Cell Division: Mitosis and Meiosis

Cell Division in Eukaryotes Cell Cycle –The cell cycle is the repeating set of events in the life of a cell. –The cell cycle consists of cell division and interphase. –Cell division in eukaryotes includes nuclear division, called mitosis, and the division of cytoplasm, called cytokinesis.

Cell Cycle

Cell Cycle and Mitosis Interphase –Interphase consists of growth (G 1 ), DNA replication, and preparation for cell division (G 2 ). Mitosis is divided into stages: prophase, metaphase, anaphase, and telophase. Mitosis results in two offspring cells that are genetically identical to the original cell.

Phases of Mitosis

Prophase –DNA coils into chromosomes –Nuclear envelope dissolves Metaphase –Chromosomes line up along center of cell Anaphase –Centromeres dissolve and chromatids separate Telophase –Nuclear envelopes reform around separated chromatids Cytokinesis

Control of Cell Division Cell division in eukaryotes is controlled by many proteins. Control occurs at three main checkpoints. –G1: Does cell need to/ is it ready to divide? –G2: Cell mature and DNA copied successfully? –Metaphase: Chromatids paired and lined up? Cancer may result if cells do not respond to control mechanisms

Cell Division and Cancer Mutations of proto-oncogenes, which regulate cell growth, or tumor-suppressor genes, which prevent cell division from occurring too often,may lead to cancer. –Proto-oncogenes are like the accelerator –Tumor-suppressor genes are like the brake pedal Cancer is the uncontrolled growth of abnormal cells.

Cell Division and Cancer

Gene Expression in Cancer –Unlike normal cells, cancer cells continue to divide indefinitely, even if they become densely packed. –Cancer cells will also continue dividing even if they are no longer attached to other cells. Causes of Cancer –A carcinogen is any substance that can induce or promote cancer. –Most carcinogens are mutagens, substances that cause mutations.

Meiosis Meiosis is a process of nuclear division that reduces the number of chromosomes in new cells to half the number in the original cell. Meiosis leads to four haploid cells (gametes) rather than two diploid cells as in mitosis.

Stages of Meiosis

Development of Gametes Spermatogenesis is the process by which sperm cells are produced. Oogenesis is the process that produces mature egg cells.

Meiosis in Male and Female Animals

Sexual Reproduction Sexual reproduction is the formation of offspring through meiosis and the union of a sperm and an egg. Offspring produced by sexual reproduction are genetically different from the parents.

When Meiosis Goes Awry Nondisjunction: the failure of homologous chromosomes to separate during Meiosis I, or the failure of sister chromatids to separate during Meiosis II Results in gametes with an abnormal number of chromosomes –Aneuploidy—cell having an abnormal number of chromosomes –Trisomy—3 copies of a chromosome –Monosomy—1 copy of a chromosome %20Meiosis%20I.htmhttp:// %20Meiosis%20I.htm

Disorders Caused by Nondisjunction Trisomy 21—Down Syndrome Trisomy 13 Syndrome (Patau Syndrome) XXY—Klinefelter’s Syndrome XYY—Jacobs Syndrome XXX—Triple X Syndrome X0—Turner’s Syndrome XXXXX syndrome

Karyotypes A micrograph taken of an organism’s chromosomes during metaphase; arranged in homologous pairs for analysis

Trisomy 21 Karyotype