University students

1. CYTOLOGY
Dr. Abdiaziiz
(MBBS, BSc PH, MPH, EMERGENECY MEDICINE RESIDENCE)
1

2. • INTRODUCTION
2

3. An Introduction to Cells
 Cell theory
• Cells are the building blocks of all organisms
• All cells come from the division of preexisting cells
• Cells are the smallest units that perform all vital
physiological functions
• Each cell maintains homeostasis at the cellular level
3

4. An Introduction to Cells
 Cytology is a branch of cell biology
• The study of cells
• Sex cells (germ cells or reproductive cells)
• Male sperm
• Female oocytes (cells that develop into ova)
• Somatic cells
• All body cells except sex cells
4

5. Structure of the cell
5

6. Figure 3–1 Anatomy of a Model
Cell (Part 1 of 7).
6
= Plasma membrane
= Nonmembranous organelles
= Membranous organelles
Secretory vesicles
Centrosome and Centrioles
Cytoplasm containing two centrioles at
right angles; each centriole is composed
of 9 microtubule triplets in a 9 + 0 array
Functions
Essential for
movement of
chromosomes
during cell division;
organization of
microtubules in
cytoskeleton
Centrosome
CYTOSOL
Centrioles
NUCLEUS

7. Figure 3–1 Anatomy of a Model
Cell (Part 2 of 7).
7
= Plasma membrane
= Nonmembranous organelles
Cytoskeleton
Proteins organized in
fine filaments or
slender tubes
Functions
Strength and
support; movement
of cellular structures
and materials
Microfilament
= Membranous organelles
Microtubule
Plasma Membrane
Lipid bilayer containing phospholipids,
steroids, proteins, and carbohydrates
Functions
Isolation;
protection;
sensitivity;
support;
controls entry
and exit of
materials
Free
ribosomes
Cytosol (distributes
materials by diffusion)

8. Figure 3–1 Anatomy of a Model
Cell (Part 3 of 7).
8
Microvilli
Microvilli are extensions of the plasma
membrane containing microfilaments.
Function
Increase surface area to
facilitate absorption of
extracellular materials
= Plasma membrane
= Nonmembranous organelles
= Membranous organelles

9. 9
Cilia
Cilia are long extensions of the
plasma membrane containing
microtubules. There are two
types: primary and motile.
= Plasma membrane
= Nonmembranous organelles
= Membranous organelles
Functions
A primary cilium acts as a
sensor. Motile cilia move
materials over cell surfaces
Proteasomes
Hollow cylinders of proteolytic
enzymes with regulatory
proteins at their ends
Functions
Breakdown and recycling
of damaged or abnormal
intracellular proteins
Ribosomes
RNA + proteins; fixed ribosomes bound
to rough endoplasmic reticulum; free
ribosomes scattered
in cytoplasm
Function
Protein synthesis

10. Figure 3–1 Anatomy of a Model
Cell (Part 5 of 7).
10
Golgi apparatus
Stacks of flattened membranes
(cisternae) containing chambers
Functions
Storage, alteration, and
packaging of secretory
products and lysosomal
enzymes
Mitochondria
Double membrane, with
inner membrane folds
(cristae) enclosing important
metabolic enzymes
Function
Produce 95% of the ATP
required by the cell
Endoplasmic reticulum (ER)
NUCLEUS
Network of membranous
channels extending
throughout the cytoplasm
Functions
Synthesis of secretory
products; intracellular
storage and transport;
detoxification of drugs or
toxins
Rough ER has
ribosomes,
and it modifies and
packages newly
synthesized proteins
SmoothER does not
have ribosomes and
synthesizes lipids
and carbohydrates
= Plasma membrane
= Nonmembranous organelles
= Membranous organelles

11. Figure 3–1 Anatomy of a Model
Cell (Part 6 of 7).
11
= Plasma membrane
= Nonmembranous organelles
= Membranous organelles
Peroxisomes
Vesicles containing
degradative enzymes
Functions
Catabolism of fats and
other organic compounds;
neutralization of toxic
compounds generated in
the process
Free
ribosomes
Lysosomes
Vesicles containing
digestive enzymes
Function
Intracellular removal
of damaged organelles
or pathogens

12. Figure 3–1 Anatomy of a Model
Cell (Part 7 of 7).
12
Chromatin
Nuclear
envelope
Nucleolus
(site of rRNA
synthesis and
assembly of
ribosomal
subunits)
NUCLEOPLASM
NUCLEUS
Nucleoplasm containing
nucleotides, enzymes,
nucleoproteins, and
chromatin; surrounded
by a double membrane,
the nuclear envelope
Functions
Control of metabolism;
storage and processing
of genetic information;
control of protein
synthesis
Nuclear
pore

13. 3-1 Plasma Membrane
 Extracellular fluid (interstitial fluid)
• A watery medium that surrounds a cell
 Plasma membrane (cell membrane) separates cytoplasm from
the extracellular fluid
13

14. 3-1 Plasma Membrane
 Functions of the plasma membrane
• Physical isolation
• Barrier
• Regulation of exchange with the environment
• Ions and nutrients enter
• Wastes eliminated and cellular products released
• Sensitivity to the environment
• Extracellular fluid composition and chemical signals
• Structural support
• Anchors cells and tissues
14

15. 3-1 Plasma Membrane
 Membrane lipids
• Phospholipid bilayer
• Hydrophilic heads—face outward on both sides, toward
watery environments
• Hydrophobic fatty-acid tails—inside membrane
• Barrier to ions and water-soluble compounds
15

16. 3-1 Plasma Membrane
 Membrane proteins
• Integral proteins
• Within the membrane
• Peripheral proteins
• Bound to inner or outer surface of the membrane
16

17. 3-1 Plasma Membrane
 Membrane carbohydrates
• Proteoglycans, glycoproteins, and glycolipids
• Extend outside cell membrane
• Form sticky “sugar coat” (glycocalyx)
• Functions of the glycocalyx
• Lubrication and protection
• Anchoring and locomotion
• Specificity in binding (function as receptors)
• Recognition (immune response)
17

18. Figure 3–2 The Plasma
Membrane.
18
Hydrophilic
heads
Hydrophobic
tails
Cholesterol
EXTRACELLULAR FLUID
Glycolipids
of glycocalyx
Plasma
membrane
Integral
protein
with channel
The phospholipid bilayer
Phospholipid
bilayer
Hydrophilic
heads
Hydrophobic
tails
Cholesterol
Gated channel
CYTOPLASM
Peripheral
proteins
= 2 nm
Integral
glycoproteins
Cytoskeleton
(Microfilaments)
The plasma membrane
b
a
a
b

19. 19
Organelles

20. 3-2 Organelles within the Cytoplasm
 Cytoplasm
• All materials inside the cell, outside of the nucleus
• Cytosol (intracellular fluid)
• Contains dissolved materials
• Nutrients, ions, proteins, and waste products
• High protein and potassium levels
• Low carbohydrate, lipid, amino acid, and sodium
levels
• Organelles
• Structures with specific functions
20

21. 3-2 Organelles within the Cytoplasm
 Organelles
• Nonmembranous organelles
• No membrane
• Direct contact with cytosol
• Include the cytoskeleton, centrioles, ribosomes,
proteasomes, microvilli, cilia, and flagella
• Membranous organelles
• Isolated from cytosol by a plasma membrane
• Endoplasmic reticulum (ER), the Golgi apparatus,
lysosomes, peroxisomes, and mitochondria
21

22. 3-2 Organelles within the Cytoplasm
 Inclusions
• Masses of insoluble materials in cells
• Some consist of glycogen or lipid droplets
 Cytoskeleton
• Structural proteins for shape and strength
• Microfilaments
• Intermediate filaments
• Microtubules
22

23. 3-2 Organelles within the Cytoplasm
 Cytoskeleton
• Microfilaments—thin filaments composed of the protein
actin
• Provide mechanical strength
• Interact with other proteins to adjust consistency of
cytosol
• Interact with thick filaments of myosin for muscle
contraction
23

24. 3-2 Organelles within the Cytoplasm
 Cytoskeleton
• Intermediate filaments—mid-sized between
microfilaments and microtubules
• Durable
• Strengthen the cell and maintain its shape
• Stabilize position of organelles
• Stabilize cell position
24

25. 3-2 Organelles within the Cytoplasm
 Cytoskeleton
• Microtubules—large, hollow tubes of tubulin proteins
• Attach to centrosome
• Strengthen cell and anchor organelles
• Change cell shape
• Move organelles within the cell with the help of motor
proteins (kinesin and dynein)
• Form spindle apparatus to distribute chromosomes
• Form centrioles and cilia of organelles
25

26. Figure 3–3a The Cytoskeleton.
26
Microvillus
Microfilaments
Plasma membrane
Terminal web
Mitochondrion
Intermediate
filaments
Endoplasmic
reticulum
Secretory
vesicle
The cytoskeleton provides strength and structural support
for the cell and its organelles. Interactions between
cytoskeletal components are also important in moving
organelles and in changing the shape of the cell.
a
a
Microtubule

27. Figure 3–3b The Cytoskeleton.
27
Microvillus
Microfilaments
Terminal web
Microfilaments and
microvilli
SEM × 30,000
The microfilaments and microvilli
of an intestinal cell. Such an image,
produced by a scanning electron
microscope, is called a scanning
electron micrograph (SEM).
b
b

28. Figure 3–3c The Cytoskeleton.
28
Microfilaments and
microvilli
LM × 3200
Microtubules (yellow) in living cells,
as seen in a light micrograph (LM)
after special fluorescent labeling.
c
c

29. 3-2 Organelles within the Cytoplasm
 Microvilli
• Increase surface area for absorption
• Attach to cytoskeleton
 Centrioles
• Form spindle apparatus during cell division
• Centrosome—cytoplasm next to the nucleus that
surrounds centrioles
29

30. 3-2 Organelles within the Cytoplasm
 Cilia (singular, cilium)
• Slender extensions of plasma membrane
• Move fluids across the cell surface
• A primary cilium is nonmotile
• Found on a variety of cells
• Senses environmental stimuli
• Motile cilia are found on cells lining the respiratory and
reproductive tracts
• Microtubules in cilia are anchored to a basal body
 Flagellum is whip-like extension of cell membrane
30

31. Figure 3–4a Centrioles and Cilia.
31
Microtubules
a Centriole. A centriole consists of nine microtubule
triplets (known as a 9 + 0 array).
A pair of centrioles oriented at right angles to one
another occupies the centrosome. This micrograph,
produced by a transmission electron microscope, is
called a TEM.
Centriole TEM × 185,000

32. Figure 3–4b Centrioles and Cilia.
32
Plasma
membrane
Basalbody
b Motile cilium. A motile cilium contains nine pairs of
microtubules surrounding a central pair (9 + 2 array). The
basal body to which it is anchored has a microtubule array
similar to that of a centriole.
Microtubules

33. Figure 3–4c Centrioles and Cilia.
33
Power stroke
c Ciliary movement. Action of a single motile
cilium. During the power stroke, the cilium is
relatively stiff. During the return stroke, it bends and
returns to its original position.
c
Returnstroke

34. 3-2 Organelles within the Cytoplasm
 Ribosomes—organelles that synthesize proteins
• Composed of small and large ribosomal subunits
• Contain ribosomal RNA (rRNA)
• Free ribosomes in cytoplasm
• Manufacture proteins that enter cytosol directly
• Fixed ribosomes are attached to ER
• Manufacture proteins that enter ER for packaging
 Proteasomes
• Organelles that contain enzymes (proteases)
• Disassemble damaged proteins for recycling
34

35. 3-2 Organelles within the Cytoplasm
 Five types of membranous organelles
1. Endoplasmic reticulum (ER)
2. Golgi apparatus
3. Lysosomes
4. Peroxisomes
5. Mitochondria
35

36. 3-2 Organelles within the Cytoplasm
 Endoplasmic reticulum (ER)
• Contains storage chambers known as cisternae
• Functions
1. Synthesis of proteins, carbohydrates, and lipids
2. Storage of synthesized molecules and materials
3. Transport of materials within the ER
4. Detoxification of drugs or toxins
36

37. 3-2 Organelles within the Cytoplasm
 Smooth endoplasmic reticulum (SER)
• No attached ribosomes
• Synthesizes
• Phospholipids and cholesterol (for membranes)
• Steroid hormones (for reproductive system)
• Glycerides (for storage in liver and fat cells)
• Glycogen (for storage in muscle and liver cells)
37

38. 3-2 Organelles within the Cytoplasm
 Rough endoplasmic reticulum (RER)
• Surface covered with ribosomes
• Active in protein and glycoprotein synthesis
• Folds proteins into secondary and tertiary structures
• Encloses products in transport vesicles for delivery to
Golgi apparatus
38

39. Figure 3–5a The Endoplasmic
Reticulum.
39
Ribosomes
Rough endoplasmic
reticulum with fixed
(attached) ribosomes
Cisternae
Nucleus
Smooth
endoplasmic
reticulum
The three-dimensional relationships between the rough
and smooth endoplasmic reticula are shown here.
a
a

40. Figure 3–5b The Endoplasmic
Reticulum.
40
Endoplasmic
reticulum
TEM × 111,000
Rough endoplasmic
reticulum and free
ribosomes in the
cytoplasm of a cell.
b
Rough endoplasmic
reticulum with fixed
(attached) ribosomes
Free
ribosomes
Smooth
endoplasmic
reticulum

41. 3-2 Organelles within the Cytoplasm
 Golgi apparatus (Golgi complex)
• Vesicles enter forming face and exit maturing face
• Functions
1. Modifies and packages secretions
• Such as hormones or enzymes, for release from cell
2. Adds or removes carbohydrates to or from proteins
3. Renews or modifies the plasma membrane
4. Packages special enzymes within vesicles (lysosomes)
for use in the cytoplasm
41

42. Figure 3–6a The Golgi Apparatus.
42
Secretory
vesicles
Secretory
product
This is a three-dimensional view of
the Golgi apparatus with a cut edge.
Transport
vesicles
a

43. Figure 3–6b The Golgi Apparatus.
43
Golgi apparatus TEM × 42,000
This is a sectional view of the Golgi
apparatus of an active secretory cell.
b

44. Figure 3–7 Protein Synthesis,
Processing, and Packaging (Part 1 of
2).
44

45. Figure 3–7 Protein Synthesis,
Processing, and Packaging (Part 2 of
2).
45
The transport
vesicles carry
the proteins and
glycoproteins
generated in the ER
toward the Golgi
apparatus. The
transport vesicles
then fuse to create
the forming cis face
(“receiving side”) of
the Golgi apparatus.
Multiple transport
vesicles combine to form
cisternae on the cis face.
Further protein and
glycoproteinmodification
and packaging occur as the
cisternae move toward the
maturing (trans) face. Small
transport vesicles return
resident Golgi proteins to the
forming cis face for reuse.
The maturing trans
face (“shipping side”)
generates vesicles that
carry modified proteins
away from the Golgi
apparatus. One type of
vesicle becomes a
lysosome, which contains
digestive enzymes.
Two other types of
vesicles proceedto
the plasma membrane:
secretory and membrane
renewal. Secretory
vesicles fuse with the
plasma membraneand
empty their products
outsidethe cell by
exocytosis.
Membrane renewal
vesicles add new
lipids and proteins to
the plasma membrane.
Cisternae
7
Lysosome Secretory
vesicle
5 6 TEM × 175,000
Exocytosis of secretory
molecules at cell surface
Forming
(cis) face
Maturing
(trans) face
Membrane
renewal
vesicle
7
Membrane
renewal
Secretory
vesicle
8
8
6
5 6 7
5 8
7
6
5
8

46. Figure 3–7 Protein Synthesis,
Processing, and Packaging (Part 1 of
9).
46
Protein synthesis
begins when a gene
on DNA produces
messenger RNA (mRNA),
the template for protein
synthesis.
Ribosome
DNA
1
mRNA
Nucleus
1
5
1

47. Figure 3–7 Protein Synthesis,
Processing, and Packaging (Part 2 of
9).
47
2
The mRNA leaves
the nucleus and
attaches to a free
ribosome in the
cytoplasm, or a fixed
ribosome on the RER.
Ribosome
Rough ER
mRNA
2
Cytosol
2
5
2

48. Figure 3–7 Protein Synthesis,
Processing, and Packaging (Part 3 of
9).
48
3
Proteins
constructed on
free ribosomes are
released into the
cytosol for use within
the cell.
Protein
released into
cytosol
Ribosome
3
3
3

49. Figure 3–7 Protein Synthesis,
Processing, and Packaging (Part 4 of
9).
49
Protein synthesis
on fixed ribosomes
occurs at the RER. The
newly synthesized
protein folds into its
three-dimensional shape.
Rough ER
Cytosol
4
4
4
4

50. Figure 3–7 Protein Synthesis,
Processing, and Packaging (Part 5 of
9).
50
The proteins are
then modified
within the ER. Regions
of the ER then bud off,
forming transport
vesicles containing
modified proteins and
glycoproteins.
Transport
vesicle
5
5
5
5

51. Figure 3–7 Protein Synthesis,
Processing, and Packaging (Part 6 of
9).
51
The transport vesicles
carry the proteins and
glycoproteins generated in
the ER toward the Golgi
apparatus. The transport
vesicles then fuse to create
the forming cis face
(“receiving side”) of the
Golgi apparatus.
Cisternae
Forming
(cis) face
6
6
6
6

52. Figure 3–7 Protein Synthesis,
Processing, and Packaging (Part 7 of
9).
52
Cisternae
Multiple transport vesicleson
combine to form cisternae on
the cis face. Further protein and
glycoprotein modification and
packaging occur as the cisternae
move toward the maturing (trans)
face. Small transport vesicles return
resident Golgi proteins to the
forming cis face for reuse.
Forming
(cis) face
Maturing
(trans) face
7
7
7
7

53. Figure 3–7 Protein Synthesis,
Processing, and Packaging (Part 8 of
9).
53
The maturing trans
face (“shipping side”)
generates vesicles that
carry modified proteins
away from the Golgi
apparatus. One type of
vesicle becomes a
lysosome, which contains
digestive enzymes.
8
Lysosome
Maturing
(trans) face
8
8
8

54. Figure 3–7 Protein Synthesis,
Processing, and Packaging (Part 9 of
9).
54
Two other types of
vesicles proceed to the
plasma membrane: secretory
and membrane renewal.
Secretory vesicles fuse
with the plasma membrane
and empty their products
outside the cell by
exocytosis. Membrane
renewal vesicles add new
lipids and proteins to the
plasma membrane.
Lysosome Secretory
vesicle
TEM × 175,000
Secretory
vesicle
Maturing
(trans) face
Membrane
renewal
vesicle
9
Exocytosis of secretory
molecules at cell surface
Membrane
renewal
9
9
9

55. 3-2 Organelles within the Cytoplasm
 Lysosomes
• Powerful enzyme-containing vesicles produced by Golgi
apparatus
• Primary lysosomes
• Contain inactive enzymes
• Secondary lysosomes
• Formed when primary lysosomes fuse with damaged
organelles and enzymes are activated
• Function to destroy bacteria, break down molecules, and
recycle damaged organelles
55

56. Figure 3–8 Lysosome Functions.
56
Golgi apparatus
Damaged
organelle
Reabsorption
Secondary
lysosome
Primary
lysosome
Autolysis
liberates
digestive
enzymes
Endosome
Secondary
lysosome
Extracellular solid or fluid
Reabsorption
Exocytosis
ejects residue
Endocytosis Exocytosis
ejects residue
Lysosome activation occurs when:
A primary lysosome
fuses with the
membrane of another
organelle, such as a
mitochondrion
A primary lysosome
fuses with an
endosome containing
fluid or solid materials
from outside the cell
The lysosomal
membrane breaks
down during autolysis
following cellular
injury or death
1
2
3
2
1 3

57. 3-2 Organelles within the Cytoplasm
 Autolysis
• Self-destruction of damaged or inactive cells
• Lysosome membranes break down
• Digestive enzymes released
• Cell is destroyed
• Cellular materials are recycled
57

58. 3-2 Organelles within the Cytoplasm
 Peroxisomes
• Small, enzyme-containing vesicles
• Produced by division of existing peroxisomes
• Break down organic compounds such as fatty acids
• Produce the free radical hydrogen peroxide (H2O2)
• Catalase converts H2O2 to oxygen and water
58

59. 3-2 Organelles within the Cytoplasm
 Mitochondria
• Smooth outer membrane
• Inner membrane has numerous folds (cristae)
• Cristae surround fluid contents (matrix)
• Take chemical energy from food (glucose)
• Produce the energy molecule ATP
59

60. 3-2 Organelles within the Cytoplasm
 Mitochondrial energy production
• Glycolysis
• Glucose to pyruvic acid (in cytosol)
• Mitochondria absorb pyruvate molecules
• Citric acid cycle (Krebs cycle, tricarboxylic acid cycle, or TCA
cycle)
• Occurs in mitochondrial matrix
• Breaks down pyruvate
• Electron transport chain
• Inner mitochondrial membrane
60

61. 3-2 Organelles within the Cytoplasm
 Mitochondrial energy production
• Aerobic metabolism (cellular respiration)
• Mitochondria use oxygen to break down food and
produce ATP
• Produces 95 percent of ATP needed to keep a cell alive
• Glucose + oxygen + ADP → carbon dioxide + water +
ATP
61

62. Figure 3–9a Mitochondria.
62
Inner membrane
Organic molecules
and O2
Outer
membrane
CO2
ATP
Matrix Enzymes
Matrix
Mitochondrial
ribosomes
Outer
membrane
Mitochondrion TEM × 46,332
This is the three-dimensional organization and a color-enhanced
TEM of a typical mitochondrion in longitudinal section.
Cytosol
Cristae
Cristae
a

63. Figure 3–9b Mitochondria.
63
CYTOSOL Glucose
Glycolysis
CO2
Pyruvate
ATP
ATP
ADP +
phosphate
H+
Citric
acid
cycle
MATRIX
Enzymes
and
coenzymes
of cristae
O2
MITOCHONDRION
H2O
This is an overview of the role of mitochondria in energy
production. Mitochondria absorb oxygen and short carbon
chains, such as pyruvate, and they generate carbon dioxide,
ATP, and water.
b

64. 3-2 Organelles within the Cytoplasm
 Membrane flow (membrane trafficking)
• A continuous exchange of membrane segments by vesicles
• Involves all membranous organelles (except
mitochondria)
• Allows adaptation and change
64

65. 3-3 Cell Nucleus
 Nucleus
• Largest organelle
• The cell’s control center
• Nuclear envelope
• Double membrane around the nucleus
• Perinuclear space
• Between the two layers of the nuclear envelope
• Nuclear pores
• Communication passages in nuclear envelope
65

66. Figure 3–10a The Nucleus.
66
Nucleoplasm
Chromatin
Nucleolus
Nuclear envelope
Nuclear pore
Nucleus TEM × 8000
Important nuclear structures
are shown here.
a

67. Figure 3–10b The Nucleus.
67
Nuclear pore
Perinuclear space
Nuclear envelope
A nuclear pore is a large
protein complex that spans
the nuclear envelope.
b

68. Figure 3–10c The Nucleus.
68
Nuclear pores
Inner membrane of
nuclear envelope
Broken edge of
outer membrane
Outer membrane of
nuclear envelope
Nuclear envelope Freeze fracture SEM × 9240
This cell was frozen and then broken apart to make the double
membrane of its nuclear envelope visible. The technique, called
freeze fracture or freeze-etching, provides a unique perspective
on the internal organization of cells. The nuclear envelope and
nuclear pores are visible. The fracturing process broke away part
of the outer membrane of the nuclear envelope, and the broken
edge of this membrane can be seen.
c

69. 3-3 Cell Nucleus
 Contents of the nucleus
• Nuclear matrix in the nucleoplasm
• Support filaments
• Nucleoli
• Nuclear organelles
• Synthesize rRNA and assemble ribosomal subunits
• Made of RNA, enzymes, and histones
• Nucleosome—DNA coiled around histones
• Loosely coiled into chromatin in non-dividing cells
• Tightly coiled chromosomes form before division
69

70. Figure 3–11 The Organization of DNA
within the Nucleus.
70
Nucleus Sister chromatids*
Kinetochore*
Supercoiled
region
Centromere*
Cell prepared
for division Visible
chromosome
Nondividing
cell
Chromatin in
nucleus
DNA
double
helix
Nucleosome
Histones

71. 3-3 Cell Nucleus
 Information storage in the nucleus
• Genetic code
• Chemical language of DNA instructions
• Sequence of bases (A, T, C, G)
• Triplet code
• Three bases = one amino acid
• Gene
• DNA instructions for one protein
• Functional unit of heredity
71

72. 3-4 Protein Synthesis
 Protein synthesis
• Assembling of functional polypeptides in the cytoplasm
 Gene activation
• Uncoiling DNA and temporarily removing histones
 Transcription
• Synthesis of RNA from DNA template
• All RNA, including messenger RNA (mRNA), is formed
through transcription of DNA
72

73. 3-4 Protein Synthesis
 Two strands of DNA
• Coding strand specifies the sequence of amino acids in
polypeptides
• Template strand is used for mRNA production
73

74. 3-4 Protein Synthesis
 Process of transcription
1. RNA polymerase binding
2. RNA polymerase nucleotide linking
• Begins at “start” signal in promoter region
• Reads DNA code
• Binds nucleotides to form mRNA in three-base
sequences known as codons
3. Detachment of mRNA
• Enzyme and mRNA strand detach from DNA at “stop”
signal
74

75. 3-4 Protein Synthesis
 RNA processing
• mRNA is “edited” before leaving nucleus
• Noncoding sequences (introns) are removed
• Coding segments (exons) are spliced together
75

76. Figure 3–12 mRNA Transcription.
76
DNA
C
T
T
A
C
T
C
A
T
G
C
C
G
A
G
C
T
A
A
1
G
A
A
T
G
A
G
T
A
2
G
A
A
T
G
C
3
G
A
A
T
G
A
G
Template
strand T
Coding
strand
RNA
polymerase
Codon
1
U mRNA
strand
Promoter A
G
T
A
C
G
Codon
2
T
Complementary
triplets
Gene C
G
G
C
T
C
G
A
T
T
Triplet 1 1
2
G
C
C
G
A
A
C
G
G
C
T
C
G
A
T
T
G
C
C
Codon
1
G
Codon
3
Triplet 2 G
C
T
C
G
A
T
T
2
Codon 4
(stop codon)
Triplet 3 3
4
RNA
nucleotide
G
Triplet 4
RNA
polymerase
KEY
After transcription, the two DNA strands reassociate
A
G
Adenine
Guanine
Cytosine
U
T
Uracil (RNA)
Thymine (DNA)
C

77. 3-4 Protein Synthesis
 Translation
• After leaving nucleus, mRNA binds to ribosomal subunits in
cytoplasm
• Each mRNA codon translates to one amino acid
• Amino acids are delivered by transfer RNA (tRNA)
• A tRNA anticodon binds to a complementary mRNA codon
• Enzymes join amino acids with peptide bonds
• At stop codon, components separate
77

78. Table 3–1 Examples of the
Genetic Code
78

79. Figure 3–13 The Process of
Translation (Part 1 of 5).
79
NUCLEUS
DNA
mRNA
1 Binding of Small Ribosomal
Subunit to mRNA
1 First amino acid
(methionine)
Transfer RNA
(tRNA)
Anticodon
Small ribosomal
subunit
Start codon
KEY
A
G
C
U
mRNA strand
Adenine
Guanine
Cytosine
Uracil
Translation begins with the small
ribosomal unit binding to an mRNA
strand. An initiator tRNA carrying a
specific amino acid, methionine,
binds to the mRNA “start” codon
AUG.
A

80. Figure 3–13 The Process of
Translation (Part 2 of 5).
80
2 Formation of Functional Ribosome
tRNA
binding sites
(E, P, and A)
Large
ribosomal
subunit
1
2
E P A
The small and large ribosomal units join
together and enclose the mRNA and tRNA.
The large ribosomal subunit has three sites
for tRNA binding, called the E site (Exit), P
site (Polypeptide building), and A site
(Arrival).
A

81. Figure 3–13 The Process of
Translation (Part 3 of 5).
81
3 Formation of Peptide
Bond
Peptide
bond
1
2
E
U C G
A second tRNA then arrives at the A
site of the ribosome, carrying amino
acid 2. Its anticodon binds to the
second codon of the mRNA strand.
Ribosomal enzymes now remove
amino acid 1 from the first tRNA in
the P site and attach it to amino
acid 2 with a peptide bond.
G
G G
C
C
C
U

82. Figure 3–13 The Process of
Translation (Part 4 of 5).
82
4 Extension of Polypeptide
3
1
2
E A
G
C
The ribosome now moves one
codon farther along the length of
the mRNA strand. A third tRNA
arrives at the A site, bearing amino
acid 3. The first tRNA then detaches
from the E site of the ribosome and
reenters the cytosol. It can pick up
another amino acid molecule in the
cytosol and repeat the process.
G C
C C
G G G
A

83. Figure 3–13 The Process of
Translation (Part 5 of 5).
83
5 Completion and Release
of Polypeptide
Large
ribosomal
subunit
E
1
2
3
4
5
6
7
P
A
Completed
polypeptide
Small ribosomal
subunit
mRNA strand
Stop
codon
Elongation continues until the ribo-
some reaches a stop codon. Termina-
tion occurs as a protein-release factor
bonds with the stop codon and the
completed polypeptide is released.
The ribosomal subunits then separate,
freeing the mRNA strand.

84. 3-4 Protein Synthesis
 DNA controls cell structure and function by directing synthesis
of specific proteins
• Changes in extracellular environment may alter
intracellular environment
• May initiate chemical signaling pathways
• Substances that cross plasma membrane may enter the
nucleus
• And bind to receptors or promoters on DNA
84

85. 3-5 Diffusion and Osmosis
 The plasma (cell) membrane is a barrier, but
• Nutrients must get in
• Products and wastes must get out
 Permeability determines what moves in and out of a cell, and
a membrane that
• Lets nothing in or out is impermeable
• Lets anything pass is freely permeable
• Restricts movement is selectively permeable
85

86. 3-5 Diffusion and Osmosis
 Plasma membrane is selectively permeable
• Allows some materials to move freely
• Restricts other materials based on their
• Size
• Electrical charge
• Molecular shape
• Lipid solubility
86

87. 3-5 Diffusion and Osmosis
 Transport through plasma membrane can involve
• Passive processes (no energy required)
• Active processes (requiring energy)
 Diffusion and osmosis (passive)
 Carrier-mediated transport (passive or active)
 Vesicular transport (active)
87

88. 3-5 Diffusion and Osmosis
 Diffusion
• Net movement of a substance from area of higher
concentration to area of lower concentration
• Ions and molecules are constantly in motion
• Molecules in solution move randomly
• Random motion causes mixing
• Concentration gradient
• Difference between high and low concentrations of a
substance
88

89. Figure 3–14 Diffusion.
89
1 Placing a colored
sugar cube into a
water-filled beaker
establishes a
steep
concen-
tration
gradient.
2 As the cube begins
to dissolve, many
sugar and dye
molecules are in
one location,
and none are
elsewhere.
3 With time, the
sugar and dye
molecules
spread through
the water.
4 Eventually, the
concentration
gradient is
eliminated and the
molecules are
evenly distributed
throughout the
solution.

90. 3-5 Diffusion and Osmosis
 Factors influencing diffusion
• Distance the particle has to move
• Ion and molecule size
• Smaller = faster diffusion
• Temperature
• More heat = faster diffusion
• Concentration gradient
• Steeper gradient = faster diffusion
• Electrical forces
• Opposites attract, like charges repel
90

91. 3-5 Diffusion and Osmosis
 Diffusion across plasma membranes
• Simple diffusion
• Lipid-soluble compounds (alcohols, fatty acids, and
steroids)
• Dissolved gases (oxygen and carbon dioxide)
• Water molecules
• Channel-mediated diffusion
• Water-soluble compounds and ions
• Affected by size, charge, and interaction with channel
walls
91

92. Figure 3–15 Diffusion across the
Plasma Membrane.
92
EXTRACELLULAR FLUID Lipid-soluble molecules
diffuse through the
plasma membrane
Plasma membrane Channel
protein
CYTOPLASM
Large molecules that cannot
diffuse through lipids cannot
cross the plasma membrane
unless they are transported
by a carrier mechanism
Small water-soluble
molecules and ions
diffuse through
membrane channels

93. 3-5 Diffusion and Osmosis
 Osmosis
• Diffusion of water across a selectively permeable
membrane
• Water molecules diffuse across a membrane toward the
solution with more solutes
93

94. Figure 3–16 Osmosis (Part 1 of 3).
94
1 Two solutions containing different
solute concentrations are separated
by a selectively permeable membrane.
Water molecules (small blue dots)
begin crossing the membrane toward
solution B, the solution with the
higher concentration of solutes (large
pink dots).
A B
Water
molecules
Solute
molecules
Selectively permeable membrane

95. Figure 3–16 Osmosis (Part 2 of 3).
95
2 At equilibrium, the solute concen-
trations on the two sides of the
membrane are equal. The volume
of solution B has increased at the
expense of that of solution A.
Volume
increased
Volume
decreased
Original
level

96. Figure 3–16 Osmosis (Part 3 of 3).
96
3 Osmosis can be prevented by resisting
the change in volume. The osmotic
pressure of solution B is equal to the
amount of hydrostatic pressure required
to stop the osmotic flow.
Applied
force
Volumes
equal

97. 3-5 Diffusion and Osmosis
 Osmotic pressure
• The force with which pure water moves into a solution as a
result of solute concentration
• Hydrostatic pressure is the pressure needed to block
osmosis
 Osmosis occurs more rapidly than solute diffusion
• Because water can cross a membrane through abundant
water channels (aquaporins)
• Aquaporins outnumber solute channels
97

98. 3-5 Diffusion and Osmosis
 Osmolarity (osmotic concentration) is the total solute
concentration in a solution
 Tonicity describes how a solution affects cells
• Depends on the nature of the solutes
• Isotonic solution (iso- = same, tonos = tension)
• Does not cause osmotic flow
• Hypotonic solution (hypo- = below)
• Lower solute concentration than the cell
• Hypertonic solution (hyper- = above)
• Higher solute concentration than the cell
98

99. 3-5 Diffusion and Osmosis
 Osmolarity and tonicity
• A cell in an isotonic solution
• Stays the same size and shape
• A cell in a hypotonic solution
• Gains water
• May rupture (hemolysis)
• A cell in a hypertonic solution
• Loses water and shrinks (crenation)
99

100. Figure 3–17a Osmotic Flow across a
Plasma Membrane.
100
a Isotonic solution
In an isotonic saline solution, no
osmotic flow occurs, and the red
blood cells appear normal in size
and shape.
Water
molecules
Solute
molecules
SEM of a normal RBC
in an isotonic solution

101. Figure 3–17b Osmotic Flow across a
Plasma Membrane.
101
b Hypotonic solution
In a hypotonic solution, the water
flows into the cell. The swelling may
continue until the plasma membrane
ruptures, or lyses.
SEM of swollen RBC in
a hypotonic solution

102. Figure 3–17c Osmotic Flow across a
Plasma Membrane.
102
In a hypertonic solution, water
moves out of the cell. The red
blood cells crenate (shrivel).
SEM of crenated RBCs
in a hypertonic solution
c Hypertonic solution

103. 3-6 Carriers and Vesicles
 Carrier-mediated transport
• Proteins transport ions or organic substrates across plasma
membrane
• Specificity
• One transport protein, one set of substrates
• Saturation limits
• Rate depends on availability of transport proteins and
substrates
• Regulation
• Cofactors such as hormones affect activity of carriers
103

104. 3-6 Carriers and Vesicles
 Carrier-mediated transport
• Symport (cotransport)
• Two substances move in the same direction at the
same time
• Antiport (countertransport)
• One substance moves in while another moves out
104

105. 3-6 Carriers and Vesicles
 Carrier-mediated transport
• Facilitated diffusion
• Passive
• Carrier proteins transport molecules too large to fit
through channel proteins (glucose, amino acids)
• Molecule binds to receptor site on carrier protein
• Protein changes shape, molecule passes through
• Receptor site is specific to certain molecules
105

106. Figure 3–18 Facilitated Diffusion.
106
EXTRACELLULAR
FLUID
Glucose
molecule
Receptor site Carrier
protein
CYTOPLASM
Glucose released
into cytoplasm

107. 3-6 Carriers and Vesicles
 Carrier-mediated transport
• Active transport
• Active transport proteins move substrates against
concentration gradients
• Requires energy, such as ATP
• Ion pumps move ions (Na+, K+, Ca2+, Mg2+)
• Exchange pumps move two ions in opposite
directions at the same time
107

108. 3-6 Carriers and Vesicles
 Carrier-mediated transport
• Primary active transport
• Pumping solutes against a concentration gradient using
ATP
• Sodium–potassium exchange pump
• One ATP powers the movement of three sodium
ions (Na+) out, and two potassium ions (K+) in
108

109. Figure 3–19 The Sodium–Potassium
Exchange Pump.
109
EXTRACELLULAR
FLUID
3 Na+
Sodium–
potassium
exchange
pump
2 K+
ATP ADP
CYTOPLASM

110. 3-6 Carriers and Vesicles
 Carrier-mediated transport
• Secondary active transport
• ATP is required to establish a concentration gradient of
one substance in order to passively transport another
• Example: Na+ concentration gradient drives glucose
transport into cells
• ATP is used to pump Na+ back out
110

111. Figure 3–20 Secondary Active
Transport.
111
Glucose
molecule
Sodium
ion (Na+) 2 K+
Na+–K+
pump
ADP
CYTOPLASM
K+
3 Na+
ATP

112. 3-6 Carriers and Vesicles
 Vesicular transport (bulk transport)
• Materials move into or out of a cell in vesicles
• Endocytosis (endo- = inside) is the importation of
extracellular materials packaged within vesicles, which
requires ATP
1. Receptor-mediated endocytosis
2. Pinocytosis
3. Phagocytosis
112

113. 3-6 Carriers and Vesicles
 Endocytosis
• Receptor-mediated endocytosis
• Receptors (glycoproteins) bind target molecules
(ligands)
• Receptors and their ligands migrate to clathrin-coated
pits of plasma membrane to enter cell
• Some receptors are associated with membrane lipids
and small indentations called caveolae
113

114. Figure 3–21 Receptor-Mediated
Endocytosis.
114
EXTRACELLULAR FLUID
Ligands binding
to receptors
Exocytosis
1
Ligands Ligands bind to receptors in plasma membrane,
which migrate to clathrin-coated pits.
1
2
Ligand
receptors
Clathrin-coated
pit
Endocytosis
2
Ligand-receptor areas form deep pockets
in plasma membrane surface.
3
3 Pockets pinch off, forming endosomes
known as clathrin-coated vesicles.
7
Clathrin
(protein)
Clathrin-
coated
vesicle
4
Clathrin recycles back to the plasma
membrane and endosomes fuse with primary
lysosomes to form secondary lysosomes.
4 5
Ligands are removed and
absorbed into the cytoplasm.
6
6
Primary
lysosome
Secondary
lysosome
The lysosomal and endosomal
membranes separate.
Ligands
removed
CYTOPLASM
5
7
The endosome fuses with the plasma
membrane, and the receptors are again
available for ligand binding.

115. 3-6 Carriers and Vesicles
 Endocytosis
• Pinocytosis
• Endosomes “drink” extracellular fluid
• Phagocytosis
• Cytoplasmic extensions called pseudopodia (pseudo- =
false, podon = foot)
• Large objects are engulfed in phagosomes
 Exocytosis (exo- = outside)
• Granules or droplets are released from the cell as a vesicle
fuses to plasma membrane
115

116. Figure 3–22 Overview of
Membrane Transport (Part 1 of 6).
116
Diffusion
Diffusion is the movement of molecules
from an area of higher concentration to
an area of lower concentration. That
is, the movement occurs down a
concentration gradient (high to low).
Factors Affecting Rate: Size of
gradient, molecular size, electric charge,
lipid solubility, temperature, and the
presence of membrane channel proteins
Substances Involved: Gases, small
inorganic ions and molecules, lipid-
soluble materials
Plasma
membrane
Extracellular
fluid
CO2
Example:
When the concentra-
tion of CO2 inside a
cell is greater than
outside the cell, the
CO2 diffuses out of the
cell and into the
extracellular fluid.

117. Figure 3–22 Overview of
Membrane Transport (Part 2 of 6).
117
Osmosis
Osmosis is the diffusion of water
molecules across a selectively
permeable membrane. Movement
occurs toward higher solute
concentration because that is where
the concentration of water is lower.
Osmosis continues until the
concentration gradient is eliminated.
Factors Affecting Rate:
Concentration gradient; opposing
pressure
Substances Involved: Water only Water
Solute
Example:
If the solute concentra-
tion outside a cell is
greater than inside the
cell, water molecules will
move across the plasma
membrane into the
extracellular fluid.

118. Figure 3–22 Overview of
Membrane Transport (Part 3 of 6).
118
Facilitated diffusion
Facilitated diffusion is the
movement of materials across a
membrane by a carrier protein.
Movement follows the
concentration gradient.
Factors Affecting Rate:
Concentration gradient,
availability of carrier proteins
Substances Involved: Simple
sugars and amino acids
Glucose
Receptor
site
Carrier
protein
Carrier protein releases
glucose into cytoplasm
Example:
Nutrients, such as glucose,
that are insoluble in lipids
and too large to fit through
membrane channels may
be transported across the
plasma membrane by
carrier proteins. Many
carrier proteins move a
specific substance in one
direction only, either into or
out of the cell.

119. Figure 3–22 Overview of
Membrane Transport (Part 4 of 6).
119
Active transport
Active transport requires carrier proteins that move specific
substances across a membrane against their concentration
gradient. If the carrier moves one solute in one direction and
another solute in the opposite direction, it is called an
exchange pump.
Factors Affecting Rate: Availability of carrier protein,
substrate, and ATP
Substances Involved: Na+, K+, Ca2+, Mg2+; other solutes in
special cases
Extracellular
fluid
Sodium–potassium
exchange pump
3 Na+
Example:
The sodium–potassium
exchange pump. One
ATP molecule is hydro-
lyzed to ADP for each 3
sodium ions that are
ejected from the cell,
while 2 potassium ions
are reclaimed.
2 K+
ATP
Cytoplasm
ADP

120. Figure 3–22 Overview of
Membrane Transport (Part 5 of 6).
120
Endocytosis Endocytosis is the packaging of extracellular materials into a vesicle for transport into the cell.
Receptor-Mediated
Endocytosis
Extracellular fluid
Target molecules
Pinocytosis
In pinocytosis, vesicles form at the
plasma membrane and bring fluids
and small molecules into the cell. This
process is often called “cell drinking.”
Pinocytic
vesicle
forming
Endosome
Phagocytosis
In phagocytosis, vesicles called
phagosomes form at the plasma mem-
brane to bring particles into the cell.
This process is often called “cell eating.”
Factors Affecting Rate: Presence of
pathogens and cellular debris
Example:
Once the
vesicle is inside
the cytoplasm,
water and small
molecules enter
the cell across
the vesicle
membrane.
Cytoplasm
Receptor
proteins
Clathrin-coated
vesicle containing
target molecules
In receptor-
mediated
endocytosis, target
molecules called ligands bind to
receptor proteins on the membrane
surface, triggering vesicle formation.
Example:
Cholesterol
and iron ions
are transported
this way.
Substances Involved: Bacteria,
viruses, cellular debris, and other
foreign material
Pseudopodium
extends to
surround object
Example:
Large particles
are brought into
the cell by
cytoplasmic
extensions
(called
pseudopodia)
that engulf the
particle and pull
it into the cell.
Cell
Cell
Factors Affecting Rate: Stimulus
and mechanism not understood
Substances Involved:
Extracellular fluid, with dissolved
molecules such as nutrients
Phagosome
Factors Affecting Rate: Number of
receptors on the plasma membrane
and the concentration of ligands
Substances Involved: Ligands

121. Figure 3–22 Overview of
Membrane Transport (Part 6 of 6).
121
Exocytosis
In exocytosis,
intracellular vesicles
fuse with the plasma
membrane to release
fluids and/or solids from
the cells.
Factors Affecting Rate: Stimulus
and mechanism incompletely
understood
Substances Involved: Fluid and
cellular wastes; secretory products
from some cells Cell
Material
ejected
from cell
Example:
Cellular wastes in
vesicles are
ejected from the
cell.

122. 3-7 Membrane Potential
 Membrane potential
• When positive and negative charges are separated, a
potential difference is created
• Unequal charge across the plasma membrane is membrane
potential
• Resting membrane potential ranges from –10 mV
to –100 mV, depending on cell type
122

123. 3-8 Cell Life Cycle
Cell life cycle
Cell division is a form of cellular reproduction
A single cell divides to produce two daughter cells
At the end of a cell’s life span, it undergoes genetically controlled
death called apoptosis
123

124. 124

125. 3-8 Cell Life Cycle
 Interphase
• Non dividing period in which somatic cells spend the
majority of their lives
• G0, G1, S, and G2 phases
125

126. 126
INTERPHASE
Most cells spend only a small part of their
time actively engaged in cell division.
Somatic cells spend the majority of their
functional lives in a state known as
interphase. During interphase, a cell
performs all its normal functions and,
if necessary, prepares for cell division.
A cell that is ready to
divide first enters the G1
phase. In this phase, the cell
makes enough mitochondria,
cytoskeletal elements,
endoplasmic reticula, ribo-
somes, Golgi apparatus
membranes, and cytosol
for two functional cells.
Centriole replication
begins in G1 and
commonly continues
until G2. In cells
dividing at top
speed, G1 may last
just 8–12 hours.
Such cells use all
their energy on mi-
tosis, and all other
activities cease.
When G1 lasts for
days, weeks, or
months, preparation
for mitosis occurs as
the cells perform their
normal functions.
G1
Normal
cell functions
plus cell growth,
duplication of
organelles,
protein
synthesis
THE
CELL
CYCLE

127. 127
When the activities of G1 have been
completed, the cell enters the S phase.
Over the next 6–8 hours, the cell
duplicates its chromosomes. This
involves DNA replication and the
synthesis of histones and other
proteins in the nucleus.
S
DNA
replication,
synthesis
of
histones
THE
CELL
CYCLE

128. Figure 3–23 Stages of a Cell’s Life
Cycle (Part 3 of 9).
128
THE
CELL
CYCLE
G2
Protein
synthesis
Once DNA
replication has
ended, there is a
brief (2–5-hour) G2
phase devoted to last-
minute protein synthesis
and to the completion of
centriole replication.

129. 129
THE
CELL
CYCLE
MITOSIS
Centrioles in
centrosome
Nucleus
M Phase–MITOSIS
and CYTOKINESIS
Interphase
During interphase,
the DNA strands are
loosely coiled as
chromatin and
chromosomes
cannot be seen.

130. 130
THE
CELL
CYCLE
G0
An interphase cell in the G0 phase is
not preparing for division, but is performing
all of the other functions appropriate for that
particular cell type. Some mature cells, such as
skeletal muscle cells and most neurons, remain in
G0 indefinitely and never divide. In contrast, stem cells,
which divide repeatedly with very brief interphase periods,
never enter G0.

131. 3-8 Cell Life Cycle
 DNA replication
• Helicases unwind the DNA strands
• DNA polymerase
1. Promotes bonding between the nitrogenous bases of
the DNA strand and complementary DNA nucleotides
dissolved in the nucleoplasm
2. Links the nucleotides by covalent bonds
131

132. 132
1 DNA replication begins when helicase enzymes
unwind the strands and disrupt the hydrogen
bonds between the bases. As the strands unwind,
molecules of DNA polymerase bind to the
exposed nitrogenous bases. This enzyme (1)
promotes bonding between the nitrogenous bases
of each separated strand with complementary DNA
nucleotides in the nucleoplasm and (2) links the
nucleotides by covalent bonds.
Leading strand
template
3
KEY
Guanine
Cytosine
Thymine
Adenine
2
4
1
9
8
7
6
5
2
DNA polymerase can work in only
one direction along a strand of
DNA. However, the two strands
in a DNA molecule are oriented
in opposite directions. The DNA
polymerase bound to the upper
leading strand template adds
nucleotides to make a single,
continuous complementary copy
called the leading strand that
grows toward the “zipper.”
Leading strand
Each DNA
molecule consists
of two nucleotide
strands joined by
hydrogen bonds
between their
complementary
nitrogenous bases.
Lagging strand
template
Segment 2
DNA nucleotide
Segment 1
8
1
2
3 4
6
7
5

133. Figure 3–24 DNA Replication (Part
2 of 3).
133
2
4
3
1
KEY
Guanine
Cytosine
Thymine
Adenine
9
8
7
6
5
Segment 2
DNA nucleotide
Segment 1
8
1
2
3 4
6
7
3
5 DNA polymerase on the other
original strand, however, can work
only away from the unzipping site.
In the lower lagging strand
template, the first DNA
polymerase to bind to it must
work from left to right, adding
nucleotides in the sequence
12345. But as the original strands
continue to unzip, additional
nucleotides are continuously
exposed. This molecule of DNA
polymerase cannot go into
reverse. It can only continue
working from left to right.
4
Thus, a second molecule of DNA polymerase must bind closer
to the point of unzipping and assemble a complementary copy
that grows in sequence until it bumps into the segment created
by the first DNA polymerase. Enzymes called DNA ligases
(liga, to tie) then splice together the two DNA segments into a
strand called the lagging strand.

134. Figure 3–24 DNA Replication (Part
3 of 3).
134
5
Eventually, the unzipping completely
separates the original strands. The copying
and last splicing are completed, and 2
identical DNA molecules are formed. Once the
DNA has been replicated, the centrioles
duplicated, and the necessary enzymes and
proteins have been synthesized, the cell
leaves interphase and is ready to proceed
to mitosis.
Duplicated DNA
double helices

135. 3-8 Cell Life Cycle
 M phase
• Mitosis
• Duplication of chromosomes in the nucleus and their
separation into two identical sets
• A continuous process consisting of several stages:
prophase, metaphase, anaphase, and telophase
• Cytokinesis
• Division of the cytoplasm
• Produces two daughter cells
135

136. Figure 3–23 Stages of a Cell’s Life
Cycle (Part 6 of 9).
136
A dividing cell held in place
by a sucker pipe to the left
and being injected with a
needle from the right.

137. 137
Centrioles
(two pairs)
Chromosome
with two sister
chromatids
Metaphase
plate
Early prophase
Prophase (PRŌ-fāz; pro-,
before) begins when the
chromatin condenses and
chromosomes become
visible as single
structures under a light
microscope. An array of
microtubules called
spindle fibers extends
between the centriole
pairs. Smaller
microtubules called
astral rays radiate into
the cytoplasm.
Late prophase
As a result of DNA replication
during the S phase, two copies
of each chromosome now
exist. Each copy, called
a chromatid (KRŌ-ma-tid),
is connected to its duplicate
copy at a single point, the
centromere (SEN-trō-mēr).
Kinetochores (ki-NĒ-tō-korz)
are the protein-bound areas
of the centromere; they attach
to spindle fibers forming
chromosomal microtubules.
Metaphase
Metaphase
(MET-a-fāz; meta-,
after) begins as the
chromatids move to a
narrow central zone
called the metaphase
plate. Metaphase
ends when all the
chromatids are
aligned in the plane of
the metaphase plate.
Astral rays and
spindle fibers
Chromosomal
microtubules

138. 138
The two daughter
chromosomes are pulled
apart and toward opposite
ends of the cell along the
spindle apparatus (the
complex of spindle fibers). Cleavage
furrow Daughter
cells
Anaphase
Anaphase (AN-a-fāz;
ana-, apart) begins
when the centromere
of each chromatid pair
splits and the
chromatids separate.
The two daughter
chromosomes are
now pulled toward
opposite ends of
the cell along the
chromosomal
microtubules.
Telophase
During telophase
(TĒL-ō-fāz; telo-, end),
each new cell prepares
to return to the
interphase state. The
nuclear membranes
re-form, the nuclei
enlarge, and the
chromosomes
gradually uncoil.
This stage marks
the end of mitosis.
Cytokinesis
Cytokinesis is the
division of the
cytoplasm into two
daughter cells.
Cytokinesis usually
begins with the
formation of a
cleavage furrow and
continues throughout
telophase. The
completion of
cytokinesis marks the
end of cell division.

139. Figure 3–23 Stages of a Cell’s Life
Cycle (Part 9 of 9).
139
Centrioles
(two pairs)
Chromosome
with two sister
chromatids
Metaphase
plate
The two daughter
chromosomes are pulled
apart and toward opposite
ends of the cell along the
spindle apparatus (the
complex of spindle
fibers). Cleavage
furrow Daughter
cells
Early prophase
Prophase (PRŌ-fāz; pro-,
before) begins when the
chromatin condenses and
chromosomes become
visible as single
structures under a light
microscope. An array of
microtubules called
spindle fibers extends
between the centriole
pairs. Smaller
microtubules called
astral rays radiate into
the cytoplasm.
Late prophase
As a result of DNA replication
during the S phase, two copies
of each chromosome now
exist. Each copy, called
a chromatid (KRŌ-ma-tid),
is connected to its duplicate
copy at a single point, the
centromere (SEN-trō-mēr).
Kinetochores (ki-NĒ-tō-korz)
are the protein-bound areas of
the centromere; they attach to
spindle fibers forming
chromosomal microtubules.
Metaphase
Metaphase
(MET-a-fāz; meta-,
after) begins as the
chromatids move to a
narrow central zone
called the metaphase
plate. Metaphase
ends when all the
chromatids are
aligned in the plane of
the metaphase plate.
Anaphase Telophase
During telophase (TĒL-
ō-fāz; telo-, end), each
new cell prepares to
return to the
interphase state. The
nuclear membranes
re-form, the nuclei
enlarge, and the
chromosomes
gradually uncoil.
This stage marks
the end of mitosis.
Cytokinesis
Cytokinesis is the
division of the
cytoplasm into two
daughter cells.
Cytokinesis usually
begins with the
formation of a
cleavage furrow and
continues throughout
telophase. The
completion of
cytokinesis marks the
end of cell division.
Anaphase (AN-a-fāz;
ana-, apart) begins
when the centromere
of each chromatid pair
splits and the
chromatids separate.
The two daughter
chromosomes are
now pulled toward
opposite ends of
the cell along the
chromosomal
microtubules.
Astral rays and
spindle fibers
Chromosomal
microtubules

140. 3-8 Cell Life Cycle
 Mitotic rate
• Rate of cell division
• Slower mitotic rate means longer cell life
• Cell division requires energy (ATP)
• Muscle cells and neurons rarely divide
• Exposed cells (skin and digestive tract) live only days or
hours—replenished by stem cells
140

141. 3-9 Regulation of the Cell Life Cycle
 Cell division
• Normally, cell division balances cell loss
• Cell division can be stimulated by
• Internal factors (M-phase promoting factor, MPF)
• Extracellular chemical factors (growth factors)
• Cell division can be inhibited by
• Repressor genes (faulty repressors cause cancers)
• Worn out telomeres (terminal DNA segments)
141

142. 3-10 Cell Division and Cancer
 Tumor (neoplasm)
• Mass produced by abnormal cell growth and division
• Benign tumor
• Contained, not life threatening unless large
• Malignant tumor
• Spreads into surrounding tissues (invasion)
142

143. 3-10 Cell Division and Cancer
 Cancer results from abnormal proliferation of cells
• Caused by mutations in genes involved with cell growth
• Modified genes are called oncogenes
• Mutagens are agents that cause mutations
• Carcinogens, including many mutagens, are cancer-causing
agents
• Metastasis is the spread of cancer to other areas
• Begins with invasion of tissues surrounding tumor
143

144. Figure 3–25 The Development and
Metastasis of Cancer.
144
Abnormal
cell Cell
divisions
Primary tumor cells
Growth of blood
vessels into tumor
Invasion
Penetration
Secondary
tumor cells
Cell
divisions
Escape
Circulation

145. 3-11 Cellular Differentiation
 All cells contain the same chromosomes and genes
 Cells undergo cellular differentiation by turning off genes not
needed by that cell
• Allows for formation of different types of cells like liver
cells, fat cells, and neurons
145