The Cell Cycle-Mitosis and Meiosis • What is the Cell Cycle? – A highly coordinated process that allows the cell to divide into 2 daughter cells in an orderly manner. • In unicellular organisms, division of one cell reproduces the entire organism • Role of the Cell Cycle in multicellular organisms is to produce cells for: a) Reproduction of the organism b) Growth & Development c) Tissue RepairCell division results in genetically identical daughter cells • Most cell division results in daughter cells with identical genetic information, DNA  A special type of division produces nonidentical daughter cells (gametes, or sperm and egg cells) • All the DNA in a cell constitutes the cell’s genome • A genome can consist of a single DNA molecule (common in prokaryotic cells) or a number of DNA molecules (common in eukaryotic cells) • DNA molecules in a cell are packaged into chromosomesConservation of genetic material • Distributes genetic material to offspring.  Needs to be a faithful copy of with few mistakes. • To ensure this occurs, the genetic material in the nucleus is duplicated.  The cell then splits into two daughter cells. • Somatic cells - All cells except reproductive cells.  During mitosis, when these cells are dividing and the chromosomes are visible, somatic cells contain 46 chromosomes in 23 pairs. • Diploid number • Gametes - reproductive cells.  During the phase when chromosomes are visible, gametes contain half that number - 23 chromosomes. • Haploid numberDistribution of Chromosomes During Eukaryotic Cell Division • Eukaryotic chromosomes consist of chromatin, a complex of DNA and protein that condenses during cell division • In preparation for cell division, DNA is replicated and the chromosomes condense • Each duplicated chromosome has two sister chromatids, which separate during cell division • The centromere is the narrow “waist” of the duplicated chromosome, where the two chromatids are most closely attachedCell Division • Eukaryotic cell division consists of:  Mitosis, the division of the nucleus  Cytokinesis, the division of the cytoplasm • Gametes are produced by a variation of cell division called meiosis  Meiosis yields nonidentical daughter cells that have only one set of chromosomes, half as many as the parent cell • The cell cycle consists of  Mitotic (M) phase (mitosis and cytokinesis)  Interphase (cell growth and copying of chromosomes in preparation for cell division) http://www2.mrc-lmb.cam.ac.ukInterphase  Interphase (about 90% of the cell cycle) can be divided into subphases:  G1 phase (“first gap”) • Cell grows and accumulates the substrates required for DNA replications.  S phase (“synthesis”) • The amount of DNA present in the cell doubles as the DNA strands are replicated. • Centrosome divides to give 2 daughter pairs.  G2 phase (“second gap”) • Cell continues to grow and produce the proteins and other molecules required for mitosis to occur. • In cells with centrosomes, these divide at this stage. • The cell grows during all three phases, but chromosomes are duplicated only during the S phaseMitosis and cytokinesis • Mitosis is conventionally divided into five phases:  Prophase  Prometaphase  Metaphase  Anaphase  Telophase • Cytokinesis is well underway by late telophaseMitosis and cytokinesisSteps in mitosis • Some definitions:  Centriole  Paired microtubular structures found in some centrosomes  Centromere  The ‘waist’ of a chromosome where microtubules (mitotic spindles) attach to the kinetochore  Centrosome  Organelle where microtubules are formed in some cells  Chromatid  One of the paired structures that constitute a chromosome after DNA replication in the S phase  Chromatin  ‘Uncondensed’ DNA, which together with histone proteins is how DNA is packaged during G0 and interphase of the cell cycle  Chromosome  The ‘condensed’ form of DNA seen during mitosis and meiosis. Can be a single structure or consist of 2 sister chromatids.G2 of Interphase Centrosomes (with centriole pairs) Chromatin (duplicated) Nucleolus Nuclear envelope Plasma membrane Early mitotic spindle Aster Centromere Chromosome, consisting of two sister chromatids Prophase Prometaphase Fragments of nuclear envelope Nonkinetochore microtubules Kinetochore Kinetochore microtubule Metaphase Metaphase plate Spindle Centrosome at one spindle pole Anaphase Daughter chromosomes Telophase and Cytokinesis Cleavage furrow Nucleolus forming Nuclear envelope formingSteps in mitosis • Prophase  Two structures are involved during prophase. 1. Chromosomes 2. Centrosomes with microtubules • Chromosomes  During prophase, chromosomes become visible as distinct structures, but are fairly elongated.  Prophase chromosomes consist of two sister chromatids held together over much of their length by proteins called cohesins.  The centromere is the region where the chromatids are the closest, and is the site where the microtubules will attach to the chromatids.  The kinetochore is a distinct structure located close to the centromere and is the site where microtubules attach to the chromatids. • Kinetochores develop in late prophase.• Centrosomes and microtubules  Polar microtubules begin to extend from each centrosome complex in a formation called an aster (‘star’).  These tubules will develop into mitotic spindles. • Microtubules are formed by addition of tubulin dimers to the end of each microtubule. • Each polar microtubule runs from a centrosome to a point where it overlaps and interacts with a microtubule from the other side. • The mitotic spindle serves as a track along which chromosomes will move later in mitosis. Steps in mitosis• Prometaphase  The nuclear lamina disintegrates and the nuclear envelope fragments to permit entry of spindle microtubules  Some of the spindle microtubules, called kinetochore microtubules begin to associate with kinetochores.  The microtubules from one pole associate with the kinetochore of one of the sister chromatids of each chromosome.  Signals from the cell poles move chromosomes toward the center of the cell—the metaphase plate. Steps in mitosis• Metaphase  The kinetochores on each chromosome are: • attached to a kinetochore microtubule • Located midway between the spindle poles