Cells, cytoplasm, and organelles: (Zellen, Zytoplasma und Organellen)

Transcription

1 Cells, cytoplasm, and organelles: (Zellen, Zytoplasma und Organellen) Cytoplasm consists of a gelatinous solution and contains microtubules (which serve as a cell's cytoskeleton) and organelles (literally 'little organs') 1

2 Cells, cytoplasm, and organelles Microfilaments are fine, thread-like protein fibers, 3-6 nm in diameter. They are composed predominantly of a contractile protein called actin, which is the most abundant cellular protein. Microfilaments' association with the protein myosin is responsible for muscle contraction. Microfilaments can also carry out cellular movements including gliding, contraction, and cytokinesis. Microtubules are cylindrical tubes, nm in diameter. They are composed of subunits of the protein tubulin. These subunits are termed alpha and beta. Microtubules act as a scaffold to determine cell shape, and provide a set of "tracks" for cell organelles and vesicles to move on. Microtubules also form the spindle fibers for separating chromosomes during mitosis. When arranged in geometric patterns inside flagella and cilia, they are used for locomotion. 2

3 Filaments Intermediate filaments are about 10 nm diameter and provide tensile strength for the cell. Examples of the cytoskeleton (in epithelial cells) In the epithelial (skin) cells of the intestine, all three types of fibers are present. Microfilaments project into the villi, giving shape to the cell surface. Microtubules grow out of the centrosome to the cell periphery. Intermediate filaments connect adjacent cells through desmosomes. 3

4 Cytoskeleton The cytoskeleton acts as a "track" on which cells can move organelles, chromosomes and other things. Some examples are: Vesicle movement between organelles and the cell surface, frequently studied in the squid axon. Cytoplasmic streaming Movement of pigment vesicles for protective coloration Discharge of vesicle content for water regulation in protozoa Cell division cytokinesis Movement of chromosomes during mitosis and meiosis Broken motors. In healthy individuals, the protein dystrophin is part of the linkage between the cellular cytoskeleton and the adhesive proteins on the outside of the cell. In Duchenne Muscular Dystrophy, however, the gene that codes for dystrophin is defective, resulting in muscle degeneration and finally death. This disease is X-linked recessive and occurs in 1 out of every 3,500 males. 4

5 Cytoskeleton Cellular motors Cells have protein motors that bind two molecules, and using ATP as energy, cause one molecule to shift in relationship to the other. Two types of these protein motors are myosin and actin, and dynein or kinesin and microtubules. These families of proteins all have a motor end, but may have several kinds of different molecular structures on the binding end. When these proteins bind, they can cause many different molecules, organelles, etc. to move. To the right is an example of the different binding ends found in the kinesin family of motors. When linked to other microtubules, protein motors can cause motion if the ends are fixed or extend the lengths of the fiber bundles if the ends are free. 5

6 External cell movement (Cellular movement) Cellular movement is accomplished by cilia and flagella. Cilia are hair-like structures that can beat in synchrony causing the movement of unicellular paramaecium. Cilia are also found in specialize linings in eukaryotes. For example, cilia sweep fluids past stationary cells in the lining of trachea and tubes of female oviduct. Flagella are whip-like appendages that undulate to move cells. They are longer than cilia, but have similar internal structures made of microtubules. Prokaryotic and eukaryotic flagella differ greatly. Both flagella and cilia have a arrangement of microtubules. This arrangement refers to the 9 fused pairs of microtubules on the outside of a cylinder, and the 2 unfused microtubules in the center. Dynein "arms" attached to the microtubules serve as the molecular motors. Defective dynein arms cause male infertility and also lead to respiratory tract and sinus problems. 6

7 Cells also contain a nucleus within which is found DNA (deoxyribonucleic acid) in the form of chromosomes plus nucleoli (within which ribosomes are formed) 7

8 The nucleus Within the nucleus is the DNA responsible for providing the cell with its unique characteristics. The DNA is similar in every cell of the body, but depending on the specific cell type, some genes may be turned on or off - that's why a liver cell is different from a muscle cell, and a muscle cell is different from a fat cell. When a cell is dividing, the DNA and surrounding protein condense into chromosomes (see photo) that are visible by microscopy. The prominent structure in the nucleus is the nucleolus. The nucleolus produces ribosomes, which move out of the nucleus to positions on the rough endoplasmic reticulum where they are critical in protein synthesis. The nucleus is the most obvious organelle in any eukaryotic cell. It is a membrane-bound organelle and is surrounded by a double membrane. It communicates with the surrounding cytosol via numerous nuclear pores. 8

9 DNA Composition In eukaryotes, chromosomes consist of a single molecule of DNA associated with: many copies of 5 kinds of histones. Histones are proteins rich in lysine and arginine residues and thus positively-charged. For this reason they bind tightly to the negativelycharged phosphates in DNA. a small number of copies of many different kinds of non-histone proteins. Most of these are transcription factors that regulate which parts of the DNA will be transcribed into RNA. 9

10 Structure For most of the life of the cell, chromosomes are too elongated and tenuous to be seen under a microscope. Before a cell gets ready to divide by mitosis, each chromosome is duplicated (during S phase of the cell cycle). As mitosis begins, the duplicated chromosomes condense into short (~ 5 µm) structures which can be stained and easily observed under the light microscope. These duplicated chromosomes are called dyads. 10

11 DNA When first seen, the duplicates are held together at their centromeres. In humans, the centromere contains ~1 million base pairs of DNA. Most of this is repetitive DNA: short sequences (e.g., 171 bp) repeated over and over in tandem arrays. While they are still attached, it is common to call the duplicated chromosomes sister chromatids, but this should not obscure the fact that each is a bona fide chromosome with a full complement of genes. The kinetochore is a complex of proteins that forms at each centromere and serves as the attachment point for the spindle fibers that will separate the sister chromatids as mitosis proceeds into anaphase. The shorter of the two arms extending from the centromere is called the p arm; the longer is the q arm. Staining with the trypsin-giemsa method reveals a series of alternating light and dark bands called G bands. G bands are numbered and provide "addresses" for the assignment of gene loci. 11

12 Chromosome Chromosome Numbers All animals have a characteristic number of chromosomes in their body cells called the diploid (or 2n) number. These occur as homologous pairs, one member of each pair having been acquired from the gamete of one of the two parents of the individual whose cells are being examined. The gametes contain the haploid number (n) of chromosomes. (In plants, the haploid stage takes up a larger part of its life cycle) 12

13 Diploid numbers of some commonly studied organisms Homo sapiens (human)46 Mus musculus (house mouse)40 Drosophila melanogaster (fruit fly)8 Caenorhabditis elegans (microscopic roundworm)12 Saccharomyces cerevisiae (budding yeast)32 Arabidopsis thaliana (plant in the mustard family)10 Xenopus laevis (South African clawed frog)36 Zea mays (corn or maize)20 Muntiacus reevesi (the Chinese muntjac, a deer)23 Muntiacus muntjac (its Indian cousin)6 Myrmecia pilosula (an ant)2 Parascaris equorum var. univalens (parasitic roundworm)2 13

14 Karyotypes The complete set of chromosomes in the cells of an organism is its karyotype. It is most often studied when the cell is at metaphase of mitosis and all the chromosomes are present as dyads. The karyotype of the human female contains 23 pairs of homologous chromosomes: 22 pairs of autosomes 1 pair of X chromosomes The karyotype of the human male contains: the same 22 pairs of autosomes one X chromosome one Y chromosome (A gene on the Y chromosome designated SRY is the master switch for making a male.) Link to a karyotype of a normal human male stained by the trypsingiemsa method. The X and Y chromosomes are called the sex chromosomes.) 14

15 Below is a human karyotype (of which sex?). It differs from a normal human karyotype in having an extra #21 dyad. As a result, this individual suffered from a developmental disorder called Down Syndrome. The inheritance of an extra chromosome, is called trisomy, in this case trisomy

16 Translocations Karyotype analysis can also reveal translocations between chromosomes. A number of these cause cancer, for example the Philadelphia chromosome (Ph1) formed by a translocation between chromosomes 9 and 22 and a cause of Chronic Myelogenous Leukemia (CML) a translocation between chromosomes 8 and 14 that causes Burkitt's lymphoma a translocation between chromosomes 18 and 14 that causes B-cell leukemia 16

17 DNA Content The molecule of DNA in a single human chromosome ranges in size from 50 x 10 6 nucleotide pairs in the smallest chromosome (stretched full-length this molecule would extend 1.7 cm) up to 250 x 10 6 nucleotide pairs in the largest (which would extend 8.5 cm). Stretched end-to-end, the DNA in a single human diploid cell would extend over 2 meters. See some of the DNA molecule released from a single human chromosome. In the intact chromosome, however, this molecule is packed into a much more compact structure. The packing reaches its extreme during mitosis when a typical chromosome is condensed into a structure about 5 µm long (a 10,000-fold reduction in length). 17

18 Burkitt's Lymphoma Burkitt's lymphoma is a solid tumor of B lymphocytes, the lymphocytes that the immune system uses to make antibodies. The genes for making antibodies are located on chromosomes 14 (the heavy [H] chains), 2 (kappa light chains), and 22 (lambda light chains). These genes are expressed only in B lymphocytes because only B cells have the necessary transcription factors for the promoters and enhancers needed to turn these antibody genes "on". In most (approximately 90%) of the cases of Burkitt's lymphoma, a reciprocal translocation has moved the proto-oncogene c-myc from its normal position on chromosome 8 to a location close to the enhancers of the antibody heavy chain genes on chromosome 14. In all the other cases, c-myc has been translocated close to the antibody genes on chromosome 2 or 22. In every case, c-myc now finds itself in a region of vigorous gene transcription, and it may simply be the overproduction of the c-myc product (a transcription factor essential for mitosis of mammalian cells) that turns the lymphocyte cancerous. Uncontrolled mitosis of this cell results in a clone of cancer cells, Burkitt's lymphoma. Many other human cancers involve chromosome aberrations, such as translocations, at the loci of known proto-oncogenes. 18

19 Endoplasmic Reticulum Throughout the eukaryotic cell, especially those responsible for the production of hormones and other secretory products, is a vast amount of membrane called the endoplasmic reticulum, or ER for short. The ER membrane is a continuation of the outer nuclear membrane and its function suggests just how complex and organized the eukaryotic cell really is. When viewed by electron microscopy, some areas of the endoplasmic reticulum look "smooth" (smooth ER) and some appear "rough" (rough ER). The rough ER appears rough due to the presence of ribosomes on the membrane surface. Smooth and Rough ER also have different functions. Smooth ER is important in the synthesis of lipids and membrane proteins. Rough ER is important in the synthesis of other proteins. Information coded in DNA sequences in the nucleus is transcribed as messenger RNA. Messenger RNA exits the nucleus through small pores to enter the cytoplasm. At the ribosomes on the rough ER, the messenger RNA is translated into proteins. These proteins are then transferred to the Golgi in "transport vesicles" where they are further processed and packaged into lysosomes, peroxisomes, or secretory vesicles. 19

20 Golgi Apparatus The Golgi apparatus is a membrane-bound structure with a double membrane. It is actually a stack of membrane-bound vesicles that are important in packaging macromolecules for transport elsewhere in the cell. The stack of larger vesicles is surrounded by numerous smaller vesicles containing those packaged macromolecules. The enzymatic or hormonal contents of lysosomes, peroxisomes and secretory vesicles are packaged in membrane-bound vesicles at the periphery of the Golgi apparatus. 20

21 Lysosomes, Peroxisomes, Secretory Vesicles Lysosomes: (common in animal cells but rare in plant cells) contain hydrolytic enzymes necessary for intracellular digestion. In white blood cells that eat bacteria, lysosome contents are carefully released into the vacuole around the bacteria and serve to kill and digest those bacteria. Uncontrolled release of lysosome contents into the cytoplasm can also cause cell death (necrosis). Peroxisomes: This organelle is responsible for protecting the cell from its own production of toxic hydrogen peroxide. As an example, white blood cells produce hydrogen peroxide to kill bacteria. The oxidative enzymes in peroxisomes break down the hydrogen peroxide into water and oxygen. Secretory Vesicles: Cell secretions - e.g. hormones, neurotransmitters - are packaged in secretory vesicles at the Golgi apparatus. The secretory vesicles are then transported to the cell surface for release. 21

22 Phagocytosis Phagocytosis is a process describing the engulfment and destruction of extracellularly-derived materials by phagocytic cells, such as macrophages and neutrophils. Five steps of phago-cytosis are illustrated in the image below. Phagocytosis of bacteria Schematic diagram of the steps in phagocytosis: 1. Attachment of the bacterium to the long membrane evaginations, called pseudopodia. 2. Ingestion of the bacterium forming a "phagosome," which moves toward the lysosome. 3. Fusion of the lysosome and phagosome, releasing lysosomal enzymes into the phagosome. 4. Digestion of the ingested material. 5. Release of digestion 22

23 Mitochondria Mitochondria provide the energy a cell needs to move, divide, produce secretory products, contract - in short, they are the power centers of the cell. They are about the size of bacteria but may have different shapes depending on the cell type. Mitochondria are membrane-bound organelles, and like the nucleus have a double membrane. The outer membrane is fairly smooth. But the inner membrane is highly convoluted, forming folds called cristae. The cristae greatly increase the inner membrane's surface area. It is on these cristae that food (sugar) is combined with oxygen to produce ATP -the primary energy source for the cell. 23

24 Mitochondria have a double-membrane: outer membrane & highly convoluted inner membrane inner membrane has folds or shelf-like structures called cristae that contain elementary particles; these particles contain enzymes important in ATP production primary function is production of adenosine triphosphate (ATP) 24

25 Ribosomes composed of rrna (ribosomal RNA) & protein may be dispersed randomly throughout the cytoplasm or attached to surface of rough endoplasmic reticulum often linked together in chains called polyribosomes or polysomes primary function is to produce proteins Ribosome Structure - non-membraneous, spherical bodies composed of RNA (ribonucleic acid) and protein enzymes Function - site of protein synthesis 25

26 The Centrosome and the Centrioles ANIMAL CELL CENTROSOME: The centrosome, also called the "microtubule organizing center", is an area in the cell where microtubles are produced. Within an animal cell centrosome there is a pair of small organelles, the centrioles, each made up of a ring of nine groups of microtubules. There are three fused in each group. The two centrioles are arranged such that one is perpendicular to the other. During animal cell division, the centrosome divides and the centrioles replicate (make new copies). The result is two centrosomes, each with its own pair of centrioles. The two centrosomes move to opposite ends of the nucleus, and from each centrosome, microtubules grow into a "spindle" which is responsible for separating replicated chromosomes into the two daughter cells. PLANT CELL CENTROSOME: Plant cells have centrosomes that function much like animal cell centrosomes. However, unlike centrosomes in animal cells, they do not have centrioles. 26

27 Flagella & cilia Smoking? We know that smoking damages the cilia lining the lungs. As a result, a smoker s lungs are not as effective at sweeping dust and bacteria out of the lungs. This animation demostrates how the cilia in the lungs of a non-smoker protect the lung. These tiny hair-like structures lining the inside of the bronchial tubes are constantly engaged in this sweeping motion, moving dust, bacteria, and viruses up and out of the lungs. Compare this to the cilia action inside the lungs of a smoker. Since the smoker s lungs are not as effective at sweeping dust, viruses, and bacteria up and out of the lungs, the smoker is more susceptible to frequent lung infections. 27

28 Villi Projections of cell membrane that serve to increase surface area of a cell (which is important, for example, for cells that line the intestine) 28