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The Cell Cycle
www.glogster.com
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
“Where a cell exists, there must
have been a preexisting cell…”
-Rudolf Virchow
bobbiblogger.wordpress.com
Roles of Cell Division
poosk.com,
www.telegraph.co.uk,
www.andersonmoores.com
Renewal and
repair
Reproduction
Growth and
development
www.rapidlearningcenter.com, www.emc.maricopa.edu
Types of cell division
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
• chromosomes
– packaged DNA
molecules in nuclei
– somatic cells
have 2 sets of
chromosomes (2N,
diploid)
– gametes have 1
set of
chromosomes (N,
haploid)
• chromatin –
unorganized mass of
DNA and proteins
that condenses
during cell division
Chromosomes vs. Chromatin
www.cnr.berkeley.edu
Unduplicated
chromosome
(End of G1)
Duplicated
chromosome
(End of S)
centromere (point of attachment)
one sister chromatid
Daughter
cells with
identical
chromosomes
(End of M)
its sister chromatid
What
happens to
the DNA
during cell
division?
Prophase
Mitosis
Eukaryotic
Cell Division
(division of the nucleus)
Metaphase
Anaphase
Cytokinesis
(division of the cytoplasm)
Telophase
Overview of cell division in animal cells
cantorsbiologyblog.blogspot.com
Overview of cell division in a plant cell
Nucleus
Nucleolus
Interphase
Chromatin
condensing
Chromosome
Prophase
Metaphase
• No centrioles/centrosomes
• Formation of a cell plate
Anaphase
Telophase and
Cytokinesis
Cell plate
formation
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
Before
chromosomes
condense
1. DNA
replicated?
2. Sufficient
growth?
1.
2.
3.
Sufficient growth?
Suitable environment?
Undamaged DNA?
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.cherifer.ph
www.vetstreet.com
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
For next meeting
• Submission of lab report on mitosis in
Allium cepa (handout and style sheet
available on the blogsite). May also be
emailed to [email protected]
• Read on MEIOSIS. We will have a small
group activity prior to discussion.
Meiosis
Reductional Division for Sexual
Reproduction
• Types of reproduction
– Asexual
– Sexual
• gene – hereditary unit
of DNA
• locus – gene’s specific
location in the
chromosome
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  “crossingover” that occurs at a
chiasma
• Does not normally
happen to sex
chromosomes
• Purpose: to increase
genetic variation
Independent assortment during Metaphase I
Meiosis II is equational cell division
How unique are you?
• Random fertilization
– 1/64 million
• Independent assortment
– 1/223
• Crossing-over
– occurs an average of 2-3 times
per chromosome pair
Gametogenesis
1) Meiosis
-
-
Gametes (n) formed from
embryonic primordial germ
cells (PGC’s) via meiosis
PGC’s (2n)  meiosis 
sex cells (n)
Spermatogonium and
oogonium
2) Maturation
–
distinctive characteristics
of sperm and egg cells are
formed
Spermatogenesis vs. Oogenesis
SPERMATOGENESIS
 process is continuous
 100-650 million sperm cells
produced
OOGENESIS
 unequal cytokinesis
 time table
 only 400 oocytes ovulated
between puberty & menopause
Spermatogenesis vs. Oogenesis
2N
Suspended in prophase I
One oocyte / month
N
N
Halted at metaphase II
until fertilization
OOGENESIS
2 million 1o oocytes in a fetus
1 million 1o oocytes in a newborn (at prophase I)
400,000 1o oocytes during puberty
(meiosis I completed in only one each month)
400 2o oocytes ovulated (at metaphase II)
between puberty and menopause
(meiosis II completed only after fertilization)
mature ovum
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:
• Pairs of
homologous
chromosomes do
not separate
normally during
meiosis
• Gametes contain
two copies or no
copies of a
particular
chromosome
Meiosis I
Nondisjunction
Meiosis II
Nondisjunction
Gametes
n+1
n+1
n
1
n–1
n+1
n –1
n
Number of chromosomes
(a) Nondisjunction of homologous
chromosomes in meiosis I
(b)
Nondisjunction of sister
chromatids in meiosis II
n
• Aneuploidy
– Results from the fertilization of
gametes in which nondisjunction
occurred
– Is a condition in which offspring
have an abnormal number of a
particular chromosome
• If a zygote is trisomic
– It has three copies of a particular
chromosome
• If a zygote is monosomic
– It has only one copy of a
particular chromosome
• Klinefelter (XXY), Turner (X0)
• The incidence of Down
syndrome in the general
population is about 1 in every
770 births.
• Among women over the age
of 35 years, however, the
incidence of delivering a child
with Down syndrome
increases.
• The correlation between
maternal age and Down
syndrome risk is striking
when the age distribution for
all mothers for all mothers is
compares to that of mothers
of Down syndrome children.
• 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