Cytolo

gy

Lecture
two
Ce
ll
 Introducti
on
• Cells are the basic structural and functional unit of the living organism showing a
variety of functional specializations which perform all the activities necessary for the
survival, growth and reproduction of the organism

• It is a self replicating, self regulating and self governing biological system

 Metabolism (absorption, synthesis, respiration and excretion),
 Growth and regeneration
 Irritability (excitability)
 Movement
 Reproduction
 Aging and death
 Cont.
…
• Number (around 100 trillion
(1012) cells
• Type (more than 250 named
cell types)
• Shape (round, oval,
columnar, multipolar,
polygonal, cylindrical, fusiform,
pyramidal, pyriform, etc.)
• Size (5µm - 120µm in
diameter)
• Functions
 Cellular Functions in Some Specialized Cells
Function Specialized Cell(s)
Movement Muscle cell
Synthesis and secretion of enzymes Pancreatic acinar cells
Synthesis and secretion of mucous Mucous-gland cells
substances
Synthesis and secretion of steroids Some adrenal gland, testis, and ovary
cells
Ion transport Cells of the kidney and salivary gland
ducts
Intracellular digestion Macrophages and some white blood
cells
Transformation of physical and chemical stimuli into Sensory cells
nervous impulses

Metabolite absorption Cells of the intestine

4/4/2023 Fikre B. 5
Cont. nucleus circle=cuboid
cell flat=squamous
cell round=cuboid cell
WBCs10,000 under normal condition
exceeds 10,000 when there's an
infection

…
 Components of
Eukaryotic
1. Cell membrane
Cell
2. Cytoplasm
3. Karyoplasm (Nucleus)
 1. Cell dont forget to search up the three types trilaminar

membrane
layer
laminar
• In electron micrographs of osmium-stained tissue, appears as trilaminar layer, each layer
s

2.5 nm in diameter
• Because all membranes have this appearance, it is called the unit membrane
 Cell
 membrane
about 7.5 nm thick
 Biochemically made by the following components
1. Lipids volunme 90% weight

2. 50%
Proteins volume 10% weight 50%

3. Carbohydrates
 Phospholip 75
%

• Such ids
as phosphatidylcholine (lecithin)
• With an amphipathic character (both hydrophobic and hydrophilic)
• Arranged in a bilayer, each with:
• Hydrophilic polar phosphate-containing head - towards water
• Hydrophobic nonpolar pair of fatty acid tails – away from water
 Choleste
• Modulate the fluidity and movement of all membrane components maintaining the
rol integrity of the plasma membrane
structural
 Protei
nsin the plasma membrane
• 50% w/w
• Most are globular proteins forming the following two groups:
1. Integral membrane proteins intrinsic

2. Peripheral membrane proteins extrinsic

 Integral membrane
proteins
• Some protrude from only one membrane surface, while others are transmembrane
proteins and protrude from both sides
 Peripheral membrane
• Are proteins
more loosely associated, mostly on the inner membrane surface
• Bound to the polar groups of the membrane phospholipids or integral membrane proteins
• Usually functions as part of the cytoskeleton or an intracellular secondary messenger
proteins of the cell

those which are found inside

the cell
 Cell membrane
proteins
• Functionally there are 6 broad categories of membrane proteins as:
1. Pumps •Pumps: These proteins actively transport ions or molecules across
the membrane against their concentration gradient, using energy
typically derived from ATP. An example is the sodium-potassium
2. Channels pump, which maintains the electrochemical gradient in cells.
•Channels: Channel proteins form pores in the membrane that allow
3. Receptors specific ions or small molecules to pass through passively, following
their concentration gradient. Ion channels, for example, facilitate the
4. Linkers movement of ions like sodium, potassium, and calcium.
•Receptors: Receptor proteins bind to specific signaling molecules
5. Enzymes (ligands/chemicals), such as hormones or neurotransmitters,
triggering a cellular response. They play a crucial role in cell
6. Structural proteins communication and signaling pathways.
•Linkers: Linker proteins connect the membrane to the cytoskeleton
or the extracellular matrix, providing structural support and helping
maintain the cell's shape. They also facilitate communication
between the inside and outside of the cell.
•Enzymes: Membrane-associated enzymes catalyze biochemical
reactions at the membrane surface. They can be involved in various
processes, such as digesting nutrients or modifying signaling
molecules.
•Structural Proteins: These proteins provide structural integrity and
stability to the cell membrane. They help maintain the shape of the
cell and organize other proteins within the membrane, contributing to
the overall architecture of the cell.
 Carbohydra
• Occur as oligosaccharide attached to plasma membrane proteins as glycoproteins and lipid
as tes
glycolipids
• Branch and project from the outer surface of the outer leaflet of the plasma membrane
• They form a cell coat called glycocalyx that participates in:
• Cell adhesion to the extracellular matrix
• Binding of enzymes and antigens to the cell surface.
• Cell-to-cell recognition and interaction
Organization of the cell
membrane
• Are asymmetrical because of:
• Different composition of lipids
• Peripheral proteins are mainly on inner leaflet
• Oligosaccharides are on outer leaflet
 Functions of the cell
1. membrane
Selective permeability & transportation
2. Communication
3. Physical barrier
4. Intercellular connections
 2.
• It isCytoplas
the cellular material outside the nucleus but within the plasma
membrane; consists of the following:
•m Cytosol - cellular fluid (mainly water) with dissolved proteins, salts, sugars, and other
solutes
• Organelles - ultramicroscopic structures that perform various cellular functions;
ribosomes, ER, mitochondria, etc.
• Cytoskeleton - protein filaments and tubules that provide support, movement within
the cell; cellular skeleton
• Inclusions - chemicals such as glycogen, fat, and pigments
Cytosol, cytoplasm matrix (or ground
substance)
• In between the three dimensional cytoskeletal structure
• Contains proteins, electrolytes, and metabolites dissolved in water (which makes 75-90%)
• Could generally be divided into an inner and outer domains
Outer Domain
•Located just beneath the cell membrane.
•Rich in cytoskeletal elements like actin filaments, which help
maintain cell shape, allow movement, and provide structural
support.
•Often interacts with the cell membrane and extracellular
matrix(ECM), contributing to cell signaling and transport processes.
Inner Domain
•Found deeper within the cell, around the nucleus and other
organelles.
•Contains fewer cytoskeletal elements but is denser with organelles
like the endoplasmic reticulum, mitochondria, and Golgi apparatus.
•Focused on facilitating metabolic activities, such as glycolysis and
protein synthesis, by providing a supportive environment for
enzymes and metabolites.
 Organell
es
• Metabolically active structures actively produce or transform energy, synthesize
• include:
molecules, or regulate biochemical reactions.

1. Endoplasmic reticulum
2. Golgi complex (bodies or apparatus )
3.
Lysosomes
4. Peroxisomes (microbodies)
5.
Mitochondria
6. Endosomes and phagosomes
7. Proteasomes
8. Ribosomes
9. Cytoskeleton
i. Microtubules
ii. Microfilaments
iii. Intermediate filaments
10. Centrioles
Metabolically inactive structures in the cytosol are cellular components that do not require energy or actively participate in
metabolic reactions. Metabolically inactive structures include inclusion bodies, pigment granules, crystals, residual bodies, and
lipid droplets. These structures serve primarily for storage or waste retention and do not require energy for maintenance.
 Endoplasmic
reticulum
• Convoluted network of anastomosing membrane channels (cisternae ) of various shapes
• Show transfer vesicles which bud and move to the Golgi complex
• Are of two types:
1. Rough or granular endoplasmic reticulum
2. Smooth or agranular endoplasmic reticulum
 Rough or granular endoplasmic
reticulum
• Show continuity (RER)
with the outer nuclear membrane
• Have docking proteins as receptors for ribosomes, and glycoprotein ribophorins
• Synthesize proteins for sequestration The RER has a system of channels called cisternae, which
transport molecules within the organelle.
• For export
• For proteins of the ER, the Golgi apparatus, Lysosomes or the cell membrane
• Transport molecules through cisternal space
• Under LM appear as basophilic patches formerly termed as:
– Ergastoplasm in glandular cells
– Nissl bodies in they synthesize proteins
including neuro

neurons transmitters
 Smooth or agranular endoplasmic
reticulum
• Have more tubular(SER)
or vesicular cisternae than the RER
• Functions in:
o Synthesis of phospholipids and steroids
o Lipid metabolism
Glycogen breakdown
o Detoxification
o Transport molecules through cisternal space
Store and release calcium ions in the striated muscle cells as sarcoplasmic reticulum
o
Cisternae are
o flattened
membrane-
bound sacs.
 Golgi complex (Golgi body or Golgi
• Stalkapparatus)
of 3-10 discrete flattened and slightly curved bag-like channels or cisternae
surrounded by vesicles
• Has the following two surfaces:
a. Forming, convex entry, cis face
, the nucleus
• Closest to
• Surrounded by small transfer vesicles (vesicles in)
b. Maturin , condensing, exit, trans face
•gUsually concav
e
• Has condensing vacuoles (vesicles out) and secretory granules
 Golgi complex (Golgi body or Golgi
apparatus)
• Functions in:
• Synthesis of polysaccharides
• Glycosylate proteins and lipid forming, respectively glycoproteins &
proteoglycans and glycolipids
• Sulfate glycosaminoglycans

• Packaging of secretory products

• Concentration and storage of secretory products

The Golgi apparatus functions by modifying proteins and lipids
received from the endoplasmic reticulum, sorting and
packaging them into vesicles for transport, creating lysosomes,
producing secretory vesicles for exocytosis, synthesizing
complex carbohydrates, and maintaining the lipid composition
of cellular membranes, acting as a crucial processing and
packaging center for cellular materials.
 Lysosom
es dense usually spherical bodies with a diameter of 0.05 - 0.5 μm
• Electron
• Contain more than 40 hydrolytic enzymes (cause hydrolysis of a chemical bonds, breaking
down bigger molecules), most commonly acid hydrolyases such as proteases, nucleases,
phosphatase, phospholipases, sulfatases, and β-glucuronidase
• Most active at an acidic pH (5.0)
 Peroxisomes
• Are (microbodies)
like the lysosomes, but slightly larger in diameter (0.2–0.5 μm)
• Contain more than 40 oxidative enzymes that:
– Produce hydrogen peroxide (H2O2) to kill microorganisms and detoxifies toxic agents
• Complement certain functions of the SER and mitochondria in the metabolism of lipids
and other molecules
• Replicate by budding of precursor vesicles from the ER fission of preexisting peroxisomes
•
or
Budding of precursor
vesicles: involves the formation
of small vesicles that pinch off
from the ER membrane and
mature into fully functional
peroxisomes.
•Fission of Existing
Peroxisomes: Existing
peroxisomes can divide to form
new ones like cell division.
 Mitochond
ria
• Large spherical, filamentous or rod-shaped with diameters of 0.5-1 μm and 2-7 μm
long
• Have doubleporous
a. Outer membranes
smooth membrane - containing many transmembrane proteins channels cal
porins that form channels
b. Inner less porous membrane
o Makes cristae
 Shelf-like in many cells
 Tubular in steroid secreting cells
• Able to rapidly changing shape, fusing together and dividing by fission
 Mitochond
ria
• Have two spaces
a. Intermembrane space (outer chamber, intracristal)
b. Matrix space (intercristal space), which contains:
o Enzymes, water, solutes, and granules that bind Ca2+ and Mg2+
o Mitochondrial ribosomes
o mRNA, tRNA and rRNA
o Circular
DNA Like those of prokaryotic cells
 Are maternal
 Endosomes and
Phagosomes are specialized vesicles formed
phagosomes
• Endosomes are membrane bound structures in the cell during phagocytosis, a process where cells
(often immune cells) engulf large particles like
• May be released at other side in transcytosis bacteria or cell debris. The cell’s membrane
extends around the particle, enclosing it in a
phagosome. This phagosome then fuses with a
• Phagosomes are found in the forms of: lysosome (another vesicle containing digestive
enzymes), forming a phagolysosome. Inside the
1. Hetrophagosomes - ingested phagolysosome, enzymes break down the
particle, allowing the cell to either use the
2. Autophagosomes – self resulting components or dispose of waste.
Phagosomes play a key role in immune defense
3. May fuse with lysosomes forming the by helping cells destroy pathogens and clean up
dead cells.
hetrophagolysosomes and autophagolysosomes Endosomes vs Phagosomes: Endosomes are
•Endocytosis is a general process where a cell takes in substances by engulfing them with
membrane-bound vesicles involved in sorting
its membrane. Types of Endocytosis: and processing smaller substances taken up by
•Phagocytosis: Often called "cell eating," this is a type of endocytosis specifically for large the cell through general endocytosis, while
particles, such as bacteria or cell debris. phagosomes are specialized vesicles formed
•Pinocytosis: Also known as "cell drinking," it involves the uptake of liquids and dissolved
substances. during phagocytosis that specifically engulf and
•Receptor-Mediated Endocytosis: In this type, cells use receptors to target specific digest large particles, such as bacteria and
molecules, allowing precise uptake of certain substances like hormones or cholesterol. cellular debris.
 Proteaso
mes
• A made
cylindrical
of four
structure
stacked rings, each composed of seven proteins
including proteases
• Degrade denatured or nonfunctional polypeptides tagged for degradation with a small
protein called ubiquitin
 ER-Attached Ribosomes:
Riboso •Mainly produce proteins for secretion outside the cell, like
hormones and digestive enzymes.
mes
• About 20 × 30 nm in size Free Ribosomes:
•Primarily produce proteins that function within the cytoplasm,
• Composed of : like metabolic enzymes and cytoskeletal proteins.
•Thus, while ER-attached ribosomes focus on proteins for
1. Several types of ribosomal RNA (rRNA)
secretion or the membrane system, free ribosomes make
2. Specific ribosomal proteins proteins mainly for intracellular functions, though exceptions
exist for both.
• Have large and small subunits
• Found in two forms: free ribosomes and polyribosomes (polysomes)
free ribosomes are individual ribosomes
that synthesize proteins for use within
the cell, while polyribosomes are
clusters of multiple ribosomes(both free
and bound to the endoplasmic
reticulum) translating the same mRNA
molecule simultaneously, increasing the
efficiency of protein production.
 Polyribosomes or
polysomes
• Clustered along a single strand of mRNA, and occur as:
1. Free polyribosomes
• Found in the cytoplasm
• Synthesize structural proteins and enzymes for intracellular use
2. Attached polyribosomes
• Attached to the outer nuclear membrane and RER
• Produce proteins to be secreted, proteins of the ER, the Golgi apparatus,
Lysosomes or the cell membrane
 Cytoskele
tonnetwork of protein filaments
• Gel-like
• Include:
1. Microtubules
2. Intermediate filaments
3. Microfilaments
 Microtubu
les
• Tubular with 24 nm outer diameter, 5 nm wall thickness and variable length up to many
micrometers
• Wall is made by tubulin heterodimers
• Each with α-tubulin and β-tubulin protein
• molecules
Arranged as thread like polymers called protofilaments
• 13 protofilaments, circumferentially form wall of a
microtubule
 Microtubu
les
• Contain microtubule-associated proteins (MAPs), Include:
a. Kinesin
o Transport vesicles towards the plus end
b. Dynein
o Move vesicles towards the minus end
c. Dynamin
o Motor for sliding microtubules in respect to each other, for activities such as
elongation of axon at growth cone
 Microtubul
es include
• Functions
• Intracellular transport
• Maintaining cell shape
• Intracellular compartmentalization
• Cell migration
 Intermediate
filaments
• Are 10-12 nm wide, between microtubules and microfilaments
• Unlike microtubules and actin filaments, are stable, conferring increased mechanical
stability to cell structure and tensile strength
• Formed of tetramers of rod-like proteins making staggering helical cable-like bundles
Intermediate
filaments
• Several types, the most common ones include:

1. Lamins in nuclear lamina of all types of cells

-
2. Keratin or Cytokeratin - in all epithelial cells

3. Vimentin - in mesenchylly derived cells

4. Desmin - in all muscle cells

5. Neurofilaments - in neurons

6. Glial fibrillary acid protein (glial filament) - in glial

cells
 Microfilaments (Actin
Filaments)
• Measure
diameter
5 -7 nm in
• Composed of polymers of globular G-actin monomers that assemble into a double-
stranded helix of filamentous F-actin
• G-actin is added to preexisting filaments for growth and branching, but new filaments can
also be formed
 Microfilaments (Actin
•
Filaments)
Have various myosin motor transport cargo along F-actin.
• Movement is usually toward the (+) ends
• Interactions between F-actin and myosins form the basis for various cell
movements, which include:
• Transport of organelles, vesicles, and granules in the process of cytoplasmic
streamin Myosin is a crucial motor protein involved in muscle
g contraction, cell movement, intracellular transport, and cell
• Cytokinesis during mitosis division, functioning primarily through its interactions with
actin filaments in the cytoskeleton and powered by ATP
• Endocytosi hydrolysis.
s Motor proteins are specialized proteins that convert
• Muscle and contractile cells contraction chemical energy, typically derived from ATP hydrolysis, into
mechanical work to produce movement within cells.
 Centriol
esat the centrosome, near the nucleus and Golgi bodies
• Located
• Cylindrical shaped, as a pair, perpendicular to each other
• Made by 9 triplets of microtubules
• Each microtubule in the triplet share portion of its neighbor’s wall
• Surrounded by pericentriolar bodies or microtubule-organizing centers (MTOCs)
• The MTOCs contain γ-tubulin rings, each of which serves as the nucleation site for the
growth of a single microtubule
 The pericentriolar material (PCM), often referred to as the pericentriolar body or pericentriolar matrix, is a
dense, amorphous collection of proteins surrounding the centrioles within the centrosome.
 A nucleation site is a specific location where the initial assembly, or "nucleation," of microtubules begins in
the pericentriolar material (PCM) surrounding the centrioles where microtubule formation is initiated.
 dont forget the amount of centrioles in the shaft of flagella and

Centriol
body of cilia
Each basal body at the base of a flagellum or cilium has just one
centriole (not a pair like in centrosomes).
•Basal Body: This is like a tiny “root” structure that anchors the
es include:
• Functions flagellum or cilium to the cell and starts the process of building its
internal structure.
• Control microtubule polymerization •Centriole: The basal body itself is made up of one centriole. This
centriole helps organize and grow the long, hair-like structure of the
• Transmit physical organizing forces flagellum or cilium.
• Control movements of organelles and vesicles
• Form the poles of mitotic spindle apparatus
• Form basal bodies of cilia and flagella
 Inclusio
ns
• Metabolically inactive materials
• Include:
1. Lipid droplets
2. Glycogen granules
3. Various types of pigments
4. Crystals
 Inclusio
1.ns
Lipid droplets
• Mainly in the adipose cells, but also in many other cells
• Store Triglycerides as energy source and store cholesterol for the synthesis of
steroids
 Granules are small, dense, membrane-bound or non-membrane-

Inclusio bound structures that often contain various substances, such as

proteins, lipids & glycogen.

2. ns granules
Glycogen
• As clusters of electron-dense bodies (rosettes) specially abundant in the hepatocytes
and muscle cells
• Converted to glucose
 Inclusio
3.ns
Pigments
a. Hemosiderin
• Brown colored inclusion that accumulates within the macrophages
• Breakdown product of hemoglobin
Inclusio
nsLipofuscin
b.
• Yellowish-brown pigment that increase with increasing age
• Are residual bodies of lysosomal activities
Inclusio
ns
c. Melanin
• Brownish pigment mainly in the cells of skin and hairs (melanocytes &
keratinocytes), but also in some neurons and in the pigment epithelium
of retina
 Inclusio
•
ns
e. Carbon particles
Mainly in macrophages located in the lungs
 Inclusio
ns
d. Carotenoid
•A yellowish-orange-red pigment obtained from vegetables and fruits
 Inclusio
4.
ns
Crystals
• May be crystalline form of certain proteins
• In humans crystals described include:
 Crystals of Reinke in the interstitial cells of Leydig of testis
 Crystals of Charcot-Böttcher in the Sertoli cells of testis
 3.
Nucleus
• Oval, elongated or flattened in shape.
• 5-10 µm in diameter
• Composed of:
1. Nuclear envelope
2. Chromatin
3. Nucleolus
4. Nucleoplasm
 Nuclear
envelope
• Double membrane separated by perinuclear cisternal space (30-50 nm
1.wide)
Outer nuclear membrane
– Shows continuities with the RER and has polyribosomes attached to it.
–occasional
The outer nuclear membrane is surrounded by a loose network of intermediate
filaments called vimentin from its cytoplasm aspect (the site of the membrane that
faces the cytoplasm).
– These filaments provide structural support to the nucleus and help maintain its shape.
 Nuclear
2. envelope
Inner nuclear membrane
– Lined internally by nuclear (fibrous) lamina: composed of intermediate filaments
called Lamins.
• Contains specific lamin receptors and several lamina-associated proteins
• Functions include:
– Serve as scaffolding for chromatin
– Involved in nuclear organization, cell-cycle regulation, differentiation, and
gene expression
 Chroma
tinand associated 5 basic proteins called histone, and nonhistone proteins
• DNA
• The 46 human DNA, forming the Chromatin together, are about 40,000 μm long in
average making a total of 1.8 - 2 meter long. The whole genetic material in the nucleus(The Chromatin) is
1.8 - 2 meter long.
• Located in a nucleus with a diameter of only 5 - 10 μm
• The DNAs went a super helical coiling to give repeating nucleosomes connected by
short connecting strands.
 Chroma
tin
• A nucleosome is made by:
• 146-166 base pairs of the DNA strand wrapped twice around a core of a pair of 4 types
of histones (H2A, H2B, H3 and H4) called an octamer
• The connecting strand as a 2-nm filament is made by 50-80 DNA base pairs with
another type of histone (H1)
 Chroma
tin further progressive super coiling and looping
• Undergo
 Chroma
tininterphase chromatin is identified as:
• During
1. Heterochromatin transcriptionally inactive
• As dense basophilic staining by LM and dense granular regions by EM
• Heterochromatin is the more condensed form of chromatin that is generally associated with inactive or
silenced genes.
• It appears darker under a microscope due to its tightly packed structure. This dense packing restricts
access to the underlying DNA, preventing transcription.
2. transcriptionally
active
Euchromatin
• Lightly stained & dispersed throughout the nucleus (not evident in the light microscope)
• Euchromatin is the less condensed form of chromatin that is typically associated with active gene expression.
• It appears lighter under a microscope due to its loose, extended structure. This loose packing allows the DNA
to be unwound and accessed by RNA polymerase and other transcription factors.
 Heterochrom
• Is found as:
atin Marginal chromatin – along the inner surface of the nuclear envelope
1.
associated with the fibrous lamina
2. Karyosomal chromatin – dispersed throughout the nucleus
3. Nucleolar associated chromatin – in the nucleolus
4. Barr body (sex chromatin) - One of the X chromosomes in females is inactivated and condensed
into a compact structure called a Barr body. It appears as a dark, drum-stick-shaped appendage in the
nuclei of neutrophils (a type of white blood cell).
 Nucleol
us
• Round basophilic & non-membrane bound bodies
• Is site of rRNA synthesis and initial assembly of ribosomes
• Has a protein named nucleostemin that regulates the cell cycle and influences cell
differentiation.
 Nucleopla
sm
• Includes nuclear components in addition to the chromatin and nucleolus
• Although appear amorphous medium under microscopy it contains:
• Many proteins and other metabolites
• Intranuclear lamin-based structures
• Protein filaments of the nuclear pore complexes
• RNA transcription and processing apparatus
 Cell
• Somatic cells may be classified according to their mitotic
Renewal
activity as:
1. Renewing cells Slowly Renewing Cells: An example is liver cells, which can regenerate but do
so gradually.
2. Rapidly Renewing Cells:
• Slowly renewing cells: these cells divide at a slower rate. E.g., Liver cells
• Rapidly renewing cells: these cells divide quickly to replace cells that are frequently lost.
E.g., Skin cells, Gut lining cells
3. Stable cells (quiescent cells): are typically in a resting phase (G0 phase of the cell cycle) and
do not divide under normal circumstances. But they can start dividing only in response to
specific signals, such as injury or tissue loss. E.g., Liver cells, Somatic cells
4. Static cells: these cells are considered “permanent” because they do not divide after
they are fully developed. Once they are mature, they remain in a resting state for life.
Damage to these cells is often irreversible. E.g., Neurons, Cardiac muscles
•Slowly renewing cells are constantly dividing, just at a slow rate, as part of
ongoing tissue maintenance.
•Stable cells are mostly inactive and don’t divide under normal conditions. They
enter the cell cycle only when triggered by specific events like injury or stress.
•LIVER CELLS:
Normal conditions → Slowly renewing. mucus cells 5-6
days skin cell up-to
In response to damage → Stable/quiescent, then activated for regeneration. a month
 Cell
Cycle
• A self-regulated sequential events that controls cell growth and cell division
• The cell cycle stops at several checkpoints (ensuring that the cell is ready to move
on to the next stage.)
• Driven by a family of cytoplasmic proteins called cyclins, which are cyclically
synthesized and degraded during each cycle in response to intracellular or
environmental signals.
 Cell
Cycle
• In somatic cell is comprised of:
1. Interphase
a. Gap1 (G1) phase
b. Synthesis (S) phase
c. Gap2 (G2) phase
2. Mitosis(For somatic
cells)
Meiosis(For Germ
cells)
 The G1 (gap 1)
phase
• Longest and most variable phase - a few hours to several days
• The cell:
• Gathers nutrients
• Synthesizes: RNA, proteins, organelles and
• Centrioles begin to duplicate.
• These components grow to the size of the parent cell
 The G1 (gap 1)
phase
• Monitored by 2 checkpoints:
1. G1 DNA-damage checkpoint
• Monitors the integrity of DNA: checks whether DNA damage or mutation occurred.
• If the DNA has irreparable damage, it will most likely undergo programmed cell
death
(apoptosis) necrosis

- The cell undergoes apoptosis to prevent the propagation of damaged DNA.

The DNA damage checkpoint in G₁ is important even
though DNA duplication (replication) hasn't occurred
yet. The G₁ DNA damage checkpoint ensures that
cells do not enter DNA replication (S phase) with
damaged DNA.
This helps maintain genetic stability by:
• Repairing any damage from past events or
environmental exposure.
• Providing a clean template for accurate DNA
replication.
• Reducing the risk of mutations and cancer.
 The G1 (gap 1)
2.phase
Restriction checkpoint
– Is the most important checkpoint in the cell cycle where the cell self-
evaluates its own replicative potential.(determines whether the cell will continue through the cycle
or exit to the G₀ phase (a resting state).)
• If requirements are met the cell enters the S phase.
• If requirements are not met cell cycle is paused and:
1. Attempt to remedy the problematic condition, or
2. Enter G0 and awaits further signals permanently (for static cell) or
temporarily (for stable cells).
 S (synthesis) Centrioles:
•G1 Phase: The two centrioles
• Takes about 7.5 to 10 hours
phase
• Undertake:
within the centrosome begin to
separate.
•S Phase: A new centriole
(procentriole) begins to form
• DNA replication and doubling perpendicular to each existing
centriole.
• Histone synthesis These procentrioles grow and
mature throughout the S and G2
• Each chromosome duplicates, forming sister chromatids. phases.
• Centrosome duplication
• Has S DNA-damage checkpoint
• Monitor quality of replicating DNA (ensures that DNA
replication proceeds accurately without errors)
 G2 (gap 2)
• Is aphase
period of :
(cell is growing because many proteins and organelles are synthesized)
1. Further cell growth and reorganization of cytoplasmic organelles occurs.
2. Examination of the replicated DNA by 2 checkpoints:
a. G2 DNA-damage checkpoint: this checkpoint ensures that the DNA has been
accurately replicated and is free of damage before the cell proceeds to mitosis (M
phase).
b. The unreplicated-DNA checkpoint: this checkpoint ensures that all DNA has
been fully replicated before the cell enters mitosis.
• Prevents entry into the M phase if DNA synthesis is incomplete

Genetically problematized cell may fix

the problem, do
apoptosis(programmed cell death) or
enter a state of senescence
(where it
remains alive/metabolically
active but no longer divides.)
Kinetochore is a protein complex located at the
centromere of a chromosome that serves as the
attachment site for spindle fibers during cell
division.
Gametogenesis: the formation of female and male reproductive cells
• It is done by Meiosis.
• Before meiosis germ cells were diploid cells, having 23 pairs of chromosomes(like somatic cells).
• After meiosis, these cells become haploid cells, but the time of formation of haploid cells is different for males and
females.
• Males: Spermatogenesis(in testes)
 Meiosis I: Is at puberty; diploid cells called Spermatogonia undergo Meiosis I.
 Meiosis II: Occurs in continuation to Meiosis I, resulting in 4 haploid spermatids that mature into sperm cells.
 Both Meiosis I and Meiosis II occur throughout male’s life.
• Females: Oogenesis(in ovaries)
 Meiosis I: Begins when females are in the womb and goes up to Prophase I and pauses after each chromosome
pair(maternal and paternal) formed their sister chromatids. The diploid cells before meiosis are called Oocytes.
 At puberty, the duplicated maternal and paternal chromosomes(2 maternal and 2 paternal sister chromatids) begin
crossing over and Meiosis I end up resulting one larger ovulated oocyte cell (secondary oocyte) (ready for
fertilization) and one smaller nonfunctional polar body(gets discarded) and two of them are haploid cells but with
different genetic material(because of crossing over).
 During Meiosis I, the division of the primary oocyte is asymmetrical. Because of this, the secondary oocyte is only
important and receives much of the cytoplasm and cellular resources, while the polar body receives very little. So,
the secondary oocyte is retained for further fertilization, whereas the polar body is discarded.
 Meiosis II: The secondary oocyte doesn’t undergo MtII until a sperm cell comes for fertilization. When the
secondary oocyte is fertilized by sperm, it completes Meiosis II, resulting in the formation of 2 haploid cells the
ovum (fertilized egg) and another polar body (which is again discarded).
 Finally, the ovum gets implanted into the Uterus.
Revision
1. Introduction to Cells
•Cells: Cells are the smallest living units that make up all living organisms. Each cell acts as a self-sufficient
unit that carries out essential life functions, such as generating energy, processing nutrients, and
reproducing. Cells are specialized in structure and function depending on their role in the body.
•Metabolism: Refers to the chemical reactions within cells that allow them to maintain life, involving
processes such as:
• Absorption: The intake of necessary molecules from the environment, like nutrients and water.
• Synthesis: The creation of complex molecules, such as proteins, that cells need for growth and
repair.
• Respiration: The process of converting glucose and oxygen into energy (ATP), mainly in
mitochondria.
• Excretion: The removal of waste products from the cell to maintain a clean internal environment.
•Growth and Regeneration: Cells have the ability to grow in size and number, and many can regenerate to
replace damaged or dead cells, ensuring tissue repair and maintenance.
•Irritability (Excitability): The cell’s ability to respond to external stimuli, such as changes in temperature,
chemical signals, or mechanical stress. This is crucial for maintaining homeostasis.
•Movement: Certain cells, like muscle cells or immune cells, are capable of moving to perform their
functions, whether it be contraction, response to infection, or other actions.
•Reproduction: Cells reproduce through mitosis (for somatic cells) or meiosis (for gametes), which is vital
for growth and maintaining genetic continuity.
•Aging and Death: Cells have a natural lifespan. Over time, cellular structures degrade, which eventually
leads to cell death through apoptosis (programmed cell death) or necrosis (cell death due to injury).
 2. Characteristics of Cells
•Number: Humans are made up of around 100 trillion cells, each performing specific functions
necessary for survival.
•Types: There are more than 250 types of cells in the human body, including red blood cells, nerve
cells, muscle cells, and more. Each type is adapted to perform unique tasks.
•Sizes: Cell size varies from about 5 µm in small cells like red blood cells to about 120 µm in egg
cells, the largest human cells.
•Shapes:
•Round Cells: These cells are circular in shape, like red blood cells, which have a slightly indented center (biconcave shape)
for flexibility and ease of passage through blood vessels.
•Oval Cells: These cells are egg-shaped or elongated spheres. An example is certain epithelial cells, which have a smooth,
slightly stretched appearance.
•Columnar Cells: Tall and slender, these cells are shaped like columns. Found in the intestines and respiratory tract, they are
typically aligned in rows to maximize surface area.
•Multipolar Cells: These cells have multiple projections extending from the main body, resembling branches. Neurons are a
prime example, with extensions for transmitting electrical signals.
•Polygonal Cells: These cells have multiple flat sides, forming irregular shapes that fit tightly together, like tiles. They are often
seen in liver tissue, where the cells connect in a continuous layer.
•Cylindrical Cells: These are tube-shaped cells, seen in muscle tissue, especially in skeletal muscle fibers, which are long,
cylindrical, and multinucleated for contraction.
•Fusiform Cells: Shaped like a spindle, these cells are thick in the middle and taper toward the ends. Smooth muscle cells are
an example, allowing for controlled contraction and relaxation.
•Pyramidal Cells: With a broad base tapering to a point, these cells are triangular or pyramid-shaped. Found in certain parts of
the brain, such as the cerebral cortex, they have a single thick dendrite that extends toward the brain surface.
•Pyriform Cells: These cells are pear-shaped, with a rounded end that tapers to a narrow point. Some neurons and gland cells
can take this shape, often having a pointed end for connection or secretion.
Components of the Eukaryotic Cell
•Cell Membrane: A flexible boundary that surrounds the cell, providing structure and regulating what
enters and exits. Composed of:
• Lipids: Mainly phospholipids arranged in a bilayer, which makes the membrane semi-permeable.
• Proteins: Integral (embedded in the membrane) and peripheral (loosely attached), responsible for
transport, signaling, and structural support.
• Carbohydrates: Often attached to proteins (glycoproteins) or lipids (glycolipids), forming the
glycocalyx, which aids in cell recognition and interaction.
•Cytoplasm: The gel-like substance within the cell membrane that holds all organelles and is the site for
numerous cellular processes. It consists of:
• Cytosol: The fluid component, rich in nutrients, salts, and proteins.
• Organelles: Specialized structures such as mitochondria, ER, and Golgi apparatus that perform
specific cellular functions.
• Cytoskeleton: A network of protein filaments and tubules that provides structure, support, and
aids in cellular movement.
•Organelles:
• Endoplasmic Reticulum (ER): Network of membranes involved in protein (rough ER) and lipid
(smooth ER) synthesis.
• Golgi Apparatus: Modifies, sorts, and packages proteins for secretion or use within the cell.
• Lysosomes and Peroxisomes: Lysosomes break down waste materials, while peroxisomes detoxify
harmful substances and break down fatty acids.
• Mitochondria: Known as the powerhouse of the cell, producing ATP, the cell’s main energy
currency.
• Ribosomes: Composed of RNA and protein, they synthesize proteins based on mRNA instructions.
Cytoskeleton Nucleus
•Microtubules: Thick, hollow tubes that maintain cell shape, •Nuclear Envelope: A double membrane structure with pores
help with intracellular transport, and form the mitotic spindle that regulate the passage of molecules in and out of the
during cell division. They are composed of tubulin subunits. nucleus.
•Microfilaments (Actin Filaments): Thin fibers that support cell •Chromatin: DNA wrapped around histones to form
shape and are involved in movement and division (cytokinesis). nucleosomes, which condense to chromosomes during cell
They are made of actin. division.
•Intermediate Filaments: Rope-like structures that provide • Heterochromatin: Densely packed, transcriptionally
tensile strength, helping cells withstand mechanical stress. They inactive DNA.
are made from various proteins, such as keratin, depending on • Euchromatin: Loosely packed, transcriptionally active
cell type. DNA, allowing gene expression.
•Nucleolus: A dense region within the nucleus where ribosomal
Cell Cycle RNA (rRNA) is synthesized and ribosomes are assembled.
•G1 Phase: The cell grows, produces proteins, and checks DNA •Nucleoplasm: The viscous fluid within the nucleus that
integrity. This phase has two checkpoints to ensure the cell is contains the nucleolus, chromatin, and other nuclear
prepared for DNA replication. components.
•S Phase: The phase of DNA replication where each
Cell Renewal
chromosome is duplicated. •Renewing Cells: Cells that continuously divide, like skin or
•G2 Phase: The cell continues to grow, and organelles
blood cells, to replace old cells.
reorganize. G2 phase checkpoints ensure DNA replication is •Stable Cells: Cells that only divide when stimulated by injury or
complete and accurate. specific needs, like liver cells.
•Mitosis (M Phase): The cell divides, with each new cell •Permanent Cells: Cells that do not divide once fully developed,
receiving an identical set of chromosomes. such as neurons and cardiac muscle cells.
No Membrane
•Ribosomes
•Centrosomes (made up of centrioles and pericentriolar
material)
•Cytoskeleton components (microtubules, intermediate
filaments, actin filaments)
Single Membrane
•Lysosomes
•Peroxisomes
•Endoplasmic Reticulum (ER)
• Rough ER
• Smooth ER
•Golgi Apparatus
•Endosomes and Phagosomes (and vesicles associated with
them)
Double Membrane
•Nucleus
•Mitochondria
4/4/2023 Fikre B. 74