The Cell Cycle

The Cell Cycle

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The Cell Cycle

Is an orderly sequence of events that occur as a cell duplicates its contents and divides (Fig. 8.1). During the cell cycle, genetic information is duplicated and the duplicated chromosomes are appropriately aligned for distribution between two genetically identical daughter cells.

The cell cycle is divided into four phases, referred to as G₁ , S, G₂ , and M. G₁ (gap 1) occurs after the postmitosis phase when DNA synthesis stops and ribonucleic acid (RNA) and protein synthesis and cell growth take place. During the S phase, DNA synthesis occurs, causing two separate sets of chromosomes to develop, one for each daughter cell. G₂ (gap 2) is the premitotic phase and is similar to G₁ in that DNA synthesis stops, but RNA and protein synthesis continue. The phases, G₁ , S, and G₂ are referred to as interphase. The M phase is the phase of nuclear division, or mitosis, and cytoplasmic division. Continually dividing cells, such as the skin’s stratified squamous epithelium, continue to cycle from one mitotic division to the next. When environmental conditions are adverse, such as nutrient or growth factor unavailability, or when cells are highly specialized, cells may leave the cell cycle, becoming mitotically quiescent, and reside in a resting state known as G₀ . Cells in G₀ may reenter the cell cycle in response to extracellular nutrients, growth factors, hormones, and other signals such as blood loss or tissue injury that trigger cell growth. Highly specialized and terminally differentiated cells, such as neurons, may permanently stay in G₀.

Mitochondrial Gene Disorders

Mitochondrial Gene Disorders

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Mitochondrial Gene Disorders

The mitochondria contain their own DNA, which is distinct from the DNA contained in the cell nucleus. There are multiple disease-affected rearrangements and point mutations. Mitochondrial DNA (mtDNA) is packaged in a double-stranded circular chromosome located inside the mitochondria. Mitochondrial DNA contains 37 genes: 2 ribosomal RNA (rRNA) genes, 22 transfer RNA (tRNA) genes, and 13 structural genes encoding subunits of the mitochondrial respiratory chain enzymes, which participate in oxidative phosphorylation and generation of adenosine triphosphate.

       In contrast to the mendelian pattern of inheritance of nuclear DNA, disorders of mtDNA are inherited on the maternal line. This can be explained by the fact that ova contain numerous mitochondria in their abundant cytoplasm, whereas spermatozoa contain few, if any, mitochondria. Thus, the mtDNA in the zygote is derived solely from the mother. The zygote and its daughter cells have many mitochondria, each of which contains multiple copies of the maternally derived mtDNA. During growth of the fetus or later, it is likely that some cells will contain only normal or mutant mtDNA (a situation called homoplasmy), whereas others receive a mixture of normal and mutant DNA (heteroplasmy). In turn, the clinical expression of a disease produced by a given mutation of mtDNA depends on the total content of mitochondrial genes and the proportion that is mutant. The fraction of mutant mtDNA  must  exceed  a  critical  value  for  a  mitochondrial disease to become symptomatic. This threshold varies in different organs and is presumably related to the energy requirements of the cells.

The Cytoskeleton

The Cytoskeleton

pediagenosis    April 05, 2021   

The Cytoskeleton

Besides its organelles, the cytoplasm contains a network of microtubules, microfilaments, intermediate filaments, and thick filaments (Fig. 4.6). Because they control cell shape and movement, these structures are a major component of the structural elements called the cytoskeleton, which participates in the movement of entire cells.

Microtubules

Microtubules are formed from protein subunits called tubulin. They are long, stiff, hollow, cylindrical structures, 25 nm in outer diameter with a lumen 15 nm in diameter. Each microtubule consists of parallel protofilaments, each composed of α- and β-tubulin dimers. Microtubules are dynamic structures that can rapidly disassemble in one location and reassemble in another. During the reassembly process the tubulin dimers polymerize in an end-to-end fashion to form protofilaments. As a result of the polymerization process, each microtubule possesses a nongrowing “minus” end and a rapidly growing “plus” end. During the disassembly process, the tubulin dimers dissociate from the protofilaments and form a pool of free tubulin in the cytoplasm. This pool is used in the polymerization process for reassembly of the protofilaments.

The Nucleus

The Nucleus

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The Nucleus

The nucleus of the cell appears as a rounded or elongated structure situated near the center of the cell (see Fig. 4.1). All eukaryotic cells have at least one nucleus (prokaryotic cells, such as bacteria, lack a nucleus and nuclear membrane). Some cells contain more than one nucleus; osteoclasts (a type of bone cell) typically contain 12 nuclei or more. The platelet- producing cell, the megakaryocyte, has only one nucleus but usually contains 16 times the normal chromatin amount.

The nucleus can be regarded as the control center for the cell. It contains the deoxyribonucleic acid (DNA) that is essential to the cell because its genes encode the information necessary for the synthesis of proteins that the cell must produce to stay alive. These proteins include structural proteins and enzymes used to synthesize other substances, including carbohydrates and lipids. Genes also represent the individual units of inheritance that transmit information from one generation to another. The nucleus also is the site for the synthesis of the three types of ribonucleic acid (messenger RNA [mRNA], ribosomal RNA [rRNA], and transfer RNA [tRNA]) that move to the cytoplasm and carry out the actual synthesis of proteins. mRNA copies and carries the DNA instructions for protein synthesis to the cytoplasm; rRNA is the site of protein synthesis; and tRNA transports amino acids to the site of proteins synthesis for incorporation into the protein being synthesized.

Chromatin is the term denoting the complex structure of DNA and DNA-associated proteins dispersed in the nuclear matrix. Depending on its transcriptional activity, chromatin may be condensed as an inactive form of chromatin called heterochromatin or extended as a more active form called euchromatin. Because heterochromatic regions of the nucleus stain more intensely than regions consisting of euchromatin, nuclear staining can be a guide to cell activity. Evidence suggests the importance that alteration in the chromatin, along with DNA hypermethylation, in neoplastic progression. It seems both of these processes work symbiotically not separately in their role regarding cancer.

Blood

Blood

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Blood

The primary function of blood is to deliver O2 and energy to the tissues, and remove CO2 and waste products. It is also important for the defence and immune systems, regulation of temperature, and trans- port of hormones and signalling molecules between tissues. Blood consists of plasma (Chapter 2) and blood cells. Red blood cells contain haemoglobin and transport respiratory gases (Chapter 28), whereas white cells form part of the defence system (Chapter 10). In adults, all blood cells are produced in the red bone marrow. Normal values for cell counts, haemoglobin and proportion of blood volume due to red cells (haematocrit or packed cell volume; estimated by centrifuging a blood sample) are shown in Figure 8a. Platelets are discussed in Chapter 9.

Plasma proteins

Plasma contains several important proteins (Fig. 8b), with a total concentration of 65–83 g/L. Most, other than γ-globulins (see below), are synthesized in the liver. Proteins can ionize as either acids or bases because of the presence of both NH2 and COOH groups. At pH 7.4 they are mostly in the anionic (acidic) form. Their ability to accept or donate H+ means they can act as buffers (Chapter 36). Plasma proteins have important transport functions, as they bind many hormones (e.g. cortisol and thyroxine) and metals (e.g. iron). They are classified into albumin, globulin and fibrinogen fractions. Globulins are further classified as α-, β- and γ-globulins. Examples and their major functions are shown in Figure 8b.

Conduction Of Action Potentials

Conduction Of Action Potentials

pediagenosis    April 05, 2021   

Conduction Of Action Potentials

The action potential described in Chapter 5 is a local event that can occur in all excitable cells. This local event is an all-or-nothing response, leading to abolishion and then reversal of the polarity from negative (−70 mV) to positive (+40 mV) on the inside of the cell with respect to the outside for a short time during the course of the action potential.

Local currents are set up around the action potential because the positive charges from the membrane ahead of the action potential are drawn towards the area of negativity surrounding the action potential (current sink). This decreases the polarity of the membrane ahead of the action potential.

This electronic depolarization initiates a local response that causes the opening of the voltage-gated ion channels (Na+ followed by K+); when the threshold for firing of the action potential is reached, it propagates the action potential and this, in turn, leads to the local depolarization of the next area, and so on. Once initiated, an action potential does not depolarize the area behind it sufficiently to initiate another action potential because the area is refractory (Chapter 5).

This successive depolarization moves along each segment of an unmyelinated nerve until it reaches the end. It is all-or-nothing and does not decrease in size (Fig. 6a).

Saltatory conduction

Conduction in myelinated axons depends on a similar pattern of current flows. However, because myelin is an insulator and because the membrane below it cannot be depolarized, the only areas of the myelinated axon that can be depolarized are those that are devoid of any myelin, i.e. at the nodes of Ranvier. The depolarization jumps from one node to another and is called saltatory, from the Latin saltare (to jump) (Fig. 6b). Saltatory conduction is rapid and can be up to 50 times faster than in the fastest unmyelinated fibres.

Neuromuscular Junction And Whole Muscle Contraction

Neuromuscular Junction And Whole Muscle Contraction

pediagenosis    April 05, 2021   

Neuromuscular Junction And Whole Muscle Contraction

Neuromuscular junction

For skeletal (voluntary) muscle to contract, there must be neuronal activation to the muscle fibres themselves from either higher centres in the brain or via reflex pathways involving either the spinal cord or the brain stem. The neurones that innervate skeletal muscles are called α-motor neurones. Each motor axon splits into a number of branches that make contact with the surface of individual muscle fibres in the form of bulb-shaped endings. These endings make connections with a specialized structure on the surface of the muscle fibre, called the motor end plate, and together form the neuromuscular junction (NMJ) (Fig. 13a).

The role of the NMJ is the one-to-one transmission of excitatory impulses from the α-motor neurone to the muscle fibres it innervates. It allows a reliable transmission of the impulses from nerve to muscle and produces a predictable response in the muscle. In other words, an action potential in the motor neurone must produce an action potential in the muscle fibres it innervates; this, in turn, must produce a contraction of the muscle fibres. The process by which the NMJ produces this one-to-one response is shown in Figures 13a and b.

The motor neurone axon terminal has a large number of vesicles containing the transmitter substance acetylcholine (ACh). At rest, when not stimulated, a small number of these vesicles release their contents, by a process called exocytosis, into the synaptic cleft between the neurones and the muscle fibres. ACh diffuses across the cleft and reacts with specific ACh receptor proteins in the postsynaptic mem- brane (motor end plate). These receptors contain an integral ion channel, which opens and allows the movement inwards (influx) of small cations, mainly Na+. There are more than 107 receptors on each end plate (postjunctional membrane); each of these can open for about 1 ms and allow small positively charged ions to enter the cell. This movement of positively charged ions generates an end plate potential (EPP). This is a depolarization of the cell with a rise-time of approximately 1–2 ms and may vary in amplitude (unlike the all- or-nothing response seen in the action potential; Chapter 6). The random release of ACh from the vesicles at rest gives rise to small, 0–4-mV depolarizations of the end plate, called miniature end plate potentials (MEPP) (Fig. 13c).

However, when an action potential reaches the prejunctional nerve terminal, there is an enhanced permeability of the membrane to Ca2+ ions due to opening of voltage-gated Ca2+ channels. This causes an increase in the exocytotic release of ACh from several hundred vesicles at the same time. This sudden volume of ACh diffuses across the cleft and stimulates a large number of receptors on the postsynaptic membrane, and thus produces an EPP that is above the threshold for triggering an action potential in the muscle fibre. It triggers a self- propagating muscle action potential. The depolarizing current (the generator potential) generated by the numbers of quanta of ACh is more than sufficient to cause the initiation of an action potential in the muscle membrane surrounding the postsynaptic junction. The typical summed EPP is usually four times the potential necessary to trigger an action potential in the muscle fibre, and so there is a large inherent safety factor.

The effect of ACh is rapidly abolished by the activity of the enzyme acetylcholinesterase (AChE). ACh is hydrolysed to choline and acetic acid. About one-half of the choline is recaptured by the presynaptic nerve terminal and used to make more ACh. Some ACh diffuses out of the cleft, but the enzyme destroys most of it. The number of vesicles available in the nerve ending is said to be sufficient for only about 2000 nerve–muscle impulses, and therefore the vesicles reform very rapidly within about 30 s (Fig. 13b).

Whole muscle contraction

As the action potential spreads over the muscle fibre, it invades the T-tubules and releases Ca2+ from the sarcoplasmic reticulum into the sarcoplasm, and the muscle fibres that are excited contract. This contraction will be maintained as long as the levels of Ca2+ are high. The single contraction of a muscle due to a single action potential is called a muscle twitch. Fibres are divided into fast and slow twitch fibres depending on the time course of their twitch contraction. This is determined by the type of myosin in the muscles and the amount of sarcoplasmic reticulum. Different muscles are made up of different proportions of these two types of fibre, leading to a huge variation in overall muscle contraction times (Fig. 3d).

The Cytoplasm and Its Organelles

The Cytoplasm and Its Organelles

pediagenosis    April 05, 2021   

The Cytoplasm and Its Organelles

The cytoplasm surrounds the nucleus, and it is in the cytoplasm that the work of the cell takes place. Cytoplasm is essentially a colloidal solution that contains water, electrolytes, suspended proteins, neutral fats, and glycogen molecules. Although not contributing to the cell’s function, pigments may also accumulate in the cytoplasm. Some pigments, such as melanin, which gives skin its color, are normal constituents of the cell. Bilirubin is a normal major pigment of bile; its excess accumulation in cells is evidenced clinically by a yellowish discoloration of the skin and sclera, a condition called jaundice.

Embedded in the cytoplasm are various organelles, which function as the organs of the cell. These organelles include the ribosomes, ER, Golgi complex, mitochondria, and lysosomes.

Ribosomes

The ribosomes serve as sites of protein synthesis in the cell. They are small particles of nucleoproteins (rRNA and proteins) that are held together by a strand of mRNA to form polyribosomes (also called polysomes). Polyribosomes exist as isolated clusters of free ribosomes within the cytoplasm (Fig. 4.2) or attached to the membrane of the ER. Whereas free ribosomes are involved in the synthesis of proteins, mainly enzymes that aid in the control of cell function, those attached to the ER translate mRNAs that code for proteins secreted from the cell or stored within the cell (e.g., granules in white blood cells).

Endoplasmic Reticulum

The ER is an extensive system of paired membranes and flat vesicles that connect various parts of the inner cell (see Fig. 4.2). Between the paired ER membranes is a fluid-filled space called the matrix. The matrix connects the space between the two membranes of the nuclear envelope, the cell membrane, and various cytoplasmic organelles. It functions as a tubular communication system for transporting various substances from one part of the cell to another. A large surface area and multiple enzyme systems attached to the ER membranes also provide the machinery for a major share of the cell’s metabolic functions.

CHEMICAL BONDS

CHEMICAL BONDS

pediagenosis    November 02, 2020    Comment   

CHEMICAL BONDS

Covalent bonds, the strongest interactions between atoms, are responsible for holding atoms together to form molecules. They form when two atoms come together and share a pair of electrons (Figure 2.1). For example, methane (CH4) is formed when four hydrogen atoms share electrons with one carbon atom (Figure 2.1A). The number of covalent bonds that an atom can form is determined by the number of unpaired electrons in its outer electron shell (its valence). Carbon has four unpaired electrons whereas hydrogen has one, so carbon can form covalent bonds with four hydrogen atoms. The other principal atoms of living organisms, oxygen and nitrogen, have two and three unpaired electrons, respectively.

CARBOHYDRATES

CARBOHYDRATES

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CARBOHYDRATES

The carbohydrates include simple sugars as well as polysaccharides. These simple sugars, such as glucose, are the major nutrients of cells. As discussed in Chapter 3, their breakdown provides both a source of cellular energy and the starting material for the synthesis of other cell constituents. Polysaccharides are storage forms of sugars and form structural components of the cell. In addition, polysaccharides and shorter polymers of sugars act as markers for a variety of cell recognition processes, including the adhesion of cells to their neighbors and the transport of proteins to appropriate intracellular destinations.

LIPIDS

LIPIDS

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LIPIDS

Lipids have three major roles in cells. First, they provide an important form of energy storage. Second, and of great importance in cell biology, lipids are the major components of cell membranes. Third, lipids play important roles in cell signaling, both as steroid hormones (e.g., estrogen and testosterone) and as messenger molecules that convey signals from cell surface receptors to targets within the cell. The simplest lipids are fatty acids, which consist of long hydrocarbon chains, most frequently containing 16 or 18 carbon atoms, with a carboxyl group (COO^–) at one end (Figure 2.8). Unsaturated fatty acids contain one or more double bonds between carbon atoms; in saturated fatty acids all of the carbon atoms are bonded to the maximum number of hydrogen atoms. The long hydrocarbon chains of fatty acids contain only nonpolar C—H bonds, which are unable to interact with water. The hydrophobic nature of these fatty acid chains is responsible for much of the behavior of complex lipids, particularly in the formation of biological membranes.

SUBCELLULAR FRACTIONATION

SUBCELLULAR FRACTIONATION

pediagenosis    October 28, 2020    Comment   

SUBCELLULAR FRACTIONATION

Although the electron microscope has allowed detailed visualization of cell structure, microscopy alone is not sufficient to define the functions of the various components of eukaryotic cells. To address many questions concerning the function of subcellular organelles, it is necessary to isolate the organelles of eukaryotic cells in a form that can be used for biochemical studies. This is usually accomplished by differential centrifugation a method developed largely by Albert Claude, Christian de Duve, and their colleagues in the 1940s and 1950s to separate the components of cells on the basis of their size and density. The first step in subcellular fractionation is the disruption of the plasma membrane under conditions that do not destroy the internal components of the cell. Several methods are used, including sonication (exposure to high-frequency sound), grinding in a mechanical homogenizer, or treatment with a high-speed blender. All these procedures break the plasma membrane and the endoplasmic reticulum into small fragments, while leaving other components of the cell (such as nuclei, lysosomes, peroxisomes and mitochondria) intact. The suspension of broken cells (called a lysate or homogenate) is then fractionated into its components by a series of centrifugations, with an ultracentrifuge used to rotate samples at very high speeds (over 100,000 rpm), producing forces up to 500,000 times greater than gravity. This force causes cell components to move toward the bottom of the centrifuge tube and form a pellet (a process called sedimentation) at a rate that depends on their size and density, with the largest and heaviest structures sedimenting most rapidly (Figure 1.40). Usually the cell homogenate is first centrifuged at a low speed, which sediments only unbroken cells and the largest sub- cellular structures the nuclei. Thus, an enriched fraction of nuclei can be recovered from the pellet of such a low-speed centrifugation while the other cell components remain suspended in the supernatant (the remaining solution). The supernatant is then centrifuged at a higher speed to sediment mitochondria, chloroplasts, lysosomes, and peroxisomes. Recentrifugation of the supernatant at an even higher speed sediments fragments of the plasma membrane and the endoplasmic reticulum. A fourth centrifugation at a still higher speed sediments ribosomes, leaving only the soluble portion of the cytoplasm (the cytosol) in the supernatant.

ELECTRON MICROSCOPY

ELECTRON MICROSCOPY

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ELECTRON MICROSCOPY

Because of the limited resolution of the light microscope, analysis of the details of cell structure has required the use of more powerful microscopic techniques namely electron microscopy, which was developed in the 1930s and first applied to biological specimens by Albert Claude, Keith Porter, and George Palade in the 1940s and 1950s. The electron microscope can achieve a much greater resolution than that obtained with the light microscope because the wavelength of electrons is shorter than that of light. The wavelength of electrons in an electron microscope can be as short as 0.004 nm about 100,000 times shorter than the wavelength of visible light. Theoretically, this wavelength could yield a resolution of 0.002 nm, but such a resolution cannot be obtained in practice, because resolution is determined not only by wave-length, but also by the properties of the microscope lens and the specimen being examined. Consequently, for biological samples the practical limit of resolution of the electron microscope is 1–2 nm. Although this resolution is much less than that predicted simply from the wavelength of electrons, it represents more than a hundredfold improvement over the resolving power of the light microscope (see Figure 1.25).

SUPER-RESOLUTION MICROSCOPY: BREAKING THE DIFFRACTION BARRIER

SUPER-RESOLUTION MICROSCOPY: BREAKING THE DIFFRACTION BARRIER

pediagenosis    October 05, 2020    Comment   

SUPER-RESOLUTION MICROSCOPY: BREAKING THE DIFFRACTION BARRIER

An exciting advance in recent years has been the development of super-resolution microscopy techniques that break the diffraction barrier and increase the resolution of fluorescence microscopy to the range of 10-100 nm, about tenfold less than the theoretical limit of resolution of the light microscope (see Figure 1.25). Several methods of super-resolution microscopy use fluorescent probes that shift the limit of resolution from the wavelength of visible light to the molecular level.

SHARPENING THE FOCUS AND SEEING CELLS IN THREE DIMENSIONS

SHARPENING THE FOCUS AND SEEING CELLS IN THREE DIMENSIONS

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SHARPENING THE FOCUS AND SEEING CELLS IN THREE DIMENSIONS

The images obtained by conventional fluorescence microscopy are blurred as a result of out-of-focus fluorescence. These images can be improved by a computational approach called image deconvolution, in which a computer analyzes images obtained from different depths of focus and generates a sharper image than would have been expected from a single focal point. Alternatively, confocal microscopy allows images of increased contrast and detail to be obtained by analyzing fluorescence from only a single point in the specimen. A small point of light, usually supplied by a laser, is focused on the specimen at a particular depth. The emitted fluorescent light is then collected using a detector, such as a video camera. Before the emitted light reaches the detector, however, it must pass through a pinhole aperture (called a confocal aperture) placed at precisely the point where light emitted from the chosen depth of the specimen comes to a focus (Figure the plane of focus is able to reach the detector. Scanning across the specimen generates a two-dimensional image of the plane of focus, a much sharper image than that obtained with standard fluorescence microscopy (Figure 1.34). Moreover, a series of images obtained at different depths can be used to reconstruct a three-dimensional image of the sample.

FOLLOWING PROTEIN MOVEMENTS AND INTERACTIONS

FOLLOWING PROTEIN MOVEMENTS AND INTERACTIONS

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FOLLOWING PROTEIN MOVEMENTS AND INTERACTIONS

A variety of methods have been developed to follow the movement and interactions of GFP-labeled proteins within living cells. One widely used method for studying the movements of GFP-labeled proteins is fluorescence recovery after photobleaching (FRAP) (Figure 1.31). In this technique, a region of interest in a cell expressing a GFP-labeled protein is bleached by exposure to high-intensity light. Fluorescence recovers over time due to the movement of unbleached GFP-labeled molecules into the bleached region, allowing the rate at which the protein moves within the cell to be determined.