Nucleus and Cytoskeleton PDF

– Lectuer (4) – Cellular organelles (nucleus, peroxisomes and endoskeleton) – Cell Fractionation The nucleus is the cell’s genetic control center ▪ The nucleus – contains most of the cell’s DNA and controls the cell’s activities by directing protein synthesis via making messen...

– Lectuer (4) – Cellular organelles (nucleus, peroxisomes and endoskeleton) – Cell Fractionation The nucleus is the cell’s genetic control center ▪ The nucleus – contains most of the cell’s DNA and controls the cell’s activities by directing protein synthesis via making messenger RNA (mRNA). DNA is associated with many proteins in structures called chromosomes. Each chromosome is composed of a single DNA molecule associated with proteins. ▪ The nuclear envelope – Is a double membrane and has pores that allow material to flow in and out of the nucleus. The nuclear envelope is attached to a network of cellular membranes called the endoplasmic reticulum ▪ The nucleolus is a prominent structure in the nucleus and the site of ribosomal RNA (rRNA) synthesis. – The Peroxisomes are specialized metabolic compartments bounded by a single membrane produce hydrogen peroxide and convert it to water. Peroxisomes perform reactions with many different functions Figure 4.5 Nucleus Two membranes of nuclear envelope Chromatin Nucleolus Pore Endoplasmic reticulum Ribosomes Centrosomes and Centrioles In many cells, microtubules grow out from a centrosome near the nucleus. The centrosome is a “microtubule-organizing center” In animal cells, the centrosome has a pair of centrioles, each with nine triplets of microtubules arranged in a ring. Centrosome Microtubule Centrioles 0.25 m Longitudinal section of one centriole Microtubules Cross section of the other centriole  The cell’s internal skeleton helps organize its structure and activities ▪ Cells contain a network of protein fibers, called the cytoskeleton, which functions in structural support and motility. Scientists believe that motility and cellular regulation result when the cytoskeleton interacts with proteins called motor proteins. ▪ The cytoskeleton is composed of three kinds of fibers. ▪ Microfilaments (actin filaments) support the cell’s shape and are involved in motility. Microfilaments that function in cellular motility contain the protein myosin in addition to actin. In muscle cells, thousands of actin filaments are arranged parallel to one another. 1. Intermediate filaments reinforce cell shape and anchor organelles. 2. Microtubules tubulin) give the cell rigidity and act as tracks for organelle movement. Figure 4.16 Nucleus Nucleus Actin subunit Fibrous subunits Tubulin subunits 7 nm 10 nm 25 nm Microfilament Intermediate filament Microtubule Figure 6.27a Muscle cell 0.5 m Actin filament Myosin filament Myosin head (a) Myosin motors in muscle cell contraction Cilia and flagella move when microtubules bend ▪ While some protists have flagella and cilia that are important in locomotion, some cells of multicellular organisms have them for different reasons. – Cells that sweep mucus out of our lungs have cilia. – Animal sperm are flagellated. ▪ Both flagella and cilia are made of microtubules wrapped in an extension of the plasma membrane. ▪ A ring of nine microtubule doublets surrounds a central pair of microtubules. This arrangement is – called the 9 + 2 pattern and anchored in a basal body with nine microtubule triplets arranged in a ring. – Cilia and flagella move by bending motor proteins called dynein feet. © 2012 Pearson Education, Inc. Outer microtubule doublet Central microtubules Radial spoke Dynein proteins Plasma membrane The extracellular matrix of animal cells functions in support and regulation ▪ Animal cells synthesize and secrete an elaborate extracellular matrix (ECM) that – helps hold cells together in tissues, protects and supports the plasma membrane. – The ECM may attach to a cell through glycoproteins that then bind to membrane proteins called integrins. Integrins span the plasma membrane and connect to microfilaments of the cytoskeleton. © 2012 Pearson Education, Inc. Glycoprotein EXTRACELLULAR FLUID complex with long polysaccharide Collagen fiber Connecting glycoprotein Integrin Plasma membrane CYTOPLASM Microfilaments of cytoskelton  Cell junctions in animal tissues ▪ Adjacent cells communicate, interact, and adhere through specialized junctions between them. – Tight junctions: prevent leakage of extracellular fluid across a layer of epithelial cells. – Anchoring junctions: Anchoring junctions are cell junctions that are anchored to one another and attached to components of the extracellular matrix. They are important in keeping the cells together and structural cohesion of tissues. They are commonly found in tissues that are prone to constant mechanical stress, e.g. skin and heart. – Gap junctions: are channels that physically connect adjacent cells, mediating the rapid exchange of small molecules, and playing an essential role in a wide range of physiological processes in nearly every system in the body, including the nervous system. Thus, altered function of gap junctions has been linked with pathological conditions. Figure 6.32 Tight junctions prevent fluid from moving Tight junction across a layer of cells TEM 0.5 m Tight junction Intermediate filaments Desmosome TEM 1 m Gap junction Ions or small molecules Space TEM between cells Extracellular Plasma membranes matrix of adjacent cells 0.1 m Cell Fractionation ▪ Cell fractionation takes cells apart and separates the major organelles from one another ▪ Centrifuges fractionate cells into their component parts ▪ Cell fractionation enables scientists to determine the functions of organelles ▪ Biochemistry and cytology help correlate cell function with structure  Homogenization Tissue cells Homogenate Centrifugation  Centrifuged at 1,000 g (1,000 times the force of gravity) for 10 min Supernatant poured into next tube Differential centrifugation 20,000 g 20 min 80,000 g 60 min Pellet rich in nuclei and cellular debris 150,000 g 3 hr Pellet rich in mitochondria (and chloro- plasts if cells are from a plant) Pellet rich in “microsomes” Pellet rich in ribosomes – Thanks