1. Chapter 9: The Cell Cycle and Cellular Reproduction (Outline)
Interphase
Mitotic Stage
Control of the Cell Cycle
Apoptosis
Mitosis and Cytokinesis
Phases of Mitosis
Cytokinesis in Animal and Plant Cells
Prokaryotic Cell Division

2. 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

3. 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

4. Interphase (cont.) G1 Phase: S (Synthesis) Phase: G2 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

5. 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

6. The Cell Cycle

7. 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)

8. 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

9. 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

10. 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

11. Apoptosis The sequence of events during 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

12. Apoptosis (cont.)

13. 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)

14. 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

15. 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

16. 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

17. 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

18. Phases of Mitosis
Mitosis is a continuous process that is divided into five stages
Prophase
Prometaphase
Metaphase
Anaphase
Telophase

19. 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?

20. 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

21. 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

22. 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

23. 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

26. 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

27. 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

28. Cytokinesis in Animal Cells

29. 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

30. Cytokinesis in Plant Cells

31. 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

32. 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

33. 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!

34. Binary Fission of Prokaryotes

35. 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)

36. Cell Division and Function