1. Introduction to Biology
The Cell Cycle
Introduction to Biology

2. The Key Roles of Cell Division
The ability to reproduce is one of the key features that separates life from non-life.
All cells have the ability to reproduce, by making exact copies of themselves.

3. In multicellular organisms, cell division is needed for:
In unicellular organisms, division of one cell reproduces the entire organism
In multicellular organisms, cell division is needed for:
Development of an embryo from a sperm/egg
Growth
Repair

4. LE 12-2 Reproduction Growth and development Tissue renewal 100 µm

5. Asexual Reproduction
Asexual reproduction is reproduction that involves a single parent producing an offspring.
The offspring produced are, in most cases, genetically identical to the single cell that produced them.
Asexual reproduction is a simple, efficient, and effective way for an organism to produce a large number of offspring.
Prokaryotic organisms (like bacteria) reproduce asexually, as do some eukaryotes (like sponges)

6. Sexual Reproduction
In sexual reproduction, offspring are produced by the fusion of two sex cells – one from each of two parents. These fuse into a single cell before the offspring can grow.
The offspring produced inherit some genetic information from both parents.
Most animals and plants, and many single-celled organisms, reproduce sexually.

7. Contrasting Reproduction Types

8. Cell Division
Cells duplicate their genetic material before they divide, ensuring that each daughter cell receives an exact copy of the genetic material, DNA.
A dividing cell duplicates its DNA, allocates the two copies to opposite ends of the cell, and only then splits into daughter cells.

9. Cellular Organization of the Genetic Material
A cell’s endowment of DNA (its genetic information) is called its genome.
DNA molecules in a cell are packaged into chromosomes.

10. Chromosomes
The genetic information that is passed on from one generation of cells to the next is carried by chromosomes.
Every cell must copy its genetic information before cell division begins.
Each daughter cell gets its own copy of that genetic information.
Cells of every organism have a specific number of chromosomes.

11. Prokaryotic Chromosomes
Prokaryotic cells lack nuclei. Instead, their DNA molecules are found in the cytoplasm.
Most prokaryotes contain a single, circular DNA molecule, or chromosome, that contains most of the cell’s genetic information.

12. Eukaryotic Chromosomes
In eukaryotic cells, chromosomes are located in the nucleus, and are made up of chromatin.

13. Chromatin is composed of DNA and histone proteins.

14. DNA coils around histone proteins to form nucleosomes.

15. The nucleosomes interact with one another to form coils and supercoils that make up chromosomes.

16. Chromosomes During Cell Division
In preparation for cell division, DNA is replicated and the chromosomes condense
Each duplicated chromosome has two sister chromatids, which separate during cell division
The centromere is the narrow “waist” of the duplicated chromosome, where the two chromatids are most closely attached

17. LE 12-4 0.5 µm Chromosome duplication (including DNA synthesis)
Centromere
Sister
chromatids
Separation
of sister
chromatids
Centromeres
Sister chromatids

18. Phases of the Cell Cycle
The cell cycle consists of
Mitotic (M) phase (mitosis and cytokinesis)
Interphase (cell growth and copying of chromosomes in preparation for cell division)
Interphase (about 90% of the cell cycle) can be divided into subphases:
G1 phase (“first gap”)
S phase (“synthesis”)
G2 phase (“second gap”)

19. INTERPHASE S (DNA synthesis)
LE 12-5
INTERPHASE S
(DNA synthesis) G1
Cytokinesis
Mitosis G2
MITOTIC
(M) PHASE

20. G1 Phase: Cell Growth
In the G1 phase, cells increase in size and synthesize new proteins and organelles.

21. S Phase: DNA Replication
In the S (or synthesis) phase, new DNA is synthesized when the chromosomes are replicated.

22. G2 Phase: Preparing for Cell Division
In the G2 phase, many of the organelles and molecules required for cell division are produced.

23. M Phase: Cell Division
In eukaryotes, cell division occurs in two stages: mitosis and cytokinesis.
Mitosis is the division of the cell nucleus.
Cytokinesis is the division of the cytoplasm.

24. Important Cell Structures Involved in Mitosis
Chromatid – each strand of a duplicated chromosome
Centromere – the area where each pair of chromatids is joined
Centrioles – tiny structures located in the cytoplasm of animal cells that help organize the spindle
Spindle – long proteins (part of the cytoskeleton) that the centrioles produce
Helps move the chromosomes into place.

26. Prophase
During prophase, the first phase of mitosis, the duplicated chromosome condenses and becomes visible.

27. Prophase
The centrioles move to opposite sides of nucleus and help organize the spindle.

28. Prophase
The spindle forms and DNA strands attach at a point called their centromere.

29. Prophase
The nucleolus disappears and nuclear envelope breaks down.

30. Metaphase
During metaphase, the second phase of mitosis, the centromeres of the duplicated chromosomes line up across the center of the cell.

31. Metaphase
The spindle fibers connect the centromere of each chromosome to the two poles of the spindle.

32. Anaphase
During anaphase, the third phase of mitosis, the centromeres are pulled apart and the chromatids separate to become individual chromosomes.

33. Anaphase
The chromosomes separate into two groups near the poles of the spindle.

34. Telophase
During telophase, the fourth and final phase of mitosis, the chromosomes spread out into a tangle of chromatin.

35. Telophase
A nuclear envelope re-forms around each cluster of chromosomes.

36. Telophase
The spindle breaks apart, and a nucleolus becomes visible in each daughter nucleus.

37. Cytokinesis Cytokinesis is the division of the cytoplasm.
The process of cytokinesis is different in animal and plant cells.

38. Cytokinesis in Animal Cells
The cell membrane is drawn in until the cytoplasm is pinched into two equal parts.
Each part contains its own nucleus and organelles.

39. LE 12-9a
100 µm
Cleavage furrow
Contractile ring of
microfilaments
Daughter cells
Cleavage of an animal cell (SEM)

40. Cytokinesis in Animal Cells
In plants, the cell membrane is not flexible enough to draw inward because of the rigid cell wall.
Instead, a cell plate forms between the divided nuclei that develops into cell membranes.
A cell wall then forms in between the two new membranes.

41. LE 12-9b
Vesicles
forming
cell plate
Wall of
parent cell
1 µm
Cell plate
New cell wall
Daughter cells
Cell plate formation in a plant cell (TEM)

42. LE 12-10 Chromatin condensing Nucleus Chromosomes Cell plate 10 µm
Nucleolus
Prophase. The
chromatin is condensing.
The nucleolus is beginning to disappear.
Although not yet visible in the micrograph, the mitotic spindle is starting to form.
Prometaphase. We
now see discrete chromosomes; each
consists of two identical sister chromatids. Later
in prometaphase, the
nuclear envelope 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 the cell as their kinetochore micro-
tubules shorten.
Telophase. Daughter nuclei are forming. Meanwhile, cytokinesis has started: The cell plate, which will divide the cytoplasm in two, is growing toward the perimeter of the parent cell.

43. LE 12-6ca
INTERPHASE
PROPHASE
PROMETAPHASE

44. LE 12-6ca
INTERPHASE
PROPHASE
PROMETAPHASE

45. LE 12-6ca
INTERPHASE
PROPHASE
PROMETAPHASE

46. LE 12-6da
10 µm
METAPHASE
ANAPHASE
TELOPHASE AND CYTOKINESIS

47. LE 12-6da
10 µm
METAPHASE
ANAPHASE
TELOPHASE AND CYTOKINESIS

48. LE 12-6da
10 µm
METAPHASE
ANAPHASE
TELOPHASE AND CYTOKINESIS

49. Virtual Onion Root Tip Mitosis Lab
Click here to start

50. Binary Fission
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

51. Chromosome replication begins. Soon thereafter,
LE 12-11_1
Cell wall
Origin of
replication
Plasma
membrane
E. coli cell
Bacterial
chromosome
Chromosome replication begins. Soon thereafter,
one copy of the origin moves rapidly toward the other end of the cell.
Two copies
of origin

52. Chromosome replication begins. Soon thereafter,
LE 12-11_2
Cell wall
Origin of
replication
Plasma
membrane
E. coli cell
Bacterial
chromosome
Chromosome replication begins. Soon thereafter,
one copy of the origin moves rapidly toward the other end of the cell.
Two copies
of origin
Origin
Origin
Replication continues. One copy of the origin is now at each end of the cell.

53. LE 12-11_3 Cell wall Origin of replication Plasma membrane
E. coli cell
Bacterial
chromosome
Chromosome replication begins.
Soon thereafter,
one copy of the origin moves rapidly toward the other end of the cell.
Two copies
of origin
Origin
Origin
Replication continues. One copy of the origin is now at each end of the cell.
Replication finishes.
The plasma membrane grows inward, and
new cell wall is deposited.
Two daughter
cells result.

54. The Evolution of Mitosis
Since prokaryotes evolved before eukaryotes, mitosis probably evolved from binary fission
Certain protists exhibit types of cell division that seem intermediate between binary fission and mitosis

55. LE 12-12 Bacterial chromosome Prokaryotes Chromosomes Microtubules
Intact nuclear
envelope
Dinoflagellates (Type of plankton)
Kinetochore
microtubules
Intact nuclear
envelope
Diatoms (Type of Algae)
Kinetochore
microtubules
Centrosome
Fragments of
nuclear envelope
Most eukaryotes

56. The Cell Cycle Control System
The sequential events of the cell cycle are directed by a distinct cell cycle control system, which is similar to a clock
The clock has specific checkpoints where the cell cycle stops until a go-ahead signal is received

57. G1 checkpoint Control system S G1 G2 M M checkpoint G2 checkpoint
LE 12-14
G1 checkpoint
Control
system S G1 G2 M
M checkpoint
G2 checkpoint

58. For many cells, the G1 checkpoint seems to be the most important one
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

59. LE 12-15 G0
G1 checkpoint G1 G1
If a cell receives a go-ahead signal at the G1 checkpoint, the cell continues on in the cell cycle.
If a cell does not receive a go-ahead signal at the G1 checkpoint, the cell exits the cell cycle and goes into G0, a nondividing state.

60. An example of external signals is density-dependent inhibition, in which crowded cells stop dividing
Most animal cells also exhibit anchorage dependence, in which they must be attached to a substratum (connective tissue) in order to divide

61. Cells anchor to dish surface and divide (anchorage dependence).
LE 12-18a
Cells anchor to dish surface and
divide (anchorage dependence).
When cells have formed a complete
single layer, they stop dividing
(density-dependent inhibition).
If some cells are scraped away, the
remaining cells divide to fill the gap and
then stop (density-dependent inhibition).
25 µm
Normal mammalian cells

62. Cancer cells do not exhibit anchorage dependence
LE 12-18b
Cancer cells do not exhibit
anchorage dependence
or density-dependent inhibition.
25 µm
Cancer cells

63. Loss of Cell Cycle Controls in Cancer Cells
Cancer cells do not respond normally to the body’s control mechanisms
Cancer cells form tumors, masses of abnormal cells within otherwise normal tissue
If abnormal cells remain at the original site, the lump is called a benign tumor
Malignant tumors invade surrounding tissues and can metastasize, exporting cancer cells to other parts of the body, where they may form secondary tumors

64. LE 12-19 Lymph vessel Tumor Blood vessel Glandular tissue Metastatic
Cancer cell
A tumor grows from a
single cancer cell.
Cancer cells invade
neighboring tissue.
Cancer cells spread
through lymph and
blood vessels to
other parts of the
body.
A small percentage
of cancer cells may
survive and establish
a new tumor in another
part of the body.