Download Mitosis - Walton High

Cells come from pre-existing cells….
Who said this and what is it a part of now?
Cell Division in Eukaryotes

As you are sitting in class now, your
cells are growing, dividing and dying.
Cuts and bruises are healing
Muscle cells are
get larger when
you exercise.
RBC’s are being
produced in your
bones at a rate of
10-15 million per sec.
Worn out cells in the palm of your hand are being replaced.
How Do Cells Increase In
Number?

All of your cells, except your sex cells, divide
by a process called mitosis.


In mitosis, a parent cell divides to form two
identical daughter cells.
The daughter cells have the same contents of the
parent’s nucleus.
Cell Cycle
Just like a living organism, a cell has a life
cycle.
 Most of the eukaryotic cell’s life is spent
in a period called interphase.
 Interphase can be divided into three
stages: G1, S, and G2

G1: a time of growth and maintenance
 S: DNA is replicated: this commits the cell
to divide
 G2: more growth as the cell prepares for
division

Cell Cycle Con’t.

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
Mitosis: division of the
nucleus: divided into
prophase, metaphase,
anaphase and telephase.
Cytokinesis: division of
the cytoplasm
After the cell divides
into 2 identical daughter
cells, the cycle starts
over again.
The cell cycle is a series
of events that takes
place in a cell from one
division to the next.
From Birth to
Rebirth, a
Cell
Progresses
Through
Characteristic
Stages That
Constitute
the Cell Cycle
In multicellular
organisms like
us, progress
through the
cell cycle is
carefully
regulated.
Cell grows,
Prepares for
mitosis
DNA replicates,
chromosomes
duplicate
Cell grows,
functions,
Organelles
duplicate
Cell Cycle Control

Cell cycle controlled by internal and
external signals

External signals
 Growth
factors
 Received at the plasma membrane
 Cause completion of cell cycle

Internal signals
 Family
of proteins called cyclins
 Increase and decrease as cell cycle continues
 Without them cycle stops at G1, M or G2
 Allows time for any damage to be repaired
Apoptosis
Programmed cell death
 Mitosis and apoptosis are opposing forces

Mitosis increases cell number
 Apoptosis decreases cell number


Cells harbor apoptosis enzymes (caspases)
Held in check by inhibitors
 Can be unleashed by internal or external
signals


Signal protein P53
Stops cycle at G1 when DNA damaged
 Initiates DNA attempt at repair

 If
successful, cycle continues to mitosis
 If not, apoptosis is initiated
Apoptosis
Which organelle is involved?
Mitosis: Preparation

DNA is in very long threads





Chromosomes
Stretched out and intertangled between divisions
DNA is associated with histone proteins
Collectively called chromatin
Before mitosis begins:


Chromatin condenses (coils) into distinctly visible
chromosomes
Each species has a characteristic chromosome number



Humans 46
Corn 20
Goldfish 94
Chromosome Number
Most familiar organisms diploid
 Have two chromosomes of each type


Humans have 23 different types of
chromosomes
 Each
type is represented twice in each body cell
(Diploid)
 Only sperm and eggs have one of each type
(haploid)

The n number for humans is n=23
 Two
representatives of each type
 Makes a total of 2n=46 in each nucleus


One set of 23 from individual’s father (paternal)
Other set of 23 from individual’s mother (maternal)
Chromosome
Numbers
of Some
Eukaryotes
Chromosome Structure


At end of S phase:
 Each chromosome duplicated
 Consists of two identical DNA chains
 Sister chromatids
 Attached together at a single point (centromere)
 Attached to each other at
During mitosis:
 Centromeres holding sister chromatids together
simultaneously break
 Sister chromatids separate
 Each becomes a daughter chromosome
 Sisters of each type distributed to opposite
daughter nuclei
Duplicated Chromosome
Interphase




Cells that no longer divide are always in G1 of
interphase. Ex: muscle and nerve cells
A copy of the cell’s DNA is made during the S
phase.
Chromatin begins to coil up into chromosomes
at the end of G2.
These duplicated chromosomes are held
together by a centromere.
In a non-dividing cell, chromosomes are
not visible by light microscopy, because
chromatin spreads throughout the
nucleus.
 During the metaphase of cell division,
the chromatin condenses and becomes
visible as chromosomes.
 At this time, each chromosome has been
duplicated.
 A chromosome becomes
two sister chromatids
attached at the
centromere.

This is one chromosome.
Sister Chromatids
DNA and Its Faithful Replication – The
Knit of Identity
Label1
Because DNA stores genetic information and is
faithfully replicated, information is passed largely
unaltered from cell-to-cell, generation-togeneration.
DNA Replication –
Simple in Principle,
Complicated in
Practice
DNA is Packaged into Chromosomes
duplicated
chromosome
chromatin
DNA in the cell is virtually always associated with proteins.
The packaging is impressive – 2 meters of human DNA fit into a sphere about
0.000005 meters in diameter.
The Link Between DNA Replication and Chromosome Duplication
DNA is Condensed into Visible Chromosomes Only For Brief
Periods in the Life of a Cell
95% of the time, chromosomes are
like this.
Easily visible chromosomes are
apparent perhaps 5% of the time in
an actively growing cell and less in a
non-growing cell.
A Karyotype is an Arranged Picture of Chromosomes At
Their Most Condensed State
Note that almost all chromosomes come in homologous pairs.
A normal
human
karyotype
Interphase
Mitosis in Animal Cells



A form of asexual
reproduction
The nucleus of a cell
divides, producing 2
nuclei that are
identical to each
other
Has 4 phases:
prophase, metaphase,
anaphase, telophase
Cell Division Demands
Coordination of DNA
Replication, Mitosis
and Cytokinesis
What’s so important
about cell division?
Cell division requires coordinated division of
chromosomes (mitosis) …..
…… and division of the
cytoplasm
(cytokinesis).
Prophase



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Duplicated chromosomes become fully visible.
Organelles called centrioles move to opposite
ends of the cell.
The nucleolus and nuclear membrane
disintegrate.
Threadlike spindles stretch across the cell
between the centrioles.
Prophase
Metaphase
Duplicated chromosomes line up
across the center, or equator, of the
cell.
 Each centromere attaches to 2
spindle fibers.

Metaphase
Anaphase

Each centromere splits and the
identical chromosomes (sister
chromatids) separate and move
towards opposite ends of the cell.
Anaphase
Telophase

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Spindle fibers disappear
Chromosomes uncoil and are harder to see
A nuclear membrane forms around each
mass of chromosomes
A new nucleolus forms in each new nucleus
Telophase
Cytokinesis
 The
cytoplasm and its contents
divide into 2 individual daughter
cells.
 Each daughter contains a nucleus
and identical chromosomes.
Cancer Is One Outcome of A Runaway Cell Cycle
Licentious division - prostate cancer cells during division.
In Animal Cells, a
Cleavage Furrow Forms
and Separates Daughter
Cells
Cleave furrow in a dividing frog cell.
Cytokinesis: Animal Cells


Division of cytoplasm
Allocates mother cell’s cytoplasm equally to
daughter nucleus

Encloses each in it’s own plasma membrane

Often begins in anaphase

Animal cytokinesis:


A cleavage furrow appears between daughter
nuclei
Formed by a contractile ring of actin
filaments

Like pulling on a draw string

Eventually pinches mother cell in two
Cytokinesis in Animal Cells
Mitosis in Action
Blue
shows
DNA,
green
shows
spindle
fibers.
The Plant Cell Wall Forces Cytokinesis to Play by
Different Rules
Cytokinesis: Plant Cells
Rigid cell walls outside plasma membrane do
not permit furrowing
 Begins with formation of a cell plate

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Many small membrane-bounded vesicles
Eventually fuse into one thin vesicle extending
across the mother cell
The membranes of the cell plate become the
plasma membrane between the daughter cells
 Contents
of vesicles become the middle lamella
between the two daughter cells
 Daughter cells later secrete primary cell walls on
opposite sides of middle lamella
Cytokinesis in Plant Cells
Mitosis in Plant Cells
 What’s
Different?
Plant Cells do not have centrioles.
 A cell plate forms between 2 new nuclei
 New cell walls form along the cell plate,
and new cell membranes form inside the
cell walls.

Plant Mitosis
The Knit of Identity - Mitosis Precisely and Evenly
Divides Duplicated Chromosomes
interphase
prophase
metaphase
Precisely dividing the duplicated chromosomes has the consequence of
providing each new cell with an identical and complete set of genetic
instructions.
Mitosis Precisely and Evenly Divides Duplicated Chromosomes
Cytokinesis is the process of dividing the cytoplasm and it is
distinct and separable from mitosis.
Cancer – uncontrolled cell growth

Illustration of a
malignant tumor
and its recruitment
of new blood
vessels. These
invasive new
blood vessels will
nourish the tumor,
allowing it to
mutate, grow in
size, invade
surrounding
tissues, and
eventually spread
via the body’s
circulatory sytem.
Asexual Reproduction

produces genetically identical offspring from
a single parent cell

mitosis is associated with asexual
reproduction and the growth and repair of
cells in sexually reproducing organisms

Asexual reproduction is sometimes called
cloning.

Cloning is the production of identical genetic
copies.
Binary Fission

Binary fission -equal division of both
the organism
cytoplasm and
nucleus to form two
identical organisms

Simplest form of
asexual reproduction.

ex: Eukaryotes amoeba, Paramecium;
Prokaryotes - bacteria
Asexual Reproduction in
Paramecium

Binary Fission in a
unicellular eukaryote
Binary Fission in Prokaryotes


DNA replicates and
attaches to the cell
membrane
The cell divides to
form two identical
organisms.
Budding


Budding -- one
parent dividing its
nucleus (genetic
material) equally,
but cytoplasm
unequally
-- ex: Fungi- yeast
Examples of Asexual
Reproduction

Budding: A new organism buds off the
side of the parent
Regeneration
(animals)

Regeneration: a whole organism grows
from a piece of an organism
A salamander can regenerate a cut off tail
Seastars can reproduce themselves from
parts that break off
Spores

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Sporulation (spore
formation) -- is
reproduction involving
specialized single cells
coming from one parent
Spores have a hard outer
coating to protect them
from the elements and
lack of water
Can exist for hundreds of
years “waiting” for right
conditions!
-- ex. Fungi -mold spores
Vegetative Propagation
(Plants)
Strawberry
plants can
reproduce by
sending out
runners. Onion
plants form
bulbs and
potato plants
form tubers.