READING ASSIGNMENTS 2:     

 Milestones in Cell Biology

1626 Redi postulated that living things do not arise from spontaneous generation.
1655 Hooke described 'cells' in cork.
1674 Leeuwenhoek discovered protozoa. He saw bacteria some 9 years later.
1833 Brown descibed the cell nucleus in cells of the orchid.
1838 Schleiden and Schwann proposed cell theory.
1855 Virchow postulated that new cells come from preexisting cells.
1857 Kolliker described mitochondria.
1869 Miescher isolated DNA for the first time.
1879 Flemming described chromosome behavior during mitosis.
1883 Germ cells are haploid, chromosome theory of heredity.
1898 Golgi described the golgi apparatus.
1926 Svedberg developed the first analytical ultracentrifuge.
1938 Behrens used differential centrifugation to separate nuclei from cytoplasm.
1939 Siemens produced the first commercial transmission electron microscope.
1941 Coons used fluorescent labeled antibodies to detect cellular antigens.
1952 Gey and co-workers established a continuous human cell line.
1953 Crick, Wilkins and Watson proposed structure of DNA double-helix.
1955 Eagle systematically defined the nutritional needs of animal cells in culture.
1957 Meselson, Stahl and Vinograd developed density gradient centrifugation in cesium chloride solutions for separating nucleic acids.
1965 Ham introduced a defined serum-free medium. Cambridge Instruments produced the first commercial scanning electron microscope.
1976 Sato and collegues publish papers showing that different cell lines require different mixtures of hormones and growth factors in serum-free media.
1981 Transgenic mice and fruit flies are produced.                               
 

15. Cell Division

Cell division is essential to growth, repair and reproduction in eukaryotic organisms. The process of dividing the genetic material amoung the daughter cells is integral to both types of cell division. In mitosis, the daughter cells receive the same number of chromosomes as the parent cell. In meiosis, a reductive cell division occurs such that the daughter cells are haploid with respect to the parent cell. The nuclear events occurring in both types of cell division have been described in some detail. Below is a summary of the major events of each stage.

Mitosis

Prophase
Chromatin condenses, each chromosome consists of two sister chromatids, cytoskeleton disassembles, and mitotic spindle begins to form from centriolar organizing centers, nucleoli disappear.

Prometaphase
Nuclear envelope dissentigrates into cytoplasmic vessicles, kinetochores form on each centromere and attach to some of the spindle microtubules . During cell division, chromosomes may move by ATP driven microtubule walking protein within the kinetochore or microtubule dissembly may drive movement. Kinetochore contains a protein with high affinity for polymerized tubulin.

Metaphase
Chromosomes are aligned in middle of spindle. At metaphase tubulin subunits are added to the plus end of microtubules at the kinetochore and are removed from the minus end at spindle pole.

Anaphase 
Paired kinetichores on each chromosome separate and each chromatid moves toward the spindle pole. At anaphase the kinetochore moves because subunits are removed from the plus ends of the microtubules.

Telophase 
New Kinetochore microtubules disappear, new nuclear envelope forms around each group of daughter chromosomes, chromatin expands and nucleoli reappear.

Cytokinesis
Cleavage furrow forms aound the equatorial middle of the cell and a cytoplasmic contractile ring contracts until remains of mitotic spindle are encountered. The spindle finally breaks and the membranes fuse leaving two separated daughter cells.

Meiosis

In meiosis, two successive cell divisions after one round of DNA replication give rise to four haploid cells from a single diploid cell. This process is necessary to the formation of gametes so that the resulting zygote is a product of the fusion of haploid maternal and paternal genes. With the exception of the sex chromosomes, a diploid nucleus contains two similar versions of each chromosome. During DNA replication the two copies of the fully replicated chromosome remain closely associated and are called sister chromatids. A haploid cell resulting from meiosis must contain only one member of each homologous pair of chromosomes and therefore only half of the original number of chromosomes. To accomplish this, the homologues must pair up before they line up on the meiotic spindle. This bivalent contains four chromatids and each daughter cell recieves two copies of one of the two homologues when the meiotic cell divides for the first time. The formation of haploid cells can now proceed via a second cell division, in which chromosomes align on a second spindle and the sister chromatids separate in division II of meiosis as in mitosis.

No two offspring of the same parents are genetically the same unless they are identical twins. This is because genetic reassortment that occurs during meiosis. Random genetic shuffling of maternal and paternal homologues to daughter haploid cells in meiosis allows for some reassortment. Another type of reassortment, chromosomal crossing over, occurs when parts of homologous chromosomes are exchanged. On average two or three crossover events occur on each pair of human chromosomes during prophase of the first meiotic division.



Cellular Communication

Cells in a multicellular organism must communicate with one another in order to direct and regulate growth, development and organization. Animal cells communicate by secreting chemicals that signal to distant cells, display cell surface chemicals that influence other cells in direct physical contact, and communicate directly via porous cellular junctions called gap junctions.

Endocrine signalling occurs when substances (hormones) are secreted by cells and travel in the bloodstream to distant target cells. In paracrine signalling, cells secrete local chemical mediators that act only on cells in the immediate environment. Paracrine signalling molecules are rapidly internalized, destroyed or immobilized such that their effects can be limited to the local environment. Synaptic signalling occurs when molecules are released vesicles at specialized neuronal cell junctions called synapses. The molecules, neurotransmitters, diffuse across the synaptic cleft and act only on the postsynaptic target cell. Protein receptor molecules on or within the target cells bind to the hormone, paracrine or neurotransmitter and a response is initiated. Often the same molecules are endocrine, paracrine or neurotransmitter, the differences lie in the rapidity and selectivity of the delivered signal.

Each eukaryotic cell has as its boundary to the outside a cell membrane (7.5 to 10 nm in thickness) that envelopes the cytoplasmic matrix containing specialized membrane-bound components called organelles. The cell or plasma membrane is a lipid bilayer containing proteins, cholesterol, and oligosaccharides that functions as a selective barrier for entry and exit of substances. The plasma membrane, by limiting the transport of some things and facilitating the movement of others helps to maintain the internal environment of the cell, which is different from the extracellular fluid. Under an electron microscope membranes appear to have a trilaminar structure. This is because the lipid bilayers are arranged such that hydrophilic phospholipid groups are oriented toward the outside of the membrane while the more hydrophobic lipid fatty acid chains form the middle of the trilaminar struc

The receptor mechanisms vary for cellular communication molecules based on their solubility in water. Those, such as neurotransmitters and proteins are water soluble and cannot cross the cell membrane without help. Others such as lipid soluble steroids can cross the lipid bilayer to bind to intracellular receptors. These hydrophobic molecules must be carried in the blood stream bound to transport proteins and therefore their half-life in the bloodstream is hours to days in contrast to hydrophillic molecules which are broken down within seconds. Therefore, water soluble signaling molecules usually mediate responses of short duration, while hydrophobic molecules mediate longer lasting responses.ture. The molecular make up of each half of the membrane is different in that different lipids and proteins are more abundant in one side over the other.

Proteins are a very important part of the cell membrane. Basically they can be classified into two groups based on physical distribution. Integral proteins are embedded within the cell membrane and may in fact pass multiple times through the membrane. Peripheral proteins are loosely associated with membrane surfaces. Carbohydrate portions of glycoproteins and glycolipids are found on the external surface of the cell membrane where they are important parts of receptor molecules. Receptors are necessary to cellular signalling, adhesion, and re

Intracellular receptors
Small hydrophobic signalling molecules (steroid and thyroid hormones ) pass through the target cell membrane to bind to intracellular receptors in the cytoplasm or nucleus. The hormone receptor complex undergoes a conformational change that increases the receptors affinity for DNA and enables it to bind to specific genes in the nucleus and regulate transcription. Binding to specific genes activates or suppresses transcription of those genes. DNA recognition sites associated with steroid-hormone-responsive genes function as receptor dependent transcriptional enhancers. The products of some of these genes may in turn activate other genes to produce a delayed secondary effect.cognition. Most proteins are fixed in place within the cell membrane by interactions with the cytoskeleton . However, some integral proteins can move about and sometimes will accumulate on one region of the membrane in a process called capping.

The cell membrane is not static. It is remodeled by the addition of new membrane vesicles from the Golgi while removal takes place in the form of endocytotic, phagocytotic and pinocytotic vesicles being formed and then fused with lysozomes for processing. Membrane receptors and membrane are often conserved and recycled to the plasma membrane. This membrane trafficking is important in the cell economy.


Cell Surface receptors
All hydrophillic molecules and the hydrophobic prostaglandins effect cellular responses via specific cell membrane receptors on the target cell. These protein receptors bind the signalling molecule with great affinity and transduce the signal into intracellular signals that affect cellular behavior. Cell surface receptors do not regulate gene expression directly. They relay a signal across the cell membrane and the response of the target cell depends on intracellular second messenger molecules such as cAMP, inositol phosphate, or calcium.

There are three families of cell surface receptors based on signal transduction mechnism.

Channel-linked receptors -- These are transmitter gated ion channels involved in rapid synaptic signalling as in nervous tissue or the neuromuscular junction . A specific transmitter can rapidly open or close ion channels upon binding to its receptor thus changing the ion permeability of the cell membrane. All of these receptors belong to a family of similar multipass transmembrane proteins.

Catalytic receptors -- These receptors behave as enzymes when activated by a specific ligand. Most of these have a cytoplasmic catalytic region that behaves as a tyrosine kinase. Target proteins are phosporylated at specific tyrosine residues thus changing their activation state. The insulin receptor functions in this way.

G-protein linked receptors -- When bound to a specific ligand these receptors indirectly activate or inactivate a separate plasma membrane bound enzyme or ion channel. The interaction between the receptor and the affected enzyme or ion channel is mediated by a GTP binding protein. G-protein linked receptors initiate a cascade of chemical events within the target cell that usually alter the concentration of small intracellular messengers such as cAMP or inositol triphosphate. These intracellular messengers in turn alter the the behaviour of other intracellular proteins. cAMP levels affect cells by stimulating cyclic AMP-dependnent protein kinase to phosphorylate specific target proteins. Calcium levels modify the activity of certain enzymes by binding to the calcium binding protein calmodulin that then activates target proteins. The effects of all these second messengers are rapidly reversible when the extracellular signal is removed. The response of cells to external signals initiates signalling cascades that can greatly amplify and regulate various inputs.asmic matrix is not an unstructured liquid gel as once was thought. The cytosol makes up some 50% of the cell volume. It contains numerous cytoskeletal elements, organelles, vesicles, metabolic enzymes and sometimes pigment deposits. The matrix coordinates the movement of intracellular organelles, and provides a framework for the organization of enzyme pathways such as those within the glycolytic series. All of the necessities of protein synthesis are found within the cytosol and it contains numerous enzymes that build large molecules and break down small molecules. Protein motors are present in the cytosol to help transport things along the cytoskeletal framework.