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

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

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

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

The Cell Life Cycle G1 S G2 M 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 G1 S G2 Gap 2 - Final preparation for division M Mitosis - Cell division

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

Stages Of Mitosis Interphase 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 Interphase G1 M G2 S

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

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

Packaging DNA T G A Histone C octomer Histone proteins 2 nm T A G C C G TA T A G C C G G C T A A T Packaging DNA Histone proteins Histone octomer 2 nm B DNA Helix

Packaging DNA T G A Histone C G C octomer Histone proteins 2 nm TA G C T A C G A T A T 2 nm C G B DNA Helix G C T A

Packaging DNA T G A 11 nm Histone C G C octomer Histone proteins TA G C T A C G Nucleosome A T A T 2 nm C G B DNA Helix G C T A

Packaging DNA A T T A G C C G C G G C T A A T

Packaging DNA A T T A G C C G C G G C T A A T

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

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

Chromosomes, Chromatids and Centromeres A packaged chromosome Chromatid Identical chromatid Replication Anaphase Chromosome arm Two identical chromosomes Centromere Chromosome arm

The End