Why Study Cell Biology? The key to every biological problem must finally be sought in the cell, for every living organism is, or at some time has been, a cell. E.B. Wilson, 1925 Cells are Us Cells are Us Cilia on a protozoan Sperm meets egg Cells are Us A person contains about 100 trillion cells. That’s 100,000,000,000,000 or 1 x 1014 cells. There are about 200 different cell types in mammals (one of us). Cells are tiny, measuring on average about 0.002 cm (20 um) across. That’s about 1250 cells, “shoulder-to-shoulder” per inch. Red and white blood cells above vesselforming cells. nerve cell The Cell Theory The cell theory (proposed independently in 1838 and 1839) is a cornerstone of biology. All organisms are composed of one or more cells. Schleiden Cells are the smallest living things. Cells arise only by division of previously existing cells. All organisms living today are descendents of an ancestral cell. Schwann A Sense of Scale and Abundance – Bacteria on the Head of a Pin Two Fundamentally Different Types of Cells A prokaryotic cell A eukaryotic cell Us vs. Them Eukaryotes and Prokaryotes An Idealized Animal Cell Major Divisions of the Eukaryotic Cell A rat liver cell (with color enhancement to show organelles) It’s Crowded In There An artist’s conception of the cytoplasm - the region of a cell that’s not in the nucleus or within an organelle. It’s Crowded In There A micrograph showing cytoskeleton (red), ribosomes (green), and membrane (blue) Animal and Plant Cells Have More Similarities Than Differences Cellular Anatomy We’ll start by seeing what role these parts play in making and moving proteins. The Nucleus Think of the nucleus as the cell’s control center. Two meters of human DNA fits into a nucleus that’s 0.000005 meters across. Ribosomes and the Endoplasmic Reticulum The Rough Endoplasmic Reticulum Functions: Protein synthesis (about half the cell’s proteins are made here). Protein movement (trafficking) Protein “proofreading” Cystic Fibrosis Click here to see the article. The Lysosome Functions: Digesting food or cellular invaders Recycling cellular components Cell suicide (suicide is bad for cells, but good for us!) (The lysosome is not found in plant cells) The Lysosome This bacterium about to be eaten by an immune system cell will spend the last minutes of its existence within a lysosome. Many Diseases are Caused by Lysosome Malfunction Cellular Anatomy The Mitochondrion Think of the mitochondrion as the powerhouse of the cell. Both plant and animal cells contain many mitochondria. (Mitochondria is the plural of mitochondrion) The Mitochondrion A class of diseases that causes muscle weakness and neurological disorders are due to malfunctioning mitochondria. Worn out mitochondria may be an important factor in aging. Mitochondrial Diseases Mitochondria and Health Animal vs. Plant Cells – Chloroplasts Are a Big Part of the Difference Two Other Unique Features of Plant Cells The central vacuole may occupy 90% of a plant cell. Cellular Anatomy The Cytoskeleton The name is misleading. The cytoskeleton is the skeleton of the cell, but it’s also like the muscular system, able to change the shape of cells in a flash. An animal cell cytoskeleton A Cytoskeleton Gallery The Cytoskeleton in Action A white blood cell using the cytoskeleton to “reach out” for a hapless bacterium. The Cytoskeleton in Action Cilia on a protozoan Beating sperm tail at fertilization Smoker’s cough is due to destruction of cilia linking the airways. Cell Connections & Junctions Definition and Classification of cell junction Cell junction is the connection between the neighbouring cells or the contact between the cell and extracellular matrix. It is also called membrane junction. Cell junction are classified into three types a-Occluding junction b-Communicating junction c-Anchoring junction. Cell Adhesion Molecules (CAMs) Important cell surface proteins molecules promoting cell–cell and cell–matrix interactions. Important for many normal biological processes -embryonic cell migration, immune system functions, wound healing. Involved in intracellular signaling pathways (primarily for cell death/survival, secretion etc.) Cell Adhesion Molecules (CAMs) Express 3 major domains: The extracellular domain allows one CAM to bind to another on an adjacent cell. The transmembrane domain links the CAM to the plasma membrane through hydrophobic forces. The cytoplasmic domain is directly connected to the cytoskeleton by linker proteins. Cell Adhesion Molecules (CAMs) Interactions between CAMs can be mediated by : Binding of an adhesion molecule on one cell to the same adhesion molecule on a second cell An adhesion molecule on one cell type binds to a different type of cell adhesion molecule on a The linker molecule in most cases is Laminin, a family of large cross shaped molecules with These cell adhesion molecules can be divided into 4 major families The cadherin superfamily The selectins The immunoglobulin superfamily and The integrins The Cadherin superfamily Cadherins are the most prevalent CAMs in vertebrates. 125 kD transmembrane glycoproteins - mediate intercellular adhesion in epithelial and endothelial cells by Ca2+ dependent homophilic adhesion. Primarily link epithelial and muscle cells to their neighbors Form desmosomes and adherens junctions Play critical role during development (cell sorting). Do not interact with extracellular matrix. The Cadherin superfamily Contain a short transmembrane domain and a relatively long extracellular domain containing four cadherin repeats (EC1-EC4), each of which contains calcium binding sequences Cadherins interact with specific cytoplasmic proteins, e.g., catenins (α, β and γ), as a means of being linked to the actin cytoskeleton. The binding of cadherins to the catenins is crucial for cadherin function. The Selectins Structural features of selectins include: NH2-terminal C-type Ca2+ dependent lectin like binding domain, which determines the ability of each selectin to bind to specific carbohydrate lingands. an epidermal growth factor-like region. a number of repeat sequences. a membrane-spanning region and a short cytoplasmic region Immunoglobulin Superfamily Molecules Consists of more than 25 molecules. Important ones being: Intracellular adhesion molecule 1(ICAM1; CD54) Intercellular adhesion molecule 2 (ICAM2), Vascular cell adhesion molecule1 (VCAM1; CD106), Platelet endothelial cell adhesion molecule 1 (PECAM 1; CD31) and the mucosal addressin cell adhesion molecule 1 (MAdCAM1). The integrins Fifteen different α and eight different β subunits give rise to over twently different heterodimeric combinations at cell surfaces. Bind epithelial and muscle cells to laminin in the basal lamina Allow platelets to stick to exposed collagen in a damaged blood vessel Allow fibroblasts and white blood cells to adhere to fibronectin and collagen as they move tissue Types of cell junction in animal Occluding Junction A cell-cell junction that seals cells together in an epithelium in a way that prevents even small molecules from leaking from one side of the sheet to the other. Tight Junction Tight Junction- occluding junctions / zonulae occludens - zonula occludens), are the closely associated areas of two cells whose membranes join together forming a virtually impermeable barrier to fluid. A type of junctional complex present only in vertebrates. Consist of linear array of several integral proteins. Junctional proteins occludins and claudins & members of IG suprfamily are transmembrane proteins. Function of Tight Junction Strength and stability Selective permeable for ions. Fencing function Maintance of cell polarity Blood-brain barrier Cludin -16 in Thick Junctions of Ascending Loop of henle. Cludin- 15 Permability of cations / anions. Adhering Junctions Desmosome- Connects intermediate filament of one cell with other cells. Claudin Hemidesmosome Desmoplakin is essential for normal desmosomal adhesion. Communicating Junction Cell junction which permit the intercellular exchange of substance are called communicating junction, these junction permit the movement of ions and molecules from one cell to another cell. a- Gap junction b- Chemical synapse Gap Junction Gap junctions are clusters of intercellular channels that allow direct diffusion of ions and small molecules between adjacent cells. At gap junctions, the intercellular space narrows from 25 nm to 3 nm. gap junctions were first discovered in myocardium and nerve because of their properties of electrical transmission between adjacent cells (Weidmann 1952; Furshpan and Potter 1957). Low resistance intercellular junction that allows passage of ions and smaller molecules between the cells. It present in heart, basal part of epithelial cell of intestinal mucosa, etc Junctional unit-Connexons- 6 connexins Connexon of one cell have allignment with connexon of other cells. Gap Junction Electron microscopy of gap junctions joining adjacent hepatocytes in the mouse. The gap junction (GJ) is seen as an area of close plasma membrane apposition Function of gap junction channel passage the substance have molecular weight less than 1000. Exchange of between cells Rapid propagation of action potential from one cell to another cell. chemical messenger Desmosomes Also known as macula adherens is a cell structure specialized for cell-to-cell adhesion. Are molecular complexes of cell adhesion proteins and linking proteins that attach the cell surface adhesion proteins to intracellular keratin cytoskeletal filaments. The cell adhesion proteins of the desmosome, desmoglein and desmocollin, are members of the cadherin family. On the cytoplasmic side of the plasma membrane, there are two dense structures called the Outer Dense Plaque (ODP) and the Inner Dense Plaque (IDP). The Outer Dense Plaque is where the cytoplasmic domains of the cadherins attach to desmoplakin via plakoglobin and plakophillin. The Inner Dense Plaque is where desmoplakin attaches to the intermediate filaments of the cell. Desmosomes Hemidesmosomes Hemidesmosomes look like half-desmosomes that attach cells to the underlying basal lamina. Rather than using desmogleins, hemidesmosomes use desmopenetrin cell adhesion proteins,which are members of Integrin family. The integrin molecule attach to one of many multi-adhesive proteins such as laminin, resident within the extracellular matrix, thereby forming one of many potential adhesions between cell and matrix. Chemical synapse Chemical synapse is the junction between a nerve fibre and a muscle fiber or between two nerve fibre ,through which signals transmitted by the release of chemical transmitter. 60 Anchoring junction. Anchoring junction are the junction ,which provides strength to the cell by acting like mechanical attachment. These junction provide firm structural attachment between two cells or between a cell and extracellular matrix Anchoring junction are responsible for structural integrity of the tissue. various cell junctions found in a vertebrate epithelial cell, classified according to their primary functions The Cell CycleMitosis and Meiosis INTERPHASE- G , S, 1 G2 MITOSIS OR MEIOSIS The Cell Cycle The sequence of growth and division of a cell Interphase = G1, S, G2 Interphase is when the cell grows, and the organelles double prior to the actual splitting of the nucleus. 93% of a cell’s life is spent in interphase. Interphase has three parts Growth 1 (G1) Synthesis (S) Growth 2 (G2) G1, S, G2 G1 is when organelles double. S when DNA is replicated. Remember each new cell needs a complete set of organelles. Each cell needs a complete and identical set of DNA G2 Proteins needed for Mitosis are produced. Mitosis The process by which the cell nucleus divides into two identical cell nuclei. In some Human cells interphases lasts 15.3 hours, while mitosis lasts only .7 hours. Occurs in a series of steps Prophase Metaphase Anaphase Telophase Cytokinesis Chromosomes Must duplicate and separate during Mitosis Structures of the tightly packaged DNA DNA is tangled up into a substance of chromatin The chromatin is packaged on the chromosome Chromosomal structure Prophase Chromosomes now called chromatids because they doubled to form short thick rods which pair up and line up in the center of the nucleus. A centromere connects the two halves of the doubled chromatids. Spindle fibers begin to form. Spindle fiber – a fibrous structure from the cytoplasm which forms to the centriole. Centrioles move to opposite sides of the cell. The nuclear membrane breaks down. Prophase Metaphase Centromeres of the chromatid pairs line up in the middle of the cell. Metaphase plate- location where the centromeres line up in the center of the cell. By the end of metaphase each chromatid has attached to spindle fibers. Metaphase Anaphase The spindle fibers pull the chromatids apart. This separates each one from its duplicate. These move to opposite sides of the cell. Now there are two identical sets of chromosomes. Anaphase Telophase When the chromosomes reach opposite sides of the cell the spindle fibers break up. The nuclear membrane begins to reform. A furrow begins to develop between the two sets of chromosomes. Telophase Cytokinesis The two identical cells completely divide and the cell membrane is completely formed. Mitosis Movie 1 Mitosis movie 2 Meiosis Diploid (2n) - A cell with two of each kind of chromosome. One chromosome from each parent. If two body cells were to combine nuclei, the number of chromosomes would double. In order for sexual reproduction to occur, each cell involved must reduce its chromosome number by half. Haploid (n)- A cell with one of each kind of chromosome. Haploid cells Haploid cells are called gametes Gametes are either sperm or eggs Organism Human Pea Fruit fly Dog diploid gamete 46 14 23 7 8 78 4 39 Homologous chromosomes Are paired chromosomes with genes for the same trait arranged in the same order. Ex. Eye color, hair color, height, one may code for blue, blonde, tall, its homolog may code for brown, blonde, short Homologous chromosomes may have different alleles on them Allele- gene form for each variation of a trait of an organism. Meiosis Meiosis is the process of cell division in which gametes are formed and the number of chromosomes is halved. So that sexual reproduction and zygote formation can occur. Zygote- Fertilized egg which has a diploid number of chromosomes. Stages of Meiosis Interphase Chromosomes replicate Each chromosome consists of 2 identical sister chromatids Prophase I Each Pair of homologous chromosomes come together to form a tetrad. Tetrad- 2 homologous chromosomes come together and the 4 chromatids overlap. Crossing over Tetrads are so tight that non-sister chromatids from the homologous pair actually exchange genetic material. Crossing over- The exchange of genetic material by non-sister chromatids during late prophase I of meiosis. Results in a new combination of alleles Metaphase I Homologous chromosomes line up together in pairs. * In mitosis homologous chromosomes line up in the middle independently of each other. Anaphase I Spindle fibers attach to the centromeres of each pair. Homologous chromosomes separate and move to opposite ends of the cell. Centromeres DO NOT split like they do in mitosis Now each cell will get one chromosome from each homologous pair. Telophase I Spindle fibers break down Chromosomes uncoil Cytoplasm divides Another cell division is needed because the number of chromosomes has not been reduced After telophase I there maybe a short interphase, but not always. It is important to note that if a cell does have a second interphase, there is No replication of chromosomes. Meiosis I Meiosis II Is basically just like mitosis, but remember the chromosomes did not duplicate in interphase II. Prophase II Chromosomes begin to line up in the middle of the cell. Spindle fibers begin to form Metaphase II Chromosomes line up on the metaphase plate Meiosis II Anaphase II Centromeres split Sister chromatids separate and move to opposite sides of the cell Telophase II Nuclei reform Spindle fibers disappear Cytoplasm divides into two. The number of chromosomes in each daughter cell has now been reduced by half. Meiosis II Regulation of Cell cycle Regulation of the cell cycle involves processes crucial to the survival of a cell, including the detection and repair of genetic damage as well as the prevention of uncontrolled cell division Two key classes of regulatory molecules, cyclins and cyclin-dependent kinases (CDKs), determine a cell's progress through the cell cycle General mechanism of cyclin-CDK interaction Upon receiving a pro-mitotic extracellular signal, G1 cyclinCDK complexes become active to prepare the cell for S phase, promoting the expression of transcription factors that in turn promote the expression of S cyclins and of enzymes required for DNA replication. The G1 cyclin-CDK complexes also promote the degradation of molecules that function as S phase inhibitors by targeting them for ubiquitination. Specific action of cyclinCDK complexes Cyclin D is the first cyclin produced in the cells that enter the cell cycle, in response to extracellular signals (e.g. growth factors). Cyclin D levels stay low in resting cells that are not proliferating. Additionally, CDK4/6 and CDK2 are also inactive because CDK4/6 are bound by INK4 family members (e.g., p16), limiting kinase activity. Meanwhile, CDK2 complexes are inhibited by the CIP/KIP proteins such as p21 and p27 Inhibitors Endogenous Two families of genes, the cip/kip (CDK interacting protein/Kinase inhibitory protein) family and the INK4a/ARF (Inhibitor of Kinase 4/Alternative Reading Frame) family, prevent the progression of the cell cycle. Because these genes are instrumental in prevention of tumor formation, they are known as tumor suppressors. The cip/kip family includes the genes p21, p27 and p57. They halt the cell cycle in G1 phase by binding to and inactivating cyclin-CDK complexes. p21 is activated by p53 (which, in turn, is triggered by DNA damage e.g. due to radiation). p27 is activated by Transforming Growth Factor β (TGF β), a growth inhibitor. The INK4a/ARF family includes p16INK4a, which binds to CDK4 and arrests the cell cycle in G1 phase, and p14ARF which prevents p53 degradation. Synthetic Synthetic inhibitors of Cdc25 could also be useful for the arrest of cell cycle and therefore be useful as antineoplastic and anticancer agents.
