Genetics - 02 Ultrastructure of the Cell

o HUMAN GENETICS AND CYTOGENETICS THE ULTRASTRUCTURE OF THE CELL LESSON 1: OVERVIEW OF CELL STRUCTURES o INTRODUCTION • • • • • • • • • • • • • • • • • • • The cell is the fundamental organizational unit of the body. Although cells come in many different forms, they share many features. Each cell consists of a number of organelles, so named because they contribute to overall cell function in as much the same way as the organs contribute to bodily function. The cell membrane determines the inside and outside of a cell. This is the customs officer of the cell, determining what enters or exits the cell. Materials move through the cytoplasm, the watery cell fluid that transports materials from one place to another, largely by diffusion. A cytoskeleton maintains cell shape and provides movement. All of the activities of the cell require chemical energy supplied as adenosine triphosphate. The mitochondria make ATP by coupling the chemical energy liberated by oxidation of foodstuffs to the synthesis of ATP. The cell also contains a variety of other organelles, including the lysosome, peroxisome, and proteasome. It is found that these structures degrade material, engulfed by the cell by endocytosis, and also degrade worn-out organelles and cell proteins. Eukaryotic cell - have membrane-bound organelles Cell wall - in plant cell Cell membrane - regulate entrance/exit o Semi-permiable Cytoplasm - fluid-like substance; where materials move o Organelles are found in cytoplasm Cytoskeleton o Maintains cell shape o Provide movement Mitochondria - powerhouse of the cell, produce energy o Cristae - for ATP production ER - transport materials Smooth - lipid synthesis Rough - with ribosome; assoc. protein synthesis Golgi Apparatus - packaging Lysosome - digestion; has hydrolytic enzymes for digestion o Suicide bags along with peroxisome Centrioles - common in animal & plants o Important in cell division for movement of chromosomes o Produce spindle fibers Vacuoles - large in plant cell; several in animal cell o Storage of water/energy o Maintain fragility of plant cells Nucleus o Membrane-bound o Nucleolus - assoc. with protein synthesis o Chromosomes - important in reproduction & heredity Under the right conditions in the body or a laboratory, stem cells divide to form more cells called daughter cells. These daughter cells either become new stem cells (self-renewal) or become specialized cells (differentiation) with a more specific function, such as blood cells, brain cells, heart muscle cells or bone cells. No other cell in the body has the natural ability to generate new cell types. WHY IS THERE SUCH AN INTEREST IN STEM CELLS? • • • Increase understanding of how diseases occur. By watching stem cells mature into cells in bones, heart muscle, nerves, and other organs and tissue, researchers and doctors may better understand how diseases and conditions develop. Generate healthy cells to replace diseased cells (regenerative medicine). Stem cells can be guided into becoming specific cells that can be used to regenerate and repair diseased or damaged tissues in people. People who might benefit from stem cell therapies include those with spinal cord injuries, type 1 Diabetes, Parkinson's disease, amyotrophic lateral sclerosis, Alzheimer's disease, heart disease, stroke, burns, cancer and osteoarthritis. Stem cells may have the potential to be grown to become new tissue for use in transplant and regenerative medicine. Researchers continue to advance the knowledge on stem cells and their applications in transplant and regenerative medicine. Test new drugs for safety and effectiveness. Before using investigational drugs in people, researchers can use some types of stem cells to test the drugs for safety and quality. This type of testing will most likely first have a direct impact on drug development first for cardiac toxicity testing. New areas of study include the effectiveness of using human stem cells that have been programmed into tissue-specific cells to test new drugs. For the testing of new drugs to be accurate, the cells must be programmed to acquire properties of the type of cells targeted by the drug. Techniques to program cells into specific cells continue to be studied. For instance, nerve cells could be generated to test a new drug for a nerve disease. Tests could show whether the new drug had any effect on the cells and whether the cells were harmed. WHERE DO STEM CELLS COME FROM? LESSON 2: UNDERSTANDING THE STEM CELL • Stem cells - body's raw materials o cells from which all other cells with specialized functions are generated. • Embryonic stem cells. These stem cells come from embryos that are three to five days old. At this stage, an embryo is called a blastocyst and has about 150 cells. These are pluripotent (ploo-RIP-uh-tunt) stem cells, • • • meaning they can divide into more stem cells or can become any type of cell in the body. This versatility allows embryonic stem cells to be used to regenerate or repair diseased tissue and organs. Adult stem cells. These stem cells are found in small numbers in most adult tissues, such as bone marrow or fat. Compared with embryonic stem cells, adult stem cells have a more limited ability to give rise to various cells of the body. Until recently, researchers thought adult stem cells could create only similar types of cells. For instance, researchers thought that stem cells residing in the bone marrow could give rise only to blood cells. However, emerging evidence suggests that adult stem cells may be able to create various types of cells. For instance, bone marrow stem cells may be able to create bone or heart muscle cells. This research has led to early-stage clinical trials to test usefulness and safety in people. For example, adult stem cells are currently being tested in people with neurological or heart disease. Adult cells altered to have properties of embryonic stem cells (induced pluripotent stem cells). Scientists have successfully transformed regular adult cells into stem cells using genetic reprogramming. By altering the genes in the adult cells, researchers can reprogram the cells to act similarly to embryonic stem cells. This new technique may allow researchers to use reprogrammed cells instead of embryonic stem cells and prevent immune system rejection of the new stem cells. However, scientists don't yet know whether using altered adult cells will cause adverse effects in humans. Researchers have been able to take regular connective tissue cells and reprogram them to become functional heart cells. In studies, animals with heart failure that were injected with new heart cells experienced improved heart function and survival time. Perinatal stem cells. Researchers have discovered stem cells in amniotic fluid as well as umbilical cord blood. These stem cells also have the ability to change into specialized cells. Amniotic fluid fills the sac that surrounds and protects a developing fetus in the uterus. Researchers have identified stem cells in samples of amniotic fluid drawn from pregnant women to test for abnormalities — a procedure called amniocentesis. More study of amniotic fluid stem cells is needed to understand their potential. WHY CAN'T RESEARCHERS USE ADULT STEM CELLS INSTEAD? • • WHAT ARE STEM CELL LINES AND WHY DO RESEARCHERS WANT TO USE THEM? • • Embryonic stem cells are obtained from early-stage embryos — a group of cells that forms when a woman's egg is fertilized with a man's sperm in an in vitro fertilization clinic. Because human embryonic stem cells are extracted from human embryos, several questions and issues have been raised about the ethics of embryonic stem cell research. The National Institutes of Health created guidelines for human stem cell research in 2009. The guidelines define embryonic stem cells and how they may be used in research, and include recommendations for the donation of embryonic stem cells. Also, the guidelines state embryonic stem cells from embryos created by in vitro fertilization can be used only when the embryo is no longer needed. A stem cell line is a group of cells that all descend from a single original stem cell and are grown in a lab. Cells in a stem cell line keep growing but don't differentiate into specialized cells. Ideally, they remain free of genetic defects and continue to create more stem cells. Clusters of cells can be taken from a stem cell line and frozen for storage or shared with other researchers. WHAT IS STEM CELL THERAPY (REGENERATIVE MEDICINE) AND HOW DOES IT WORK? • • • • Stem cell therapy, also known as regenerative medicine, promotes the repair response of diseased, dysfunctional or injured tissue using stem cells or their derivatives. It is the next chapter in organ transplantation and uses cells instead of donor organs, which are limited in supply. Researchers grow stem cells in a lab. These stem cells are manipulated to specialize into specific types of cells, such as heart muscle cells, blood cells or nerve cells. The specialized cells can then be implanted into a person. For example, if the person has heart disease, the cells could be injected into the heart muscle. The healthy transplanted heart muscle cells could then contribute to repairing defective heart muscle. Researchers have already shown that adult bone marrow cells guided to become heart-like cells can repair heart tissue in people, and more research is ongoing. HAVE STEM CELLS ALREADY BEEN USED TO TREAT DISEASES? WHY IS THERE A CONTROVERSY ABOUT USING EMBRYONIC STEM CELLS? • Although research into adult stem cells is promising, adult stem cells may not be as versatile and durable as are embryonic stem cells. Adult stem cells may not be able to be manipulated to produce all cell types, which limits how adult stem cells can be used to treat diseases. Adult stem cells also are more likely to contain abnormalities due to environmental hazards, such as toxins, or from errors acquired by the cells during replication. However, researchers have found that adult stem cells are more adaptable than was first thought. • • Yes. Doctors have performed stem cell transplants, also known as bone marrow transplants. In stem cell transplants, stem cells replace cells damaged by chemotherapy or disease or serve as a way for the donor's immune system to fight some types of cancer and bloodrelated diseases, such as leukemia, lymphoma, neuroblastoma and multiple myeloma. These transplants use adult stem cells or umbilical cord blood. Researchers are testing adult stem cells to treat other conditions, including a number of degenerative diseases such as heart failure. WHAT ARE THE POTENTIAL PROBLEMS WITH USING EMBRYONIC STEM CELLS IN HUMANS? • For embryonic stem cells to be useful in people, researchers must be certain that the stem cells will differentiate into the specific cell types desired. • • • Researchers have discovered ways to direct stem cells to become specific types of cells, such as directing embryonic stem cells to become heart cells. Research is ongoing in this area. Embryonic stem cells can also grow irregularly or specialize in different cell types spontaneously. Researchers are studying how to control the growth and differentiation of embryonic stem cells. Embryonic stem cells might also trigger an immune response in which the recipient's body attacks the stem cells as foreign invaders, or the stem cells might simply fail to function normally, with unknown consequences. Researchers continue to study how to avoid these possible complications. WHAT IS THERAPEUTIC CLONING, AND WHAT BENEFITS MIGHT IT OFFER? • • • Therapeutic cloning, also called somatic cell nuclear transfer, is a technique to create versatile stem cells independent of fertilized eggs. In this technique, the nucleus, which contains the genetic material, is removed from an unfertilized egg. The nucleus is also removed from the cell of a donor. This donor nucleus is then injected into the egg, replacing the nucleus that was removed, in a process called nuclear transfer. The egg is allowed to divide and soon forms a blastocyst. This process creates a line of stem cells that is genetically identical to the donor's cells — in essence, a clone. Some researchers believe that stem cells derived from therapeutic cloning may offer benefits over those from fertilized eggs because cloned cells are less likely to be rejected once transplanted back into the donor and may allow researchers to see exactly how a disease develops. • Meiosis o Formation of gametes (egg & sperm) o Reduce the number of chromosomes by half o Diploid -> Haploid o Creates 4 haploid cells o Has 2 divisions (Meiosis I & Meiosis II) o Produces genetical variation o Each daughter cell is genetically distinct from parent o Combination of paternal & maternal o Reduction division LESSON 4: CELL CYCLE AND CELL DIVISION CELL TYPES • • Somatic cell o from the body of organisms o maintain life processes Reproductive cell o production of offspring o animals : gametes (eggs & sperm) • HAS THERAPEUTIC CLONING IN PEOPLE BEEN SUCCESSFUL? • • No. Researchers haven't been able to successfully perform therapeutic cloning with humans despite success in a number of other species. However, in recent studies, researchers have created human pluripotent stem cells by modifying the therapeutic cloning process. Researchers continue to study the potential of therapeutic cloning in people. LESSON 3: THE NATURE OF THE CELL UNDERGOING THE CELL CYCLE • • • All cells arise from pre-existing cells through the process of cell division. Cell division produce new cells Mitosis o Direct cell division o Occur in somatic/body cells o Within nucleus containing chromosomes o Chromosomes are visible during cell division o Has genes carrying the DNA o Ensures chromosome number of daughter cell is same with parent cell o Guarantee genetic consistency from one generation to another o Produces two diploids CELL TYPES BASED ON CHROMOSOME NUMBER • Diploid o 2N diploid o specific for each organism o Humans = 2N = 46 o Fruitfly = 2N = 8 o Onion = 2N = 16 CELL CYCLE • • • • life of a cell from its origin in the division of parent cell until its own division into two cells Why do cells divide? o Asexual reproduction o growth o renewal and repair Cell Cycle Stages o Interphase ▪ Gap 1 (G1) ▪ S (synthesis) phase ▪ Gap 2 (G2) ▪ Gap 0 (GO) o Mitotic phase ▪ Karyokinesis ▪ Cytokinesis • CHROMOSOMES AND CHROMATIDS • INTERPHASE • • • • • G1 phase S G2 G0 accounts for about 90% of the time during each cycle • • • Which parts of the cell are involved or undergo changes during cell division? o Centriole o Nuclear Membrane o Nucleolus o Chromosome Chromosomes o Can be double or single stranded o Chromatids joined together by a centromere o w/ locus (pl. loci) where genes are found Types of Chromosomes o As to position of the centromere: ▪ Metacentric ▪ Submetacentric ▪ Acrocentric ▪ Telocentric o As to size: ▪ Small ▪ Medium ▪ Large Human Chromosomes Gap 0 o cell will leave the cycle and quit dividing o may be temporary resting period or more permanent o Example: a cell that has reached an end stage of development and will no longer divide (e.g. neuron) MITOTIC PHASE • • STAGES OF MITOTIC PHASE Mitosis o division of the nucleus Cytokinesis o division of the cytoplasm o Shortest part of the cycle o Mitosis has 4 subphases: P, M, A, T • Mitosis/Karyokinesis o Nuclear division o Cell growth stops o Produces two similar daughter cells (2n) o Asexual reproduction • Prophase o Chromatin thickens into chromosomes o Centriole pairs move to opposite ends of the cell o Spindle fibers begin to form o Nuclear membrane & nucleoli disintegrates • Metaphase o Chromosomes align along the metaphase plate o Chromosomes attached to the spindle fiber through kinetochores • Anaphase o Paired chromosomes separate and move to opposite sides of the cell o Separation of centromeres o Separated chromosomes are called daughter chromosomes • Telophase o Chromatids arrive at opposite poles of cell o Spindle fibers disintegrate o Nuclear membrane reforms o Nucleoli reappear o Plant cells: cell plate o Animals: cleavage furrow o • Cell Plate Animal cell vs Plant cell o Animal Cell Cleavage furrow Centrioles Plant Cell Cell plate No centrioles CYTOKINESIS • • Cytoplasm divides Produces two daughter cells, each with one nucleus RESULT OF CELL CYCLE • • • two new daughter cells o identical to the parent cell in chromosome number and chromosome type net gain of only one cell two new daughter cells will enter interphase and the cycle will continue… WHAT IS CANCER? • • • Stages of Prophase I a group of many related diseases that deal with cells Cancer Cells o cells that are not normal, grow and spread very fast o divide out of control and don't die o usually group or clump together to form tumors that can destroy the normal cells GAMETOGENESIS CELL TYPES BASED ON CHROMOSOME NUMBER • • Diploid cell - somatic cells Haploid cell - reproductive cells MEIOSIS • • • • • For sex cells (haploid) Meiosis I: reduction division Meiosis II: equational division Results to 4 haploid cells Long Prophase I CELL CYCLE CHECKPOINTS AND REGULATION (VIDEO) • • • • • • • Cell cycle o Three states: quiescent, interphase, and mitosis o Five phases: G not (G0), G1, S, G2, M Quiescent state o also known as a senescent state o comprises of the G0 phase o example: neurons, cardiac muscle and RBC's Interphase state o Consist of G1, S, G2 phases Mitosis o Consist of Prophase, metaphase, Anaphase, Telophase During this phase the three major cell cycle checkpoints are o the end of g1 o the end of g2 o during metaphase of the M phase Progression through checkpoints is regulated by o cyclins o CDKs (cyclin dependent kinases) o Tumor suppressors During the g1 phase cell cycle progression is controlled by a variety of different cellular mechanisms such as tumor suppressors, genes that code for transcription factors, proteins that detect DNA damage, enzymes that allow the • • • cell to progress to the S phase, and signaling molecules that activate those enzymes o Gene E2F - regulates expression of transcription genes and CDK 2, Cyclin E, and Cyclin A o If there is significant DNA damage, P53 stimulates production of P21 o P21 binds and inhibits all Cyclin-CDK complexes which leads to arrest of cell cycle until the DNA damage is repaired and P21 levels drop o Tumor Suppressor Retinoblastoma Protein (pRb) inhibits E2F expression o Cyclin D-CDK4 and Cyclin D-CDK6 complexes phosphorylate pRb that leads to inactivation of pRb and expression of E2F o the expression of E2F then leads to the expression of transcription genes and formation of Cyclin E-CDK2 o Therefore, the cell is pushed into the S phase Cell cycle regulation in S and M phase o Cyclin A-CDK2 is required for DNA synthesis o Cyclin A-CDK1 and Cyclin B-CDK1 promote the events of mitosis o Towards the end of mitosis, the Anaphase Promoting Complex (APC) causes ubiquitination and destruction of Cyclin A-CDK1 and Cyclin B-CDK1 that leads to termination of M phase with anaphase, telophase and cytokinesis The most radiosensitive phases are the G2 phase and mitosis the least radiosensitive phase is the latter part of the S phase MEIOSIS