Download Chromosomes, Cell Cycle & Cell Division Chapter 9

Cell Cycle
Cell Cycle
• describes the life cycle of a cell
• differs fundamentally between prokaryotes &
eukaryotes
• shares four features among all cell types
– reproductive signal
– DNA replication
– genome segregation
– cytokinesis
• produces a new individual or new parts
Prokaryotic Cell Cycle
• one circular chromosome
– packed on a protein frame in the nucleoid
– anchored to the plasma membrane
• chromosome replication yields two anchored
circles
• cell growth separates anchored circles
• annular pinching separates daughter cells
Binary Fission
binary fission
in a
bacterium
Figure 9.2
chromosome numbers vary widely
Table 9.1
Eukaryotic Cell Cycle
• multiple linear chromosomes
– wrapped on a protein frame
– contained within the nucleus
• replication yields two complete sets of
chromosomes
• mitosis segregates one set of chromosomes to
each end of the cell
• cytokinesis divides the cytoplasm to form two
new cells
Eukaryotic Cell Cycle
• mitosis, cytokinesis (M phase) and interphase
• interphase represents most of the cell cycle
growth
&
biosynthesis
eukaryotic
cell
cycle
Figure 9.3
growth
&
biosynthesis
Cell Cycle Decision Making
• G1 => S transition & G2 => M transitions
require signals
• CDKs signal cell cycle transitions
– Cyclin-Dependent Kinases
• kinases phosphorylate target proteins
• phosphorylation activates (some) proteins
– Cyclins activate CDKs by allosteric binding
• Cyclins cycle
“Go!”
“Don’t go!”
“Go!”
Replicate!
eukaryotic
cell cycle
regulation
Figure 9.4
“Okay, go!”
“Radiation
damage!
Don’t go!”
“Go!”
external signals stimulate cell division
• growth factors stimulate cell division
– platelet-derived growth factor
• wound healing
– interleukins
• immune system function
– erythropoietin
• red blood cell production
• each provides information to the cell regarding
the needs of the body
Eukaryotic Chromosomes
• chromsomes consist of chromatin
– double stranded DNA
– protein “spools”
• chromatin is ~1/2 DNA & 1/2 protein
• chromatin is highly organized
– DNA wraps around histone proteins to form
nucleosomes
– during cell division, nucleosomes coil
tightly to form highly-condensed chromatin
organization of chromatin
Figure 9.5
organization of
chromatin
Figure 9.6
Replication
• during S phase
– partially wound DNA is replicated to form
two identical copies of each chromosome
– two sister chromatids remain attached at the
centromere
• each chromosome enters M phase as two
linked sister chromatids
• mitosis separates the chromatids and
distributes one to each daughter cell
mitosis: chromatin & cytoskeleton
Figure 9.8
events of mitotic phases
• Prophase
– chromosomes condense
– nucleolus disorganizes
– spindle apparatus begins to organize
– kinetochores forms
mitosis: cartoon version
Figure 9.8
Mitotic spindle apparatus
Figure 9.7
events of mitotic phases
• Prometaphase
– nuclear envelope is fragmented
– spindle fibers bind kinetochores
– chromosomes begin to migrate to equatorial
plate
events of mitotic phases
• Metaphase
– chromosomes are aligned at equatorial plate
• Anaphase
– sister chromatids separate
– daughter chromosomes migrate to poles
• Telophase
– prophase is reversed
Animal
Cytokinesis
Figure 9.10
Plant
Cytokinesis: Division of Cytoplasm
• Animals
– annular pinching by actin & myosin ring
• Plants
– deposition of cell plate by Golgi vesicles
• Organelles are distributed to daughter cells
~randomly
Modes of Reproduction
• asexual reproduction
– production of genetic clones through mitotic
cell divisions
– common among plants (vegetative
propagation) and unicellular eukaryotes
– eliminates costs & risks associated with
sexual reproduction
– offspring lack genetic variability
Modes of Reproduction
• sexual reproduction
– offspring exhibit genetic variability
• each bears a unique combination of
parental genetic contributions
– requires
• meiosis - reduction of chromosome
number from 2n (diploid) to 1n (haploid)
• fertilization - combination of 1n parental
contributions to produce 2n offspring
Fungal/animal life cycles
Figure 9.12
Modes of Reproduction
• meiosis produces
– gametes - animals, some protists
– spores - fungi, plants, some protists
• produce 1n adults
• produce gametes
• fertilization (gamete fusion) produces
– zygotes
• produce 2n adults and/or
• undergo meiosis
Modes of Reproduction
• meiosis
– two divisions
– reduces 2n parent cell to 1n products
– always produces 4 haploid products
• begins with 4 homologous chromatids
– recombination produces novel chromatids
– phases resemble mitotic phases, except
• meiosis I - homologs pair at prophase
• meiosis I - homologs separate at anaphase
Meiosis I: cartoon version
Figure 9.14
Modes of Reproduction
• meiosis I
– prophase I
• chromosomes condense, spindle forms
• nuclear envelope fragments
• homologs pair (synapsis)
• crossing over
crossing- over
occurs during
prophase I
Figure 9.15
Figure 9.16
Modes of Reproduction
• meiosis I
– metaphase I
• homolog pairs align at equatorial plate
– anaphase I
• homologs separate to poles
– telophase I
• may or may not happen
Modes of Reproduction
• interkinesis
– period between telophase I and prophase II
– no replication occurs
Modes of Reproduction
• meiosis II
– prophase II
• reverses telophase I
– metaphase II
• chromosomes align at equatorial plate
– anaphase II
• sister chromatids separate to poles
– telophase II
• reverses prophase
mitosis => identical
daughters
meiosis => variable
daughters
Figure 9.17
nondisjunction
at anaphase I
of meiosis
Figure 9.18
Errors of Reproduction
• meiotic errors may result in chromosomal
disorders
– aneuploidy - incorrect genetic complement
• nondisjuction
• translocation
Errors of Reproduction
• meiotic errors may result in chromosomal
disorders
– polyploidy
• 3, 4, or more sets of chromosomes
• can perform mitosis (reproduce asexually)
• 1, 3,5, etc. cannot perform meiosis
Cell Death
• death by necrosis
– damage, poison, starvation
• death by apoptosis (programmed cell death)
– discards un-needed or old cells
– signals are common in many organisms
– many cancers result from failed apoptosis
apoptosis:
programmed
cell death
Figure 9.18
cell death mechanisms
Table 9.2