All daughter cells are haploid.

A．Although all cell types can divide,only stem cells differentiate after division．

B．Stem cells are able to divide to produce populations of cells that differentiate into a related set of cells．

C．Stem cells divide asymmetrically to give rise to one daughter cell that remains a stem cell, and a second daughter cell that differentiates into one or more cell types．

D．Stem cells have a special type of cell division in which they form small stems that then pinch off to give rise to the daughter cell．

A、Although all cell types can divide, only stem cells differentiate after division.

B、Stem cells are able to divide to produce populations of cells that differentiate into a related set of cells.

C、Stem cells divide asymmetrically to give rise to one daughter cell that remains a stem cell, and a second daughter cell that differentiates into one or more cell types.

D、Stem cells have a special type of cell division in which they form small stems that then pinch off to give rise to the daughter cell.

A.a

B.b

C.c

D.d

A hematopoietic stem cell is defined as

A．a cell with extensive self-renewal and proliferative potential

B．a cell only to produce daughter cells with identical characteristics

C．a cell with the capacity to differentiate into the progenitors of all the blood cell lineages

D．a cell With the capacity to produce some hematopoietic growth factors such as SCF，GM-CSFs，et．a1．

E．a cell with the capacity to express some specific surface antigens such as CD34

A．One of the simplest and most important is through a positive feedback loop, where a master transcription regulator activates transcription of its own gene,in addition to that of other cell-typespecific genes. Each time a cell divides, the regulator is distributed to both daughter cells, where it continues to stimulate the positive feedback loop.？The continued stimulation ensures that the regulator will continue to be produced in subsequent cell generations.？Positive feedback is crucial for establishing the “self-sustaining” circuits of gene expression that allow a cell to commit to a particular fate—and then to transmit that decision to its progeny.

B．Although positive feedback loops are probably the most prevalent way of ensuring that daughter cells remember what kind of cells they are meant to be,there are other ways of reinforcing cell identity. One involves the methylation of DNA．In vertebrate cells, DNA methylation occurs on certain cytosine bases. This covalent modification generally turns off the affected genes by attracting proteins that bind to methylated cytosines and block gene transcription. DNA methylation patterns are passed on to progeny cells by the action of an enzyme that copies the methylation pattern on the parent DNA strand to the daughter DNA strand as it is synthesizeD

C．Another mechanism for inheriting gene expression patterns involves the modification of histones. When a cell replicates its DNA,each daughter double helix receives half of its parent’s histone proteins, which contain the covalent modifications that were present on the parent chromosom

E．Enzymes responsible for these modifications may bind to the parental histones and confer the same modifications to the new histones nearby. It has been proposed that this cycle of modification helps reestablish. the pattern of chromatin structure found in the parent chromosome

D．Because all of these cell-memory mechanisms transmit patterns of gene expression from parent to daughter cell without altering the actual nucleotide sequence of the DNA,they are considered to be forms of epigenetic inheritanc

E．These mechanisms, which work together, play an important part in maintaining patterns of gene expression, allowing transient signals from the environment to be remembered by our cells—a fact that has important implications for understanding how cells operate and how they malfunction in diseas

E．

A．One of the simplest and most important is through a positive feedback loop, where a master transcription regulator activates transcription of its own gene,in addition to that of other cell-typespecific genes. Each time a cell divides, the regulator is distributed to both daughter cells, where it continues to stimulate the positive feedback loop.？The continued stimulation ensures that the regulator will continue to be produced in subsequent cell generations.？Positive feedback is crucial for establishing the “self-sustaining” circuits of gene expression that allow a cell to commit to a particular fate—and then to transmit that decision to its progeny.

B．Although positive feedback loops are probably the most prevalent way of ensuring that daughter cells remember what kind of cells they are meant to be,there are other ways of reinforcing cell identity. One involves the methylation of DNA．In vertebrate cells, DNA methylation occurs on certain cytosine bases. This covalent modification generally turns off the affected genes by attracting proteins that bind to methylated cytosines and block gene transcription. DNA methylation patterns are passed on to progeny cells by the action of an enzyme that copies the methylation pattern on the parent DNA strand to the daughter DNA strand as it is synthesizeD

C．Another mechanism for inheriting gene expression patterns involves the modification of histones. When a cell replicates its DNA,each daughter double helix receives half of its parent’s histone proteins, which contain the covalent modifications that were present on the parent chromosom

E．Enzymes responsible for these modifications may bind to the parental histones and confer the same modifications to the new histones nearby. It has been proposed that this cycle of modification helps reestablish. the pattern of chromatin structure found in the parent chromosome

D．Because all of these cell-memory mechanisms transmit patterns of gene expression from parent to daughter cell without altering the actual nucleotide sequence of the DNA,they are considered to be forms of epigenetic inheritanc

E．These mechanisms, which work together, play an important part in maintaining patterns of gene expression, allowing transient signals from the environment to be remembered by our cells—a fact that has important implications for understanding how cells operate and how they malfunction in diseas

E．

ge, choose the best answer to each question. Answer all questions following the passage on the basis of what is stated or implied in the passage.

The complex life cycle of the Plasmodium protozoan, the causative agent of malaria, has contributed to the difficulty of devising effective public health measures to combat the disease. It took scientists centuries to deconstruct the basic relationship between protozoan, mosquito vector, and human host. Modern physiologists and epidemiologists are still working out the intricacies of malarial infection.

The disease is transmitted by the bite of a female Anopheles mosquito infected with the Plasmodium parasite. Only Anopheles mosquitoes are capable of transmitting the disease, and only females take blood meals from humans. To become infected with Plasmodium, the female mosquito takes a blood meal from a human carrying the parasite in his or her blood. Once ingested, the parasite matures in the mosquito's gut for approximately a week, after which it migrates to the insect's salivary glands. By mixing with the mosquito's saliva, the parasite facilitates its transmission to a human host when the mosquito bites that human.

Once in a human's bloodstream, the parasite travels to the human's liver. At this initial stage, the Plasmodium parasite is called a sporozoite. Within the liver, the sporozoite can form. 30,000 to 40,000 daughter cells, called merozoites, which are released into the host's bloodstream at a later date, sometimes within a week of the initial infection and sometimes as much as several months later. The merozoites seek out and attach themselves to red blood cells, in which they incubate 8 to 24 daughter cells over the next two days. When the daughter cells are mature, the red blood cell ruptures and the new parasites are released into the bloodstream to seek out red blood cells of their own. Some of the new merozoites become male and female gametocytes; if these gametocytes are ingested by a mosquito feeding on the host's blood, they will fertilize in the mosquito's gut to produce new sporozoites, and the cycle will continue.

The symptoms that we associate with malaria--a high, recurring fever; joint pain; a swollen spleen--are caused by toxins released from the red blood cells ruptured by merozoites. The human spleen can destroy these infected blood cells, but the Plasmodium parasite counters this effect by increasing the stickiness of proteins on the blood cells' surfaces so that the cells stick to the walls of blood vessels. If the sticky surface proteins affect a particularly large number of cells, the malaria can trans-form. into a hemorrhagic fever, the most deadly form. of malaria.

A further complicating factor in the natural history of malaria is the many variants of the Plasmodium protozoan. Scientists now recognize that malaria is caused by at least six different species: P. falciparum, P. vivax, P. ovale, P. malariae, P. knowesli, and P. semiovale. Of these species, P. falciparum accounts for the majority of infections and approximately 90 percent of malarial deaths in the world.

The passage is primarily concerned with which of the following？

A．Describing the life cycle of the Plasmodium protozoan as it relates to the disease malaria

B．Comparing and contrasting the life cycles of the six variants of the Plasmodium protozoan known to cause malaria

C．Addressing the public health implications of the life cycle of the Plasmodium parasite

D．Providing information on how a person can avoid infection with malaria

E．Describing the life cycle of the Anopheles mosquito as it relates to the transmission of the Plasmodium protozoan to humans

A.5

B.10

C.20

D.40

A.a

B.b

C.c

D.d