©1999 Timothy G. Standish Mitosis Cell Division Timothy G. Standish, Ph. D. Arr. Mr. Markley

©1999 Timothy G. Standish In The Beginning One Most of the organisms we see started out as one cell Humans start out as a single cell, the zygote, formed by uniting a sperm and egg The zygote divides to make approximately one trillion cells During the process of dividing, cells become specialized to function in the various tissues and organs of the body Mitosis is the process of cell division in eukaryotic cells

©1999 Timothy G. Standish Why Cells Must Divide In multi-celled organisms (like humans) cells specialize for specific functions thus the original cells must divide to produce different kinds of cells Cells can only take in nutrients and excrete waste products over the surface of the membrane that surrounds them. The surface to volume ratio decreases with the square of the volume (unless special accommodations are made) 2 cm Surface 24 cm 2 / volume 8 cm 3 = 3 1 cm Surface 6 cm 2 / volume 1cm 3 = 6

©1999 Timothy G. Standish The Cell Lifecycle The cell lifecycle is well defined and can be divided into four stages: –Gap 1 (G1) - The growth phase in which most cells are found most of the time –Synthesis (S) - During which new DNA is synthesized –Gap 2 (G2) - The period during which no transcription or translation occurs and final preparations for division are made –Mitosis - Cell division

©1999 Timothy G. Standish G1 M M G2 S S The Cell Life Cycle Gap 1 - Doubling of cell size. Regular cellular activities. transcription and translation etc. Synthesis of DNA - Regular cell activities cease and a copy of all nuclear DNA is made Gap 2 - Final preparation for division Mitosis - Cell division

©1999 Timothy G. Standish Stages Of Mitosis During mitosis an exact copy of the genetic material in the “mother” cell must be distributed to each “daughter” cell Each stage of mitosis is designed to achieve equal and exact distribution of the genetic material which has been copied during the S phase of the cell cycle

©1999 Timothy G. Standish Stages Of Mitosis Interphase - The in between stage - Originally metaphase was thought to be a resting stage now we know that this is the stage most cells spend their time in doing the things that cells do and, if they are preparing to divide, growing and replicating their DNA G1 M M G2 S S Interphase

©1999 Timothy G. Standish Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. INTERPHASE: Cells have two copies of each chromosome (here, one pair of little chromosomes, and one pair of big chromosomes). WATCH CAREFULLY! Nuclear envelope Chromosome

©1999 Timothy G. Standish Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. G 2, Late interphase Cell checks for complete DNA replication. Nucleolus Nuclear envelope Chromatid Centrioles

©1999 Timothy G. Standish Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Prophase Chromosomes condense, become visible. Spindle apparatus forms. Condensed replicated chromosome Spindle fibers

©1999 Timothy G. Standish Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Metaphase Chromosomes align along equator of cell.

©1999 Timothy G. Standish Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Anaphase Sister chromatids separate to opposite poles of cell.

©1999 Timothy G. Standish Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Telophase Nuclear membranes assemble around two daughter nuclei. Chromosomes decondense. Spindle disappears.

©1999 Timothy G. Standish Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cytokinesis Division of the cytoplasm into two cells. THIS IS NOT A STAGE, BUT AN EVENT THAT MARKS THE END OF TELOPHASE

©1999 Timothy G. Standish Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Interphase of daughter cells. Cells resume normal functions or enter another division cycle.

©1999 Timothy G. Standish Steps of Mitosis I P M A T Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Late interphase Cell checks for complete DNA replication. Prophase Chromosomes condense, become visible. Spindle apparatus forms. Metaphase Chromosomes align along equator of cell. Anaphase Sister chromatids separate to opposite poles of cell. Telophase Nuclear membranes assemble around two daughter nuclei. Chromosomes decondense. Spindle disappears. Early interphase of daughter cells. Cells resume normal functions or enter another division cycle.

©1999 Timothy G. Standish Stages Of Mitosis Prophase - The beginning phase - DNA which was unraveled and spread all over the nucleus is condensed and packaged Metaphase - Middle stage - Condensed chromosomes line up along the equator of the cell Anaphase - One copy of each chromosome moves to each pole of the cell Telophase - End stage - New nuclear membranes are formed around the chromosomes and cytokinesis (cytoplasm division) occurs resulting in two daughter cells

©1999 Timothy G. Standish Stages Of Mitosis Interphase Anaphase Telophase Metaphase Mitotic spindle Prophase Nucleus with un- condensed chromosomes Equator of the cell Condensed chromosomes Disappearing nuclear membrane Poles of the cell Mother cell Two daughter cells

©1999 Timothy G. Standish

Controller of mitosis: CYCLIN

©1999 Timothy G. Standish Immortality through MITOSIS? Mitosis allows cells to make duplicates of themselves essentially allowing them to cheat death. But live forever? Nope: Telomeres – Protective ends of the chromosomes shorten with each division. Shrinking of telomeres is thought to be linked to aging. Hayflick limit – Leonard Hayflick determined that each cell can divide between times before the telomeres have shrunk too small to allow the cell to live. Telomeres are elongated/maintained by an enzyme known as Telomerase.