Cell Cycle, Mitosis and Meiosis

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Cell Cycle, Mitosis and Meiosis
Covered in these slides, in the
Concepts & Connections book- Chapter 8 to page 143
In Raven and Johnson Book- Chapter 10 pages192-204
And in your packet on The Cell Cycle
The cell cycle multiplies cells
 The cell cycle is an ordered sequence of events
that extends
– from the time a cell is first formed from a dividing
parent cell
– until its own division.
© 2012 Pearson Education, Inc.
Cell division is a continuum of dynamic
changes
 During cytokinesis, the cytoplasm is divided into separate
cells.
 The process of cytokinesis differs in animal and plant cells.
Cytokinesis
Cleavage
furrow
Contracting ring of
microfilaments
Daughter
cells
Cleavage
furrow
Anchorage, cell density, and chemical growth
factors affect cell division
 The cells within an organism’s body divide and
develop at different rates.
 Cell division is influenced externally by
– the presence of essential nutrients,
– growth factors, proteins that stimulate division, there are
over 50 different growth factors which work for one or
more cell type
– density-dependent inhibition, in which crowded cells
stop dividing,
– anchorage dependence, the need for cells to be in
contact with a solid surface to divide.
Growth factors signal the cell cycle control
system
 The cell cycle control system is a cycling set of
molecules in the cell that
– triggers and
– coordinates key events in the cell cycle.
 Checkpoints in the cell cycle can
– stop an event or
– signal an event to proceed.
Growth factors signal the cell cycle control
system
 There are three major checkpoints in the cell cycle.
G1- commitment to divide, growth
factors present?, Size of cell ok?,
G2- check for proper DNA replication
M- all chromosomes attached to
spindle fibers
Cell Cycle progresses by action of Cdks
Cyclins
proteins produced by the cell during cell division
Cyclin-dependent kinases (Cdk)
cyclin is required to activate these enzymes
activates cell proteins by phosphorylating them
(proteins needed for S phase)
needed to go through G1 checkpoint
MPF
Maturation-promoting factor (mitosis promoting factor)
aka Mitosis- promoting factor is a cyclin-Cdk complex
phosphorylates proteins needed for mitosis
needed to go through G2 checkpoint
Rate of Cell Division
• Differs from one cell type to the next
– Examples:
• red bone marrow cells divide every 12 hours to replace RBCs that
wear out
• Cells at tip of root divide about every 19 hours.
• Neurons (nerve cells) normally never divide again once brain is fully
formed in utero
• Control of Division, lost = CANCER
–
–
–
–
Cancer is different depending on the tissue affected
Common theme is lack of control over cell division
Abnormal, uncontrolled cell division
Mutation in genes (including p53) that target and control
abnormal cells.
– Abnormal cells impede functioning of normal cells
p53 gene ( tumor suppressor gene)
• Key role in G1 checkpoint
• P53 protein monitors DNA
• Found absent or damaged in most cancer cells
Cancer is failure of
cell cycle control
• Tumor suppressor genes- prevents the
development of mutated cells, prevents
cancer/tumors
• Oncogenes- cancer causing genes
• Proto-oncogenes- normal genes that
become mutated
Meiosis
• Production/formation of __________
• Basis of sexual reproduction
• Only germ cells undergo meiosis
Haploid gametes (n  23)
n
Egg cell
n
Sperm cell
Meiosis
Ovary
Fertilization
Testis
Diploid
zygote
(2n  46)
2n
Key
Multicellular diploid
adults (2n  46)
Mitosis
Haploid stage (n)
Diploid stage (2n)
How meiosis halves chromosome number…
MEIOSIS I
INTERPHASE
MEIOSIS II
Sister
chromatids
2
1
A pair of
homologous
chromosomes
in a diploid
parent cell
A pair of
duplicated
homologous
chromosomes
3
MEIOSIS I: Homologous chromosomes separate
INTERPHASE:
Chromosomes duplicate
Centrosomes
(with centriole
pairs)
Prophase I
Metaphase I
Sites of crossing over
Spindle microtubules
attached to a kinetochore
Centrioles
Anaphase I
Sister chromatids
remain attached
Spindle
Tetrad
Nuclear
envelope
Chromatin
Sister
chromatids
Fragments
of the
nuclear
envelope
Centromere
(with a
kinetochore)
Metaphase
plate
Homologous
chromosomes
separate
MEIOSIS II: Sister chromatids separate
Prophase II
Metaphase II
Anaphase II
Sister chromatids
separate
Telophase II
and Cytokinesis
Haploid daughter
cells forming
Meiosis Leads to Genetic Diversity
 Three ways genetic diversity is increased by
meiosis:
1. 2 parents contribute ½ of the genetic material to offspring
2. Crossing-over in Prophase I
3. Chromosome Alignment in Metaphase I
 Meiosis produces cells that are NOT identical,
unique gametes
C
E
c
e
Crossing Over
increases genetic
diversity by
producing “new”
chromosomes.
1
Breakage of homologous chromatids
C
E
c
e
2
Tetrad
(pair of homologous
chromosomes in synapsis)
Joining of homologous chromatids
E
C
Chiasma
c
e
3
Separation of homologous
chromosomes at anaphase I
C
E
C
c
e
E
c
e
4
Separation of chromatids at
anaphase II and
completion of meiosis
C
E
C
e
c
E
c
e
Parental type of chromosome
Recombinant chromosome
Recombinant chromosome
Parental type of chromosome
Gametes of four genetic types
Independent orientation of chromosomes in
meiosis and random fertilization lead to
varied offspring
Independent orientation at metaphase I
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