SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
Intro Molecular Cell Biology (BIO 210); Instructor: Elmar Schmid, Ph.D.
Chapter 8: Growth, cell division, chromosomes,
cell cycle & Mitosis
- Part I INTRODUCTION

Growth and development is one of the hall mark features and characteristics of all
forms of life on planet Earth

In order for single-celled organisms, such as bacteria or protists to increase in
numbers or for multi-cellular biological organism to grow in size, its cells have to
multiply to increase the amount of cells, roots, stem, tissue, organ and other body
materials

Multi-cellular organisms, such as fungi, plants or animals grow, because their cells
exponentially increase in numbers by repeating a very coordinated and synchronized
cellular program, called the cell cycle, in a process called cell division

In this (first) part of chapter 8 we will therefore have to have a closer look at the
cellular basis of growth to fully appreciate this amazing biological process
HISTORY

Long before man knew about the existence of genes, chromosomes and the
underlying principles of cell growth and inheritance, so-called breeding was done with
life stock or certain plants to bring out a wished phenotype
 e.g. pigs with more meat mass or corn with larger cobs

1865: the Augustinian monk Gregor Mendel publishes his ‘Principles of Heredity’,
which he discovered while conducting his famous breeding experiments with the
common garden pea
 without knowing about the substance of genes he speaks already of
‘heritable factors’

at the end of the 19th century, the German physician Rudolf Virchov states that ‘all
cells come from cells’ and that cellular growth and reproduction requires a process
called cell division

Mendel’s principles were not anticipated and understood before cell biologists worked
out and discovered the cellular process of mitosis and meiosis in more detail

Around 1910, scientists developed the chromosome theory of inheritance which
states that the heritable factors (= genes in modern terminology) which are
responsible for the expression of heritable traits are located on chromosomes
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
Intro Molecular Cell Biology (BIO 210); Instructor: Elmar Schmid, Ph.D.
The Cell division & The cell cycle

In order to for a multicellular organism to grow in size or to increase its mass, cell
division is necessary

Cell division is the biological process of splitting of a unique form of cell, a so-called
stem cell, into two identical daughter cells usually after stimulation of the parent cell
with so-called growth factors
- important growth factors are peptide molecules such as insulin-like growth factor
(IGF), EGF, PDGF or NGF
- they bind to antennae-like, so-called receptor proteins located on the surface of
the cell
- some cells in our human body divide continuously to form new cells, such as:
1. Myelogenic stem cells in our red bone marrow
 to form new red blood cells, platelets and immune cells
2. Keratinocytes and fibroblasts in our skin
 to form new protective skin layers
3. Osteoblasts in our bone marrow
 to build up new calcified bone material
 some cells start to divide only after loosing their cell-to-cell contact with
neighboring cells, e.g. after tissue or cell damage
e.g. liver cells after intoxication, injury or after surgical removal
e.g. skin fibroblasts after injury

Most cells of biological organisms, after reaching the adult stage, do not divide at all
e.g. nerve cells in our brain

Before a cell divides it duplicates (= copies) or enlarges all of its cellular constituents,
including organelles, membranes, cytoskeleton

Especially the exact copying (= replication) and separation of the cell’s chromosomal
genetic material in identical portions into each of the two new cells is of prime
importance to assure a loss or disproportion of genetic material during growth

The highly coordinated and orchestrated process of cell division assures that cells,
especially cells with large amount of genetic material, such as eukaryotic cells, will
produce new daughter cells with equal amounts of chromosomal material as the
parental cell
 e.g. a dividing human cell has to copy and handle chromosomal DNA comprised of
6 billion base pairs!! Certainly not an easy task!

Biological organisms with less genetic material, such as bacteria, perform a less
complex cell division process called binary fission which we will look up in the last
section of this chapter
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
Intro Molecular Cell Biology (BIO 210); Instructor: Elmar Schmid, Ph.D.

In non-dividing eukaryotic cells, e.g. a human cell, genetic material appears as socalled chromatin fibers located within the nucleus of the cells
Electron microscopic picture of chromatin

Chromatin fibers consist of DNA and certain, tightly associated proteins, called
histones
 DNA strands are wrapped around the histone proteins
 these DNA/protein structures, or also called nucleosomes, can
be seen through an electron microscope
 when a cell prepares to divide, the cellular chromatin fibers start to fold and coil
to end up forming a X-shaped chromosome structure

A chromosome is a cellular structure which contains the genes for genetic traits of
biological organisms; it is made up of very long strands of chromatin, which (after
heavy folding and coiling) during certain stages of the cell cycle (see sections ‘mitosis’
and meiosis’ below) can be seen with the help of a light microscope
 the total cellular DNA of a species is called the genome, which is split into a
certain number of chromosomes
 the DNA of the complete genome is visible as a distinct number of chromosomes
which is unique for each species
e.g. human cells have 46 chromosomes which are visible during certain
stages before and along cell division (see Graphic below)

Chromosomes are visible only during certain stages of the cell cycle (see sections
‘mitosis’ and meiosis’ below)

The total cellular DNA of a species (= genome) is split into a certain number of
chromosomes
- the DNA of the complete genome is visible as a distinct number of chromosomes
which is unique for each species
- e.g. human cells have 46 chromosomes which are visible during certain stages
before and along cell division
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
Intro Molecular Cell Biology (BIO 210); Instructor: Elmar Schmid, Ph.D.
TThhee 4466 hhuum
maann cchhrroom
moossoom
meess

The process which is responsible for the complete duplication of the whole cellular
genetic material is called DNA replication ( see Chapter 10 for more details);
during DNA replication, a cell duplicates its whole set of DNA or genome

Shortly after completion of DNA replication sets of X-shaped chromosomes become
visible within eukaryotic cells
 chromosomes become only visible to us shortly before a cell enters mitosis
 the chromosomes can be seen through a light microscope as X-shaped bodies
which show a typical banding pattern (so-called G-bands) after staining with
certain dyes (see Graphic below)

Each chromosome is formed by two so-called sister chromatids,
 each sister chromatid contains genes coding for identical traits at the same spot
(= locus)
 two sister chromatids are joined together in the so-called centromere region, which
is the most narrow part of the of a metaphase chromosome
 the centromere plays an important role in the separation of the two sister
chromatids during anaphase of mitosis and meiosis;
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
Intro Molecular Cell Biology (BIO 210); Instructor: Elmar Schmid, Ph.D.
Structure and important regions of a typical metaphase chromosome
Coiled euchromatin
(DNA + histones)
Centromere
Sister
Metaphasechromosome
Sister
Centromeric
heterochromatin
Telomeric end

Eukaryotic chromosomes are much more complex than prokaryotic chromosomes
 eukaryotic chromosomes contain so-called scaffolding proteins which help to
organize and maintain the complex 3-dimensional chromosome structure
 some of these proteins are play a role in the control of gene activity (see Chapter
10)

The degree of folding of coiling of chromatin fibers is not equal within a chromosome;
scientists discovered areas which are made up form highly compacted chromatin, the
so-called heterochromatin regions, and more loosely arranged, open chromatin
regions, the so-called euchromatin regions (see Graphic above)

Each eukaryotic chromosome consists of one long DNA double helix which codes for
thousands of genes

A gene is a segment on the DNA strand of the genome which codes for a distinct
protein or enzyme

The long DNA double helix of each eukaryotic chromosome codes for thousands of
genes; an average gene is about 1000 nucleotide base pairs (bp) long

Almost all genes which make up an eukaryotic organism are found in the cell nucleus
 some genes are located on the so-called extra-chromosomal DNA which is located
in mitochondria

eukaryotic chromosomes contain much more genes and are much more complex
than prokaryotic chromosomes
 e.g. a human cell has about 35,000 – 40, 000 genes, while the
genome of a bacterium harbors about 3000 genes
 eukaryotic chromosomes contain proteins which help to
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
Intro Molecular Cell Biology (BIO 210); Instructor: Elmar Schmid, Ph.D.
organize the complex 3-dimensional (X-shaped) structure
 some of these proteins are play a role in the control of gene
activity

The sequence of nucleotides (see Graphic below) or the so-called letter code which
makes up a gene, determines the later shape and function of the gene product; the
gene product can either be a protein, which helps to build up the cell structure or an
enzyme, which regulates essential part of the cell’s biochemical pathways
The DNA sequence of a typical gene
(= gene of the human enzyme superoxide dismutase)
SOURCE
human.
ORGANISM Homo sapiens
Eukaryota; Metazoa; Chordata; Craniata; Vertebrata; Euteleostomi;
Mammalia; Eutheria; Primates; Catarrhini; Hominidae; Homo.
REFERENCE 1 (bases 1 to 560)
AUTHORS Sherman,L., Dafni,N., Lieman-Hurwitz,J. and Groner,Y.
TITLE Nucleotide sequence and expression of human chromosome 21-encoded
superoxide dismutase mRNA
JOURNAL Proc. Natl. Acad. Sci. U.S.A. 80 (18), 5465-5469 (1983)
BASE COUNT
158 a
ORIGIN (human)
108 c
160 g
134 t
bp1 A
ATTG
GGCGACGA AGGCCGTGTG CGTGCTGAAG GGCGACGGCC
CAGTGCAGGCATCATCAATTTCGAGCAGA AGGAAAGTAA TGGACCAGTG
AAGGTGTGGGAAGCATTAA AGGACTGACTGAAGGCCTGC ATGGATTCCTGTTCATGAG
TTTGGAGATAATACGGCAGCTGTACCAGTGCAGGTCCTCACTTTAATCCTCTATCCAGAAAAC
ACGGTGGGCCAAAGGATGAAGAGAGGCATGTTGGAG ACTTGGGCAA TGTGACTGCT
GACAAAGATG GTGTGGCCGATGTGTCTATTGAAGATTCTG
TGATCTCACTCTCAGGAGACCATTGCATCATTGGCCGCACACTGGTGGTCCATGAAAAAGCA
GATGACTTGGGCAAAGGT GGAAATGAAGAAAGTACAAAGACAGGAAACGCTGGAAGTC
GTTTGGCTTG TGGTGTAATT GGGATCGCCCAATAAACATT CCCTTGGATGTAGTCTGAGG
CCCCTTAACT CATCTGTTAT CCTGCTAGCT GTAGAAATGT ATCCTGATAAACATTAAACA
CTGTAATCTT bp561
(from: NIH/NCBI Entrez Nucleotide data base)
Nucleotide abbreviation:
A = Adenine T = Thymine G = Guanine C = Cytosine

ATG = Start codon
Almost all genes which make up an eukaryotic organism are found in the cell nucleus
 e.g. a human cell has about 100,000 genes, while a bacterium harbors about 3000
genes
 some genes are located on the so-called extra-chromosomal DNA which is located
in mitochondria or chloroplasts
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
Intro Molecular Cell Biology (BIO 210); Instructor: Elmar Schmid, Ph.D.

Cell division of somatic cells in multi-cellular organisms not only plays a role in normal
cell growth and cell repair, but also in a unique form of reproduction, so-called
asexual reproduction, which leads to new daughter organisms (or organism body
parts) with identical DNA molecules (= genotype) and identical appearance (=
phenotype); many biological organisms, such as fungi and certain plants, rely on this
form of reproduction to quickly thrive in relatively stable environments; some animals,
such as lizards, can replace lost body parts, e.g. a tail, regrowing a new one

During each round of cell division, a cell runs through a clock work-like program, the
so-called cell cycle which steers and coordinates all important cellular events while
the parent cell prepares and executes its division into to equal daughter cells

Today, we begin to understand the enormously intricate and complex nature and
involved components of the cell cycle
- the cell cycle is dependent on many proteins, so-called cyclins, and enzymes,
such as the cyclin-dependent kinases
- the cell cycle is a strictly controlled biological clock work which involves many
proteins such as p53 and Rb
(for more details on the cell cycle see separate chapter)
C
Ceellll ccyyccllee &
&M
Miittoossiiss

Whenever eukaryotic cells decide, i.e. become the signal, to divide, they undergo an
orderly sequence of events which is also called a cell cycle; a cell cycle spans from
the time a cell divides to form a daughter cell to the time point this daughter cell itself
starts to divide again
TThhee ddiiffffeerreenntt pphhaasseess ooff tthhee cceellll ccyyccllee
G0
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
Intro Molecular Cell Biology (BIO 210); Instructor: Elmar Schmid, Ph.D.
-

performance of specific cell
functions
e.g. protein production,
metabolism
The cell cycle consists of two major stages:
11.. IInntteerrpphhaassee

A cell spends most of the time of the cell cycle (90%) in this stage the interphase is
subdivided into the three sub-phases:
G0-phase
- the resting, non-dividing cell which performs the “routine” metabolic or
cellular activities, e.g. enzyme or hormone synthesis
G
G11--pphhaassee
S
S--pphhaassee
G
G22--pphhaassee

The cell has a high metabolic activity to be able to perform the various, “normal”
functions
G
G11--pphhaassee:
(‘G’ refers to the gap between cell division and the occurrence of DNA synthesis)
 upon activation by an out-side trigger, e.g. a docking growth factor at a surface
receptor, the cell increases its supply of proteins and increases the number of its
organelles (mitochondria, golgi, peroxisomes, etc.)
 the cell grows in size
S
S--pphhaassee:
- DNA synthesis (= DNA replication) occurs and the cell duplicates its complete set of
Chromosomes (for details see separate Chapter 10-1)
“Licensing” of DNA for DNA replication & Regulation
Preparing DNA for the necessary DNA copying process (“DNA licensing”) is a very critical
regulatory step during cell division. In recent years scientists discovered that this
important step requires a functional “origin recognition complex” (ORC) which is the
fist component for the assembly of the “pre-replicative protein complex” at each origin of
replication on the DNA. Other important cell regulators involved in the control of DNA
replication are the cell cycle proteins Cyclin E, Cyclin A and cdk2.
- the cell synthesizes most of the proteins and enzymes it needs for DNA synthesis
during the S-phase, e.g. histones, DNA polymerase, Helicase, etc.
- at the end of the S-phase two sister chromatids are formed by the cell
- the transition from S to G2 marks the so-called mitotic checkpoint of the cell cycle
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
Intro Molecular Cell Biology (BIO 210); Instructor: Elmar Schmid, Ph.D.
- many regulator or check point proteins, e.g. the breast cancer susceptibility gene
product BRCA-1 control at this point of the cell cycle
G
G22--pphhaassee:
the cell starts to synthesize proteins which are essential for cell division and shows
a high metabolic activity again
especially proteins needed for the following M-phase, such as tubulin, actin, are
produced in high amounts
M
Miittoottiicc oorr M
M -- pphhaassee

at the end of the G2-phase the cell installed a second checkpoint, which regulates
the entering into the M-phase;
so-called key-proteins of this important cellular checkpoint is the CDC25B protein and
the so-called cyclins
The mitotic phase is subdivided into 2 processes
M
Miittoossiiss
 Duplicated chromosomes (= sister chromatids) divide and are
evenly distributed into two daughter cells
C
Cyyttookkiinneessiiss
 the cytosol is divided two; two identical daughter cells are formed
Let’s have a closer look at the very dynamic cellular processes which occur during mitosis
M
Miittoossiiss

Even though mitosis is continuous, dynamic cellular process, 4 phases are usually
distinguished by biologists; the 4 phases of mitosis
1. Prophase
Summary of changes which occur in the nucleus and in the cytosol:
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
Intro Molecular Cell Biology (BIO 210); Instructor: Elmar Schmid, Ph.D.
Nuclleeuuss::
- the chromatin fibers located in the nucleus of the cell now become highly coiled
and folded by a process called DNA condensation (see Graphic below); DNA
condensation leads to highly compacted chromtin fibers, which can be seen
through a high magnifying light microscope;
The way from DNA, chromatin to a metaphase chromosome
(Understanding DNA condensation during prophase of mitosis)
-
towards the end of prophase, discrete chromosomes can be seen; each duplicated
chromosome appears as two identical ssiisstteerr cchhrroom
maattiiddss; the two sister
chromatid arms are hold together by proteins called cohesins
10
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
Intro Molecular Cell Biology (BIO 210); Instructor: Elmar Schmid, Ph.D.
-
the nuclear envelope breaks apart
chromosomes are moved to the center of the cell by forces exerted by ATPconsuming protein motors
C
Cyyttoossooll::
- the cell begins to duplicate a large protein-made cell structure, called the
centrosome to form two microtubule organizing centers (MTOCs); at the center
of each MTOC one finds a pair of short microtubule triplets, called centrioles (see
Graphic below) surrounded by other proteins called pericentriolar material
(PCM);
- the centrosome is important to organize microtubules in animal and
human cells and for the correct assembly of the bipolar microtubular
spindle apparatus as you will see further below
- in order to assure that the centrosome is only copied ONCE per one round of
cell division, cells installed critical regulatory steps to assure the correct copy
number; recently, scientists showed that an “origin recognition complex” protein
called Orc1 plays an important role in the control of the correct centriole and
centrosome copy number in human cells
- Orc1 controls the (cell cycle protein) Cyclin E-dependent centrosome copy
number by directly inhibiting the (otherwise dangerous) reduplication of
of the centrioles right after they have been copied
(Hemerly A.S., et al., Science 323(5915): 789-793 (2009)
In the recent years scientists identified important protein components of the MTOC, most
importantly the MTOC proteins nucleophosphmin, pericentrin and INCENP. Pericentrin
has been shown to play an important role in cell division and is an integral component of
the centrosome. Scientists further unraveled that the protein kinase Aurora plays an
important role in the separation of the two centrosomes during prophase of mitosis.
In 2008, scientists reported that mutations in the gene PCNT which encodes the
centrosome protein pericentrin are responsible for Majewski Osteodysplastic
Primordial Dwarfism type II (MOPD II) and Seckel syndrome, two rare autosomal
recessive human genetic disorders [Delaval B. & Doxsey S., Science 319: 732-33
(2008)]. Both disorders share as clinical features a small body size and smaller brain. On
the cellular level both forms of dwarfism share cellular abnormalities including loss of
pericentrin, centrosomes and visible mitotic spindle defects (see section below).
-
next, the 2 centrosomes slowly move to the opposite cell poles
hollow protein fibers, the so-called microtubules grow out of from each of the two
MTOC to form the mitotic or microtubule spindle apparatus by polymerization
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
Intro Molecular Cell Biology (BIO 210); Instructor: Elmar Schmid, Ph.D.
Centrosome duplication and separation during early prophase of mitosis
cdk2
Pericentrin
INCENP
Nek2
ATP
Cell
Cycle
Phosphorylation
Separation
Dissociation
P
P
Nucleophosphmin
Centrosome
Duplication & Assembly
ATP
P
P
Aurora kinase
2 centrioles
Centrosomal
proteins
Centrosome
Graphic©E.Schmid/2002
Immunofluorescence image
of cancer cell with 4 instead of
2 centrosomes
2. Metaphase
-
during this important and visually most dramatic phase of mitosis, the
microtubular spindle apparatus is fully formed
 the kinetochore microtubules made contact with protein-made structures, the
so-called kinetochores, which are located in the centromere region of each
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
Intro Molecular Cell Biology (BIO 210); Instructor: Elmar Schmid, Ph.D.
-
metaphase chromosome (see Graphic below)
chromosomes convene in line on the (virtual) metaphase plate in the center of the
cell
for each chromosomes the kinetochores of the two sister chromatids face
opposite poles
 each replicated chromosome possesses two discrete "sister" kinetochores
that are positioned on the opposite sides
 during mitosis, sister kinetochores firmly attach to the centromere region of
their associated chromosome
 they are important in the formation of the spindle apparatus by capturing the
ends of the microtubules arriving from the MTOCs of the cell poles
 a kinetochore in vertebrate cells is a disk-shaped protein structure,
which is composed of three distinct plate-like domains
The role and location of kinetochores and cohesins in metaphase of mitosis
Metaphase
Chromosome
(2 sister chromatids)
Microtubule
Cohesin
Sister
kinetochores
(“opposite orientation”)
Graphic©E.Schmid/2003
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
Intro Molecular Cell Biology (BIO 210); Instructor: Elmar Schmid, Ph.D.
Mitosis & Cancer Treatment
Many plant-derived molecules, such as colchicine, taxol or vinblastine, have been
shown to block mitosis at this very critical stage; they work as so-called spindle poisons
or mitosis blockers, since they interfere with the proper formation or disassembly of the
microtubules and arrest a dividing cell in this stage (see Graphic below). Taxol stabilizes
the microtubule fibers by blocking its degradation (= depolymerization); it “freezes” a
mitotic cell in metaphase. As a consequence, the chromosomes cannot migrate toward
the cell poles. Taxol has been approved by the FDA for treatment of certain forms of
cancers in humans, such as mammary carcinomas and ovarian cancer.
2 examples of known mitosis-blocking molecules
1. Taxol
 Isolated from the Pacific Yew tree
2. Laulimalide
 Isolated from the marine sponge
Fasciospongia rimosa
- mitosis blocker which kills cancer cells by
blocking mitosis & triggering apoptosis
- binds to polymerized tubulin and prevents
the disassembly of microtubules
- also binds to bcl-2 and prevents its anti-apoptotic
- inhibits many different cancer cell types
- is even active against cancer cells that are
resistant to Taxol
 “Multi-drug-resistant cell types”
cellular function
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
Intro Molecular Cell Biology (BIO 210); Instructor: Elmar Schmid, Ph.D.
3. Anaphase
- the mitotic cell proceeds into anaphase, which is visually one of the most dynamic
phases of mitosis
- the hallmark event of anaphase is that the ssiisstteerr cchhrroom
maattiiddss become separated into
full-fledged daughter chromosomes
Anaphase & Regulation
In yeast, for the onset of anaphase to happen, a phosphatase called cdc14 is necessary
and has to be activated. Cdc14 is activated by a cellular protease called separase.
Dcd14 assures that the - until this point very dynamic and flexible - microtubules become
more stable which assures the directed movement of the separated sister chromatids
towards the cell poles.
- the microtubules forming the spindle apparatus elongate or shorten
 the microtubules which make contact with the kinetochores of the
sister chromatids (= kinetochore microtubule) start to shorten (= depolymerization)
and perform a force in the direction of the MTOCs located at the cell poles
-
however, some microtubules, the so-called spindle microtubules, (coming from the
opposite poles) which do not make contact with the chromosomal kinetochores,
continue to polymerize until they meet in the middle of the cell
-
the spindle microtubules make contact with each other in a cell central area , the socalled spindle midzone; they continue to elongate due to continuous polymerization
which leads to the gradual stretching of the cell into an oblong, “football-shaped” cell
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
Intro Molecular Cell Biology (BIO 210); Instructor: Elmar Schmid, Ph.D.
In the recent years, cell biologists identified two important centralspindulin proteins of
the mitotic spindle midzone; one is a protein of the kinesin family (ZEN-4 in the worm C.
elegans and MKLP-1 in humans) and the other is a microtubule cross-linking protein
(CYK-4 in C. elegans and MgcRac-GAP in humans). Modification of the centralspindulin
complex by phosphorylation prevents the tight assembly of the polar microtubules in the
spindle midzone ( regulation). Another important protein which plays an important role
in the regulation and orchestration of the sister chromatid separation during anaphase is
MKLP-1.
-
due to the continued polymerization of the polar microtubules, the cell poles are
moving farther apart and the cell begins to elongate
The dynamic role of microtubules during anaphase
Spindle midzone
Polar microtubule
Telophase and cytokinesis
-
-
The sister chromatids finally arrive at the MTOC (centrosomal region) positioned at the
cell poles
Cell elongation continues and daughter nuclei appear at the cell poles. The cell reestablishes a new nuclear envelope around the sister chromatid DNA material at each
cell poles
Another crucial molecular event happening during mitotic telophase and cytokinesis is
the formation of the so-called ‘cleavage furrow’;
The cleavage furrow is a circumformed indentation of the cell membrane in the
equatorial cell region which involves crucial elements of the cytoskeleton, most
importantly a contracting actin ring
16
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
Intro Molecular Cell Biology (BIO 210); Instructor: Elmar Schmid, Ph.D.
The contractile actin (or actomyosin) ring is formed under neath the cell membrane
and involves a series of proteins, such as actin, the actin-binding protein anillin, septins,
myosin II and several tyrosine phosphorylated proteins (regulation?). It has been shown
that the non-muscle myosin II motor protein provides the mechanical force for
successful furrow ingression. The septins seems to play a role in the formation of the
final phospholipid septum that closes the remaining cell hole.
-
cytokinesis, the final cellular (molecularly vastly unknown) process, finally pinches the
parental cell into two daughter cells and mitotic cell division is completed
Cleavage furrow
Actomyosin ring
“Assembly and activation of the actomyosin contractile ring underneath the cell
membrane of the cleavage furrow are key events during cytokinesis…”
-
The ring of contracting microfilaments made up of the protein monomer actin forms
a groove in the cell surface and begins to pinch the cell into two daughter cells (=
cytokinesis)
-
Cytokinesis is a critical cellular phase of mitosis which can be blocked (inhibited) by a
series of drugs of which Blebbistatin is a very prominent one (see Figure below)
 Blebbistatin is a molecule that blocks non-muscle myosin protein in its natural
function and prevents proper cleavage furrow ingression during mitosis
17
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
Intro Molecular Cell Biology (BIO 210); Instructor: Elmar Schmid, Ph.D.
Chemical structure of the cytokinesis blocker Blebbistatin
O
OH
H3C
N
N
Blebbistatin
(Inhibitor of non-muscle myosin II protein)
Drug; inhibits cytokinesis phase of mitosis
 Arrests cleavage furrow ingression
-
The very late stage of cytokinesis requires the formation of a functional elongated
microtubular structure, called the midbody, which helps to pinch the elongated cell
into two daughter cells (see Figure below)
-
At the end of cytokinesis finally two daughter cells arise
Molecular cell biology of cytokinesis, role of BRCA protein & Cancer
In recent years a series of proteins have been shown to be critical for proper execution of
cytokinesis of mitosis. Key proteins which play a role in cytokinesis include the inner
centromere protein INCENP, Aurora B kinase, survivin and surprisingly the breast
cancer gene product BRCA2 protein. Mitotic cells deficient in BRCA2 protein – besides
other chromosomal anomalies, such as translocations and large DNA deletions - have
dysfunctional cytokinesis and show alterations in chromosome numbers in the resulting
daughter cells (= aneuploidy). Studies showed that cytokinesis is delayed or even
prevented in the absence of BRCA2 protein in cells.
It’s been long known that germ-line mutations of the BRCA2 gene which lead to an
inactive BRCA2 protein predispose humans to breast, ovarian and other aggressive
epithelial cancers and aneuploidy is often observed in cancer cells from BRCA2 mutation
carriers.
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
Intro Molecular Cell Biology (BIO 210); Instructor: Elmar Schmid, Ph.D.
Role of microtubules in late cytokinesis
 Fluorescence microscopic image
Future
Daughter Cell
Future
Daughter Cell 2
Midbody
Myosin
-

DNA
Microtubules
 Tubulin
Cytokinesis in ppllaanntt cceelllss occurs by forming a so-called cell plate
 each cell plate is formed in the middle of the plant cell by fusion of
many membranous vesicles which are filled with cell wall-material
 as more vesicles fuse together the cell plate grows outwards
and its membranes finally fuse with the plasma membrane
 the cell plate content joins the parental cell wall
At the end of mitosis the chromatin fibers of each sister chromatid uncoil
 they turn into microscopic invisible chromosomal chromatin fibers
Mitosis is unique to eukaryotic organisms

Mitosis is a cellular mechanism in eukaryotes which guaranties the equal distribution
of identical copies of the large amounts of genetic (= DNA) material
 the genome is portioned into a definite number of chromosomes

Any defect during mitosis which leads to the unequal distribution of the genetic
material between the two new arising daughter cells leads to alterations in
chromosomes in the daughter cells; this condition, which is often observed in
cancerous cells, is referred to as aneuploidy

Mitosis is the evolutionary solution to the problem of allocating identical copies of
large amounts of genetic (= DNA) material into two new cells
 the genome is portioned into a definite number of chromosomes, which the
eukaryotic cell duplicate and separate in an extremely accurate mechanism called
mitosis; e.g. in yeast, errors in chromosomal distribution occur only once in
100,000 cell divisions!!
19
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
Intro Molecular Cell Biology (BIO 210); Instructor: Elmar Schmid, Ph.D.
Cell division, cell cycle & Control

Cell division and the events which enable the cell to enter mitosis are tightly
controlled

The cell division events are controlled by installation of two so-called mitotic check
points along the cell cycle

Two check points are positioned at:
 the entry into the S-phase to control DNA replication
 the entry into the M-phase to control chromosomal condensation and the mitosis
events
 a protein complex, called maturation promoting factor (MPF) is
involved in M-phase control

Several factors influence the onset and accurate timing of cell division and cell growth
1. Growth factors (GF)
 amino acid-made growth factors, such as epithelial-derived growth factor (EGF)
or fibroblast-derived growth factor (FGF) or platelet-derived growth factor
(PDGF), are the main signals for the cell to promote cell division and are
produced by specialized cells within the body
 there are so-called autocrine (= stimulate the same cell) and paracrine
(stimulate surrounding or distant cells) growth factors known
 growth factors dock on so-called receptor proteins on their target cells and
trigger a so-called signaling cascade (see also Chapter 10 for more details)
 at the end of this cascade the nucleus receives the signal of the stimulated cell
 the cell enters the cell cycle and cell division begins
 they trigger the sequential events of the cell cycle by activating a set of
proteins in the cell which are part of the so-called cell cycle control system and
which control the so-called check points of the cell cycle
 the three key check points of the cell cycle are located in the G1, G2 and Mphase; at these points the cell build in brakes that block the cell cycle from
running; therefore a cell remains non-dividing or quiescent in the so-called
Go-phase until it receives a go-ahead signal by a docking growth factor
2. Contact- or anchorage-dependent cell division
 certain cells don’t divide in suspension and need a solid surface onto which
they can adhere
e.g. fibroblasts need contact with so-called extracellular matrix
proteins, such as collagen or fibronectin, to be able to divide
 many of these extracellular matrix proteins recognize structures (= receptors)
on the surface of the so-called adherent cell
 usually cells show so-called density-dependent growth inhibition; they stop
growing after getting in close contact with other cells
 cells growing in cell culture usually stop growing after they formed a so-called
monolayer = each cell got in contact with another cell
20
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
Intro Molecular Cell Biology (BIO 210); Instructor: Elmar Schmid, Ph.D.

Under certain conditions cell growth can become dys-regulated and may lead to
development of cancer
 cancer cells usually show a loss of cell cycle control due to mutations
of components of the cell signaling cascade or of members of the
cell cycle control system
 this leads to an uncontrolled cell division and an excessive formation of cells, or
also called a tumor
 most tumors are benign (= friendly) and its cells remain at the same site where the
tumor formation started; they can be successfully removed from the body by
surgery
 in so-called malignant tumors, the cells leave the tumor formation and spread into
other tissues or parts of the body in a process called metastasis; malignant cancers
are difficult to treat and the outcome for the cancer patient is usually fatal

Different kind of cancers are classified dependent on the site of the body where they
originated
1. Carcinomas
 originate in the exterior or interior coverages of the body
e.g. skin ( melanoma) or intestine ( colon cancer)
2. Sarcomas
 originate in tissues which support the body
e.g. bone ( osteosarcoma)
muscle or connective tissue
3. Leukemias and Lymphomas
 originate in cells of the blood forming system (in the bone marrow, spleen and
lymphnodes)
e.g. B-lymphocytes

Cancer cells usually are not prone to cell-density dependent growth inhibition
(= when cells get in touch with another cell) or also called contact-inhibition
 they therefore reach high cell densities and, when grown in cell cultures, pile up in
so-called foci

Many cancer cells bear mutations within their genetic code and express mutated
proteins or enzymes
 e.g. p53 protein, BCRA-1, ras protein
Several anti-cancer therapies are able to halt the cancer cells from spreading

1. Radiation therapy
 the affected tissue is irradiated with strong electromagnetic waves which
mostly destroy the fast-growing cancer cells, while non-dividing, normal
cells are not affected
2. Chemotherapy
 uses drugs that disrupt cell division by inhibiting proteins or enzymes which
play an important role in cell activation or cell cycle regulation
 most of them are so-called antimitotic drugs isolated from plants which
interfere with the formation of the mitotic spindle apparatus
21
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
Intro Molecular Cell Biology (BIO 210); Instructor: Elmar Schmid, Ph.D.
e.g. vinblastine
taxol
colchicines
3. Antisense therapy
 short pieces of DNA or RNA (ribozymes) which base-pair with the mRNA of
the mutated gene and prevents further expression of the cancer-causing protein
 this future anti-cancer strategy is very selective and is expected to have less
side-effects for the patient
4. Gene therapy
 also there is no ‘magic bullet’ for fighting cancer in near sight, this future
cancer therapy holds great promise and has the goal to correct the mutated and
cancer-causing gene within the patients cells
 attenuated viruses e.g. adeno-associated virus are expected to shuttle the
corrected gene into the affected cancerous tissue of the patient
Binary Fission

Prokaryotic cells, i.e. bacteria and most protists divide by a cellular process called
binary fission, a process which is much faster than mitosis and less complex
 the prokaryotic chromosome is a single DNA molecule
 after replication each copy is attached to a different part of the
bacterial cell membrane
 the cell begins to pull apart, and the copied and original
chromosome is separated into opposite directions
Binary fission of the coccoid-shaped bacterium
Enterococcus faecium
Mitosis can be inhibited and destroyed by many drugs and poisons

Cell division, the cell cycle and mitosis are enormously complicated cellular processes
involving many steps, protein components and enzymes

Many synthetic and nature-derived molecules are known to interfere with distinct
steps of the cell cycle and the mitosis process
22
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
Intro Molecular Cell Biology (BIO 210); Instructor: Elmar Schmid, Ph.D.

Many of these molecules, such as the plant-derived colchicine, are known poisons
and toxins to humans and animals

Molecules which are able to interfere with the normal process of mitosis are referred
to as mitotis blockers or so-called spindle poisons,

All these mitosis blockers or inhibitors have in common that they prevent cells from
dividing and from forming new daughter cells

Due to this many of the known mitosis poisons are used today to treat forms of
human cancers, which all are characterized by continuously and fast dividing socalled cancerous cells

cancerous cells have literally lost their molecular cell cycle brakes and
therefore continue to remain in the cell cycle and undergo mitosis

Important mitosis blocking molecules (= mitosis blocker) are:
1. C
Coollcchhiicciinnee,, vviinnbbllaassttiinnee
- these plant-derived molecules block mitosis by preventing the formation of the
mitotic spindle
- they inhibit the build-up (= polymerization) of tubulin into functional
microtubules
2. TTaaxxooll,, rrhhaazziinniillaam
m
- stop the mitosis by stabilizing the microtubule polymer
- since the degradation (= depolymerization) of tubulin is blocked, the
chromosomes cannot migrate toward the poles
Taxol and Vinblastine are potent anti-cancer drugs used in modern chemo-therapy
Taxol was isolated 1967 from the bark of the Pacific yew tree (Taxus brevifolia)
 1992: the FDA approved Taxol for refractory ovarian
cancer and in 1994 for treatment of refractory
breast cancer
 since the Pacific Yew is a slow growing tree and 2 grams of Taxol
is needed for treatment of one cancer patient, scientists are
currently looking for alternative sources and develop biotechnological approaches to meet the demand in the hospitals
23
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
Intro Molecular Cell Biology (BIO 210); Instructor: Elmar Schmid, Ph.D.
TThhee cchheem
miiccaall ssttrruuccttuurree ooff tthhee m
miittoossiiss bblloocckkeerr TTaaxxooll
the mitosis blocker Vinblastine (and vincristine) are so-called plant alkaloids
 both molecules are isolated from the Madagascar periwinkle plant
(Catharanthus roseus or formerly classified as Vinca rosea)
 these Vinca alkaloids bind to tubulin, thus preventing the cell from
forming the spindle apparatus it needs to move its chromosomes
e.g. the cell poison cycloheximide blocks the activity of the DNA polymerase and
prevents the duplication of the chromosomes
M
Miittoossiiss &
&C
Caanncceerr

the different body parts, tissues and cells of multi-cellular organisms are the result of
multiple cell divisions

for the body to function normally, the organs and tissues must communicate to
control the development of each of its cells and tissues

if uncontrolled cell growth in a part of a tissue or body happens, due to the
continuous cell divisions of one (!!) cancer cell, normal functions of the individual can
be seriously impaired

Uncontrolled cell growth leads to tumors; two types of tumors are observable in
biological organisms
Malignant tumors
 can induce secondary tumors (= Metastases) by the release of
metastatic cells, which lodge and begin to grow in other parts of
the body
Benign tumors
 formed by non-aggressive cancerous cells that remain in the initial
24
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
Intro Molecular Cell Biology (BIO 210); Instructor: Elmar Schmid, Ph.D.
location

Uncontrolled cellular growth and cancer have been shown to be the result of
mutations of so-called oncogenes

Cancer cells usually show a loss of cell cycle control due to mutations of proteins
of the cell cycle control system
 e.g. many women with breast and ovarian cancer carry a mutation
in the so-called cancer susceptibility gene, BRCA1 & 2
 e.g. a single nucleotide change in the ras oncogene, located on
human chromosome 11, is frequently associated with bladder
cancer
 e.g. many human cancers are caused by mutations of so-called
Tumor suppressor genes, such as p53 or Rb
Retinoblastoma
 a juvenile eye cancer
 caused by a mutation in the Rb gene located on human
chromosome 13
 since the gene is dominant inherited, both alleles must
be mutant for the cancer to develop
 the Rb gene product interacts with a protein called
E2F, which plays a role in the control of cellular
replication during the S phase of the cell cycle
p53
 this tumor suppressor gene is located on the human chromosome 17
 a single amino acid substitution, e.g. at amino acids 175, 248, and 273, lead to loss
of function of the gene product
 about 50% of human cancers are associated with p53 mutations, including cancers
of the bladder, breast, cervix, colon, lung, liver, prostate, and skin
 p53 related cancers are very aggressive and lead to a high degree of mortality in
affected individuals
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