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

©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 multicelled 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 Lifecycle 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 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 A T T A G C C G G C TATA T A G C C G G C T A A T Packaging DNA Histone proteins Histone octomer B DNA Helix 2 nm

©1999 Timothy G. Standish A T T A G C C G G C TATA T A G C C G G C T A A T Packaging DNA Histone proteins B DNA Helix Histone octomer 2 nm

©1999 Timothy G. Standish A T T A G C C G G C TATA T A G C C G G C T A A T Packaging DNA Histone proteins Histone octomer Nucleosome 11 nm B DNA Helix 2 nm

©1999 Timothy G. Standish Packaging DNA A T T A G C C G G C T A A T

©1999 Timothy G. Standish Packaging DNA A T T A G C C G G C T A A T

©1999 Timothy G. Standish Packaging DNA A T T A G C C G G C T A A T Protein scaffold 11 nm “Beads on a string” 30 nm Tight helical fiber Looped Domains 200 nm

©1999 Timothy G. Standish Packaging DNA G C A T Protein scaffold Metaphase Chromosome 700 nm 11 nm 30 nm 200 nm 2 nm Looped Domains Nucleosomes B DNA Helix Tight helical fiber

©1999 Timothy G. Standish Replication Chromosomes, Chromatids and Centromeres Centromere Chromosome arm Identical chromatid Chromatid Anaphase A packaged chromosome Two identical chromosomes

©1999 Timothy G. Standish