THE CELL CYCLE

1
THE CELL CYCLE
2
ANNOUNCEMENTS

• Exam 1 Key is Posted on line.
• Check your answers!
• Grades will be posted by early next week (M or Tu)

3
OBJECTIVES

• Understand phases of cell cycle
• Particularly Synthesis, Mitosis, and Cytokinesis
• Know cellular events in ea. phase of Mitosis
• Understand what regulates cell cycle and when
  regulation occurs

4
THE CELL THEORY

• All organisms consist of cells and arise from
  preexisting cells

5
THE BIG PICTURE

• You and all other multicellular species got here
  through cell division
• Human fertilized egg divides 65 trillion cells

6
Sexual Reproduction
Once an egg becomes fertilized, cellular
divisions begins, eventually producing a whole
organism
All cells derived from the zygote contain the
same genetic material
An Integrated Organism
7
HOW ARE NEW CELLS MADE?

• Meiosis
• Process by which gametes are generated for
  sexual reproduction
• Mitosis
• Process by which somatic cells are created
• Some organisms may reproduce asexually by mitosis

8
AN EARLY VIEW OF MITOSIS
Paired threads
In late 1800s, scientists observed thread-like
structures in nuclei of cells about to undergo
division.
Term mitosis originated from mitos which
translates to thread in Greek.
9
MITOSIS

• Parent cell splits to form 2 identical
  (daughter) cells

• Parent cell
• original cell within body

• Daughter cells
• precise duplicates of parent cell
• (including duplicate chromosomes)

10
MITOSIS

• Occurs In All Eukaryotes
• Plants
• Occurs primarily at tips of roots and shoots in
  region called meristems
• Animals
• occurs in many cells in the body

11
AN OVERVIEW OF THE CELL CYCLE
Parent cell
Process of DNA Synthesis And Cellular Division
DNA Synthesis
Mitosis Cytokinesis
Two daughter cells
Parent daughter cells contain same complement
of chromosomes.
12
CHROMOSOMES

• Chromosomes
• Composed of DNA
• AND protein
• May include 1000s of genes
• Number of chromosomes per cell depends on
  species
• Fruit fly 8
• Horse 64
• Human 46

T- A
G -C
C-G

• Single chromosome
• DNA double helix
• Protein

13
A KARYOTYPE
14
DIFFERENCES IN DNA SEQUENCES ACCOUNT FOR VARIETY
OF LIFE ON EARTH
5
3
3
5
5
3
T
T
A
A
C
G
C
G
C
G
G
C
A
T
A
T
T
T
A
A
C
C
G
G
G
C
G
A
T
A
T
A
T
T
A
C
G
C
G
T
A
A
T
G
C
T
A
A
T
G
G
C
C
A
T
T
A
3
5
3
3
5
5
Cartoon of base pairing
Cartoon of double helix
Space-filling model of double helix
15
LIVING THINGS ARE DIVERSE!
16
THE CELL CYCLEThe Cells Time Clock

• Cells replicate for growth,
• replacement, and tissue repair
• Cell Division Requires
• Replication
• DNA cellular organelles
• Called Interphase
• Division
• Nucleus genetic material
• Called Mitosis
• Cytoplasm cell membrane
• Called Cytokinesis

17
THE CELL CYCLE

• Cell Cycle Consists of
• Interphase
• Non dividing cell
• Majority of cell cycle
• Mitosis
• Small portion of cell cycle
• In humans approx. 1/24 hr

18
INTERPHASE

• Interphase
• Cell performs normal function
• Three sub phases
• G1
• Cell duplicates most organelles
• S
• Quantity of DNA in cell is doubled
• Each chromosome now consists of a pair of sister
  chromatids
• G2
• Chemical components stockpiled
• Proteins synthesized

19
THE CELL CYCLE

• Some cells do not go through the cell cycle!
• These cells do not undergo DNA replication or
  cellular division
• Called G0

20
DNA REPLICATION (THE S PHASE)

• S Phase
• DNA replication must precede Mitosis
• Daughter cells receive same complement of
  chromosomes
• as parent

21
PRE SYNTHESIS (S PHASE)
PARENT CELL
Chromosome
Unreplicated chromosomes
Number of chromosomes 4
22
POST SYNTHESIS (S PHASE)
Chromosome
Centromere Region
PARENT CELL
Sister chromatids
Replicated chromosomes
Number of chromosomes 4
23
POST SYNTHESIS
Chromosome
A DAUGHTER CELL
Number of chromosomes 4
24
MORE SPECIFICALLY.

• DURING S PHASE
• Each strand of the chromosome is used as a
  template to produce a new double helix

25
Single chromosome (one chromatid)
Two double helices
Protein that copies DNA
Two sister chromatids

Each double helix has one original strand
SINGLE DOUBLE HELIX
26
After S-phase

• One chromosome consists of 2 sister chromatids

• Each chromatid is made of one original and one
  duplicate strand
• of DNA

centromere
2 sister chromatids
27
Cell
Before S-phase
Nucleus
After s-phase
28
GAP PHASES

• G Phases
• Gap phases separate Mitosis from S Phase
• Molecular signals mediate switch in cellular
  activity

29
MITOSIS
DIVISION

• Mitosis
• Involves separation of copied chromosomes into
  separate cells

M
Mitosis
DNA synthesis
S
INTERPHASE
30
MITOSIS

• Mitotic events categorized based on what is
  happening to cell
• Stages include
• Prophase
• Metaphase
• Anaphase
• Telophase

31
PHASES OF MITOSIS
PRIOR TO MITOSIS
MITOSIS
Chromosomes replicate,forming sister chromatids
Sister chromatids separate
Kinetochore
Early mitotic spindle
Centrosomes

• 3.Metaphase
• Chromosome
• migrate to middle
• of cell

• 2. Prophase
• Chromosomes condense
• Mitotic spindle begins forming
• Nuclear envelope breaks down

• 1. S phase
• Chromosomes replicate
• in parent cell

32
PHASES OF MITOSIS

• 6. Cytokinesis
• The cell
• divides

• 4. Anaphase
• Sister chromatids separate
• Chromosomes are pulled to
• opposite poles of cell

5. Telophase Nuclear envelope reforms
33
CYTOKINESIS IN ANIMALS

• Cytokinesis
• Furrow forms b/w
• animal cells as they split
• Cells pinch off from
• one another
• Use actin myosin

34
CYTOKINESIS IN PLANTS

• Cytokinesis
• In plants, cell wall material forms plate b/w two
  cells
• Eventually it separates them

35
BATHROOM BREAK
36
CELL CYCLE REGULATION

• Cell cycle events are triggered by cell-cycle
  control system
• Set of molecules found in cytoplasm
• Affected by internal and external controls

37
CELL CYCLE REGULATION

• Certain molecules trigger cells to enter a stage
  in the cell cycle
• Ex Mitosis

38
M phase cells (mitotic cells) induce interphase
cells to begin M phase
G1 phasecell
S phasecell
G2 phasecell
Mitotic cell
Mitotic cell
Mitotic cell
Fuse cells
Fuse cells
Fuse cells
Chromosomes from G1 phase cell condense
Chromosomes from G2 phase cell condense
Chromosomes from S phase cell condense
39
CELL CYCLE REGULATION

• What are these chemical signal molecules?
• Mitosis Protein Factors (MPF)
• Proteins trigger interphase cells to get ready
  for mitosis
• Composed of subunits
• Protein Kinase
• Protein Cyclin

40
CELL CYCLE REGULATION

• Cyclin
• Cytoplasmic protein that fluctuates in
  concentration
• Presence causes kinase proteins to trigger
  mitosis
• Protein Kinases
• Activate other proteins in cell by phosphorylation

41
Cyclin concentrations regulate MPF activity.
M phase
M phase
MPF activity
Cyclinconcentration
Time
42
Protein kinase
Targetprotein
Targetprotein
ATP
Pi
Pi
ADP
Protein kinases catalyze theaddition of a
phosphate group to a target protein.
As a result, the targetprotein is activated
ordeactivated.
43
Activated cyclin-dependent kinase has an array of
effects.
Phosphorylate chromosomal proteins initiate M
phase
Activated MPF
Phosphorylate microtubule- associated proteins.
Activate mitotic spindle?
CdK
CdK
Phosphorylate an enzyme that degrades cyclin
cyclin concentrations decline

Cyclin
Cyclin-dependent kinase
44
CELL CYCLE REGULATION

• Checkpoints
• G2
• Metaphase
• G1

45
Cell-Cycle Checkpoints
G2 checkpoint
Pass this checkpoint if cell size is
adequate chromosome replication is
successfully completed
M
Mitosis
G2
Second gap
Metaphase checkpoint
Pass this checkpoint if all chromosomes are
attached to mitotic spindle
G1
First gap
G1 checkpoint
DNA synthesis
Pass this checkpoint if cell size is
adequate nutrient availability is sufficient
growth factors (signals from other cells) are
present
S
46
WHAT IS CANCER?

• Cancer is excessive cell division during the cell
  cycle

47
CANCER

• Cell cycle checkpoints normally insure that DNA
  replication and mitosis occur only when
  conditions are favorable and the process is
  working correctly

48
CANCER

• Mutations in genes that encode cell cycle
  proteins can lead to unregulated growth,
  resulting in tumor formation and ultimately
  invasion of cancerous cells to other organs

49
CANCER SPREADS