The Cell Cycle

Introduction: Key Roles of Cell Division The ability of organisms to reproduce their kind is one characteristic that best distinguishes living things from nonliving matter.

The continuity of life from one cell to another is based on the reproduction of cells via cell division. This division process occurs as part of the cell cycle, the life of a cell from its origin in the division of a parent cell until its own division into two. Introduction: Key Roles of Cell Division

Cell division requires coordinated division of chromosomes (mitosis) ….. …… and division of the cytoplasm (cytokinesis).

Introduction: Key Roles of Cell Division In unicellular organisms, division of one cell reproduces the entire organism i.e. cloning Multicellular organisms depend on cell division for: – Development – Growth – Repair

Cellular Organization of the Genetic Material A cell’s endowment of DNA ( genetic information) is called its genome DNA molecules in a cell are packaged into units called chromosomes –Every eukaryotic species has a characteristic number of chromosomes in the nucleus. –Human somatic cells (body cells) have 46 chromosomes, called the diploid number or 2n Human gametes, sperm or egg, have 23 chromosomes, or half the diploid number (n; haploid)

Chromosome Anatomy Chromatin, a complex of DNA and proteins that condenses during cell division Sister chromatids contains identical copies of the chromosome’s DNA. Centromere, region where the strands connect shrinks to form a narrow site

25 µm 1888 W. Waldeyer was the first to introduce the term chromosome meaning “colored body”.

Cell Division Eukaryotic cell division consists of 2 phases: –karyokinesis, division of the nucleus –Cytokinesis, division of the cytoplasm Gametes = meiosis Meiosis yields non-identical daughter cells

In 1882, the German anatomist Walther Flemming developed dyes to observe chromosomes during mitosis and cytokinesis To Flemming, it appeared that the cell simply grew larger between one cell division and the next Today we know that many critical events occur during this stage in a cell’s life Mitosis =Greek for “thread”Mitosis

P rophase - chromatin condenses into chromosomes M etaphase - chromosomes align at equator homologs align independently of each other A naphase - MT attached to kinetochore; chromatids are pulled apart & poles move apart T elophase - chromosomes at opposite poles; daughter cells form by cytokinesis

The Mitotic Spindle: A Closer Look The spindle includes the centrosomes, the spindle microtubules, and the asters Some spindle microtubules attach to a structure the kinetochores of chromosomes and move the chromosomes to the metaphase plate A centrosome is an organelle that serves as the main microtubule organizing center. An aster is a cellular structure shaped like a star, formed around each centrosome during mitosis in an animal cell.

Microtubules Chromosomes Sister chromatids Aster Centrosome Metaphase plate Kineto- chores Kinetochore microtubules 0.5 µm Overlapping nonkinetochore microtubules 1 µm Centrosome The Mitotic Spindle: A Closer Look

Cytokinesis: A Closer Look In animal cells, cytokinesis occurs by a process known as cleavage, forming a cleavage furrow In plant cells, a cell plate forms during cytokinesis

Cleavage furrow 100 µm Contractile ring of microfilaments Daughter cells Cleavage of an animal cell (SEM)

Nucleus Cell plate Chromosomes Nucleolus Chromatin condensing 10 µm Prophase. The chromatin is condensing. The nucleolus is beginning to disappear. Although not yet visible in the micrograph, the mitotic spindle is starting to form. Prometaphase. We now see discrete chromosomes; each consists of two identical sister chromatids. Later in prometaphase, the nuclear envelope will fragment. Metaphase. The spindle is complete, and the chromosomes, attached to microtubules at their kinetochores, are all at the metaphase plate. Anaphase. The chromatids of each chromosome have separated, and the daughter chromosomes are moving to the ends of the cell as their kinetochore micro- tubules shorten. Telophase. Daughter nuclei are forming. Meanwhile, cytokinesis has started: The cell plate, which will divide the cytoplasm in two, is growing toward the perimeter of the parent cell.