Cell Division IB Topic 2.5 Chapter 12 Campbell New cells arise by division of existing cells .

Overview Multicellular organisms begin life as a single cell which grows and divides It is important that division of a nucleus produces two new cells (daughter cells) that contain identical sets of chromosomes Achieved by mitosis

The cell division cycle The cycle of growth and division of a cell See Fig. 12.4 in text Three main stages 1. interphase 2. division of the nucleus by mitosis 3. cytokinesis (cell division)

Interphase Longest part of the cell cycle Not resting!! Consists of three phases: 1. G1: new organelles formed, growth 2. S: cell copies all genetic material (duplicate) 3. G2: more growth

Mitosis Division of a eukaryotic nucleus into two genetically identical nuclei. 4 phases (PMAT) 2 copies of each chromosome are needed Each chromosome initially consists of a single DNA molecule; this needs to be replicated Two identical DNA molecules called sister chromatids (see next slide)

Prophase Chromosomes become visible as long threads They shorten and thicken by supercoiling Figure 12.3 in textbook Only at the end of prophase is it possible to see the two chromatids held together at the centromere Nuclear membrane breaks down Spindle microtubles are growing

Metaphase Centrioles move to opposite ends of the cell (poles) Spindle microtubules form and attach to the centromeres of each pair of chromatids Chromosomes are moved to the equator of the cell

Anaphase Centromeres divide Spindle fibers shorten Chromatids are pulled by their centromeres to opposite poles Once separated, the chromatids are referred to as chromosomes

Telophase Nuclear envelope (or membrane) reforms around both groups of chromosomes Chromosomes decondense by uncoiling, becoming chromatin again

Cytokinesis Division of the cytoplasm Cell organelles are evenly distributed between cells Pinching of cytoplasm (cleavage) in half Two daughter cells enter interphase again

Images and animations links http://www.uoguelph.ca/zoology/devobio/210labs/mitosis1.html http://www.biology.arizona.edu/cell_bio/tutorials/cell_cycle/main.html

The significance of mitosis Daughter cells produced by mitosis have a set of chromosomes identical to each other and to the parent from which they are from This occurs because: Exact copy of each chromosome is made Chromatids remain attached during metaphase Centromeres divide New nuclei Division of cytoplasm

The significance of mitosis (cont.) Growth and development of an embryo Repair of damage or worn out cells Growth, development, and repair Asexual reproduction Why is mitosis critical for items stated above?

Cancer … Cells divide by mitosis repeatedly without control or regulation, forming a tumor Benign vs. malignant Metastasize Caused by damage to DNA of chromosomes  different types of mistakes, which explain why cancer is not one single disease Environmental, genetic, and viral influence

Cancer

Test yourself In each of the following combinations of words or phrases, one word or phrase does not belong. Identify which one it is and explain your answer. 1. spindle microtubule formation, replication, growth 2. spindle microtubule formation, supercoiling of chromosomes, attachment of microtubules to centromeres 3. maintenance, repair, formation of gametes