Cell Division

Overview: Cell Division Cell Division: New cells arise from the division of pre-existing cells Most cell division results in daughter cells with identical genetic information, DNA Integral part of the cell cycle, the life of a cell from formation to its own division Three types of cell division: Binary fission (observed in asexually reproducing organisms) Mitosis (refer to next slide) Meiosis (observed in sexually reproducing organisms)

Mitosis is seen in: Unicellular (definition: one –cell) organism Reproduction of an entire organism Multicellular (definition: more than one/multiple cell) organisms Development from a fertilized cell Growth Repair 100 µm 200 µm 20 µm (a) Reproduction (b) Growth and development (c) Tissue renewal

Mitosis and the Cell cycle Interphase G1: growth phase, making cell organelles S phase: DNA synthesis (replication) G2 phase: preparation for cell division, mitosis and cytokinesis Mitotic phase: creates two identical daughter cells Mitosis: Division of the nucleus Cytokinesis: Division of the cytoplasm

Cellular Organization of the Genetic Material IN EUKARYOTES….. Chromatin: -complex of DNA and associated proteins (histones) -Condenses during the cell division

chromosomes VERSUS chromatid During DNA synthesis, one chromosome duplicates to result in two sister chromatids During cell division, the two sister chromatids separate into two daughter cells The centromere is the narrow “waist” of the duplicated chromosome, where the two chromatids are most closely attached

Cellular Organization of the Genetic Material In prokaryotes (example: bacteria): One chromosome made of a single circular molecule of DNA (a plasmid). In eukaryotes (example: humans): Variable number of chromosomes (characteristic of species) made of molecules of long linear DNA. Example: humans have 46 chromosomes

Types of cell division Eukaryotic cells: Two types of cell division for Two types of cells Mitosis occurs in somatic cells (body cells) -Mitosis results in genetically identical cells Meiosis occurs in reproductive cells called gametes (eggs and sperm) -Meiosis is a way to combine genetic traits during reproduction Prokaryotic cells: Binary fission

MITOSIS Mitosis is conventionally divided into the phases (P-M-A-T): Prophase Metaphase Anaphase Telophase Cytokinesis is well underway by late telophase NOTE: In some textbooks, interphase is considered part of mitosis….hence in that case it is IPMAT

MITOSIS

Cytokinesis Accumulation of vesicles from Golgi apparatus, carrying cell wall materials. → Cell plate Use a network of microtubules (tubulin) to align vesicles Formation of cleavage furrow Contractile ring made of microfilaments (actin) associated with myosin

Does mitosis occur in all cells? No… -Sex cells (gametes) do not undergo mitosis Why? reproduction involves the combining of traits from two organisms - cannot combine/fuse any two random cells

Sexual Reproduction Meiosis = specialized cell division so you have only one of each chromosome, called Gametes: made only in gonad (testis, ovary) n (23) n (23) + 2n = 46

Why cannot we have mitosis for sex cells (gametes)? Sex cells (gametes) allow traits to be combined from two organisms. Can’t just fuse any two random cells. 2n (46) 2n (46) + A1 A2 A3 A26 B35 B44 B41 B35 4n = 92 too many

Chromosome numbers Diploid: possess two sets of each chromosome; one from the father and one from the mother. Human: 46 chromosomes or 23 pairs of homologous chromosomes (2 sets; 2n). In somatic cells, during the cell cycle: 46 chromosomes are duplicated during the S phase Each daughter cell receives an identical copy of each of the 46 chromosomes. Gametes, obtained via meiosis, are haploid Yield non-identical cells containing 23 chromosomes (1 set; n) Fusion of two gametes (fertilization) returns the number of chromosomes to 46.

Mitosis versus Meiosis Diploid somatic cell gamete precursor MITOSIS division duplication Diploid Haploid division Meiosis has 2 divisions: Meiosis I and Meiosis II

The Cell Cycle Control System The sequential events of the cell cycle are directed by a distinct cell cycle control system, which is similar to a clock The cell cycle control system is regulated by both internal and external controls The clock has specific checkpoints where the cell cycle stops until a go-ahead signal is received Monitor integrity of genome Prevent the cell from progressing through the cell cycle if abnormalities are detected Correct size Type and amount of specific proteins

For many cells, the G1 checkpoint seems to be the most important one If a cell receives a go-ahead signal at the G1 checkpoint, it will usually complete the S, G2, and M phases and divide If the cell does not receive the go-ahead signal, it will exit the cycle, switching into a nondividing state called the G0 phase

Loss of Cell Cycle Controls in Cancer Cells Do not respond normally to the body’s control mechanisms Exhibit neither density-dependent inhibition nor anchorage dependence May not need external growth factors to grow and divide: They may make their own growth factor They may convey a growth factor’s signal without the presence of the growth factor They may have an abnormal cell cycle control system

Development of cancer Transformation: a normal cell is converted to cancer cell in a tissue; normally degraded by immune system. Tumor: mass of abnormal cells within a tissue Benign tumor: abnormal cells remain in the tissue Malignant tumor: abnormal cells; become invasive; impair the function of one or more organs → Cancer Metastasis: spreading of cancer cells to location(s) distant from original site where they may proliferate and create secondary tumors. Through fluids such as blood and lymph STUDENTS WERE TOLD THAT THEY ARE NOT RESPONSIBLE FOR THIS SLIDE