Transcript 01 The Cell Cycle

The Cell Cycle
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.
Keywords
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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
The Cell Cycle
Roles of Cell Division
• Reproduction
• Growth and
development
• Renewal and
repair
The Cell Cycle
Types of cell division
• Mitosis
– May have evolved
from binary fission
in prokaryotes
– Method of asexual
reproduction in
unicellular
eukaryotes
• Meiosis
– Responsible for
production of
gametes in
multicellular
eukaryotes
The Cell Cycle
Stages of the cell cycle
Important terms
• genome - genetic material of a cell
• chromatin – unorganized mass of DNA
and proteins that condense 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)
• interphase – preparation for cell
division
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cell grows
DNA is replicated
centrosomes are replicated*
chromosomes condense
Eukaryotic cell division
• 1 chromosome  2
sister chromatids,
connected at a
centromere, which
separate during cell
division
• mitosis – division of
the nucleus
• cytokinesis – division
of cytoplasm
Mitosis consists of five distinct phases
– Prophase
– Prometaphase
PROPHASE
G2 OF INTERPHASE
Centrosomes
(with centriole pairs)
Nucleolus
Chromatin
(duplicated)
Nuclear
envelope
Plasma
membrane
Early mitotic
spindle
Aster
Chromosome, consisting
of two sister chromatids
Centromere
PROMETAPHASE
Fragments
of nuclear
envelope
Kinetochore
Nonkinetochore
microtubules
Kinetochore
microtubule
– Metaphase
– Anaphase
– Telophase
METAPHASE
ANAPHASE
Metaphase
plate
Spindle
Centrosome at
one spindle pole
TELOPHASE AND CYTOKINESIS
Cleavage
furrow
Daughter
chromosomes
Nuclear
envelope
forming
Nucleolus
forming
Mitosis in a plant cell
Nucleus
1
Chromatin
Nucleolus condensing
Prophase.
The chromatin
is condensing.
The nucleolus is
beginning to
disappear.
Although not
yet visible
in the micrograph,
the mitotic spindle is
staring to from.
2
Chromosome
Prometaphase.
We now see discrete
chromosomes; each
consists of two
identical sister
chromatids. Later
in prometaphase, the
nuclear envelop will
fragment.
3 Metaphase. The
5
4
Telophase. Daughter
Anaphase. The
spindle is complete,
nuclei are forming.
chromatids of each
and the chromosomes,
Meanwhile, cytokinesis
chromosome have
attached to microtubules
has started: The cell
separated, and the
at their kinetochores,
plate, which will
daughter chromosomes
are all at the metaphase
divided the cytoplasm
are moving to the ends
plate.
in two, is growing
of cell as their
toward the perimeter
kinetochore
of the parent cell.
microtubles shorten.
Regulation of
the Cell Cycle
• Events in the cell cycle
triggered and
coordinated by a
molecular control
system
• Checkpoints – critical
control points where
stop and go-ahead
signals can regulate
the cycle
Restriction Point
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
cyclically fluctuates in the cell
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Cyclin-dependent kinases
(Cdks) – activate other
proteins in the presence of
cyclin
e.g. Maturation-promoting
factor (MPF)
–
M-phase promoted
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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
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proteins that stimulate other cells to divide
promote the binding of cyclin to cdks
ex. platelet-derived growth factor (PDGF)
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
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
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Surgery
Radiation
Chemotherapy
Immunotherapy
and Gene therapy
Meiosis
Reductional Division for Sexual
Reproduction
• Types of reproduction
– Asexual
– Sexual
• Genes – hereditary
units of DNA
• Locus – gene’s specific
location in the
chromosome
Meiosis in
Sexual Life Cycles
• Generation-to-generation
sequence of stages in the
reproductive history of an
organism
• Homologous chromosomes
– pair that has the same
length, centromere position,
staining pattern
• 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 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?
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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