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

©1999 Timothy G. Standish

MitosisMitosisCell DivisionCell Division

Timothy G. Standish, Ph. D.


In The Beginning OneIn 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 DivideWhy 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 cmSurface 24 cm2/volume 8 cm3 = 3

1 cm

Surface 6 cm2/volume 1cm3= 6


The Cell LifecycleThe 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


G1G1

MM

G2G2

SS

The Cell LifecycleThe Cell LifecycleGap 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


Stages Of MitosisStages Of MitosisDuring 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 MitosisStages 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

G1G1

MM

G2G2

SS

Interphase


Stages Of MitosisStages 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 MitosisStages 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


A TT AG CC

G

G C

TA

T

AG

C

C G

G C

T A

A T

Packaging DNAPackaging DNA

Histone proteins

Histoneoctomer

B DNA Helix 2 nm


A TT AG CC

G

G C

TA

T

AG

C

C G

G C

T A

A T


Histone proteins

B DNA Helix

Histoneoctomer

2 nm


A TT AG CC

G

G C

TA

T

AG

C

C G

G C

T A

A T


Histone proteins

Histoneoctomer

Nucleosome

11 nm

B DNA Helix 2 nm



A TT AC G

C G

G C

T A

A T



A TT AC G

C G

G C

T A

A T



A TT AC G

C G

G C

T A

A T

Protein scaffold

11 nm“Beads on a string”

30 nm

Tight helical fiber

Looped Domains200 nm



G

C

A

T

Protein scaffold

Metaphase Chromosome

700 nm

11 nm

30 nm200 nm

2 nm

Looped Domains

Nucleosomes

B DNA Helix

Tight helical fiber


Replication

Chromosomes, Chromatids Chromosomes, Chromatids and Centromeresand Centromeres

Centromere

Chromosome arm

Chromosome arm

Identicalchromatid

Chromatid

Anaphase

A packaged chromosome

Two identical chromosomes


Documents

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