Transcript Mitosis
How cells reproduce? • Reproduction – Produces a generation of individuals like parents • Cell division – Bridges two generations • Each daughter cell receives – A required number of DNA molecules – Some cytoplasm Eukaryotes and Prokaryotes • Eukaryotic cells undergo mitosis and/or meiosis one chromatid its sister chromatids – Separates duplicated chromosomes of parent cell into two daughter nuclei – Another mechanism divides cytoplasm • Prokaryotic cells divide by a different process one chromosome (unduplicated) one chromosome (duplicated) Mitosis and Meiosis • Mitosis – Basis of growth, cell replacements, and tissue repair in multicelled species – Basis of asexual reproduction in many single-celled and multicelled species • Meiosis – Basis of sexual reproduction – Precedes formation of gametes or sexual spores Chromosome structure • Eukaryotic chromosome – Association of DNA, histones, and other proteins – Proteins structurally organize the chromosome and affect access to its genes • Nucleosome – Smallest unit of organization – Double-stranded DNA looped twice around a spool of histones Introducing the cell cycle • Cell cycle – Starts when a new cell forms – Runs through interphase – Ends when cell reproduces by nuclear and cytoplasmic division Interphase • Most cellular activities occur in interphase – G1: Cell grows in mass, doubles number of cytoplasmic components – S: DNA replication duplicates chromosomes – G2: Cell prepares for division Chromosome number • Sum of all chromosomes in cells of a given type • In human body cells, chromosome number is 46 • Body cells are diploid (have two of each kind of chromosome) Mitosis and chromosome number • Mitosis maintains parental chromosome number from one generation to the next – Bipolar spindle divides sister chromatids A closer look at mitosis • Mitosis – A nuclear division mechanism that maintains the chromosome number • Mitosis proceeds in four stages: – – – – Prophase Metaphase Anaphase Telophase Prophase • Duplicated chromosomes become threadlike as they start to condense • Microtubules form a bipolar spindle • Nuclear envelope starts to break apart Transition to Metaphase • Microtubules from one spindle pole harness one chromatid of each chromosome – Microtubules from the opposite spindle pole harness its sister chromatid • Other microtubules extend from both poles and grow until they overlap at the spindle’s midpoint Metaphase • All chromosomes become aligned midway between the two spindle poles – Chromosomes in most condensed forms Anaphase • Sister chromatids detach from each other – Spindles move them toward opposite poles • Microtubules that overlap at spindle’s midpoint slide past each other, push poles farther apart • Motor proteins drive movements Telophase • Two identical clusters (one chromosome of each type) reach opposite spindle poles • Nuclear envelope forms around each cluster • Both new nuclei have the parental chromosome number At the end! • Interphase – Two daughter cells Fig. 8.7h, p.130 Cytoplasmic Division Mechanisms • Mechanisms of cytoplasmic division differ in plant and animal cells • In animal cells – A contractile ring of microfilaments (part of cell cortex) contracts and pulls the cell surface inward until the cytoplasm is divided Cytoplasmic Division in Plant Cells • In plant cells – A band of microtubules and microfilaments forms around the nucleus before mitosis starts – Marks site where cell plate will form • Cell plate becomes a cross-wall that partitions the cytoplasm Controls Over Cell Division • Products of checkpoint genes control cell cycle – Kinases – Growth factors When control is lost • Mutant checkpoint genes can cause tumors by disrupting normal controls Cancers • Altered cells grow and divide abnormally – Malignant cells may metastasize (break loose and colonize distant tissues) benign tumor malignant tumor