Cell biology (cytology)
Cell theory
Hooke (1665): described tiny square boxes of a thin slice of
cork called them cells.
Leeuwenkoek (1675): described the 1st living cells.
Brown (1831): described the presence of a central body
in each cell & called it the nucleus.
Schleiden (1838): showed all plants are composed of cells.
Schwaan (1839): showed all animals are composed of cells
++ animal cell lacks cell wall that found in plant cell.
Watson & Crick (1953): developed the model of DNA
which is the hereditary material.
Cell theory:
It states:
1- All living organisms are made of cells.
2- The cell is the smallest living thing that can perform all
the functions of life.
3- All cells must come from preexisting cells.
Types of cells
There are two basic types of cells (according to
internal complexity) which are:
A- Prokaryotes:
Main characteristics of prokaryotes:
1- They are the smallest, most primitive and most diverse.
2- They are mainly unicellular.
3- They have cell walls above the cell membrane.
4- They do not have a nuclear membrane.
5- They lack membranous organelles
6- Ribosomes are slightly smaller than those found in
eukaryotes.
7- They have a faster rate of division.
8- They never form tissues.
The classic example of prokaryotes is Bacteria
General plan of prokaryotic cell
* Single strand
* Circular
* Attached to cell
membrane
* Attached with small
amount of protein
Bacteria
e.g. Bacteria
2- Eukaryotes
The main
characters of
eukaryotic
cells are:
* Include
complex forms.
* The presence
of nuclear
membrane
(nucleus).
•The presence
of
membranous
organelles.
An eukaryotic animal cell
e.g. Animal
and plant cells
Another
shape of
eukaryotic
animal cell
An animal cell
Another shape of eukaryotic animal cell
Another shape of eukaryotic animal cell
An eukaryotic plant cell
mitochondrion
Central vacuole
nucleus
nuclear
envelope
chromatin
microtubules
nucleolus
microfilaments
rough endoplasmic
reticulum (ER)
chloroplast
smooth ER
plasmodesmata
peroxisome
ribosomes
cell wall
Golgi apparatus
plasma membrane
Another shape of eukaryotic plant cell
The difference between plant cells & animal cells where:
1- The plant cells lack centrioles which involved in
mitotic cell division.
2. Plant cells have chloroplasts (site of photosynthesis).
3- Plant cells have cell wall (composed cellulose, pectin
or both of them). ???!!!!!!
4- Plant cells have large central vacuole. ???!!!!
Cell shape
Variable: oval, spindle, amoeboid, flat, polyhedral, spherical,
square, columnar….etc.
Cell size
Variable: related to the function.
* The smallest is red blood cell (RBC).
* The largest is the ovum (egg) {ostrich egg = 0.5 kg , 30 cm).
* The longest is the nerve cell (1 m).
The cell
Cytoplasm
Organelles (organoids)
Nucleus
Inclusions
1- Nuclear membrane
2- Nuclear sap
Membranous org. Non-membranous org.
3- Nucleoli
4- Chromatin network
1- Ribosomes
2- Microtubules
1- Cell membrane
3- Centrioles
1- stored food
2- Mitochondria
2- secretory granules
3- Endoplasmic reticulum
3- colored pigments
4- Golgi apparatus
5- Lysosomes
4- Crystals
6- Microbodies (peroxisomes)
Cytoplasmic organelles
A- Membranous organelles
1- The plasma (cell) membrane (plasmalemma):
It is very difficult to seen by light microscope (80-100 Angstrom).
By using electron microscope, it shows three layers model
Dark layer
Light layer
Dark layer
Three layers (trilamellar) model
Molecular structure of cell membrane :
It is made of
1) Lipid component:
i- Phosopholipid molecules:
aHeads:
(phosphate
groups)
(hydrophilic) (polar) (charged).
b- Tails: (fatty acids) (hydrophobic)
(non-polar) (non-charged).
Dark
layer
Cytoplasm
Phosphate
polar heads
Light
layer
Fatty acids
non-polar
tails
Dark
layer
Exracellular
(intercellular)
fluid
Phospholipid bilayer (Trilamellar membrane)
Extracellular fluid
Hydrophillic heads (phosphate groups)
(polar)
Bilipid
layer
Hydrophobic tails
(fatty acid tails)
(non-polar)
Phospholipid molecule
Hydrophillic heads
Cytoplasm
So, phospholipids are arranged into two layers i.e. form a bilipid
layer.
Also, it is arranged in trilamellar membrane (dark, light and dark
layers).
Molecular structure of cell membrane (continue):
ii- Cholesterol molecules:
a- Hydroxyl radicals: (hydrophilic).
b- Steroid nuclei: (hydrophobic).
Note: Cholesterol is found in the hydrophobic tails of
phospholipid especially to the inner cytoplasmic ones.
2) Protein component:
i- Integral (intrinsic) protein:
a- Small molecules: embedded in the lipid bi-layer.
b- Large molecules: in the center & extended from both
surfaces.
ii- Peripheral (extrinsic) protein: loosely attached to both
surfaces.
Small molecule
Large molecule
3) Carbohydrate component
It is polysaccharides. It may be attached to:
i- Protein forming glycoproteins.
ii- Phospholipid forming glycolipids.
Both glycoproteins & glycolipids are called glycocalyx (cell coat).
The following structure of plasma membrane form what is known
as:
fluid-mosaic model
which states that:
The cell membrane is phospholipid bilayer with protein
molecules partially or wholly embedded.
The following diagrams represent this model.
Plasma (cell) membrane
(fluid mosaic model)
glycoprotein
glycolipid
carbohydrate
Extracellular
fluid
protein
cholesterol
phospholipid
filaments of cytoskeleton
cytoplasm
Functions of the protein in the plasma membrane:
1) Acts as channels.
2) Acts as enzymes.
3) Acts as receptors
4) Acts as markers (cell identification markers):
5) Acts for cell adhesion:
6) Determine the ABO blood grouping (typing).
Functions of plasma membrane proteins
(1)
(4)
(2)
(5)
(3)
(5)
Functions of the cell membrane
Different substances can pass into and out of cells at different
rates is partly due to the properties of the particles and the
structure of the plasma membrane.
Movement into and out of the cell happens in many different ways
which are:
1- Passive transport:
 The cell membrane is referred to as selective permeable
(semi-permeable).
1) It does not require energy. It is achieved by the kinetic
energy of the molecules.
2) It takes place according to (= with) the concentration
gradient.
It continues until the concentration of the molecules is the same
on both sides of a membrane i.e. equilibrium.
The passive transport comprises:
a) Simple diffusion:
It transports solutes such as O2, CO2, peptides, cholesterol and
small hydrophobic molecules (i.e. non-polar solutes).
Note: A polar solute cannot pass through the membrane because it
cannot pass through the non-polar lipid core of the membrane.
The rate of diffusion depends on temperature and size.
Molecules diffuse faster at higher temperatures.
Smaller molecules diffuse faster.
b) Facilitated diffusion:
 It transports solutes such as Glucose.
 It is facilitated because a transport protein in the membrane enhances
(increases) the transport of the substance across the membrane.
 It take place through pores and gated channels.
Outside the cell
A transport protein
Low
concentration
of solutes
Outside the cell
High
concentration
of solutes
Inside the cell
Inside the cell
Two models for facilitated diffusion
(A) pores
(B) gated channels
c) Osmosis:
It is the diffusion of water
(solvent) from an area of high
water concentration (hypotonic
solution) (less solute) to an area
of lower water concentration
(hypertonic solution) (more
solute) .
i.e. The transport is achieved
according to the concentration
gradient i.e. from higher water
concentration
to
lower
concentration (of water).
Also, it needs no energy.
Osmotic relationships in cells:
When the cell is placed in:
1) A hypertonic solution
Water diffuses out of the cell till equilibrium is reached.
It will shrink and die.
This condition is called plasmolysis.
2) A hypotonic solution
Water diffuses into the cell
till equilibrium is reached.
It causes it to swell and
often burst.
This condition is called
cytolysis.
3) An isotonic solution
2- Active transport:
It takes place against the concentration gradient.
It uses energy (in the form of ATP).
Also, it uses membrane proteins.
An example of this type of active transport is the sodium-potassium pump.
The sodium-potassium pump is formed from:
1) Carrier proteins; each has 3
receptor sites for Na+ (inside of
the cell) and 2 receptor sites for
K+ (on the outside).
2)
Adenosine
triphosphatase
(ATPase) (enzyme) adjacent
(near) to the Na+ binding sites.
3) ATP that pumps Na+ out of the
cell and K+ into the cell.
Mechanism of sodium-potassium pump:
So, an electrical gradient across the cell membrane was
achieved i.e. the outside of the membrane becomes
positively charged and the inside of the membrane
becomes negatively charged .
This unbalanced charge is important for conduction of
nerve impulses, muscle contraction, … etc.
Summary
Simple
H2 O
+
Osmosis
3Bulk
transport
(vesicle-mediated
(endocytosis & exocytosis):
 It needs energy like active transport.
 It transports large molecules through vesicles.
It comprises:
a) Endocytosis:
It moves large molecules into the cell.
It includes three different
processes which are:
transport)
i- Phagocytosis (cell eating):
When the formed vesicle encloses
solid food particles (such as
bacteria, damaged cells, large
food particles or whole cells) with
little extracellular fluid.
i- Phagocytosis
ii- Pinocytosis (cell drinking):
When the formed vesicle encloses mainly extrcellular fluid i.e.
liquid
iii- Receptor-mediated endocytosis :
When specific molecules - such as microbes - in the extracellular
fluid bind to sites on the plasma membrane.
Note
Endocytosis removes membranes from cell surface to form vesicles.
iii- Receptor-mediated endocytosis
ii- Pinocytosis
Summary
Endocytosis
b) Exocytosis
It is applied when the transportation is out of the cell.
It transports secretory products such as mucous and enzymes or
waste products made in the cell.
Note
Exocytosis adds membranes to the cell surface form vesicles.
Exocytosis
2- Mitochondria
It is found in all nucleated cells, (absent in RBCs).
Mitochondrial structure:
They are bounded by a double
membrane; smooth outer
membrane and folded inner
membrane.
The folds of the inner
membrane is called cristae that
increase the inner membrane’s
surface.
The distance between both
membranes is called inter
membrane space.
The matrix contains DNA (found in the nucleus), ribosome
(found in the cytoplasm), granules and ATP synthase particles.
2- Mitochondria (continue)
Notes
 The mitochondria are found in
a great number in the cells
with high activity e.g. muscle
and liver cells.
 The number of cristae depends
on the activity of the cell. i.e.
The cell with high activity has
numerous close cristae.
:‫الخالصة أن الميتوكوندريا ثشبه النواة فى‬
.‫(أنها تحاط بغشائين‬1)
.‫) الموجود فى النواة والذى يكون الجينات المكونة للكرموزومات‬DNA( ‫( أنها تحتوى على الــ‬2)
.‫) فتوجد فى السيتوبالزم‬Ribosomes( ‫** أما الريبوسومات‬
It is responsible for formation of energy (ATP) from nutrients,
hence they are called the powerhouse of the cell.
ATP is required in different vital activities such as muscle
contraction, protein synthesis, active transport, …etc.
3- Endoplasmic reticulum (ER)
ER occurs in all kinds of nucleated cells.
It a system of hollow network of branched and joined tubules .
Note: 1 1 cm3 (mL) of liver tissue contains about 11 m2 of ER.
There are 2 types of
ER which are:
1) Rough (granular)
ER which covered
by ribosomes.
2)
Smooth
(agranular)
ER
which
lacks
ribosomes.
3- Endoplasmic reticulum (ER) (continue)
Note
Both types may be connected in the same cell.
Also, one type may be changed to the other depending on the need of the cell.
Functions of ER:
1- Helps molecules to transport
through the cell and from one cell
to another (both rough & mooth
ER).
2- Involved in the synthesis of
proteins due to the presence of
ribosomes (rough ER).
3- Involved in the synthesis of
steroids (smooth ER).
4- Helps to regulate calcium levels
in muscle cells (smooth ER).
5- Helps in the break down of
toxic substances in the cell
(smooth ER).
4- Golgi apparatus (Golgi body) (Golgi complex)
It was found in eukaryotic cells.
The Golgi apparatus is
made up of:
1- A stack of flattened
elongated sacs called
cisternae.
The cristernae have:
i) A cis (immature) face
{directed towards the ER
and nucleus},
ii) The medial region {in
the middle} and
iii) The trans (mature)
face {directed towards the
plasma membrane}.
ER & Nucleus
Plasma membrane
4- Golgi apparatus (Golgi body) (Golgi complex) {continue)
2- Vesicles: Which may be:
a) Incoming transport
vesicles (microvesicles)
(transferring
vesicles)
which are detached from
rough ER. They move
towards the cis-face of
cisternae. These vesicles
contain
the
newly
synthesized protein.
b) Outgoing transport vesicles (large vesicles) which are
detached from the trans face of cisternae. These vesicles are filled
with protein.
c) Intermediate vesicles which are found in large number close
to the periphery of the medial region of sacs 9 cisternae).
Functions of Golgi apparatus:
1) Storge: Proteins that formed by
ribosomes migrate as incoming transport
vesicles (microvesicles) to fuse with the
membrane of cis-face where they are
collected, condensed and then enclosed by
membranes forming outgoing transport
vesicles (large vesicles) that contain
secretory granules. These vesicles move to
the plasma membrane where they release
their contents by exocytosis.
2) Packing: It forms lipoproteins by
bounding both lipids (from smooth ER) and
proteins (from rough ER) inside a
membrane. The formed lipoprotein granules
release from trans-face of Golgi apparatus.
3) Secretion: Such as hormones (by
endocrine glands), enzymes (by exocrine
glands), mucous (by goblet cells).
4) It helps in the formation of the acrosome of the sperm which has the ability
to penetrate the membrane of the ovum
5- Lysosomes
They are saclike structure
surrounded by a single
membrane. It contains
powerful digesting enzymes
such as acid phosphatase,
deoxyribonuclease,
ribonuclease, … etc.
Their number is affected by
different physiological and
pathological changes .
Decrease their number
during fasting and ageing.
Functions of lysosomes:
Lysosomes are responsible for digestion of biological compounds.
This digestion may be one of the following:
i) Intracellular digestion: This takes place inside the
cytoplasm which may be:
a) Exogenic origin: They digest the taken substances by
endocytosis in a process known as heterophagy. The engulfed
material is then digested by the enzymes into small molecules.
b) Endogenic origin:
They digest some part of
the
cytoplasm
e.g.
mitochondria by a process
known as autophagy.
Note
If digestion is completed,
residual bodies may be
formed which may be go
out by exocytosis or may be
remain in the cell.
These remaining residuals
represent an index of cell
ageing.
Heterophagy
Autophagy
??!!!
ii) Extracellular digestion:
 Lysosomal enzymes discharge (= go) outside the cell to destroy
some surrounding structures.
This explains how the sperm can penetrate the protecting coat of
the ovum during fertilization.
iii) Autolysis:
 It is a process in which the cell is self-destructed.
When cells approach death, lysosomes rupture in the surrounding
cytoplasm causing the digestion of the whole cell.
This action is not accidental but it is regulated by signals that
scientists do not fully understand.
6- Peroxisomes (microbodies)
They are about the same size, or slightly larger than lysosomes.
They contain enzymes such as that involved in the degradation of
fatty acids and amino acids and catalase.
Peroxisome function:
Peroxisomes contain enzymes that degrade fatty acids and amino
acids.
In doing so they produce hydrogen peroxide (H2O2).
H2O2 is very toxic because it is unstable and spontaneously degrades
to produce compounds called free radicals.
Free radicals are very reactive because they have unpaired electrons
and will react with a variety of cellular macromolecules and alter
their structure.
Fortunately peroxisomes contain the enzyme called catalase that
degrades hydrogen peroxide to the less dangerous oxygen and water.
catalase
O2 + 2(H2O)
H2O2
B- Non-membranous organelles
1- Ribosomes
They are found in both prokaryotes and eukaryotes
but they are larger in eukaryotes.
They are formed in the nucleolus then pass through
the nuclear pores to the cytoplasm.
Each ribosome is composed of 2 subunits, a small
subunit and a large subunit. Between them there is a
small cleft in which a central growing polypeptide
chain is present.
Chemically, they are consisted of * ribosomal RNA
(rRNA) (65%) and * proteins (35%) i.e.
ribonucleoprotein.
Ribosomes are found in 3 different places or cases in
cells which are:
1. Free floating in cytoplasm as individual subunits or dimers.
2. Membrane bound on outer surface of rough ER.
3. Attached to mRNA molecule in a polysome (polyribosome).
Function of ribosomes:
 Ribosomes are the site of protein synthesis.
The mechanism
They receive amino acids (the building units of protein),
grouping them into peptide chains by interaction between
transfer RNA (tRNA) which carries the amino acids and
messenger RNA (mRNA) which carries the specific
genetic code from DNA in the nucleus.
2- Microtubules
The microtubule is a long
cylindrical structure with a cavity.
It is elastic and capable to bend
without breaking.
Chemically, it is made of dimmers
of alpha and beta tubulin (a type of
protein).
Functions of microtubules:
1- Microtubules form centrioles, cilia, flagella and microvilli.
2- They facilitate the transport of various particles inside the
cytoplasm.
3- They share in the formation of cytoskeleton of the cell.
Note
The
cytoskeleton
determines the shape
and
provides
mechanical support
to the cell.
It is formed from:
1) Microfilaments,
2) Intermediate
filaments and
3) Microtubules.
3- Centrioles
Centrioles are short hollow cylindrical
tubules that found near the nucleus.
There are two centrioles at right angles to each
other .
Centrosome
Each centriole consists of 9 peripheral sets of
microtubules arranged in a pin-wheel of 3
microtubules (triplet) in each set.
Thus, each centriole consists of 27 (3x9)
microtubules in the configuration of (9+0).
Functions of centrioles:
1- They play an important role in the process of
cell division where they form spindle fibers.
2- They are able to replicate giving identical
structures that migrate towards the plasma
membrane to form basal bodies from which
cilia or flagella.
3- They are involved in the cytoplasmic
movement.
Basal bodies:
So, the basal bodies and centrioles are
homologous structures with the same
configuration (9+0). Each cilium or
flagellum has a basal body located at the
base.
Flagella and cilia:
Cilia and flagella are hair-like structures
projecting from the basal bodies (that
found in the cytoplasm) and enclosed
(covered) by the plasma membrane.
Eukaryotes have 9 doublets (pairs) of microtubules
arranged in a circle around 2 central microtubules i.e.
(9 + 2).
Cilia are being much shorter than cilia.
Many unicellular organisms such as Paramecium
move by cilia.
Many unicellular organisms such as Euglena move by
flagella.
The 9+2 arrangement of microtubules in
a flagellum or cilium.
The upper respiratory tract have cilia while sperms use flagella to
move.
Microvilli
They are formed from microtubules covered by cell membrane.
They are finger like structures projecting from the surfaces of
some cells of intestine or kidney.
They increase the surface area for absorption.
Nucleus
The nucleus occurs only in eukaryotes.
It has a role in controlling the shape and features of the cell.
When a cell has grown to a certain size it divides into two cells.
It is composed of:
1- Nuclear membrane
(nuclear envelope),
2- Nuclear sap,
3- Nucleolus and
4- Chromatin network.
Nuclear sap
1- The nuclear membrane (envelope)
It appears as a double
membrane (outer and inner);
each is similar in structure to
the plasma membrane.
Numerous nuclear pores occur
on it, allowing RNA and other
chemicals to pass while DNA
can not go out through it.
Structure of the nuclear envelope and
Functions:
nuclear pores
It was used to protect DNA (genetic material that found in the
nucleus forming the chromosomes) from reactions that occur in
the cytoplasm which could damage it.
2- The nuclear sap (nucleoplasm):
It is a colloidal clear medium in which all the contents of the
nucleus are embedded
It contains lipoproteins, ions, enzymes … etc.
3- Nucleolus
There are one or more nucleoli in each nucleus.
It is involved in the formation of ribosomal RNA (rRNA), which
is responsible for protein synthesis in ribosomes.
4- Chromatin network:
The material of chromatin network is formed mainly from DNA
as a double helix around a core of protein called histone.
DNA form the genes of chromosomes.
Chromatin network is found in two forms which are:
1- Euchromatin (active chromatin) (extended chromatin):
They found in active cells.
They appear as thin threads.
They are involved in protein synthesis.
2- Heterochromatin (inactive chromatin) (condensed chromatin):
They are not involved in protein synthesis.
Heterochromatin appears as:
1- Peripheral chromatin:
when they are attached to the inner nuclear membrane
(nuclear envelope).
2- Chromatin islands:
when they are scattered as granules in the nuclear sap.
Functions of chromatin network:
1) It carries genetic information.
2) It directs protein synthesis by coding the DNA bases
to form mRNA.
Nucleic acids
They include DNA and RNA.
They are composed of repeated units called
nucleotides.
Each nucleotide is composed of:
1- A nitrogenous base,
2- A pentose sugar and
3- A phosphate group.
A nucleotide
The nitrogenous base may be:
i- Pyrimidines: They include:
Cytosine (C), Thymine (T) and Uracil (U).
i) Pyrimidine
ii- Purines: They comprise:
Adenine (A) and Guanine (G).
ii) Purine
The nitrogenous bases of DNA & RNA
Both DNA and RNA contain adenine and guanine (purine bases) and cytosine
(pyrimidine bases).
Thymine is found in DNA while uracil is found in RNA.
There are two major pentoses in nucleic acids: deoxyribose in DNA and ribose in
RNA.
The phosphate group is found in the nucleotide of both DNA and RNA.
Nucleotides are linked together in both DNA and RNA via covalent bonds that
found between phosphate group and pentose sugar.
Nitrogenous bases (purine or pyrimidine) are joined by glycosidic bonds to
pentose sugar of a repeating sugar-phosphate backbone.
RNA is usually a single-stranded, whereas DNA is usually a double-stranded
helix.
In DNA, the nitrogenous bases of the two strands are connected together via
hydrogen bonds.
Adenine binds to thymine through two hydrogen bonds while cytosine binds to
guanine by three hydrogen bonds.
The sequence of a nucleic acid is usually read from 5' (the end that has the
phosphate group) to 3' (the end which has not phosphate group).
The two strands of DNA run in opposite directions i.e. 5' end of one strand is
opposite 3' end of the other strand.
A single strand
of DNA
3
Nucleotides
A single strand
of RNA
A double strand
of DNA
DNA is found mainly in the nucleus. Very small amount is found
in the mitochodria.
RNA is formed in the nucleus and pass to the cytoplasm carrying
informations about the structure of protein which will synthesized
in the ribosomes .
There are different types of RNA; the most famous of them are
messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal
RNA (rRNA).
Notes
Genes
DNA
Enzymes
Transcription
DNA sequence
Triplet sequence in DNA
(TAC)
RNA
Metabolism
Translation
RNA sequence
Codon in mRNA
(AUG)
Protein
amino acid sequence
Amino acid in protein
(Met.)
Replication is the
copying of DNA
into DNA.
Transcription is
the copying of
DNA
sequence
into RNA.
Translation is the
copying of RNA
sequence
into
protein.
Triplet sequence in DNA is the genetic word called codon
i.e. 3 nucleotides equal to 1 codon which again equal to 1
amino acid.
The Size of human genome is ≈ 3,000,000,000 base
pairs ≈ 500,000,000 possible codons (words or amino
acids).
Humans, mice and indeed all mammals have roughly the
same number of nucleotides in their genomes (about 3
billion base pairs).
CYTOGENETICS
Cell division
Cell division in prokaryotes
Prokaryotes such as bacteria use a relatively simple form of cell
division called binary fission.
Typically bacterial chromosomes consist of a single loop of DNA
often called circular DNA but eukaryotes have a linear DNA
molecule.
When the prokaryote reaches to a level to be dividing, the circular
chromosome attaches to the cell membrane at a certain point.
Bacterial chromosome replicates leading to two identical
chromosomes which are attached to separate points.
The cell begins to divide giving two daughter cells which are
identical to the parent cell.
Bacteria can divide every 20 -30 minutes.
This gives bacteria a remarkable power of multiplication where each
cell gives 2.81 x 1014 bacteria after one day.
Cell division in eukaryotes
There are two types of cell divisions which are mitosis and meiosis.
The cell cycle
There are two main stages in the cell cycle:
I) Interphase:
It is the part of the cell cycle when the cell is doing its normal job.
Generally, there are one or more nucleoli in each cell which are the sites of
ribosomal RNA synthesis.
Interphase has three big phases which are:
1) G1 phase
◙ In this phase, the cell is doing its normal (everyday) job.
◙ ◙ At this time, chromosome (2n) are called unduplicated or unreplicated
chromosomes.
$ Usually, G1 is the longest period of the cell cycle.
$ However, in some embryonic cells that are rapidly divided, G1 might only last a
few minutes i.e. very short.
$ Some cells, like nerve cells never leave G1 and this is sometimes called a G0
state (phase).
$ G1 prepares the cell to undergo the next stage (S phase).
2) S phase
◙ All chromosomes are duplicated where DNA is replicated.
◙ ◙ New proteins are synthesized to assemble with new DNA forming new
chromosomes.
The time necessary to complete S phase varies between different life stages and
between species.
During S phase, the entire cell's DNA is duplicated resulting in 4 copies of each
gene instead of the normal 2 in a diploid cell.
3) G2 phase
◙ Cell prepares itself for mitosis by synthesizing needed components.
◙ ◙ Some cells remain in interphase (G1 + S + G2) their whole life because they do
not divide e.g. nerve cells and adult muscle cells.
The result of cell cycle is the cell proliferation (division) while any uncontrolled
proliferation leads to cancer.
Notes
☼ Cells spend most of their time in this intermediate non-mitotic state (interphase).
☼ ☼ Interphase is not a part of mitosis but it is stage between two successive
mitotic divisions.
II) Mitosis:
It takes place in somatic cells .
It is an asexual reproduction for grow and replace damaged cells .
It is differentiated into the following phases (stages): .
1- Prophase
@ Chromatin begins to coil and
condense to form chromosomes which
become visible.
@ The nuclear membranes disappear.
@ The nucleolus or nucleoli have
disappeared.
Paired centrioles (centrosomes) move to opposite ends of the cell.
As they move apart, the mitotic spindle are formed.
The mitotic spindle consists of:
1) The asters which radiate in a star like pattern away from each
centrosome, and
2) The spindle fibers which go toward the equator of the cell.
2- Metaphase
Spindle fibers grow and form
attachments to the chromosomes at
the centromeres.
Chromosomes move to an equatorial
plate (metaphase plate) which is
formed along the midline of the cell
between the poles.
Chromosomes are found in the most condensed state.
Remember that the chromosomes are
still duplicated during metaphase.
3- Anaphase
Centromeres are divided leading to the
formation of daughter chromosomes.
Spindle fibers shorten and the
daughter (sister) chromosomes are
drawn to the opposite poles of the cell.
4- Telophase
Nuclear membrane (envelope) is
reformed (reassembled) and surrounds
each set of daughter chromosomes.
Nucleolus or nucleoli reappear inside
the newly formed nucleus.
Remember that the chromosomes are
still duplicated during metaphase
Chromosomes are decondensed in the daughter cells to become
chromatin and the cells are once again in interphase.
Cytokinesis (division of the cytoplasm):
It is the division of the cytoplasm.
The result of mitosis plus cytokinesis is typically two genetically
identical daughter cells.
Both daughter cells are smaller than the original parent cell and
have unduplicated chromosomes.
Interphase
Metaphase
Prophase
Anaphase
Prometaphase
Telophase
Meiosis:
Meiosis is the process by which haploid cells are produced
from diploid cells.
Meiosis has several functions:
@ Reduce the chromosome number from the diploid
number (2n) to the haploid number (n).
@ This guarantees the male and female gametes share in
the hereditary characters of the formed zygote in sexual
reproduction.
Prophase I
Meiosis I
Metaphase
I
Meiosis II
Prophase II
Telophase
II
Anaphase I
Metaphase II
Telophase II
Telophase I
Binary
fission
Comparison between mitosis & meiosis
Mitosis
Meiosis
It is an indirect division.
It is a reduction division.
It occurs in somatic cells.
It occurs in germ cells of gonads (testes
or ovaries).
Four daughter cells are produced with
haploid number of chromosomes (n).
Crossing over takes place.
Two daughter cells are produced with
diploid number of chromosomes (2n).
No crossing over takes place.
Gametogenesis (creation of gametes)
The formation of sperms in the testes is called spermatogenesis.
The formation of eggs (ova) in the ovaries is called oogenesis.
Gametogenesis includes three successive phases which are:
I- Multiplication phase, II- Growth phase and III- Maturation phase.
Oogenesis
Spermatogenesis
Primordial germ cell
2n
2n
2n
2n
2n
2n
I- Multiplication phase
By repeated mitotic cell division 2n
(i.e. by mitosis)
2n SpermatogoniumOogonium 2n
2n
2n
2n
2n
2n
II- Growth phase
By growing
1ry
spermatocyte
1ry oocyte
2n
2n
1st polar
2ry
spermatocyte
st
2ry
oocyte
st
1 meiotic division
1 meiotic division
n body
n
III- Maturation phase
n
n
2nd meiotic division
2nd meiotic division
By meiosis
Spermatid
n
n
n
n
n
n
n
n
Spermatozoon nd
Mature ovum 2 polar body 2 polar bodies
So, each primary spermatocyte (or spermatogonium) gives four
sperms.
Also, each primary oocyte (or oogonium) gives one ovum (egg) and
three polar bodies.
The formed three polar bodies are degenerated (disintegrated).
At puberty, a male will produce approximately 1000 sperm per
second .
Each ejaculation should contain 200-300
million sperms.
When the sperms are formed, they are
moved into the epididymis where they Neck
become mature then stored.
Tail
From puberty of a female to menopause,
one egg is normally formed per month.
Fertilization
It the fusion of two haploid gametes
(sperm and egg) to produce a diploid
zygote.
Mature
human
Sequence of fertilization
1- The acrosomes of thousands of sperms release their enzymes that
destroy the protective barrier (a gelatinous material) around the
ovum and clear a pathway (is called fertilization pathway) for other
sperms to follow.
2- At the point of contact between the sperms and the ovum, the egg
surface produces a conical projection known as the entrance.
3- Although thousands of sperms work to clear the fertilization
pathway, only one sperm actually enters the ovum. This successful
sperm binds with a receptor on the cell membrane of the ovum. So,
the successful sperm is engulfed and enter the ovum..
4- A biochemical changes occur that inhibit other sperms from
penetration.
5- A change in the surface layer of the egg that preventing the
entrance of other sperms.
Note:
During fertilization, the head and the middle piece (midpiece) of the
sperm pass into the cytoplasm of the ovum while the tail is cut off
and remains outside.
Embryonic development
The embryonic development of any animal starts from the fertilized egg (zygote)
which usually passes through three main stages which are:
1) Cleavage, 2) Gastrulation and 3) Organ formation
(organogenesis).
1) Cleavage:
After fertilization, the zygote divides repeatedly by a series
of mitotic divisions.
Zygote
at right angle to the
1st division
vertical
2-blastomere stage
horizontal
4-blastomere stage
double vertical
16-blastomere stage
8-blastomere stage
double horizontal
32-blastomere stage (morula)
128-blastomere stage
64-blastomere stage
A blastula
The blastula
@ It is a hollow structure formed at the end of cleavage process.
@ Its wall is consisted of a single layer of cells.
These cells are differentiated into micromeres at the animal pole and
macromeres at the vegetal pole.
@ The fluid filled cavity in its center is termed blastocoel.
This blastocoel is not connected to the exterior.
2) Gastrulation:
The gastrula
@ It is an elongated structure formed at the end of gastrulation by flattening
and invagination of macromeres of blastula.
Invagination continues until the macromeres come in direct contact with
micromeres.
So, the blastocoel is disappeared while a new cavity (archenteron) is formed.
@ Its wall is formed from a double layers of cells.
The outer layer which is formed from micromeres (is known as the ectoderm)
while the inner layer is formed from the macromeres (the endoderm forms).
@ It has a cavity that called archenteron which is connected to the exterior
through an opening called a blastopore.
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