Transcript General Biology I (BIOLS 102)

Chapter 9: The Cell Cycle and
Cellular Reproduction (Outline)
 The Cell Cycle
 Interphase
 Mitotic Stage
 Control of the Cell Cycle
 Apoptosis
 Mitosis and Cytokinesis
 Phases of Mitosis
 Cytokinesis in Animal and Plant Cells
 Prokaryotic Cell Division
The Cell Cycle
 An orderly set of stages and substages
between one division and the next
 Just prior to the next division:
 The cell grows larger
 The number of organelles doubles
 The amount of DNA is doubled as DNA is
replicated
 The two major stages of the cell cycle:
 Interphase
 Mitotic stage
Interphase
 Most of the cell cycle is spent in interphase
 Cells perform normal functions, depending on
body location
 Interphase time varies widely
 Nerve and muscle cells are permanently arrested (i.e.
cell reaches end stage of development and no longer
divide); G0 stage
 Embryonic cells complete entire cycle in few hours
 In adult mammalian cells, interphase lasts for 20
hours (i.e. 90% of cell cycle)
 Consists of three phases; G1, S and G2
Interphase (cont.)
 G1 Phase:
 Between the end of mitosis and beginning of S phase
 Cell increases in size and doubles its organelles
 Accumulates raw materials for DNA synthesis
 S (Synthesis) Phase:
 Growth and DNA synthesis or replication occurs
 Chromosomes are duplicated with 2 identical chromatids
 G2 Phase:
 Between DNA replication and the onset of mitosis
 Protein synthesis increases in preparation for division
M (Mitotic) Stage
 Involves two main processes:
 Mitosis (karyokinesis)
 Nuclear division
 Daughter chromosomes are identical to parental
nuclei and distributed to two daughter nuclei
 Cytokinesis
 Division of the cell cytoplasm
 Results in two genetically identical daughter cells
The Cell Cycle
Control of the Cell Cycle
 Signal – an agent that influences the
activities of a cell
 Growth factors
 Signaling proteins received at the plasma
membrane
 Ensure that the stages follow one another in
the normal sequence
 Cause completion of cell cycle (even cells
arrested in G0)
Cell Cycle Checkpoints
 Internal signals
 Family of proteins called cyclins
 Cell cycle stops at the G2 checkpoint if DNA
replication not completed
 Prevents initiation of the M stage before
completion of the S stage
 Allows time for any DNA damage (i.e.
exposure to solar radiation or X-rays) to be
repaired
Cell Cycle Checkpoints (cont.)
 Signal protein p53 in mammalian cells
 Stops cycle at G1 when DNA is damaged
 Initiates DNA attempt at repair
 If successful, cycle continues to mitosis
 If not, apoptosis is initiated
 RB (retinoblastoma) protein responsible for
interpreting growth signals and nutrient
availability signals
Cell Cycle Checkpoints (cont.)
 There is also a cell cycle checkpoint that occurs
during the mitotic stage
 The cell cycle stops if the chromosomes are not
distributed accurately to the daughter cells
Apoptosis
 Often defined as programmed cell death
 Cells harbor apoptosis enzymes (caspases)
 Ordinarily held in check by inhibitors
 Can be unleashed by internal or external signals
 The sequence of events during apoptosis
 Cell rounds up and loses contact with its
neighbors
 Nucleus fragments and plasma membrane
blisters
 Cell fragments are engulfed by white blood cells
Apoptosis (cont.)
Apoptosis and Cell Division
 Mitosis and apoptosis are opposing forces
 Cell division (mitosis) increases the number of
somatic (body) cells
 Apoptosis decreases the number of somatic cells
 Example:
 Tadpole tail disappears (apoptosis) to become a frog
 Webbed fingers and toes of human embryos
disappear through apoptosis
 Both mitosis and apoptosis are normal parts of
growth and development (homeostasis)
Eukaryotic Chromosomes
 DNA in the chromosomes of eukaryotes is
 Associated with histone proteins
 In very long threads and collectively called
chromatin
 Before mitosis begins:
 Chromatin condenses (coils) into distinctly
visible chromosomes
 Each species has a characteristic chromosome
number
 Example: humans 46, corn 20, goldfish 94
Chromosome Number
 Most familiar organisms are diploid
 Have two chromosomes of each type
 Humans have 23 different types of
chromosomes
 Each type is represented twice in each body cell
(diploid)
 Only sperm and eggs have one of each type
(haploid)
 The n number for humans is n=23
 Two representatives of each type
 Makes a total of 2n=46 in each nucleus
Chromosome Duplication
 Dividing cell is called the parent
cell; the resulting cells are called
the daughter cells
 At the end of S phase:
 Each chromosome is internally
duplicated
 Consists of two identical DNA chains
 Sister chromatids
 Attached together at a single point
(centromere)
 Protein complex (kinetochores) form
on both sides of the centromere
Division of the Centrosome
 Centrosome – the main microtubule-organizing
center of the cell
 Consists of a pair of barrel-shaped
organelles – centrioles
 Present in animal cells only
 Organizes the mitotic spindle
(composed of microtubules)
 Microtubules of the cytoskeleton
disassemble when spindle fibers
form
Phases of Mitosis
 Mitosis is a continuous process that is
divided into five stages
 Prophase
 Prometaphase
 Metaphase
 Anaphase
 Telophase
Phases of Mitosis:
Prophase
 Chromatin condensed and
chromosomes are visible
 Nucleolus disappear
 Nuclear envelope fragments
 Spindle begins to assemble
 The 2 centrioles migrate in
opposite poles
 Asters -arrays of microtubules
radiate from centrioles (in
animal cells)
 Chromosomes have no
orientation, why?
Phases of Mitosis:
Prometaphase (Late Prophase)
 Preparation for separation of
sister chromatids
 Kinetochores appear on each
side of the centromere
 Kinetochores attach sister
chromatids to the kinetochore
spindle fibers → chromatids
are pulled toward opposite
poles
 Chromosomes are still not in
allignment
Phases of Mitosis:
Metaphase
 Centromeres of chromosomes
are now in alignment at the
center of the cell
 This center is called
metaphase plate
 Polar spindle – non attached
spindle fibers
 Polar fibers reach beyond
metaphase plate and overlap
Phases of Mitosis:
Anaphase
 Kinetochore spindle fibers pull
sister chromatids as they
disassemble at the kinetochore
 Two sister chromatids separate
at centromere to the opposite
poles
 Poles move further apart due to
spindle fibers sliding past one
another
 Microtuble proteins (kinesin and
dynein) are involved
 Anaphase is the shortest phase
Phases of Mitosis:
Telophase
 Spindle disappears
 New nuclear envelopes form
around the daughter
chromosomes
 Nucleolus appears in each
daughter nucleus
 Ruminants of polar spindle
fibers are still visible between
the 2 nuclei
 Chromosomes become
chromatin fibers
Cytokinesis
 Division of the cytoplasm to yield two daughter
cells
 Allocates mother cell’s cytoplasm equally to
daughter nucleus
 Encloses each in it’s own plasma membrane
 Often begins in anaphase, continues during
telophase
Cytokinesis:
Animal Cells
 During animal cytokinesis:
 A cleavage furrow appears between daughter
nuclei
 Formed by a contractile ring of actin filaments
 Like pulling on a draw string
 Narrow bridge between 2 cells during telophase
 Eventually pinches mother cell in two
Cytokinesis in Animal Cells
Cytokinesis:
Plant Cells
 In plant cells, cytokinesis begins by forming a
cell plate
 Many small membrane-bounded vesicles
originating in the Golgi complex
 Eventually fuse into one thin vesicle extending
across the mother cell
 The membranes of the cell plate become the
plasma membrane between the daughter cells
 Contents of vesicles become the middle lamella
between the two daughter cells
 Daughter cells later secrete primary cell walls on
opposite sides of middle lamella
Cytokinesis in Plant Cells
The Functions of Mitosis
 Maintenance of tissue
 Cut finger or skin
 Broken bone
 Replacement of damaged cells (i.e. accidents)
 Growth of multicellular organisms
 We all start out as a single cell…?
 Fertilized egg (zygote)
 Meristematic tissue (shoot tips) in plants
Stem Cells
 Stem cells – adult mammalian cells that retain
the ability to divide
 Red bone marrow
stem cells used for
therapeutic cloning;
used to produce
human tissue
 Embryonic stem
cells in reproductive
cloning; used to
produce new individual
Prokaryotic Cell Division
 Asexual reproduction is the formation of new
individuals identical to the original parent cell
 Prokaryotic chromosome a ring of DNA




Folded up in an area called the nucleoid
1,000 X length of cell
Replicated into two rings prior to division
Replicate rings attach to plasma membrane
 Binary fission



Splitting in two between the two replicate chromosomes
No spindle apparatus is formed
Example: E. coli living in the intestines have a
generation time of about 20 minutes!
Binary Fission of Prokaryotes
Comparing Prokaryotes and
Eukaryotes
 Both binary fission & mitosis result in daughter
cells that are identical to the parent cell
 Cellular division in unicellular organisms
produces 2 new individuals – form of asexual
reproduction
 Prokaryotes (bacteria & archea)
 Protists (algae & protozoans)
 Yeasts
 Cell division in multicellular organisms is part
of the growth process and important for
renewal & repair (e.g. plants & animals)
Cell Division and Function