Chapter 1: Cells: The Fundamental Units of Life
What is the name of the process by which eukaryotic cells engulf material captured from an external medium? Choose one: A. endocytosis B. endomitosis C. exocytosis D. endosymbiosis E. cytokinesis
Endocytosis. Endocytosis is the name of the process by which eukaryotic cells engulf material captured from an external medium. This is accomplished when a portion of the plasma membrane invaginates and then pinches off, forming a transport vesicle. In the reverse process, termed exocytosis, a transport vesicle fuses with the plasma membrane and in so doing, releases its contents outside the cell. Together, endocytosis and exocytosis represent an important import and export mechanism for the cell, especially for large sized or large quantities of materials.
What organisms commonly colonize the hot acid of volcanic springs, the airless depths of marine sediments, the sludge of sewage treatment plants, the pools beneath the frozen surface of Antarctica, as well as the acidic, oxygen-free environment of a cow's stomach where they break down ingested cellulose and generate methane gas? Choose one: A. eukaryotes B. bacteria C. archaea D. all of these
Archaea. Archaea are prokaryotic cells that can survive in these rather "extreme" environmental conditions. Archaea are found not only in these habitats but also in environments that are too hostile for most other cells. Many of these extreme environments resemble the harsh conditions that must have existed on the primitive Earth, a time when living things first evolved before the atmosphere became rich in oxygen.
Is the following statement true, false, or impossible to determine? One thing all cells have in common is an ability to colonize any environment on Earth. Choose one: true false impossible to determine
False. No cell is capable of colonizing any environment on Earth. Although living cells have expanded to fill every conceivable habitat on Earth, most cells are capable of thriving only in a single, specific environment. A bacterium adapted for life in the human gut, for example, would not survive in a hot spring or an Antarctic lake.
Which is not a function of proteins? Choose one: A. providing cells with structural support B. acting as molecular motors C. encoding genetic information D. encasing certain viruses E. catalyzing biochemical reactions
Encoding genetic information. Encoding genetic information is not a cellular function of proteins; rather, nucleic acids perform this function. A cell's genome—that is, the entire sequence of nucleotides in an organism's DNA—provides the information that instructs a cell how to produce all of the proteins needed to function. However, DNA codes for proteins indirectly, with messenger RNA molecules serving as the link between the DNA sequence of a gene and the amino acid sequence of the protein. Proteins carry out a wide range of tasks and the function of a cell is determined, in large part, by which proteins it produces.
Identify the three fundamental processes that are involved in the central dogma of molecular biology by dragging the three labels to their targets.
In all living cells, genetic information flows from DNA to RNA in a process called transcription and from RNA to protein in a process called translation—an arrangement of highly efficient template-based processes known as the central dogma. Prokaryotic cells like bacteria perform these processes as well as much more highly evolved eukaryotic cells like those of humans. When DNA is to be replicated, then DNA polymerase uses the information of DNA as a template to synthesize more DNA.
Which is the smallest object that can be seen using an electron microscope? Choose one: A. individual electron B. individual hydrogen atom C. individual large organelle D. individual nanometer E. individual molecule
Individual molecule. An electron microscope allows visualization of structures that are as small as a few nanometers and provides the greatest magnification and resolution currently possible. An individual molecule that is only a few nanometers is therefore the smallest object that can be seen using an electron microscope. An example would be a phospholipid that makes up the plasma membrane or even nucleotides that make up DNA. An electron microscope uses a beam of electrons rather than a beam of light to "illuminate" a sample. Because electrons have a much shorter wavelength than light, electron microscopes greatly extend our ability to see the fine details of cells and even render some of the larger molecules visible individually. Their resolving power is therefore much better than that of the compound light microscope. Only larger cell organelles can be observed with a compound light microscope.
What is a drawback to using electron microscopy? Choose one: It requires the use of fluorescent probes.It can be used only to view samples that are sliced very thinly.It cannot be used to view a whole cell or organism.It cannot be used to view living cells.It can be used only to view atomic details in structures larger than a ribosome.
It cannot be used to view living cells. Electron microscopy can reveal details down to a few nanometers, but it cannot be used to view living cells. Electron microscopy is carried out in a vacuum and bombards samples with electrons, conditions that would destroy living cells. Transmission electron microscopy requires samples to be thinly sliced, but scanning electron microscopy can be used to view the surface details of whole samples. For both types of electron microscopy, the sample must be incubated and heated with heavy metals. These conditions would be far too harsh for living cells.
When comparing liver cells and kidney cells within an organism, many differences can be observed and documented. Which of following is not a difference between liver cells and kidney cells in the same animal? Choose one: A. The different cells are generated during the animal's development. B. The different cells produce different proteins. C. The different cells have different roles in the body. D. The different cells express different genes. E. The different cells have different DNA.
The different cells have different DNA. The statement that these different cells have different DNA is NOT true about liver cells and kidney cells in the same animal. Liver cells and kidney cells have the same genome, but they use the encoded information differently, expressing different genes and producing different proteins. The different cells have different roles in the body and are generated during the animal's embryological development.
Is the following statement true, false, or impossible to determine? At a molecular level, the members of the two domains of prokaryotes—the archaea and bacteria—differ as much from each other as either does from the eukaryotes. Choose one: true false impossible to determine
True. Currently, biologists classify all living organisms into three domains: eukaryotes, bacteria, and archaea. Eukaryotic cells, which are bigger and more complex than bacteria and archaea, have long been recognized as a distinct cell type. However, because bacteria and archaea are similar in appearance, they were previously grouped together as prokaryotes. Recent discoveries achieved through DNA sequencing have revealed that bacteria and archaea are actually quite evolutionarily distinct, and diverged from a common prokaryotic ancestor approximately 3.5 billion years ago. Thus, bacteria and archaea are just as distinct from each other as either is from eukaryotic cells.
Which organelle's ancestor was likely engulfed by primitive eukaryotes to help the cell survive in an oxygen-rich atmosphere? Choose one: A. lysosome B. endoplasmic reticulum C. mitochondrion D. peroxisome E. Golgi apparatus
Mitochondrion. The mitochondrion is an organelle that contains a double membrane—that is, two lipid bilayers that surround it. This organelle's ancestor was likely engulfed by primitive eukaryotes to help the cell survive in an oxygen-rich atmosphere. Thus, the inner mitochondrial membrane derived from the plasma membrane of the engulfed cell, and the outer mitochondrial membrane derived from the evolving eukaryotic cell's plasma membrane. The Golgi apparatus, endoplasmic reticulum, lysosome, and peroxisome are not surrounded by a double membrane.
What do eukaryotic cells have that prokaryotes lack? Choose one: A. a cell wall B. a means of using chemical energy C. a nucleus and membrane-bound organelles D. ribosomes E. nucleic acids
A nucleus and membrane-bound organelles. All cells have a means of converting energy from one form to another, whether from food, inorganic minerals, or directly from sunlight. Additionally, all cells contain nucleic acids (DNA as their genome) and ribosomes (for creating protein), and some eukaryotic cells like plants and fungi have cell walls. However, only eukaryotic cells have cell nuclei and other membrane-bound organelles; these features are not observed in prokaryotic cells. DNA is contained in the eukaryotic cell nucleus and each membrane-bound organelle carries out a specific cellular function.
Which statement represents the cell theory? Choose one: A. All cells require a continual input of energy to sustain life. B. All cells contain DNA. C. All cells can be seen using a microscope. D. All cells are formed by the growth and division of existing cells. E. All cells resemble square or rectangular chambers.
All cells are formed by the growth and division of existing cells. Observations made with the light microscope in the nineteenth century by biologists, including the seminal work by Matthias Schleiden and Theodor Schwann, documented that all plant and animal tissues were composed of cells. With repeated examination, a concept slowly emerged that all living organisms are composed of cells and that new cells are formed by the division of existing cells. Later experiments by Louis Pasteur helped confirm the cell theory and dispel the competing idea of spontaneous generation.
Which of the following is a model plant used by scientists? Choose one: A. Homo sapiens B. Caenorhabditis elegans C. Saccharomyces cerevisiae D. Homo floresiensis E. Arabidopsis thaliana
Arabidopsis thaliana. Arabidopsis thaliana, the common wall cress, is a model plant that is favored by plant biologists for a number of reasons: it can be grown indoors and reproduces quickly, with one plant giving rise to thousands of offspring in just 8-10 weeks, and it is related to plants of agricultural value, making research with it relevant to the production of crops that feed humans. A. thaliana, like S. cerevisiae, C. elegans, and other model organisms, has its strengths and limitations, but all are indispensable in helping researchers understand the cellular and molecular processes that govern life.
Which structure could not be seen using an electron microscope? Choose one: A. individual cell B. DNA C. electron D. ribosomes E. plasma membrane
Electron. An electron microscope uses a beam of electrons to "illuminate" a sample, and it is the sample that is visualized. However, no one has ever "seen" an individual electron. The electron microscope allows biologists to visualize very small structures, including DNA, ribosomes, and the plasma membrane. This represents the highest magnification and resolution capacity at the biologist's disposal when observing cells.
Genetic changes followed by selection are best described as the fundamentals of what process? Choose one: A. DNA replication B. genetic drift C. evolution D. regeneration E. reproduction
Evolution. Genetic changes followed by selection are the fundamentals of evolution, and this process is responsible for the stunning variety of organisms on this planet. With a deeper understanding of cells, we can begin to tackle the grand historical problems of life on Earth: its mysterious origins, its stunning diversity produced by billions of years of evolution. All living things on Earth share a common cell ancestor that lived on Earth at least 3.5 billion years ago. Over the course of time, driven by evolution, this ancestral cell gave rise to the great diversity of life we see around us today, yet it also explains why all living things are united by the cell as the fundamental unit of life.
Which of the following is a stack of flattened membrane-enclosed sacs that receives molecules made in the endoplasmic reticulum, often chemically modifies them, and then directs them to the exterior of the cell or to various locations inside the cell? Choose one: A. Golgi apparatus B. endoplasmic reticulum C. ribosome D. peroxisome E. lysosome
Golgi apparatus. The Golgi apparatus functions to modify and package molecules and appears like a stack of large, flattened vesicles with smaller vesicles that have budded from the Golgi stack nearby. These vesicles contain molecules that are being transported to other parts of the cell or are destined to be excreted. The Golgi apparatus is one of a group of membrane-bound organelles including the endoplasmic reticulum, lysosomes, and peroxisomes, which all work together in a dynamic fashion to synthesize, break down, modify, and transport molecules within the cell.
Which of these cannot be resolved with a conventional light microscope? Choose one: A. ribosome B. cell nucleus C. embryonic cell D. mitochondrion E. bacterium
Ribosome. A conventional light microscope has a resolution of about 0.2 µm. A ribosome is approximately 20 nanometers (nm) in size, which is 0.02 µm, and below the resolution of a light microscope. If an object is below the limit of resolution, then that object cannot be individually seen with that instrument. Mitochondria, bacterial cells, cell nuclei, and embryonic cells are all larger than the limit of resolution (about 0.2 µm) of the conventional light microscope and therefore these structures can be observed.
The electron micrograph below shows a portion of the cell. Use the included drawing to help you label the components shown.
The nucleus contains most of the DNA in a eukaryotic cell. In the electron micrograph, the nucleus is directly in the center and is immediately surrounded by the nuclear envelope. Individual chromosomes in the nucleus are not visible because at this stage of the cell-division cycle, the DNA molecules are dispersed as fine threads throughout the nucleus—DNA only condenses into readily identifiable chromosomes during cell division. The liquid surrounding the nuclear envelope is the cytoplasm in which all organelles are suspended. In addition to the nucleus, a mitochondrion is also visible in this micrograph. Like the double lipid bilayer of the nuclear envelope, a mitochondrion also has a double membrane and is the organelle where the citric acid cycle and electron transport chain of cellular respiration take place.
The bacterial cell micrograph in Part I is most likely a __________ electron micrograph. (transmission or scanning)
Transmission. The bacterial cell micrograph in Part I is most likely a transmission electron micrograph. An electron micrograph of a longitudinal section is shown here, and a scanning electron microscope does not involve a thin slice of the specimen to reveal internal cell features. In contrast, the scanning electron microscope is used to create high-resolution three-dimensional images of the surface of structures.
Is the following statement true, false, or impossible to determine?An individual ribosome can be seen with a fluorescence microscope. Choose one: true false impossible to determine
True. Fluorescence microscopes use sophisticated methods of illumination and electronic image processing to see fluorescently labeled cell components in much finer detail. Some modern, super-resolution fluorescence microscopes can push the limits of resolution to about 20 nanometers (nm). That is the size of a single ribosome, a large macromolecular complex in which RNAs are translated into proteins. These microscopes rely on cells incubated with a fluorescently labeled molecule that will bind to the intracellular target.
Why do cell biologists study yeast? Choose one or more: A. Yeast is a bacterium and therefore very easy to grow. B. Yeast carries out all the basic tasks that a eukaryotic cell must perform. C. Yeast is as closely related to animals as it is to plants. D. Yeast lacks a nucleus, so it is easy to genetically manipulate.
B & C. Yeasts do not lack a nucleus. While they may be easy to genetically manipulate, yeasts are eukaryotic and yeast cells therefore have very defined nuclei that contain the cellular DNA. Yeasts are unicellular eukaryotic fungi. S. cerevisiae is a small, single-celled fungus that is at least as closely related to animals as it is to plants. Like other fungi, it has a rigid cell wall, is relatively immobile, and possesses mitochondria but not chloroplasts. When nutrients are plentiful, S. cerevisiae reproduces almost as rapidly as a bacterium, yet it carries out all the basic tasks that every eukaryotic cell must perform. Genetic and biochemical studies in yeast have been crucial to understanding many basic mechanisms in eukaryotic cells, including the cell-division cycle—the chain of events by which the nucleus and all the other components of a cell are duplicated and parceled out to create two daughter cells.
In the electron micrograph, identify the bacterial cell features by dragging the labels to their targets.
Beyond their smaller size, the most notable difference of a prokaryotic cell compared to a eukaryotic cell is the absence of a nucleus and other membrane-bound organelles. The interior of a prokaryotic cell is a single compartment containing the cytoplasm and DNA, appearing with a varied texture. Additionally, some bacteria, including the one shown here, have a tough cell wall in addition to the plasma membrane. The bacterium shown in this question, Escherichia coli, also has an outer membrane that surrounds the cell wall.
Mitochondria are essentially the same in all eukaryotes, including plants, animals, and fungi. Based on this observation, how were mitochondria most likely acquired? Choose one: A. by an ancestral eukaryotic cell and then replaced by chloroplasts in the line that led to plant cells B. by an ancestral prokaryote and then lost in the line that led to archaea C. by a prokaryotic cell approximately 1000 years ago D. from a free-living, photosynthetic bacterium E. by an ancestral eukaryotic cell before the lines that led to animal cells, plant cells, and fungi diverged
By an ancestral eukaryotic cell before the lines that led to animal cells, plant cells, and fungi diverged. Currently, the most favored explanation for the appearance of mitochondria in eukaryotic cells is the establishment of an endosymbiotic relationship between a primitive eukaryotic cell and a free-living aerobic bacterium. The proto-eukaryote, which gave rise to plants, animals, and fungi, would have engulfed the bacterial cell whole, allowing it to persist inside itself where it eventually became what we now call the mitochondrion. Several lines of evidence support this idea, including the presence of predatory protozoans such as Didinium, which are capable of engulfing other cells whole, and the presence of ribosomes and mitochondrial DNA inside of mitochondria, which highlights their potential prokaryotic origins.
Which statement is not true of chloroplasts? Choose one: A. Chloroplasts absorb light and generate oxygen and carbohydrate. B. Chloroplasts are present in essentially all eukaryotic cells and in certain photosynthetic bacteria. C. Chloroplasts contain their own DNA. D. Chloroplasts are thought to have originated from bacteria. E. Each has an internal stack of membranes and is enclosed by two membranes.
Chloroplasts are present in essentially all eukaryotic cells and in certain photosynthetic bacteria. A chloroplast is the organelle that captures energy from sunlight and converts it into chemical potential energy in the form of sugar molecules and releases oxygen as a by-product, a process known as photosynthesis. Not all eukaryotic cells have this capability. Only plants, protozoans, and algae contain chloroplasts and can perform photosynthesis. In fact, even photosynthetic bacteria do not have chloroplasts. The components required for photosynthesis reside in the cell membrane of these photosynthetic prokaryotes. The origin of chloroplasts in eukaryotes is believed to have arisen from an endosymbiotic event between a photosynthetic bacterium and a predatory eukaryotic cell, occurring relatively recently in evolutionary history after plants, animals, and fungi diverged. This theory explains why chloroplasts have their own DNA and why only some eukaryotic cells have chloroplasts.
Consider this evolutionary tree and then answer the questions. Scientists study model organisms within these groups to gain insight into these types of organisms and their genetic heritage. Is the following statement true, false, or impossible to determine? Photosynthetic bacteria contain chloroplasts. Choose one: A. true B. false C. impossible to determine
False. Photosynthetic bacteria do not contain chloroplasts. Photosynthetic bacteria are prokaryotic, and prokaryotic cells do not contain any membrane-bound organelles. The chloroplast is a membrane-bound organelle that is part of some eukaryotic cells. For photosynthetic bacteria, the photosystem complexes that absorb light energy are embedded in the plasma membrane of the cell; this functions analagously to the inner membrane of the chloroplast of eukaryotic cells.
Is the following statement true, false, or impossible to determine? Like the differentiated cells in an individual plant or animal, all bacteria have the same DNA. Choose one: A. true B. false C. impossible to determine
False. Unlike the differentiated cells in an individual plant or animal, all bacteria do not have the same DNA. Each bacterial species has its own characteristic nucleotide sequence. The specialized cells within a multicellular organism develop from a single fertilized egg and thus share the same DNA. Bacteria belong to different species, each of which has evolved independently, becoming gradually modified and adapted to suit specific environments.
In 1970, Frye and Edidin published research describing the mobility of plasma membrane proteins. They fused mouse and human cells together, creating a hybrid cell, and then examined the localization of mouse and human proteins over time. Initially mouse and human proteins were each restricted to one-half of the heterokaryon, but over time the mouse and human proteins mixed, with each being present over the entire cell surface. What technique did Frye and Edidin likely use to examine the mouse and human proteins? Choose one: A. interference contrast light microscopy B. scanning electron microscopy C. fluorescence microscopy D. transmission electron microscopy
Fluorescence microscopy. Frye and Edidin tagged antibodies that recognized mouse proteins with the fluorescent protein rhodamine and antibodies that recognized human proteins with fluorescein. The location of mouse and human proteins could then be observed over time with a fluorescent microscope, as shown in the schematic below.
Identify the eukaryotic cellular locations of the fundamental processes that are involved in the central dogma of molecular biology by dragging the correct labels to their targets.
In all living cells, genetic information flows from DNA to RNA in a process called transcription and from RNA to protein in a process called translation -- an arrangement of highly efficient template-based processes known as the central dogma. In eukaryotic cells, transcription must occur in the nucleus and translation must occur in the cytoplasm.
What is a drawback to using light microscopy? Choose one: A. It cannot be used to view a whole cell or organism. B. It requires the use of fluorescent probes. C. It cannot be used to view structures smaller than a bacterium. D. It can be used only to view samples that are sliced very thinly. E. It cannot be used to view living cells.
It cannot be used to view structures smaller than a bacterium. One drawback of light microscopy is that it cannot be used to view structures smaller than a bacterium. Although the limit of resolution for light microscopy is about 0.2 µm, in practice structures smaller than bacteria or mitochondria (which are about 0.5 µm wide) are difficult to resolve without the use of special stains or fluorescent dyes. Unlike electron microscopy, light microscopy can be used to view living specimens. For example, in an introductory biology class, students commonly watch amoebas, paramecia, and other unicellular organisms swim in a sample of pond water. Light microscopy is ideal for viewing individual cells and single-celled organisms like these on slides that are called a "wet mount." Although samples viewed with a light microscope must be thin enough for light to pass through them, many unicellular organisms can be viewed without any more advanced preparation. Techniques such as confocal microscopy can be used to view the three-dimensional architecture of a cell or tissue that is too thick for light microscopy.
Is the following statement true or false, and why? Within a developed multicellular organism, all cells possess the ability to divide and do so regularly. Choose one: A. It is true, because all cells can always split into two cells in the same manner. B. It is false, because some cells lose the ability to replicate their DNA and divide. C. It is true, because all cells can always replicate their DNA in the same manner. D. It is false, because most multicellular organisms cannot reproduce at all.
It is false, because some cells lose the ability to replicate their DNA and divide. The ability to reproduce themselves is the hallmark of a living organism. However, in a multicellular organism, this does not necessarily mean that all cells in that organism undergo continual cell division. Multicellular organisms have many different types of specialized cells, each performing a specific function. In certain cases, such as epithelial cells that line body cavities, that function requires continual cell division and replacement. However, in other cases—including red blood cells, nerve cells, skeletal muscle cells, and gametes—the cells are no longer capable of further cell division. Such cells are said to be "terminally differentiated."
Identify the structures in the image of a mitotic cell by dragging the labels to their correct targets.
Microtubules help segregate the chromosomes in a dividing animal cell. Referred to as the mitotic spindle, the microtubules originate at the centrioles and attach to the duplicated chromosomes. On the left, a transmission electron micrograph shows the duplicated chromosomes attached to the microtubules of a mitotic spindle, and this is also represented in the illustration to the right of the figure. When a cell divides, the DNA condenses into visible chromosomes, each of which has already duplicated to form a pair of conjoined chromosomes that will ultimately be pulled apart into separate daughter cells by the spindle microtubules as the dividing cell progresses through mitosis.
Which is not evidence for the endosymbiotic origins of mitochondria and chloroplasts? Choose one: A. Mitochondria and chloroplasts are each surrounded by a double membrane. B. Mitochondria and chloroplasts have similar DNA. C. Mitochondria and chloroplasts resemble bacteria. D. Mitochondria and chloroplasts reproduce by dividing in two. E. Mitochondria and chloroplasts contain their own DNA.
Mitochondria and chloroplasts have similar DNA. Mitochondria and chloroplasts do not have similar DNA. The endosymbiotic origins of mitochondria and chloroplasts suggest that these organelles arose from separate instances of the developing/evolving primitive eukaryotic cell engulfing a smaller bacterial cell. The inner membrane would be derived from the engulfed cell membrane and the outer membrane would be derived from the primitive eukaryotic cell membrane. Because these are separate instances of endosymbiosis, there would be no expectation that the mitochondria and the chloroplast would have similar DNA. Mitochondria and chloroplasts resemble bacteria and have DNA that has evolved from those very different forms of bacteria—thus, they have unique DNA.
Which statement is not true of mitochondria? Choose one: A. Mitochondria are thought to have originated from bacteria. B. Mitochondria contain their own DNA. C. Mitochondria are involved in the chemical energy cycle of the cell. D. Mitochondria have an inner and outer membrane. E. Mitochondria are not present in plant cells.
Mitochondria are not present in plant cells. Plant cells undergo cell respiration just like animal cells, and thus they have mitochondria where bioenergetic reactions of the citric acid cycle and electron transport chains take place. All mitochondria have both an inner and an outer membrane. The proteins of the electron transport chain are embedded in the inner mitochondrial membrane and this double-membrane system is integral to the function of this organelle. Mitochondria contain their own DNA as well. This genome is referred to as mitochondrial DNA, which is circular just like most types of prokaryotic DNA. Mitochondria are thought to have originated from bacteria. The endosymbiotic theory of organelle development suggests that the evolving eukaryotic cell engulfed a smaller bacterial cell, and coevolution allowed these to rely on one another to sustain life. This evidently created a symbiotic relationship in which the host eukaryote and the engulfed bacterium helped each other to survive and reproduce.
The genome of the bacterium E. coli contains 4.6 million (4.6 ×× 106) nucleotide pairs, whereas the human genome contains some 3200 ×× 106 nucleotide pairs. What can be concluded based on these numbers? Choose one: A. E. coli are unicellular, whereas humans are multicellular. B. Human cells are larger than E. coli cells. C. Human cells have 700 times more genes than E. coli. D. Humans are a more complex life-form than E. coli. E. All of the above conclusions are drawn correctly. F. None of the above conclusions are drawn correctly.
None of the above conclusions are drawn correctly. None of the choices are correct in the context of the information about the human genome and the E. coli genome. Generally speaking, the more "complex" an organism, the larger its genome. But this relationship does not always hold true. The human genome, for example, is 60 times smaller than that of certain amoebas, which are single-celled protozoa. And though our genome is 700 times larger than that of E. coli, we have only about 4.4 times as many genes. It is true that E. coli are unicellular, whereas humans are multicellular, and humans are larger and lead more complex lives compared to E. coli, but these facts are not determinable from genomic size alone.
Which cellular component separates the DNA of eukaryotic cells from the cytoplasm? Choose one: A. nuclear membrane B. cell wall C. plasma membrane D. smooth endoplasmic reticulum
Nuclear membrane. The DNA of eukaryotic cells is contained within the cell nucleus, and the nucleus is separated from the cytoplasm by the nuclear envelope. The nucleus is enclosed within two concentric membranes that form the nuclear envelope, and it contains molecules of DNA: extremely long polymers that encode the genetic information of the organism. All eukaryotic cells contain an endoplasmic reticulum and plasma membrane, but these are not the barriers between eukaryotic DNA and the cytoplasm.
Consider this evolutionary tree and then answer the questions. Scientists study model organisms within these groups to gain insight into these types of organisms and their genetic heritage. Which of these statements is consistent with the data presented in the evolutionary tree? Choose one: A. Saccharomyces cerevisiae do not contain chloroplasts. B. Drosophila melanogaster do not contain mitochondria. C. Caenorhabditis elegans contain chloroplasts. D. All of these choices are correct regarding these organelles.
Saccharomyces cerevisiae do not contain chloroplasts. The yeast Saccharomyces cerevisiae is a model eukaryote. These organisms are unicellular fungi, and according to the evolutionary tree, fungi are not derived from the genetic lineage from where the chloroplast developed. Chloroplasts almost certainly evolved from engulfed photosynthetic bacteria. The bacteria are thought to have been taken up by early eukaryotic cells that already contained mitochondria, but this happened along a different evolutionary path from which fungi had already separated. Drosophila melanogaster is an animal and all animal cells have mitochondria. Like D. melanogaster, Caenorhabditis elegans is an animal and animal cells do not contain chloroplasts.
Consider this image of a unicellular free-living eukaryote. Which model organism is represented in this microscopic image? Choose one: A. Caenorhabditis elegans B. Escherichia coli C. Saccharomyces cerevisiae D. Drosophila melanogaster E. Arabidopsis thaliana
The brewer's yeast, Saccharomyces cerevisiae, is an important model system that has been instrumental in understanding basic aspects of processes, including cell division and cell-cycle control. As a single-celled organism, S. cerevisiae can be grown more quickly and easily than multicellular organisms. Because it is a eukaryote, many of its cellular mechanisms are more closely related to humans than bacterial model systems like Escherichia coli.
Scientists were studying yeast cells in which a mutation inactivated a gene required for cell division. Without this gene—and the protein it encoded—these mutant yeast cells were unable to divide normally. But the scientists discovered that introducing a related protein from human cells could "rescue" these mutant yeast cells, allowing the cells to resume normal division. Based on this finding, what is the most likely conclusion you could make? Choose one: A. Yeast cells require human proteins to divide. B. Yeast and humans diverged from a common ancestor much more recently than previously thought, perhaps fewer than a million years ago. C. The proteins that control cell division in yeast and humans are functionally equivalent and have been conserved, almost unchanged, for more than a billion years. D. Modern humans most likely obtained their cell-division proteins from ancient yeast.E. Yeast cells most likely obtained their cell-division proteins from the cells of early humans.
The proteins that control cell division in yeast and humans are functionally equivalent and have been conserved, almost unchanged, for more than a billion years. The experimental finding presented in this question leads to the conclusion that the human and yeast proteins are functionally equivalent. The most likely explanation for two very different species to have genes with such similar function is that this gene was found in the common ancestor cell from which these organisms diverged. In the case of humans and yeast, this common ancestor cell existed approximately 1.5 billion years ago. That the human and yeast genes and the proteins that they encode remain functionally equivalent is a testament to how fundamentally important this gene is in the cell-division process in all eukaryotic cells. There has been strict selective pressure against mutations in this gene and they have therefore retained a high degree of sequence similarity over the millennia. More than a hypothetical experiment, this question highlights a Nobel prize-winning experiment conducted by Paul Nurse, Lee Hartwell, and colleagues that examined the ability of a human cell-cycle gene, Cdc2, to rescue cell cycle mutant yeast. Human Cdc2, did indeed rescue the mutant yeast and examining the amino sequence of Cdc2 and the yeast equivalents revealed, as predicted, that they have very similar sequences, as shown below.
