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2016-­‐06-­‐13 •  In unicellular organisms, division of one cell
reproduces the entire organism
•  Multicellular organisms depend on cell division
for
Cell cycle & Mitosis •  Development from a fertilized cell
•  Growth
•  Repair
Review •  Cell division is part of the cell cycle, the life of
a cell from formation to its own division
Cellular Organization of the Genetic
Material
•  All the DNA in a cell is called the genome
•  A genome can consist of a single DNA
molecule (common in prokaryotic cellsbacteria) or a number of DNA molecules
(common in eukaryotic cells-membrane bound
organelles)
•  DNA molecules in a cell are packaged into
chromosomes
Figure 12.4
•  Eukaryotic chromosomes consist of chromatin
•  a complex of DNA and protein that
condenses during cell division
•  Every eukaryotic species has a characteristic
number of chromosomes in each cell nucleus
•  Somatic cells (body cells) have two (2n) sets
of chromosomes
•  Gametes (reproductive cells) have half (n) as
many chromosomes as somatic cells
Figure 12.5-3
Chromosomes
1
•  During cell division, the two sister
chromatids of each duplicated
chromosome separate and move into
two nuclei
•  Once separate, the chromatids are
called chromosomes
Chromosomal
DNA molecules
Centromere
Chromosome
arm
Chromosome duplication
(including DNA replication)
and condensation
2
Centromere
Sister
chromatids
Separation of sister
chromatids into
two chromosomes
3
Sister
chromatids
1 2016-­‐06-­‐13 Phases of the Cell Cycle
•  Eukaryotic cell division consists of
•  Mitosis, the division of the genetic material in the
nucleus
•  Cytokinesis, the division of the cytoplasm
•  Gametes are produced by a variation of cell division
called meiosis
•  Meiosis yields nonidentical daughter cells that have only
one set of chromosomes, half as many as the parent cell
•  The cell cycle consists of
•  Mitotic (M) phase (mitosis and cytokinesis)
•  Interphase (cell growth and copying of chromosomes in
preparation for cell division)
Figure 12.6
INTERPHASE
•  Interphase (about 90% of the cell cycle) can be divided into
subphases
•  The cell grows during all three phases, but chromosomes are
duplicated only during the S phase
S
(DNA synthesis)
G1
es
kin
to
Mi
to
sis
•  G1 phase ( first gap )
•  S phase ( synthesis )
•  G2 phase ( second gap )
y
is
G2
M C
(M) ITOTIC
PHA
SE
Cell Cycle Control System
•  G1 Phase •  make addi5onal cytoplasm and organelles •  S Phase •  Checkpoint = control point where stop/go signals regulate the
cell cycle
•  DNA and centrisomes replicate •  cell complement 2n •  G2 Phase •  cell growth con5nues •  enzymes and proteins needed for cell division are created © 2011 Pearson Education, Inc.
2 2016-­‐06-­‐13 Major Checkpoints
G1 Checkpoint
1.  G1 checkpoint (Most important!)
Controlled by cell size, growth factors, environment
“Go” à completes whole cell cycle
“Stop” à cell enters nondividing state (G0 Phase)
• 
• 
• 
• 
Nerve, muscle cells stay at G0; liver cells called back from G0
2.  G2 checkpoint
•  Controlled by DNA replication completion, DNA
mutations, cell size
3.  M-spindle (Metaphase) checkpoint
• 
Check spindle fiber (microtubule) attachment to chromosomes at
kinetochores (anchor sites)
M-spindle Checkpoint:
Mitotic spindle at metaphase
•  For many cells, the G1 checkpoint seems to be the most
important
•  If a cell receives a go-ahead signal at the G1 checkpoint, it will
usually complete the S, G2, and M phases and divide
•  If the cell does not receive the go-ahead signal, it will exit the
cycle, switching into a nondividing state called the G0 phase
Kinetochore = proteins associated with DNA at centromere Prophase •  Mitosis is conventionally divided into five phases
• 
• 
• 
• 
• 
Prophase
Prometaphase
Metaphase
Anaphase
Telophase
•  Cytokinesis overlaps the latter stages of mitosis
•  The chromosomes of the cell get ready to be moved around by coiling themselves up into 5ght liIle packages. (During interphase, DNA is spread throughout the nucleus of the cell in long thin strands). •  The chromosome coil up and become visible •  The nuclear membrane breaks down •  The mito5c spindle forms and aIaches to the chromosome •  The nucleoli break down and become visible © 2011 Pearson Educaton, Inc.
3 2016-­‐06-­‐13 Metaphase Anaphase • The chromosomes are tugged by the mito5c spindle un5l they are all lined up in the middle of the cell. • This phase is dis5nct with a metaphase plate. •  The replicated chromosomes are separated so that the two sister chroma5ds (iden5cal halves) from each replicated chromosome go to opposite sides. This way each new cell has one copy of each DNA molecule from the parent cell when cell division is over. Telophase •  The cell gets ready to divide into two by forming new nuclear membranes around the separate sets of chromosome. The two daughter nuclei each have a copy of every chromosome that was in the parent cell. •  New nuclear membrane form around the two sets of chromosomes •  The chromosomes uncoil and spread throughout the nucleus •  The mito5c spindle breaks down •  The nucleoli reform and become visible again •  In animal cells, cytokinesis occurs by a process known
as cleavage, forming a cleavage furrow
•  In plant cells, a cell plate forms during cytokinesis
Animation: Cytokinesis
Figure 12.12-4
Binary Fission in Bacteria
•  Prokaryotes (bacteria and archaea) reproduce by a type of cell
division called binary fission
•  In binary fission, the chromosome replicates (beginning at the
origin of replication), and the two daughter chromosomes
actively move apart
•  The plasma membrane pinches inward, dividing the cell into two
Origin of
replication
E. coli cell
1 Chromosome
replication
begins.
2 Replication
continues.
Cell wall
Plasma membrane
Bacterial chromosome
Two copies
of origin
Origin
Origin
3 Replication
finishes.
4 Two daughter
cells result.
4 2016-­‐06-­‐13 Figure 12.10a
(a) Cleavage of an animal cell (SEM)
Cleavage furrow
Contractile ring of
microfilaments
100 µm
Daughter cells
5