Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
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) 1 cm Surface 6 cm2/ volume 1cm3= 6 2 cm Surface 24 cm2/ volume 8 cm3 = 3 ©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 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 S G1 G2 M 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 S Interphase G2 G1 M ©1999 Timothy G. Standish INTERPHASE: Cells have two copies of each chromosome (here, one pair of little chromosomes, and one pair of big chromosomes). WATCH CAREFULLY! Nuclear Chromosome envelope Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ©1999 Timothy G. Standish Centrioles Nucleolus G2, Late interphase Cell checks for complete DNA replication. Nuclear envelope Chromatid Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ©1999 Timothy G. Standish Condensed replicated chromosome Prophase Chromosomes condense, become visible. Spindle apparatus forms. Spindle fibers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ©1999 Timothy G. Standish Metaphase Chromosomes align along equator of cell. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ©1999 Timothy G. Standish Anaphase Sister chromatids separate to opposite poles of cell. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ©1999 Timothy G. Standish Telophase Nuclear membranes assemble around two daughter nuclei. Chromosomes decondense. Spindle disappears. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ©1999 Timothy G. Standish Cytokinesis Division of the cytoplasm into two cells. THIS IS NOT A STAGE, BUT AN EVENT THAT MARKS THE END OF TELOPHASE Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ©1999 Timothy G. Standish Interphase of daughter cells. Cells resume normal functions or enter another division cycle. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ©1999 Timothy G. Standish Steps of Mitosis Telophase I P M A T Nuclear membranes Late interphase Cell checks for complete DNA replication. Metaphase Chromosomes align along equator of cell. Prophase Chromosomes condense, become visible. Spindle apparatus forms. Anaphase Sister chromatids separate to opposite poles of cell. assemble around two daughter nuclei. Chromosomes decondense. Spindle disappears. Early interphase of daughter cells. Cells resume normal functions or enter another division cycle. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ©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 Mother cell Stages Of Mitosis Nucleus with uncondensed Condensed chromosomes Equator chromosomes of the cell Interphase Disappearing Prophase nuclear membrane Mitotic spindle Poles of the cellTwo daughter cells Metaphase Anaphase Telophase ©1999 Timothy G. Standish ©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 40-60 times before the telomeres have shrunk too small to allow the cell to live. Telomeres are elongated/maintained by an enzyme known as Telomerase. ©1999 Timothy G. Standish