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    CELL BIOLOGY

    Cells are the structural and functional unit of all living organisms. Some organisms, such as bacteria, are unicellular, consisting of a single cell. Other organisms, such as humans, are multicellular, or have many cells—an estimated 100,000,000,000,000 cells! Each cell can take in nutrients, convert these nutrients into energy, carry out specialized functions, and reproduce as necessary. Even more amazing is that each cell stores its own set of instructions for carrying out each of these activities.

    It is important to know what organism the cell comes from. There are two general categories of cells: prokaryotes and eukaryotes. Prokaryotes are capable of inhabiting almost every place on the earth, from the deep ocean, to the edges of hot springs, to just about every surface of our bodies. Prokaryotes also lack any of the intracellular organelles and structures that are characteristic of eukaryotic cells. Most of the functions of organelles, such as mitochondria and the Golgi apparatus, are taken over by the prokaryotic plasma membrane. Eukaryotes are about 10 times the size of a prokaryote and can be as much as 1000 times greater in volume. The major and extremely significant difference between prokaryotes and eukaryotes is that eukaryotic cells contain membrane-bounded compartments in which specific metabolic activities take place, and have small specialized structures called organelles that are dedicated to performing certain specific functions. Most important among these is the presence of a nucleus, a membrane-delineated compartment that houses the eukaryotic cell’s DNA.

    The outer lining of a eukaryotic cell is called the plasma membrane. This membrane serves to separate and protect a cell from its surrounding environment and is made mostly from a double layer of proteins and lipids, fat-like molecules. Embedded within this membrane are a variety of other molecules that act as channels and pumps, moving different molecules into and out of the cell. A form of plasma membrane is also found in prokaryotes, but in this organism it is usually referred to as the cell membrane.

    The cytoskeleton is an important, complex, and dynamic cell component. It acts to organize and maintain the cell's shape; anchors organelles in place; helps during endocytosis (the uptake of external materials by a cell); and moves parts of the cell in processes of growth and motility. There are a great number of proteins associated with the cytoskeleton, each controlling a cell’s structure by directing, bundling, and aligning filaments.

    Inside the cell there is a large fluid-filled space called the cytoplasm, sometimes called the cytosol. In prokaryotes, this space is relatively free of compartments. In eukaryotes, the cytosol is the "soup" within which all of the cell's organelles reside. It is also the home of the cytoskeleton. The cytosol contains dissolved nutrients, helps break down waste products, and moves material around the cell. The nucleus often flows with the cytoplasm changing its shape as it moves. The cytoplasm also contains many salts and is an excellent conductor of electricity, creating the perfect environment for the mechanics of the cell. The function of the cytoplasm, and the organelles which reside in it, are critical for a cell's survival.

    Two different kinds of genetic material exist: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Most organisms are made of DNA, but a few viruses have RNA as their genetic material. The biological information contained in an organism is encoded in its DNA or RNA sequence.

    Prokaryotic genetic material is organized in a simple circular structure that rests in the cytoplasm. Eukaryotic genetic material is more complex and is in units called genes. The nuclear genome is divided into 24 DNA molecules, each contained in a different chromosome.

    The human body contains many different organs, such as the heart, lung, and kidney, with each organ performing a different function. Cells also have a set of "little organs", called organelles, which are adapted and/or specialized for carrying out one or more vital functions. Organelles are found only in eukaryotes and are always surrounded by a protective membrane. It is important to know some basic facts about the following organelles.

    The nucleus is the most conspicuous organelle found in a eukaryotic cell. It houses the cell's chromosomes and is the place where almost all DNA replication and RNA synthesis occurs. The nucleus is spheroid in shape and separated from the cytoplasm by a membrane called the nuclear envelope. The nuclear envelope isolates and protects a cell's DNA from various molecules that could accidentally damage its structure or interfere with its processing.

    Ribosomes are found in both prokaryotes and eukaryotes. The ribosome is a large complex composed of many molecules, including RNA and proteins, and is responsible for processing the genetic instructions carried by mRNA. Protein synthesis is extremely important to all cells, and therefore a large number of ribosomes—sometimes hundreds or even thousands—can be found throughout a cell.

    Ribosomes float freely in the cytoplasm or sometimes bind to another organelle called the endoplasmic reticulum.

    Mitochondria are self-replicating organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells. Mitochondria contain their own genome that is separate and distinct from the nuclear genome of a cell. Mitochondria have two functionally distinct membrane systems separated by a space: the outer membrane, which surrounds the whole organelle; and the inner membrane, which is thrown into folds or shelves that project inward. These inward folds are called cristae. The number and shape of cristae in mitochondria differ depending on the tissue and organism in which they are found, and serve to increase the surface area of the membrane. Mitochondria play a critical role in generating energy in the eukaryotic cell, and this process involves a number of complex pathways. They are the powerhouses of the cell.

    Source : www.bu.edu

    Living Environment

    New York high school regents january 2012 living environment past exam questions, answers and solutions.

    Living Environment - New York Regents January 2012 Exam

    Formats View Examination Paper with Answers Solve Examination Paper Questions Review

    Part A

    Answer all questions in this part. [30]

    Show / Hide Section Details

    1 Which statement describes an activity of a

    decomposer?

    (1) A mushroom digests and absorbs nutrients

    from organic matter.

    (2) A sunflower uses nutrients from the soil to

    make proteins.

    (3) A snail scrapes algae off rocks in an aquarium.

    (4) A hawk eats and digests a mouse.

    Answer:

    2 The calcium concentration in the root cells of

    certain plants is higher than in the surrounding

    soil. Calcium may continue to enter the root

    cells of the plant by the process of

    (1) diffusion (3) active transport

    (2) respiration (4) protein synthesis

    Answer:

    3 Homeostasis is maintained in a single-celled

    organism by the interaction of

    (1) organs (3) tissues

    (2) systems (4) organelles

    Answer:

    4 Within which structure of an animal cell does

    DNA replication take place?

    (1) vacuole (3) nucleus

    (2) cell membrane (4) ribosome

    Answer:

    5 The shape of a protein is originally determined

    by the

    (1) size of the protein molecule

    (2) location of the protein within the cell

    (3) arrangement of amino acids in the protein

    (4) function the protein must carry out

    Answer:

    6 Plant cells can synthesize energy-rich organic

    molecules, and later break them down to extract

    that energy for performing life processes. These

    activities require direct interaction between the

    (1) chloroplasts and vacuoles

    (2) cell walls and ribosomes

    (3) chloroplasts and mitochondria

    (4) ribosomes and mitochondria

    Answer:

    7 Selective breeding has been used for thousands

    of years to

    (1) develop bacteria that produce human insulin

    (2) clone desirable plant varieties

    (3) develop viruses that protect against diseases

    (4) produce new varieties of domestic animals

    Answer:

    8 A deletion of a DNA segment alters a gene in a

    single skin cell of an individual. Which statement

    best describes a result of this mutation?

    (1) Any cell produced from this skin cell will

    have the same mutation.

    (2) All offspring of the individual will have a skin

    cell mutation.

    (3) The mutation will spread into other types of

    cells.

    (4) The gametes of this individual will have the

    same mutation. Answer:

    9 Some goats have been genetically modified with

    a human gene that codes for a blood anticlotting

    factor. The anticlotting factor can then be

    extracted from the goat milk and used during

    surgery. To produce these genetically modified

    goats, scientists most likely

    (1) injected the anticlotting factor into the milkproducing glands of the animals

    (2) added modified DNA into the milk of the

    animals

    (3) inserted the human gene into the egg cells of

    goats

    (4) altered the nutritional requirements of

    newborn goats Answer:

    10 Which characteristic is necessary for natural

    selection to occur in a species?

    (1) stability (2) variation

    (3) complex cellular organization

    (4) a very low mutation rate

    Answer:

    11 Researchers use a variety of techniques to learn

    more about the function of a specific gene in an

    organism. In one type of experiment, called a

    loss-of-function experiment, the gene being

    investigated is eliminated. In a gain-of-function

    experiment, extra copies of the gene being

    investigated are inserted. The cell process most

    directly affected in both experiments is

    (1) protein synthesis

    (2) waste disposal

    (3) transport of materials

    (4) breakdown of nutrients

    Answer:

    12 Plants are green because they contain the

    protein chlorophyll. A bucket was left on the

    lawn for one week. When the bucket was

    removed, the grass under the bucket had turned

    from green to a yellowish white color. This

    change is due to the interaction between the

    grass and

    (1) decomposer organisms in the soil, an abiotic

    factor

    (2) the amount of sunlight, an abiotic factor

    (3) increased moisture under the bucket, a biotic

    factor

    (4) the metal composition of the bucket, a biotic

    factor Answer:

    13 Which statement describes a function of the

    human male reproductive system?

    (1) It produces gametes in testes.

    (2) It supplies a fluid that protects the fetus.

    (3) It provides support for the development of

    the embryo.

    (4) It provides nutrient materials through a

    placenta. Answer:

    14 Exposure to toxins during early stages of

    pregnancy is more likely to cause birth defects

    than exposure in late pregnancy because

    (1) essential organs form during early

    development

    (2) the uterus provides more protection in late

    pregnancy

    (3) the placenta forms during late pregnancy

    (4) meiosis occurs rapidly during early

    development Answer:

    Source : www.syvum.com

    The Eukaryotic Cell Cycle

    The division cycle of most cells consists of four coordinated processes: cell growth, DNA replication, distribution of the duplicated chromosomes to daughter cells, and cell division. In bacteria, cell growth and DNA replication take place throughout most of the cell cycle, and duplicated chromosomes are distributed to daughter cells in association with the plasma membrane. In eukaryotes, however, the cell cycle is more complex and consists of four discrete phases. Although cell growth is usually a continuous process, DNA is synthesized during only one phase of the cell cycle, and the replicated chromosomes are then distributed to daughter nuclei by a complex series of events preceding cell division. Progression between these stages of the cell cycle is controlled by a conserved regulatory apparatus, which not only coordinates the different events of the cell cycle but also links the cell cycle with extracellular signals that control cell proliferation.

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    The Eukaryotic Cell Cycle

    The division cycle of most cells consists of four coordinated processes: cell growth, DNA replication, distribution of the duplicated chromosomes to daughter cells, and cell division. In bacteria, cell growth and DNA replication take place throughout most of the cell cycle, and duplicated chromosomes are distributed to daughter cells in association with the plasma membrane. In eukaryotes, however, the cell cycle is more complex and consists of four discrete phases. Although cell growth is usually a continuous process, DNA is synthesized during only one phase of the cell cycle, and the replicated chromosomes are then distributed to daughter nuclei by a complex series of events preceding cell division. Progression between these stages of the cell cycle is controlled by a conserved regulatory apparatus, which not only coordinates the different events of the cell cycle but also links the cell cycle with extracellular signals that control cell proliferation.

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    Phases of the Cell Cycle

    A typical eukaryotic cell cycle is illustrated by human cells in culture, which divide approximately every 24 hours. As viewed in the microscope, the cell cycle is divided into two basic parts: mitosis and interphase. Mitosis (nuclear division) is the most dramatic stage of the cell cycle, corresponding to the separation of daughter chromosomes and usually ending with cell division (cytokinesis). However, mitosis and cytokinesis last only about an hour, so approximately 95% of the cell cycle is spent in interphase—the period between mitoses. During interphase, the chromosomes are decondensed and distributed throughout the nucleus, so the nucleus appears morphologically uniform. At the molecular level, however, interphase is the time during which both cell growth and DNA replication occur in an orderly manner in preparation for cell division.

    The cell grows at a steady rate throughout interphase, with most dividing cells doubling in size between one mitosis and the next. In contrast, DNA is synthesized during only a portion of interphase. The timing of DNA synthesis thus divides the cycle of eukaryotic cells into four discrete phases (Figure 14.1). The M phase of the cycle corresponds to mitosis, which is usually followed by cytokinesis. This phase is followed by the G1 phase (gap 1), which corresponds to the interval (gap) between mitosis and initiation of DNA replication. During G1, the cell is metabolically active and continuously grows but does not replicate its DNA. G1 is followed by S phase (synthesis), during which DNA replication takes place. The completion of DNA synthesis is followed by the G2 phase (gap 2), during which cell growth continues and proteins are synthesized in preparation for mitosis.

    Figure 14.1

    Phases of the cell cycle. The division cycle of most eukaryotic cells is divided into four discrete phases: M, G1, S, and G2. M phase (mitosis) is usually followed by cytokinesis. S phase is the period during which DNA replication occurs. The cell grows (more...)

    The duration of these cell cycle phases varies considerably in different kinds of cells. For a typical rapidly proliferating human cell with a total cycle time of 24 hours, the G1 phase might last about 11 hours, S phase about 8 hours, G2 about 4 hours, and M about 1 hour. Other types of cells, however, can divide much more rapidly. Budding yeasts, for example, can progress through all four stages of the cell cycle in only about 90 minutes. Even shorter cell cycles (30 minutes or less) occur in early embryo cells shortly after fertilization of the egg (Figure 14.2). In this case, however, cell growth does not take place. Instead, these early embryonic cell cycles rapidly divide the egg cytoplasm into smaller cells. There is no G1 or G2 phase, and DNA replication occurs very rapidly in these early embryonic cell cycles, which therefore consist of very short S phases alternating with M phases.

    Figure 14.2

    Embryonic cell cycles. Early embryonic cell cycles rapidly divide the cytoplasm of the egg into smaller cells. The cells do not grow during these cycles, which lack G1 and G2 and consist simply of short S phases alternating with M phases.

    In contrast to the rapid proliferation of embryonic cells, some cells in adult animals cease division altogether (e.g., nerve cells) and many other cells divide only occasionally, as needed to replace cells that have been lost because of injury or cell death. Cells of the latter type include skin fibroblasts, as well as the cells of many internal organs, such as the liver, kidney, and lung. As discussed further in the next section, these cells exit G1 to enter a quiescent stage of the cycle called G0, where they remain metabolically active but no longer proliferate unless called on to do so by appropriate extracellular signals.

    Source : www.ncbi.nlm.nih.gov

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