Transcript Cell Cycle, Mitosis, and Meiosis

Cell Cycle and Mitosis
Cell Division = Nuclear Division + Cytokinesis
Mitosis – produces 2 identical daughter cells with
same genetic material as parent cell
Meiosis – “reduction division”, daughter cells
contain half the genetic material of parent cell
Chromosome Structure
•
•
•
•
Chromatin condenses
Forms chromosomes
2 sister chromatids
Connected by
centromere
• Each chromatid is one
coiled molecule of DNA
Chromosome Structure
The Cell Cycle
Interphase
G1+S+G2
M phase
Mitosis
+
Cytokinesis
Interphase
• G1: growth
• S: growth and
duplication of DNA
– Creates 2 sister
chromatids
• G2: growth and
preparation for
division
Mitosis: Part One
Prophase
• Nucleoli disappear
• Chromatin condenses
• Nuclear envelope breaks
down
• Mitotic spindle assembles
– MTOC’s move to opposite
sides of cell
– Microtubules form (tubulin)
between
– MT attach to centromere at
kinetochore
Mitosis: Part Two
Metaphase
• Chromosomes distribute
along metaphase plate
• Ends with MT pulling
chromosomes apart (2
chromatids then called
chromosomes, each with
its own centromere and
kinetochore)
Anaphase
• MT shorten, pulling
chromosomes towards
opposite sides of cell
(tubulin uncoupled at
chromosome end)
• Other MT (not attached to
chromosomes) interact to
push MTOC’s apart
Telophase
• Nuclear envelope
develops around each
pole (forms 2 nuclei)
• Chromosomes disperse
into chromatin
• Nucleoli reappear
Cytokinesis – Animal cells
• microfilaments
form ring inside
membrane
between 2 nuclei
• MF shorten to
pinch off cell into
2
• Groove =
cleavage furrow
Cytokinesis – Plant cells
• Vesicles from Golgi migrate
to plane inbetween 2
nuclei
• Vesicles fuse to form cell
plate
• Cell plate becomes
membrane for daughter
cells
• Cell walls develop between
membranes
Cell Cycle Regulation
Why divide?
• Surface-to-volume Ratio
– Volume increases faster than surface area
– At some point, the cell would be unable to
exchange enough materials to maintain cell
function
• Genome-to-volume Ratio
– Limited capacity to control cell processes
– At some point, cell would exceed ability of
genome to regulate activities
• Cell must then stop growing or divide
Checkpoints
• At specific points in
cell cycle
• Cell evaluates
internal & external
conditions
• Determines whether
to continue through
cell cycle
Cyclin-dependent Kinases (Cdk’s)
• Phosphorylate
proteins that
regulate the cell
cycle
• Activated by cyclin
(regulatory protein)
Cyclin
• S-cyclin builds up
during G1 to trigger
DNA replication
• M-cyclin builds up
during G2 to trigger
mitosis
• Cdk is recycled, but
cyclin is degraded
MPF = Mitosis-promoting factor
(M-cyclin + Cdk)
Growth factors
• Receptors on outside
of cell membrane for
external molecules
(growth factors) that
stimulate cell division
• Ex: for wound healing,
growth factors are
secreted by damaged
cells and platelets to
stimulate growth of
scar tissue
Growth factors, Cdk, & G1 Checkpoint
Anchorage Dependence
• Most cells only
divide when
attached to a
substrate
– Flat external
surface
– Neighboring cell
– Culture dish
Density-dependent Inhibition
• Cells stop
dividing when
surrounding
cell density
reaches a
certain
maximum
Cancer
• Uncontrolled cell growth and division
• Transformed cell = cell that has become cancerous
• Proliferate (divide) without regard to:
–
–
–
–
Cell cycle checkpoints
Density-dependent inhibition
Anchorage dependence
Other regulatory mechanisms