1. 12
The Cell Cycle
Lecture Presentation by Cindy S. Malone, PhD, California State University Northridge

2. Please highlight all the terms in blue
Please highlight all the terms in blue. When you are done with the notes, read chapter 12.

3. Introduction to Cell Division
Cells arise through the division of preexisting cells.
There are two types of cell division:
meiosis and mitosis
Both forms of cell division are accompanied by cytokinesis.
The cytoplasm of the cell divides into two distinct daughter cells

4. Contrasting Mitosis and Meiosis
Meiosis leads to:
Production of gametes (eggs and sperm).
Daughter cells have half the amount of genetic material as the parent cell.
Mitosis leads to:
The production of all other cell types referred to as somatic cells.
Genetic material is copied and then divided equally.
Daughter cells are genetically identical to the parent cell.

5. Functions of Mitosis
Mitosis and cytokinesis are responsible for three key events in multicellular eukaryotes:
Growth
Wound repair
Asexual reproduction

6. What Is a Chromosome? DNA encodes the cell’s genetic information.
A gene is a section of DNA that encodes a specific RNA and therefore a specific protein.
Chromosomes:
Contain a single long double helix of deoxyribonucleic acid (DNA) and are wrapped around proteins.

7. Chromosomes Change before and during Mitosis
The purpose of mitosis is to distribute chromosomes to daughter cells during cell division.
Each chromosome is replicated before mitosis.
Chromosomes condense from long, thin filaments into compact structures that can be moved around the cell.
At the end of mitosis, one of the chromosome copies is distributed to each of two daughter cells.

8. Chromosome Replication
Before mitosis, each chromosome is replicated.
Each of the DNA copies in a replicated chromosome is called a chromatid.
Chromatids:
Are joined together along their entire length
As well as at a specialized region of the chromosome called the centromere.

9. Chromosome Replication
Chromatids from the same chromosome are referred to as sister chromatids.
A replicated chromosome consists of two chromatids.
It is still considered a single chromosome.

10. M Phase and Interphase Growing cells cycle between two phases:
A dividing phase called the mitotic (M) phase
A nondividing phase called interphase

11. Interphase: S Phase The cell cycle is: The orderly sequence of events
That occurs from the formation of a eukaryotic cell
Through the duplication of its chromosomes
To the time it undergoes cell division

12. Interphase: S Phase
Chromosome replication occurs only during interphase.
It does not occur during M phase.
The stage in which DNA replication occurs is called the synthesis (S) phase.

13. Interphase: Gap Phases
Interphase also includes two gap phases, during which no DNA synthesis occurs
G1 phase:
Is the first gap
Occurs before the S phase
G2 phase:
Is the second gap
Occurs between S phase and mitosis

14. Interphase: Gap Phases
During the gap phases:
Organelles replicate
Additional cytoplasm is made in preparation for cell division.
It takes a cell about 24 hours to complete one cell cycle.
G1 phase lasts 7–9 hours
S phase lasts 6–8 hours
G2 phase lasts 4–5 hours

15. The Cell Cycle There are a total of four phases in the cell cycle:
M phase
An interphase consisting of the G1, S, and G2 phases.
Gap phases allow the cell to:
Grow large enough
Synthesize enough organelles to ensure the daughter cells.
Be normal in size and function

16. (M) Mitosis Second gap First gap DNA synthesis (G1  S  G2) DIVISION
Figure 12.3
DIVISION
(M)
Mitosis
Second gap
First gap
Figure 12.3 The Cell Cycle Has Four Phases.
DNA synthesis
INTERPHASE
(G1  S  G2) 16

17. Mitosis Overview Mitosis results in:
The division of replicated chromosomes
Formation of two daughter nuclei
With identical chromosomes and genes
Mitosis is usually accompanied by cytokinesis.
Every species:
Has a characteristic number of chromosomes
Humans have 46 chromosomes

18. Chromosomes Change during the Cell Cycle
Eukaryotic chromosomes consist of DNA associated with histone proteins.
In eukaryotes, this DNA-protein material:
Is called chromatin
Is “relaxed” or uncondensed
Forms long, threadlike strands

19. Chromosomes Change during the Cell Cycle
After replication during S phase, each chromosome consists of:
Two genetically identical sister chromatids
Attached at the centromere
At the start of mitosis, the replicated chromosomes condense.

20. Events in Mitosis During mitosis:
The two sister chromatids separate to form independent chromosomes.
One copy of each chromosome goes to each of the two daughter cells.
Each daughter cell receives a copy of the genetic information that is contained in each chromosome.

21. Events in Mitosis
Mitosis (M phase) is a continuous process with five subphases based on specific events:
Prophase
Prometaphase
Metaphase
Anaphase
Telophase

22. Figure 12.4
INTERPHASE
M PHASE
Daughter cells
G1 PHASE
S PHASE
G2 PHASE
Parent cell
Parent cell
Parent cell
Sister chromatids
4 unreplicated chromosomes (chromosomes are shown partially condensed to make them visible)
4 replicated chromosomes, each consisting of two sister
chromatids
At start of mitosis, replicated chromosomes condense.
Figure 12.4 An Overview of the Cell Cycle.
During mitosis, sister chromatids separate. Two daughter cells are formed by cytokinesis. 22

23. Prophase During prophase: The mitotic spindle: Chromosomes condense
First become visible in the light microscope
The mitotic spindle:
Is made up of microtubules called spindle fibers
Forms from a microtubule-organizing center
Polar microtubules push the poles of the cell away from each other during mitosis.
Kinetochore microtubules pull chromosomes to the poles of the cell during mitosis.

24. Prophase
The microtubule-organizing center is a centrosome which contains a pair of centrioles.

25. Prometaphase During prometaphase:
The nuclear envelope breaks down
The nucleolus disappears
Kinetochore microtubules from each mitotic spindle attach to one of the sister chromatids of each chromosome.
Attachment occurs in the centromere region at the kinetochore.

26. Metaphase During metaphase:
Formation of the mitotic spindle is completed
Motor proteins on the kinetochore microtubules:
Pull each chromosome in opposite directions
Causing the chromosomes to line up in the middle of the cell.
The imaginary plane formed by metaphase is called the metaphase plate.

27. Anaphase During anaphase:
Centromeres split
Sister chromatids are pulled by the spindle fibers toward opposite poles of the cell
Replicated chromosomes split into two identical sets of unreplicated chromosomes.
As soon as they are no longer attached at the centromere sister chromatids become daughter chromosomes.

28. Telophase During telophase:
A new nuclear envelope begins to form around each set of chromosomes.
The mitotic spindle disintegrates.
The chromosomes begin to decondense.
Mitosis is complete when two independent nuclei have formed.

29. Cytokinesis Cytokinesis typically occurs immediately after mitosis.
During this process:
The cytoplasm divides to form two daughter cells.
Each daughter cell has its own nucleus and complete set of organelles.

30. 4. Metaphase: Chromosomes complete migration to middle of cell.
Figure
Sister chromatids separate; one chromosome copy goes to each daughter nucleus.
Sister chromatids
Kinetochore
Centrioles
Kinetochore microtubules
Centrosomes
Chromosomes
Early spindle apparatus
Polar microtubules
Astral microtubules
Figure Mitosis and Cytokinesis.
1. Interphase: After chromosome replication, each chromosome is composed of two sister chromatids.
2. Prophase: Chromosomes condense, and spindle apparatus begins to form.
3. Prometaphase: Nuclear envelope breaks down. Microtubules contact kinetochores.
4. Metaphase: Chromosomes complete migration to middle of cell. 30

31. 8. Cell division is complete: Two daughter cells form.
Figure
Cytoplasm is divided.
Figure Mitosis and Cytokinesis.
5. Anaphase: Sister chromatids separate into daughter chromosomes and are pulled apart.
6. Telophase: The nuclear envelope re- forms, and chromosomes de-condense.
7. Cell division begins: Actin–myosin ring causes the plasma membrane to begin pinching in.
8. Cell division is complete: Two daughter cells form. 31

32. Different Cell Types Undergo Cytokinesis Differently
Cytokinesis in plants occurs:
As vesicles are transported from the Golgi apparatus to the middle of the dividing cell.
These vesicles fuse to form a cell plate.
Cytokinesis in animals, fungi, and slime molds occurs:
When a ring of actin and myosin filaments contracts inside the cell membrane.
Causing it to pinch inward in a cleavage furrow.

33. (a) Cytokinesis in plants (b) Cytokinesis in animals
Figure 12.8
(a) Cytokinesis in plants
(b) Cytokinesis in animals
Microtubules direct vesicles to center of spindle where they fuse to divide the cell in two
Actin–myosin interactions pinch the membrane in two
Microtubule
Cell plate
Figure 12.8 The Mechanism of Cytokinesis Varies among Eukaryotes.
Cleavage furrow
5 m
100 m 33

34. Different Cell Types Undergo Cytokinesis Differently
Bacteria do not undergo cytokinesis:
Instead, they divide via fission
This is a process similar to animal cytokinesis

35. 1. DNA is copied and protein filaments attach.
Figure 12.9
New DNA
Figure 12.9 Bacterial Cells Divide but Do Not Undergo Mitosis.
Original DNA
Replication enzymes
1. DNA is copied and protein filaments attach.
2. DNA copies are separated; ring of protein forms.
3. Ring of protein draws in membrane.
4. Fission complete. 35

36. Summary Table 12.1
Summary Table 12.1 Structures Involved in Mitosis 36

37. Cell-Cycle Checkpoints
Many protein complexes are involved in regulating the cell cycle.
There are three distinct cell-cycle checkpoints:
During the four phases of the cell cycle
Interactions among regulatory molecules at each checkpoint allow a cell to “decide” whether to proceed with division.
If these regulatory molecules are defective the checkpoint may fail and cells may start dividing in an uncontrolled fashion.

38. 1. chromosomes have attached to spindle apparatus
Figure 12.12
G2 checkpoint
M-phase checkpoints
Pass checkpoint if:
Pass checkpoints if:
chromosomes have replicated successfully
DNA is undamaged
activated MPF is present
1. chromosomes have attached to spindle apparatus
2. chromosomes have properly segregated and MPF is absent
Mitosis
Second gap
First gap
Figure The Four Cell-Cycle Checkpoints.
DNA synthesis
G1 checkpoint
Pass checkpoint if:
Mature cells do not pass this checkpoint (they enter G0 state)
cell size is adequate
nutrients are sufficient
social signals are present
DNA is undamaged 38

39. Cancer: Out-of-Control Cell Division
Is a common, sometimes lethal disease
Affects many humans
Is a complex family of diseases caused by cells that:
Grow in an uncontrolled fashion
Invade nearby tissues
Spread to other sites in the body

40. Cancer: Out-of-Control Cell Division
Cancers vary widely in:
Time of onset
Growth rate
Seriousness
Cause
Despite their differences, all cancers arise from cells in which cell-cycle checkpoints have failed.

41. Please highlight all the terms in blue
Please highlight all the terms in blue. When you are done with the notes, read chapter 12.