Transcript Mitosis Notes

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
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©1999 Timothy G. Standish
Centrioles
Nucleolus
G2, Late
interphase
Cell checks for
complete DNA
replication.
Nuclear
envelope
Chromatid
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©1999 Timothy G. Standish
Condensed
replicated
chromosome
Prophase
Chromosomes
condense, become
visible. Spindle
apparatus forms.
Spindle
fibers
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©1999 Timothy G. Standish
Metaphase
Chromosomes
align along
equator of cell.
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©1999 Timothy G. Standish
Anaphase
Sister chromatids
separate to
opposite poles of
cell.
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©1999 Timothy G. Standish
Telophase
Nuclear membranes
assemble around
two daughter nuclei.
Chromosomes
decondense.
Spindle disappears.
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©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
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©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