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©1999 Timothy G. Standish
MitosisMitosisCell DivisionCell Division
Timothy G. Standish, Ph. D.
©1999 Timothy G. Standish
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
©1999 Timothy G. Standish
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
©1999 Timothy G. Standish
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
©1999 Timothy G. Standish
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
©1999 Timothy G. Standish
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
©1999 Timothy G. Standish
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
©1999 Timothy G. Standish
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
©1999 Timothy G. Standish
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
©1999 Timothy G. Standish
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
©1999 Timothy G. Standish
A TT AG CC
G
G C
TA
T
AG
C
C G
G C
T A
A T
Packaging DNAPackaging DNA
Histone proteins
B DNA Helix
Histoneoctomer
2 nm
©1999 Timothy G. Standish
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
Nucleosome
11 nm
B DNA Helix 2 nm
©1999 Timothy G. Standish
Packaging DNAPackaging DNA
A TT AC G
C G
G C
T A
A T
©1999 Timothy G. Standish
Packaging DNAPackaging DNA
A TT AC G
C G
G C
T A
A T
©1999 Timothy G. Standish
Packaging DNAPackaging DNA
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
©1999 Timothy G. Standish
Packaging DNAPackaging DNA
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
©1999 Timothy G. Standish
Replication
Chromosomes, Chromatids Chromosomes, Chromatids and Centromeresand Centromeres
Centromere
Chromosome arm
Chromosome arm
Identicalchromatid
Chromatid
Anaphase
A packaged chromosome
Two identical chromosomes
©1999 Timothy G. Standish