilize different resources in the habitat Behavioral isolation Species have different mating rituals Temporal isolation Mating or flowering occur during different seasons or at different times of the day Mating Mechanical isolation Structural differences prevent mating or pollen transfer Prevention of gamet...
ffect. (a) Mimulus lewisii (b) Mimulus cardinalis types would be retained as polymorphisms within a single population. For this reason, most biologists consider sympatric speciation a rare event. Nonetheless, in recent years, a number of cases have appeared that appear difficult to interpret in any way other than sympa...
the Hawaiian Islands, probably from a single common ancestor, is one of the most remarkable examples of intensive species formation found anywhere on earth. Chapter 22 The Origin of Species 469 Lake Victoria Cichlid Fishes Lake Victoria is an immense shallow freshwater sea about the size of Switzerland in the heart of ...
produced a wide variety of organisms, including elephants, tigers, whales, and humans. Indeed, a general observation is that biological diversity tends to rebound quickly after mass extinctions, reaching comparable levels of species richness, even if the organisms making up that diversity are not the same. A Sixth Exti...
Species maintain their genetic distinctiveness through barriers to reproduction. • Among the factors that separate populations and species are geographical, ecological, temporal, behavioral, and mechanical isolation, as well as factors that inhibit the fusion of gametes or the normal development of the hybrid organisms...
b) FIGURE 23.4 New and Old World monkeys. (a) New World monkeys, such as this golden lion tamarin, are arboreal, and many have prehensile tails. (b) Old World monkeys lack prehensile tails, and many are ground dwellers. Chapter 23 How Humans Evolved 479 How the Apes Evolved The First Hominoid The other African anthropo...
been reported. These seven species provide ample evidence that australopithecines were a diverse group, and additional species will undoubtedly be described by future investigators. The evolution of hominids seems to have begun with an initial radiation of numerous species. Early Australopithecines Were Bipedal We now...
s, H. rudolfensis, because of its substantial brain expansion. Homo ergaster Some of the early Homo fossils being discovered do not easily fit into either of these species (figure 23.11). They tend to have even larger brains than H. rudolfensis, with skeletons less like an australopithecine and more like a modern human...
H. sapiens (Cro-Magnon) H. neanderthalensis FIGURE 23.13 (continued) Chapter 23 How Humans Evolved 489 Our Own Species: Homo sapiens H. sapiens is the only surviving species of the genus Homo, and indeed is the only surviving hominid. Some of the best fossils of Homo sapiens are 20 well-preserved skeletons with skulls...
were they called? What distinguished them from the apes? • Evolution of Primates 23.3 The genus Homo evolved in Africa. • Hominids with an enlarged brain and the ability to use tools belong to the genus Homo. Species of early Homo appeared in Africa about 2 million years ago and became extinct about 1.5 million years a...
on of individuals in a population can be clumped, random, or even. Metapopulations. Sometimes, populations are arranged in networks connected by the exchange of individuals. 24.2 Population dynamics depend critically upon age distribution. Demography. The growth rate of a population is a sensitive function of its age s...
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most inclusive level, we can study the population as a whole to determine whether it is increasing, decreasing, or remaining constant. Populations grow if births outnumber deaths and shrink if deaths outnumber births. Understanding these trends is often easier if we break a population down into its constituent parts an...
will decrease the number of cones it can produce in the future. Similarly, birds that have more offspring each year have a higher probability of dying during that year or producing smaller clutches the following year (figure 24.11). Conversely, individuals that delay reproduction may grow faster and larger, enhancing ...
t. FIGURE 24.16 An example of a rapidly increasing population. European purple loosestrife, Lythrum salicaria, became naturalized over thousands of square miles of marshes and other wetlands in North America. It was introduced sometime before 1860 and has had a negative impact on many native plants and animals. of Nort...
les are nowhere near as dramatic in amplitude as the snowshoe hare and lynx cycles. Density-dependent effects are caused by factors that come into play particularly when the population size is larger; density-independent effects are controlled by factors that operate regardless of population size. Chapter 24 Population...
f 1996 Population Data in Developed and Developing Countries United States (highly developed ) Brazil (moderately developed ) Ethiopia (poorly developed ) Fertility rate Doubling time at current rate (yr) Infant mortality rate (per 1000 births) Life expectancy at birth (yrs) Per capita GNP (U.S. $; 1994) 2.0 114 7.5 76...
fferent way. The niche an organism occupies is the sum total of all the ways it utilizes the resources of its environment. A niche may be described in terms of space utilization, food consumption, temperature range, appropriate conditions for mating, requirements for moisture, and other factors. Niche is not synonymous...
or other resources (figure 25.4). This pattern of resource partitioning is thought to result from the process of natural selection causing initially similar species to diverge in resource use in order to reduce competitive pressures. Evidence for the role of evolution comes from comparison of species whose ranges are o...
rld. When the predatory Didinium is added to a Paramecium population, the numbers of Didinium initially rise, while the numbers of Paramecium steadily fall. When the Paramecium population is depleted, however, the Didinium individuals also die. capture should be strongly favored. In the next three pages, we discuss a n...
ally is the distasteful model. Many of the best-known examples of Batesian mimicry occur among butterflies and moths. Obviously, predators in systems of this kind must use visual cues to hunt for their prey; otherwise, similar color patterns would not matter to potential predators. There is also increasing evidence ind...
o the sea anemone to have particles of food removed from its tentacles; it may then be better able to catch other prey. Similarly, while often thought of as commensalism, the association of grazing mammals and gleaning birds is actually an example of mutualism. The mammal benefits by having parasites and other insects ...
nt and then switch to species B when A is rare. Similarly, a given prey species may be a primary source of food for increasing numbers of species as it becomes more abundant. In this way, superior competitors may be prevented from outcompeting other species. Such patterns are often characteristic of biological communit...
spitable. In some cases, however, as ecosystems mature, more K-selected species replace r-selected ones, and superior competitors force out other species, leading ultimately to a decline in species richness. Communities evolve to have greater total biomass and species richness in a process called succession. (a) (b) (c...
lf. The study of the behavior of nonhuman animals has led to the identification of general principles of behavior, which have been applied, often controversially, to humans. This has changed the way we think about the origins of human behavior and the way we perceive ourselves. Behavior can be defined as the way an org...
ities, even though the twins were often raised in very different environments. These similarities indicate that genetics plays a role in determining behavior even in humans, although the relative importance of genetics versus environment is still hotly debated. 536 Part VII Ecology and Behavior Parental generation Firs...
let but not its taste. Similarly, pigeons can learn to associate food with colors but not with sounds; on the other hand, they can associate danger with sounds but not with colors. These examples of learning preparedness demonstrate that what an animal can learn is biologically influenced—that is, learning is possible ...
oral development. Physical contact plays an important role in the development of psychological well-being and growth. Chapter 26 Animal Behavior 541 The Physiology of Behavior The early ethologists’ emphasis on instinct sometimes overlooked the internal factors that control behavior. If asked why a male bird defends a ...
e perceived, and identifying the ecological roles they play and their evolutionary origins. Courtship During courtship, animals produce signals to communicate with potential mates and with other members of their own sex. A stimulus-response chain sometimes occurs, in which the behavior of one individual in turn release...
was resolved (in the minds of most scientists) in the mid-1970s by the creative research of James L. Gould. Gould devised an experiment in which hive members were tricked into misinterpreting the directions given by the scout bee’s dance. As a result, Gould was able to manipulate where the hive members would go if they...
adults who have made the migratory journey before. Starlings were captured in Holland, halfway along their full migratory route from Baltic breeding grounds to wintering grounds in the British Isles; these birds were transported to Switzerland and released. Experienced older birds compensated for the displacement and ...
result of close genetic relationship of society members. 27.4 Vertebrates exhibit a broad range of social behaviors. Vertebrate Societies. Many vertebrate societies exhibit altruism. Human Sociobiology. Human behavior, like that of other vertebrates, is influenced by natural selection. FIGURE 27.1 A snake in the throes...
ended by displays that advertise that the territories are occupied and by overt aggression. A bird sings from its perch within a territory to prevent a takeover by a neighboring bird. If an intruder is not deterred by the song, it may be attacked. However, territorial defense has its costs. Singing is energetically exp...
considerably larger than females, for the simple reason that the largest males are the ones that get to mate. Such differences between the sexes are referred to as sexual dimorphism. In other species, males have evolved structures used for fighting, such as horns, antlers, and large canine teeth. These traits are also ...
athered by a male other than the female’s mate is sometimes surprisingly high. Why do individuals have extra-pair copulations? For males, the answer is obvious: increased reproductive success. For females, it is less clear, as in most cases, it does not result in an increased number of offspring. One possi- bility is t...
to support copies of themselves in other individuals. Hamilton’s kin selection model predicts that altruism is likely to be directed toward close relatives. The more closely related two individuals are, the greater the poten- tial genetic payoff. This relationship is described by Hamilton’s rule, which states that alt...
istic than insects. Nevertheless, in some complex vertebrate social systems individuals may be exhibiting both reciprocity and kin-selected altruism. But vertebrate societies also display more conflict and aggression among group members than do insect societies. Conflict in vertebrate societies generally centers on acc...
important in determining which sex exhibits mate choice? 4. In birds, how does the amount of parental care required by the offspring affect the evolution of a species’ mating system? 27.3 There is considerable controversy about the evolution of social behavior. • Bioethics Case Study: Behavior Disordered Students • On ...
opulations have been experiencing deformities and markedly declining populations—had less than one-third the photolyase activity seen in Hyla, and were strongly affected by UV-B radiation, showing significant increases in mortality rates when exposed to UV-B radiation. These results suggest that increased level of UV-B...
forest in Colombia prevented water from returning to the atmosphere and created a semiarid desert. It is a tragedy of our time that just such a transformation is occurring in many tropical areas, as tropical and temperate rainforests are being clear-cut or burned in the name of “development” (figure 28.3). Much of Mada...
tal methodology that is being applied successfully to the study of other ecosystems. Hubbard Brook is the central stream of a large watershed that drains a region of temperate deciduous forest. To measure the flow of water and nutrients within the Hubbard Brook ecosystem, concrete weirs with V-shaped notches were built...
ductivity by herbivores is approximately an order of magnitude less than the primary productivity upon which it is based. Where does all the energy in plants that is not captured by herbivores go (figure 28.10)? First, much of the biomass is not consumed by herbivores and instead supports the decomposer community (bact...
fish Trout No fish Trout FIGURE 28.14 Trophic cascades. Streams with trout have fewer herbivorous invertebrates and more algae than streams without trout. • • Fish added No fish added • • • • •• • • • • • • ) 300 200 100 0 60 50 40 30 20 10 0 5000 4000 3000 2000 1000 0 Sample 1 (June 5) Sample 2 (June 22) Chapter 28 D...
Effect of species richness on ecosystem stability. In the Cedar Creek experimental fields, each square is a 100-square-foot experimental plot. Experimental plots with more plant species seem to show less variation in the total amount of biomass produced, and thus more community stability. • • 0.40 0.35 0.30 0.25 0.20 ...
pecies-area relationship was a result of the effect of area on the likelihood of species extinction and colonization. smaller. Also, we would expect fewer colonizers to reach islands that lie farther from the mainland. Thus, small islands far from the mainland have the fewest species; large islands near the mainland ha...
acoasts. Green and dark green areas on land are dense forests, while orange areas like the deserts of western South America are largely barren of life. 591 29.1 Organisms must cope with a varied environment. The Environmental Challenge How Environments Vary The nature of the physical environment in large measure determ...
tly as a result of human activities, poses serious ecological problems. Why the Tropics Are Warmer Equator Sunlight Sunlight (a) Vernal equinox (sun aims directly at equator) 60 30 Equator 30 Summer solstice (northern hemisphere tilts toward the sun) (b) 60 30 Equator 30 231/2 Sun 60 30 Equator 30 Winter solstice (nort...
area. As a result, mean temperatures are lower. At temperate latitudes, temperature variation increases because of the increasingly marked seasons. Seasonal changes in wind circulation produce corresponding changes in ocean currents, sometimes causing nutrient-rich cold water to well up from ocean depths. This produces...
s and cool winters) and plentiful rains promote the growth of deciduous (hardwood) forests in Eurasia, the northeastern United States, and eastern Canada (figure 29.14). A deciduous tree is one that drops its leaves in the winter. Deer, bears, beavers, and raccoons are familiar animals of the temperate regions. Because...
occur above. The supply of oxygen can often be critical in the ocean, and as water temperatures become warmer, the water holds less oxygen. For this reason, the amount of available oxygen becomes an important limiting factor for organisms in warmer marine regions of the globe. Carbon dioxide, in contrast, is almost nev...
ense clusters of large animals living on geothermal energy at a depth of 2500 meters. These deep-sea oases occur where seawater circulates through porous rock at sites where molten material from beneath the earth’s crust comes close to the rocky surface. A series of these areas occur on the Mid-Ocean Ridge, where basal...
ining wetlands. The most productive freshwater ecosystems are wetlands. Most lakes are far less productive, limited by lack of nutrients. Chapter 29 The Biosphere 609 Chapter 29 Summary www.mhhe.com/raven6e www.biocourse.com Questions Media Resources 29.1 Organisms must cope with a varied environment. • Organisms emplo...
be growing rapidly. Population Growth Rate Starting to Decline The United Nations has announced that the world population growth rate continues to decline, down from a high of 2.0% in the period 1965–1970 to 1.4% in 1998. Nonetheless, because of the larger population, this amounts to an increase of 77 million people pe...
terms of energy. Mismanagement of fisheries, mainly through overfishing, local pollution, and the destruction of fish breeding and feeding grounds, has lowered the catch of fish in the sea by about 20% from maximum levels. Many fishing areas that were until recently important sources of food have been depleted or clos...
have warming, or “greenhouse,” effects similar to those of carbon dioxide. One, methane, increased from 1.14 ppm in the atmosphere in 1951 to 1.68 ppm in 1986—nearly a 50% increase. Major problems associated with climatic warming include rising sea levels. Sea levels may have already risen 2 to 5 centimeters from globa...
, and Styrofoam containers, were percolating up through the atmosphere and reducing O3 molecules to O2. One chlorine atom from a CFC molecule could destroy 100,000 ozone molecules in the following mechanism: UV radiation causes CFCs to release Cl atoms: Although other factors have also been implicated in ozone depletio...
ve to illustrate the point. The Nashua River. Running through the heart of New England, the Nashua River was severely polluted by mills established in Massachusetts in the early 1900s. By the 1960s, the river was clogged with pollution and declared ecologically dead. When Marion Stoddart moved to a town along the river...
Factors Responsible for Extinction. Most recorded extinctions can be attributed to a few causes. Habitat Loss. Without a place to live, species cannot survive. Case Study: Overexploitation Case Study: Introduced Species Case Study: Disruption of Ecological Relationships Case Study: Loss of Genetic Variation Case Study...
tion Biology 627 Species Endemism and Hot Spots A species found naturally in only one geographic area and no place else is said to be endemic to that area. The area over which an endemic species is found may be very large. The black cherry tree (Prunus serotina), for example, is endemic to all of temperate North Americ...
dscape (see chapter 24). Each individual subpopulation may be quite small and in real threat of extinction, but the metapopulation may be quite safe from extinction so long as individuals from other populations repopulate the habitat patches vacated by extinct populations. The extent of this rescue effect is an importa...
dian governments have closed the fishery, but no one can predict if the fish populations will recover. The Atlantic bluefin tuna has experienced a 90% population decline in the last 10 years. The swordfish has declined even further. In both cases, the drop has led to even more intense fishing of the remaining populatio...
imal, each individual captured is of significant commercial value. Right whales were the first to bear the brunt of commercial whaling. They were called right whales because they were slow, easy to capture, and provided up to 150 barrels of blubber oil and abundant whalebone, making them the “right” whale for a commerc...
airie chicken (Tympanuchus cupido pinnatus) is a showy 2-pound wild bird renowned for its flamboyant mating rituals (figure 31.18). Abundant in many midwestern states, the prairie chickens in Illinois have in the last six decades undergone a population collapse. Once, enormous numbers of birds covered the state, but wi...
n option. Species, and in some cases whole communities, have disappeared or have been irretrievably modified. The clear-cutting of the temperate forests of Washington State leaves little behind to conserve; nor does converting a piece of land into a wheat field or an asphalt parking lot. Redeeming these situations requ...
great cost to survival. (a) Equator Black African Rhino Present distribution Former distribution (b) FIGURE 31.23 Sustaining genetic diversity. The black rhino (a) is highly endangered, living in 75 small, widely separated populations (b). Only about 2400 individuals survive in the wild. Chapter 31 Conservation Biology...
e restoration of a habitat may be attempted, but removing introduced species, rehabilitating the habitat, and cleaning up the habitat may be more feasible. • Captive propagation, sustaining genetic variability, and preserving keystone species have been effective in preserving biodiversity. • Megareserves have been succ...
itute a worldwide moratorium on the commercial killing of whales? a. 1935 b. 1946 c. 1952 d. 1974 e. 1986 e. none of them should experience an edge 26. All of the following endangered species have effect 20. The single greatest factor threatening species today and causing their extinction is a. loss of genetic variabil...
out the rest of the virus genome from infected cells. From the virus genome, it was a straightforward matter to develop a diagnostic antibody test for the presence of the HCV virus. Using the diagnostic test, researchers found hepatitis C to be far more common than had been supposed. This is a problem of major proport...
with spear-shaped, smooth leaves with absolutely no teeth along the margins”). For the common red oak of eastern temperate North America (figure 32.2b), Linnaeus devised a new name, Quercus foliis obtuse-sinuatis setaceo-mucronatis (“oak with leaves with deep blunt lobes bearing hairlike bristles”). For each of these s...
representative samples of organisms from different environments, such as the upper branches of tropical trees or the deep ocean, scientists have estimated the total numbers of species that may actually exist to be about 10 million, about 15% of them marine organisms. Most Species Live in the Tropics Most species, perha...
nal power and judgment of the biologist is brought to bear—and also any biases he or she may have. For example, in classifying the terrestrial vertebrates, traditional taxonomists place birds in their own class (Aves), giving great weight to the characters that made powered flight possible, such as feathers. However, c...
GURE 32.11 A tree of life. This phylogeny, prepared from rRNA analyses, shows the evolutionary relationships among the three domains. The base of the tree was determined by examining genes that are duplicated in all three domains, the duplication presumably having occurred in the common ancestor. When one of the duplic...
taining two sets of chromosomes, during some part of their life cycle. A few eukaryotes complete their life cycle in the haploid condition, with only one set of chromosomes in each cell. As we have seen, in diploid cells, one set of chromosomes comes from the male parent and one from the female parent. These chromosome...
all organisms. Able to reproduce only within living cells, viruses are not themselves alive. 33.2 Bacterial viruses exhibit two sorts of reproductive cycles. Bacteriophages. Some bacterial viruses, called bacteriophages, rupture the cells they infect, while others integrate themselves into the bacterial chromosome to b...
e translated into proteins by the cell’s genetic machinery. These proteins lead to the production of more viruses. Viral Shape Most viruses have an overall structure that is either helical or isometric. Helical viruses, such as the tobacco mosaic virus, have a rodlike or threadlike appearance. Isometric viruses have a ...
V very difficult to control. The reason why HIV remains hidden for so long seems to be that its infection cycle continues throughout the 8- to 10-year latent period without doing serious harm to the infected person. Eventually, however, a random mutational event in the virus allows it to quickly overcome the immune def...

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