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Core Concepts • Cell division is necessary for reproduction, repair and growth.
• The cell cycle is a continuum of processes undergone by cells
during their lifetime, which involves growth and functioning,
and culminates in division.
• Mitosis produces two new identical cells.
• Interactions of physical and chemical signals control the events
of the cell cycle.
• Cancer results from abnormal or lacking control signals of the
cell cycle.
• Meiosis is a special kind of division that produces four (4)
haploid, non-identical cells.
• Errors may occur during cell division, producing cells with
abnormal chromosome number. www.brookes.ac.uk
Keywords
• anaphase
• centromere
• chromatin
• chromosome
• crossing-over
• cytokinesis
• diploid
• G1
• G2
• genome • haploid • homologue • interphase • kinetochore • meiosis • metaphase • mitosis
• non-disjunction • prophase • sister chromatid
• spindle • synthesis • telophase • tetrad
www.futura-sciences.com
Cell
Reproduction
bobbiblogger.wordpress.com
“Where a cell exists, there must
have been a preexisting cell…”
-Rudolf Virchow
Roles of Cell Division
Reproduction
Growth and development
Renewal and repair
poosk.com,
www.telegraph.co.uk,
www.andersonmoores.com
Types of cell division
www.rapidlearningcenter.com, www.emc.maricopa.edu
The Cell Cycle -ordered series of events that leads to
cell growth, division, and replication
www.hartnell.edu
interphase
–preparation for cell division
–cell grows
–DNA is replicated
–centrosomes are replicated*
7
• chromatin –
unorganized mass of
DNA and proteins
that condenses
during cell division
• chromosomes – packaged DNA
molecules in nuclei
– somatic cells
have 2 sets of
chromosomes (2N,
diploid)
– gametes have 1
set of
chromosomes (N,
haploid)
Chromosomes vs. Chromatin
What
happens to
the DNA
during cell
division?
Unduplicated
chromosome
(End of G1)
Duplicated
chromosome
(End of S)
Daughter
cells with
identical
chromosomes
(End of M)
its sister chromatid
one sister chromatid
centromere (point of attachment)
www.cnr.berkeley.edu
Eukaryotic Cell Division
Mitosis (division of the nucleus)
Prophase
Metaphase
Anaphase
Telophase Cytokinesis
(division of the cytoplasm)
cantorsbiologyblog.blogspot.com
Overview of cell division in animal cells
Overview of cell division in a plant cell
Nucleus
Nucleolus Chromosome Chromatin
condensing
Cell plate
formation
• No centrioles/centrosomes
• Formation of a cell plate
Interphase Prophase Metaphase Anaphase Telophase and
Cytokinesis
Regulation of the Cell Cycle
• Molecular control
system
• Checkpoints – critical
control points where
stop and go-ahead
signals can regulate
the cycle
Restriction Point
point of no return
1. Sufficient growth?
2. Suitable environment?
3. Undamaged DNA?
Before
chromosomes
condense
1. DNA
replicated?
2. Sufficient
growth?
Factors that control the cell cycle
1. Telomeres
– Repeated DNA
sequences at tips of
chromosomes
– TTAGGG sequences lost
every time a cell divides
– Restored to their
original length by
telomerase (normally
found in gametes)
Factors that control the cell cycle
2. Regulatory proteins
• Cyclins - concentration
fluctuates in the cell
• Cyclin-dependent
kinases (Cdks) –
activate other proteins in the
presence of cyclin
• e.g. Maturation-promoting
factor (MPF)
– M-phase promoted
• Chromatin condensation
• Mitotic spindle formation
• Degradation of nuclear
envelope
– Deactivated when proteolytic
enzymes digest the cyclin
Factors that control the cell cycle 3. Growth Factors
• proteins that stimulate other cells to divide
• promote the binding of cyclin to cdks
• ex. platelet-derived growth factor (PDGF),
epidermal growth factor (EGF)
www.vetstreet.com
www.cherifer.ph
Factors that control the cell cycle
4. Density-dependent
inhibition
5. Anchorage dependence • Most animal cells must be
attached to a substrate before
they can grow
Cancer Uncontrolled cell division
• Cells do not heed
normal signals to STOP
cell division
• Can invade neighboring
cells and interfere with
normal body function
• “immortal” – can keep
dividing as long as
nutrient supply is kept
constant
http://upload.wikimedia.org/wikipedia/en/1/1f/Normal_cancer_cell_differences_from_NIH.png
Transformation
• Numerous diverse
causes
• Cancer cell
tumor
– Benign
– Malignant
• Metastasis
Causes
• Oncogenes – Gene that enables
transformation when mutated
or expressed in high levels
• Viruses and bacteria – e.g. HPV and cervical cancer;
Hep B and C and liver cancer;
H.pylori and stomach cancer
• Ionizing and UV
radiation
• Carcinogens
Treatment
• Surgery
• Radiation
• Chemotherapy
• Immunotherapy
and gene therapy
Meiosis Reductional Division for Sexual
Reproduction
• Types of reproduction
– Asexual
– Sexual
• gene – hereditary unit
of DNA
Meiosis in
Sexual Life Cycles
• Homologous
chromosomes
– pair that has the same
length, centromere position,
staining pattern
– contain similar sets of
information
• Humans
– 22 pairs of autosomes
– 1 pair of sex
chromosomes
Meiosis involves 2
stages of nuclear
division
• Interphase
–G1, S, G2
• Meiosis
–Meiosis I
–Meiosis II
Meiosis I Separating
homologous
chromosomes
Meiosis I Separating
homologous
chromosomes
Meiosis I Separating
homologous
chromosomes
Meiosis I Separating
homologous
chromosomes
Meiosis I is reductional cell division
Crossing-over during Prophase I Exchange of segments between homologous pairs
• Homologues pair up
tetrad
• Synapsis
“crossing-over” that
occurs at a chiasma
• Does not normally
happen to sex
chromosomes
• Purpose: to increase
genetic variation
Independent assortment during Metaphase I Random separation of homologous chromosomes
Meiosis II is equational cell division
Property Mitosis Meiosis
DNA replication Interphase before mitosis Interphase before Meiosis I
Number of divisions One, including PMAT Two, each including PMAT
Synapsis of homologous chromosomes
Does not occur Prophase I; along with crossing-over
Number of daughter cells and genetic composition
Two, each diploid (2n), genetically identical to parent cell
Four, each haploid (n), half as many chromosomes as parent cell, genetically unique
Role in the animal body
Adult from zygote, produces cells for growth, repair; asexual reproduction
Produces gametes and introduces genetic variability among gametes
How unique are you?
• Random fertilization
– 1/2,000,000 egg cells x
1/4,000,000,000 sperm cells
• Independent assortment
– 1/223 unique chromosome
combination
• Crossing-over
– occurs an average of 2-3 times
per chromosome pair
Human gametogenesis Production of gametes
Mitosis (1 diploid cell 2 diploid cells)
Meiosis I (1 diploid cell 2 haploid cells)
Meiosis II (2 haploid cells 4 haploid cells)
Maturation and differentiation (haploid cells undergo extensive changes and organization)
www.thetimes.co.uk
Primordial germ cell in embryo (2N)
Spermatogonium (2N) Oogonium (2N)
Primary spermatocyte (2N)
Primary oocyte (2N)
Secondary spermatocytes (N)
Secondary oocyte (N)
Spermatids (N) Ootid (N)
Spermatozoa (N) Ovum (N)
Spermatogenesis Oogenesis
Spermatogenesis
• Equal cytokinesis
• Continuous
• 100-650 million sperm cells
produced
Oogenesis
• Unequal cytokinesis
• Time table
• 400 oocytes ovulated
between puberty and
menopause 2 million 1 million
400,000 Suspended in prophase I before birth
One oocyte / month completes meiosis I
400 ovulated from
puberty to
menopause Halted at metaphase II until
fertilization
Only present after
fertilization
Questions:
• How many sets of chromosomes are present in each of
the following cell types?
– an oogonium
– a 1o spermatocyte
– a spermatid
– a cell during anaphase I, from either sex
– a cell during anaphase II, from either sex
– a 2o oocyte
– a polar body derived from a 1o oocyte
• Why is it extremely unlikely that a child will be
genetically identical to a parent?
Questions:
• How do the structures of the male and female gametes
aid in their functions?
• A woman who is about 4 weeks pregnant suddenly
begins to bleed and pass some tissue through her
vagina. After a physician examines the material, he
explains to her that a sperm fertilized a polar body
instead of an ovum, and an embryo could not develop.
What has happened? Why do you think a polar body
cannot support the development of an embryo, whereas
an ovum, which is genetically identical to it, can?
Errors in cell division Chromosomal aberrations
Nondisjunction
• Failure to separate
normally
– pairs of homologous
chromosomes during
meiosis I
– sister chromatids fail
to separate in meiosis
II and mitosis
• Gametes contain two
copies or no copies of
a particular
chromosome
• What happens when
these gametes are
fertilized?
Aneuploidy
– offspring have an abnormal number
of a particular chromosome
– fertilization of gametes in which
nondisjunction occurred
• Trisomic zygote
• three copies
of a
particular
chromosome
(e.g.
Trisomy-21,
Trisomy-X)
• Monosomic
zygote
• one copy of
a particular
chromosome
(e.g.
Monosomy
X)
• Incidence of Down syndrome
1 in every 770 births.
• Among women over the age
of 35 years, incidence of
delivering a child with Down
syndrome increases.
Can sometimes occur in the early stages of
embryonic development
2N 2N 2N
2N 2N+1 2N-1
Polyploidy
– Extra sets of chromosomes
(3n, 4n, 5n, 6n, 8n, 10n, 12n)
– Caused by nondisjunction of
ALL chromosomes
• Rare, usually fatal in animals
• Common in plants (30-80%)
– Polyploids often thrive
better and grow taller
– Solution to hybrid sterility
– May be preferred because of
sterility