1. Cell Cycle and Mitosis
Honors Biology

2. Why is Cell Division Important?
Unicellular organisms
Reproduce by cell division  increasing the population.
100 µm
(a) Reproduction. An amoeba, a single-celled eukaryote, is dividing into two cells. Each new cell will be an individual organism (LM).
Figure 12.2 A

3. Why Do Multicellular Organisms Depend on Cell Division?
Development & Growth
Repair (ex: tissue renewal)
Maintenance
20 µm
200 µm
(b) Growth and development This micrograph shows a sand dollar embryo shortly after the fertilized egg divided, forming two cells (LM).
(c) Tissue renewal. These dividing bone marrow cells (arrow) will give rise to new blood cells (LM).

4. Cell Division (aka Mitosis)
Makes 2 genetically identical daughter cells from 1 parent cell
Before cells divide
They duplicate their genetic material  ensures that each daughter cell receives an exact copy of the genetic material, DNA

5. What is the structure of a chromosome?
Where in a cell is the genetic material/chromosomes located?
Nucleus
Chromatin is an uncoiled mass of DNA and histone proteins
Exists in this form the majority of the time!
Histones are proteins that help DNA
condense
As a cell prepares to divide it coils
up/condenses:
We call this CHROMOSOMES (condensed
DNA)

6. DNA Molecules DNA (in nucleus of eukaryotes) can be in 2 forms
Chromatin : DNA is not tightly packed together (loosely coiled)
Occurs during interphase
Chromosomes : tightly packed together (TIGHTLY coiled)
Occurs during mitosis (cell division)

8. Genes Segments of DNA (that make up the chromosome) are called genes
A gene is a piece/segment of DNA that stores genetic information

9. What happens to chromosomes during cell division?
What needs to be done to a chromosome before it can divide?
It must DUPLICATE! (DNA Replication)
After duplication each chromosome consists of 2 identically joined copies  Sister Chromatids
Sister Chromatids are held together by centromeres
Chromatin
Chromatid
Sister
Chromatids (condensed, duplicated chromosome)
Recall that each chromosome has already been copied (s phase). Thus, the chromosomes look similar to an “X” in which the L and R chromatids are idential DNA double helixes

10. Double Chromosome Structure
Kinetochore attaches to spindle fibers
Sister

11. Chromosomes
Every eukaryotic species has a characteristic, unique # of chromosomes in EACH cell nucleus
Ex: Humans have 46 chromosomes
# of chromosomes does NOT necessarily equal complexity

12. The Cell Cycle
The mitotic phase alternates with interphase in the cell cycle
Interphasemitosisinterphasemitosis

13. What is Mitosis!?
Mitosis is the process where cells divide to produce new cells
- Occurs in healing (Ex: if you cut yourself)
New cells are also produced as you grow
- Ex: Day-to-day life (new skin cells!)
ALL eukaryotic organisms produce new cells through mitosis

14. Cell Cycle Consists of 2 broad stages
1. Growing Stage called Interphase
2. Cell Division called Mitotic Phase (M Phase)
The majority of the cell cycle (90%) is spent in Interphase

15. Phases of the Cell Cycle
INTERPHASE G1
S (DNA synthesis) G2
Cytokinesis Mitosis
MITOTIC (M) PHASE
Figure 12.5

16. Interphase can be divided into subphases
G1 phase (GAP 1 phase)
cell grows in size
varies most in length from cell to cell
S phase (synthesis phase)
DNA is copied (DNA replication)
Single  Double
Each chromosome is single
DNA replication occurs
Chromosomes have doubled  each consisting of two sister chromatids
G2 phase (GAP 2 phase)
More growth and preparation (make proteins) for mitosis

19. Mitotic Phase After Interphase, Mitotic Phase begins
Two parts of M Phase:
Mitosis (division of the nucleus)
2) Cytokinesis (division of the cytoplasm)

20. M Phase
Mitosis – the nucleus and duplicated chromosomes divide and create two identical daughter cells
Cytokinesis – the process by which the cytoplasm is divided in two.
Cytokinesis usually begins before Mitosis is completed.

21. Refresher……. The Cell Cycle: G1 phase: Growth S phase: DNA replication
G2 phase: Preparation for cell division
M phase: Mitosis and Cytokinesis

22. I Passed My Accelerated Tough Class
Remember….
Interphase
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis
IPMATC
I Passed My Accelerated Tough Class

23. Interphase Consists of G1, S, G2 Occurs BEFORE Mitosis begins
chromosome
Consists of G1, S, G2
Occurs BEFORE Mitosis begins
During S phase, the cell copies its DNA
Chromosomes appear as threadlike coils
Made of Chromatin, a combination of DNA and protein molecules
As the cell prepares to divide, its chromatin fibers condense, becoming the compact structure we call a chromosome.
Chromosomes are copied (# doubles)
Condensed, duplicated chromosome

24. Interphase
Each chromosome has now been condensed and duplicated and consists of 2 sister chromatids
The region where the two chromatids are joined tightly together is called the centromere.

25. Interphase: Animal Cell

26. Mitosis Continuous pathway (Early, Mid, & Late)
Consists of 4 phases and cytokinesis
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis

27. Prophase (X’s) “Pack Together”
First phase of Mitosis:
Chromatin becomes tightly coiled = chromosomes
DNA “packs” together
Spindle Fibers (made by the centrioles) begins to form in the cytoplasm
3. Nuclear envelope breaks down
Late: Nucleus and nucleolus disappear

28. Prophase:
Centrioles
Spindles

29. Prophase:
2. Centrioles move
DNA supercoils into chromosomes

30. Metaphase (X’s) “Meet in the Middle”
Second phase of Mitosis:
Chromosomes attach to the spindle at the centromeres
Chromosomes line up in the middle of cell
Called equatorial or metaphase plate
Spinder fibers pull and tug chromosomes to line up

32. Metaphase:

33. Anaphase (V’s) “Adios and Away”
Third phase of Mitosis:
Spindle pulls apart chromosomes
SISTER CHROMATIDS separate athe the centromere and begin moving to opposite ends (poles) of the cell
Each chromatid is now considered its own chromosome

35. ***Remember that each chromatid has the same DNA so each is now its own chromosome***
Anaphase:

36. Telophase (V’s) “Two New Cells”
Fourth phase of Mitosis:
Chromosomes reach end of spindle
Spindle breaks down (disappear)
Cleavage furrow begins to form
4. Nuclear membrane begins to reform
5. 2 daughter nuclei
6. Chromosomes  chromatin

37. Telophase:
Spindle fall apart
Cleavage furrow

38. Cytokinesis “Division of the Cytoplasm”
Occurs in Late telophase
In animal cells
a cleavage furrow forms, which pinches the cell in two.
In plant cells
produce a cell plate at the middle of the cell
At the end of cytokinesis, there are two distinct IDENTICAL daughter cells.

39. Cytokinesis Final Phase of Cell Division/M Phase
Cleavage furrow pinches all the way through
Result is two new cells
2 cells then enter  Mitosis begins again!
G1, S, G2 (Interphase)
PMAT & Cytokinesis
Each new cell at the end of mitosis is DIPLOID
has a full set of chromosomes

40. Cytokinesis: A Closer Look
Cleavage furrow
Contractile ring of microfilaments
Daughter cells
100 µm
(a) Cleavage of an animal cell (SEM)
Figure 12.9 A
In animal cells
Cytokinesis occurs by a process known as cleavage, forming a cleavage furrow

41. In plant cells, during cytokinesis A cell plate forms
Daughter cells
1 µm
Vesicles forming cell plate
Wall of patent cell
Cell plate
New cell wall
(b) Cell plate formation in a plant cell (SEM)
Figure 12.9 B

42. Figure 12.6 G2 OF INTERPHASE PROPHASE PROMETAPHASE
Centrosomes (with centriole pairs)
Chromatin (duplicated)
Early mitotic spindle
Aster
Centromere
Fragments of nuclear envelope
Kinetochore
Nucleolus
Nuclear envelope
Plasma membrane
Chromosome, consisting of two sister chromatids
Kinetochore microtubule
Figure 12.6
Nonkinetochore microtubules

43. Figure 12.6 Centrosome at one spindle pole Daughter chromosomes
METAPHASE
ANAPHASE
TELOPHASE AND CYTOKINESIS
Spindle
Metaphase plate
Nucleolus forming
Cleavage furrow
Nuclear envelope forming
Figure 12.6

45. Mitosis in a plant cell Nucleus Chromatine condensing Chromosome1
Prophase. The chromatin is condensing. The nucleolus is beginning to disappear. Although not yet visible in the micrograph, the mitotic spindle is staring to from.
Prometaphase. We now see discrete chromosomes; each consists of two identical sister chromatids. Later in prometaphase, the nuclear envelop will fragment.
Metaphase. The spindle is complete, and the chromosomes, attached to microtubules at their kinetochores, are all at the metaphase plate.
Anaphase. The chromatids of each chromosome have separated, and the daughter chromosomes are moving to the ends of cell as their kinetochore microtubles shorten.
Telophase. Daughter nuclei are forming. Meanwhile, cytokinesis has started: The cell plate, which will divided the cytoplasm in two, is growing toward the perimeter of the parent cell. 2 3 4 5
Nucleus
Nucleolus
Chromosome
Chromatine condensing
Figure 12.10

46. Cell Cycle and Mitosis Animations