BIO 102: Biology And Society Cell Cycle: Mitosis and Meiosis PDF

BIO 102: Biology and Society Topic 6 Cell Cycle: Mitosis and Meiosis “All cells arise from preexisting cells” This fundamental principle, known as the cell doctrine, was originally postulated by Rudolf Virchow in 1858, and it provides the basis for the continuity of lif...

BIO 102: Biology and Society Topic 6 Cell Cycle: Mitosis and Meiosis “All cells arise from preexisting cells” This fundamental principle, known as the cell doctrine, was originally postulated by Rudolf Virchow in 1858, and it provides the basis for the continuity of life. The Cell Cycle A finely controlled sequence of steps that results in identical distribution of genetic material from a parent cell to two daughter cells. The entire cell cycle consists of duplication of the genome, synthesis of constituents of the cell and eventually the division into two identical daughter cells The dividing cell: – Precisely replicates its DNA – Allocates the two copies of DNA to opposite ends of the cell – Separates into two daughter cells containing identical hereditary information Eukaryotic Genomes The term “chromosome” is used to refer to a nucleic acid molecule that is the repository of genetic information in a virus, a bacterium, a eukaryotic cell, or an organelle. Somatic eukaryotic cells have diploid genomes Each chromosome is paired with a homologous chromosome Humans have 46 chromosomes Gametes have haploid genomes, as they receive half of the chromosomes by a process called meiosis  DNA must be tightly packaged in order to fit inside the nucleus Chromosomes undergo multiple levels of compaction and condensation Each chromosome consists of the following: – A single, long, double-stranded molecule of DNA, segments of which are called genes  The chromosomes exist in different states at different stages of the cell cycle. - During interphase, the chromosomes are loosely folded; cannot be seen with a light microscope - During the mitotic phase, chromosomes are highly folded and condensed; can be seen with a light microscope The Cell Cycle The Cell Cycle During interphase, the cell grows while also preparing for cell division; DNA is replicated during this phase (recall eukaryotic DNA replication) During the mitotic phase, the replicated DNA is separated (mitosis), and then the cytoplasmic contents are separated (cytokinesis) Interphase Divided into three phases: G1 Phase: Little observable change, but cell gathers the building blocks of chromosomal DNA, the associated proteins, and the energy required for DNA replication S Phase: DNA replicates; each chromosome divides into identical sister chromatids that are linked at the centromere G2 Phase: Protein synthesis, energy accumulation, and cytoskeleton dismantlement for the mitotic phase Mitosis Mitosis Cytokinesis This is the process of cytoplasmic division; begins during the telophase of mitosis During cytokinesis in animal cells, a ring of actin filaments forms at the metaphase plate The ring contracts, forming a cleavage furrow, which divides the cell in two Cytokinesis G0 Phase (Quiescent Stage) Cells in G0 phase are not actively preparing to divide The cell is in an inactive state that occurs when the cells exit the cell cycle Some cells enter the G0 temporarily until an external signal triggers the onset of G1 Other cells that never or rarely divide, such as mature cardiac muscle and nerve cells, remain in G0 permanently Cell Cycle Regulation The length of the cell cycle is highly variable depending on both external and internal signals In rapidly dividing human cells with a 24 hour cell cycle, the G1 phase lasts approximately nine hours, the S phase lasts 10 hours, the G2 phase lasts about four and one-half hours, and the M phase lasts approximately one-half hour  Initiation and inhibition of the cell cycle can both be influenced by external events Death of a neighboring cell, or the presence of growth hormone can stimulate cell division Crowding of cells, or the absence of growth hormone can inhibit cell division Regulation at Internal Checkpoints Regulation at Internal Checkpoints The G1 checkpoint: A point at which the cell irreversibly commits to the cell division process. In addition to adequate reserves and cell size, there is a check for genomic DNA damage at the G1 checkpoint. The cell can halt the cycle and attempt to remedy the problematic condition, or the cell can advance into G0 and await further signals when conditions improve The G2 checkpoint: As at the G1 checkpoint, cell size and protein reserves are assessed. However, the most important role of the G2 checkpoint is to ensure that all of the chromosomes have been replicated and that the replicated DNA is not damaged The M checkpoint: Determines whether all the sister chromatids are correctly attached to the spindle microtubules Prokaryotic Cell Division Prokaryotic Cell division Meiosis Meiosis generates germ cells with haploid genomes, ensuring the production of haploid phase in the life cycle of sexually reproducing organisms Germ or reproductive cells fuse to form diploid offspring in a process called fertilization Homologous chromosomes are separated during meiosis Meiosis Meiosis