Part II: Genetics – The basis of heredity
• FACT: Related individuals resemble each other more than
randomly chosen individuals do
– Within populations
– Among populations within species
– Within species among species
– Etc.
…WHY??
Resemblance among relatives…”heredity”
• Information is passed from parent to offspring
• Information is passed largely intact
• That information encodes the phenotype
– Phenotype is any property of an organism that can be
attributed to that organism (height, weight, eye color,
obnoxiousness, etc.)
Outline of today’s lecture – (Ch. 12)
• Review of (some) Eukaryotic cell structures
• Schematic overview of mitosis
• The mitotic spindle
• Cytokinesis
• Cell cycle control
Review of Eukaryotic cell structures
• Nucleus
– Surrounded by membrane
– Contains the genetic material
(DNA)
– During interphase, DNA uncoiled,
complexed with proteins, called
“chromatin”
– In mitosis, condenses into
chromosomes
Review of Eukaryotic cell structures
• Nucleolus
– Point of synthesis of ribosomal RNA
– Ribosomal subunits assembled
Nucleolus
Review of Eukaryotic cell structures
• Centrosomes
– Region near nucleus that is the
cellular “microtubule
organizing center”
– Associated with formation of
spindle fibers
– Form “spindle poles” during
mitosis
Review of Eukaryotic cell structures
• Cytoskeleton
– Provides mechanical
support to the cell
– Dynamic (disassemble and
reassemble)
– Constructed of
microtubules,
microfilaments,
intermediate filaments
Microtubules: Structure and Function
• Composed of polymers of
dimers of α-tubulin, βtubulin
• Lengthen by adding dimers,
shorten by losing dimers
• “Molecular motors” can
move along microtubules
Mitosis and the Cell Cycle
• All organisms must grow and reproduce
• During organismal growth, all cells must grow and
divide
• Genetic information must be faithfully passed
between parent cell and daughter cells
• MITOSIS is the process by which genetic information
is passed from parent to daughter cells
The Cell Cycle – A schematic overview
Some notation
• Chromosome
• Centromere
• Spindle pole (centrosome)
• Spindle fiber
• Nuclear envelope
Schematic drawing of a chromosome
G1 (Interphase)
• Each chromosome is a
single, unreplicated double
strand of DNA
• One chromosome from each
parent (Male, Female) forms
a Homologous Pair (=
“homologs”)
F
M
• Chromosomes in nucleus,
surrounded by nuclear
membrane
• Single centrosome
S-phase (DNA synthesis)
• After DNA replication,
TWO “sister chromatids”
are present for each
homolog
• Each sister chromatid is
the SAME doublestranded DNA molecule
• Attached by proteins,
tightly at centromere and
more loosely throughout
their length
F
F
M
M
G2 (Interphase)
• Duplication of
centrosomes
• Cell receives signal to
enter mitosis
F
F
M
M
M1 (Early Prophase)
• Chromosomes condense
• Mitotic spindle forms
from centrosome
F
F
M
M
• Centrosomes begin to
migrate to poles of cell
• Nucleoli disappear
M2 (Mid-prophase)
• Chromosomes fully
condensed
• Centrosomes complete
migration to the poles
• Nuclear envelope begins to
degrade
• Spindle fibers enter nuclear
area from the pole
• Kinetochores form at
centromeres
F
F
M
M
M3 (Prometaphase)
• Nuclear envelope
completely degraded
• Kinetochores form at
centromeres
• Some spindle fibers
attach at kinetochores
• Sister chromatids
attached to opposite
poles
F
F
M
M
M4 (Metaphase)
• Spindle fibers attached to
centromere at
kinetochore
• All sister chromatids
attached to opposite
poles
• Chromosomes migrate to
center plane of cell,
“metaphase plate”
F
F
M
M
M5 (Early Anaphase)
• Protein bond between
sister chromatids
degrades
• Sister chromatids
separate, begin migration
toward opposite poles
• Poles move farther apart
as non-kinetochore
spindle fibers lengthen
M5 (Late Anaphase)
• Chromosomes (no longer
“chromatids”) have
reached poles
M6 (Telophase)
• Non-kinetochore spindle
fibers continue to elongate
cell
• Nuclear envelopes begin to
form at poles
• Chromosomes de-condense
back into chromatin
• Nucleoli re-form, cytokinesis
begins
The mitotic spindle: Structure and Function
• Made of microtubules,
associated proteins
• Cytoskeleton partially
disassembles to provide
materials
• Assembly starts in
centrosomes
• Centrosomes “pushed”
away from each other as
microtubules grow
The mitotic spindle: Structure and Function
• Some spindle fibers grow
and attach to the
kinetochore while sister
chromatids are still attached
• “Equilibrium” reached when
sister chromatids are
midway between the poles
(metaphase plate)
Microtubules: Structure and Function
• Composed of polymers of
dimers of α-tubulin, βtubulin
• Lengthen by adding dimers,
shorten by losing dimers
• “Molecular motors” can
move along microtubules
The mitotic spindle: Structure and Function
• Hypothesized
mechanism for
chromosome
movement
The mitotic spindle: Structure and Function
• Experimental Test of
the hypothesized
mechanism for
chromosome
movement
• What is an obvious
alternative hypothesis?
Cytokinesis – Animal cells
• Cleavage furrow formed
by contraction of a ring
of microfilaments
• Associated with actin
and myosin (motor
protein system)
• Forms along metaphase
plate
Cytokinesis – Plant cells
• Plants have cell walls
• Vesicles (derived from Golgi)
move along microtubules to
the middle of the cell
• Vesicles coalesce to form a
“cell plate”; membrane
derived from vesicles
• Cell plate membrane fuses
with outer cell membrane,
forms daughter cells
Cell Division in Prokaryotes - Binary fission
• Prokaryotes preceded
Eukaryotes by billions of
years
• Bacterial chromosome is
circular DNA molecule
• Origin of replication
Cell Division in Prokaryotes - Binary fission
• As DNA replication
proceeds, one copy of the
origin migrates to each end
of the cell
Cell Division in Prokaryotes - Binary fission
• Cell membrane grows
inward
• New cell wall forms
• Two daughter cells result
• Migration of origin(s) is
similar to migration of
centromeres
Cell cycle - Regulation
• Timing and rate of cell division are critical
• Chemical signals regulate “checkpoints” in the cell cycle
• RESULTS of certain chemical processes are “checked”
• If results not appropriate, cell division does not proceed
• Evidence from cell fusion experiments
Cell cycle - Regulation
Cell cycle - Regulation
• G1 checkpoint: Cell will not enter into
S phase unless appropriate signal is
received
– Many cells in adult mammals are in G0
and thus do not divide (e.g., nerve)
• If not, enters non-dividing “G0” phase
• Variety of external chemical cues to
trigger entry into G1 and G2 phase (or
not)
Cell cycle - Regulation
• Additional Regulatory Checkpoints
– Cell will not proceed through S-phase
unless DNA synthesis completed
– M-checkpoint: Cell will not enter
anaphase until chromosomes lined up
on metaphase plate
Cell cycle - Regulation
• Cell cycle controlled by
two types of proteins,
kinases and cyclins
• Kinases active when
attached to cyclins
• E.g., “Maturationpromoting factor” MPF
Molecular control of the cell cycle - MPF
Cell cycle regulation – internal and external controls
• M-checkpoint: if kinetochores not attached to spindle
microtubules, sister chromatids remain attached
• G1-checkpoint: growth factors / receptors
– E.g., platelet-derived gf (PDGF), healing wounds
– Density-dependent inhibition (nutrient concentration below
a certain level)
– Anchorage-dependence
For Wednesday: Ch. 13
• Meiosis
• Sexual Life Cycles