University of Tripoli
Faculty of Science / Zoology Department
Lecture notes in General Zoology course Zo 105
Part one: cytology
_____________________________________________________________________________________
Textbook: Kenneth Saladin. Anatomy and Physiology: the unity of form and function. 1st ed. 1998.
McGraw Hill.
Lecture notes
Cytology is a branch of biology, which deals with the study of cells in terms of structure, function and
chemistry.
Cell Discovery
In 1591 the Dutch merchant Anton van Leewenhoek invented single lense microscope. He was the first
man to witness live microscopic organisms, named them animalcules “little animals”.
The invention of the microscope led to the discovery of cells. In 1665 the English scientist Robert Hooke
introduced the term “cell” while examining very thin slices of cork using a compound microscope. He
observed a multitude of pores that looked like the wall compartments of a honey comb, he called them
cells.
Three German scientists developed the concept of cell theory. The botanist Matthias Schleiden (in 1838)
was the first to clearly state that all plants are made of cells. The zoologist Theodor Schwann (in 1839)
was the first to clearly state that all animals are made of cells. The pathologist Rudolf Virchow (in 1855)
was the first to identify that the nucleus controls the cell activities and cell come from pre existing cells.
Modern Cell Theory
1)
2)
3)
4)
5)
6)
All living things are made of cells
The cell is the smallest fundamental unit of life
All cells arise from pre existing cells through cell division
Cells contain genetic material, which they pass to daughter cells during cell division
All cells have a similar chemical composition (proteins, nucleic acids, carbohydrates, lipids)
The metabolic processes associated with life occur within cells.
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬- ‫ كلية العلوم‬-‫حقوق الطبع و النسخ محفوظة لقسم علم الحيوان‬
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Cell Definition
The cell is the fundamental structural and functional unit of all living organisms
Classification of cells
There are two major categories or types of cells: prokaryotic and eukaryotic.
The main difference between these two cell types is that Prokaryotic cells do not have a nuclear
membrane. The nuclear material consists of a single chromosome and lies in the cytoplasm. The nuclear
region in the cytoplasm is called nucleoid. Membrane-bound organelles are absent. Prokaryotic cells are
found in bacteria and cynobacteria (blue-green algae). Eukaryotic cells include: all other cells, such as
protista, fungal, plant and animal cells. The nucleus in a eukaryotic cell is bound by a nuclear envelope
and contains nucleoplasm. The cytoplasm, found between the plasma membrane and the nucleus,
consists of fluid and the organelles.
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬- ‫ كلية العلوم‬-‫حقوق الطبع و النسخ محفوظة لقسم علم الحيوان‬
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Structural and functional differences between Prokaryotes and Eukaryotes:
Prokaryotic
Eukaryotic
Features
Plant
Animal
Size(diameter)
0.5 - 5 µm
40 µm
15 µm
Cell wall
Yes (contains peptidoglycan)
Yes (contains cellulose)
No
Genetic
Material
DNA is naked. A single circular
molecule
DNA linear, associated with histones
(proteins), in a nucleus, surrounded by a
nuclear envelope.
Ribosomes
70S ribosomes (smaller)
80S ribosomes (larger)
ER, Golgi
apparatus
No
Yes
Mitochondria
No(respiration occurs on an infolding
of the cell membrane called the
mesosome.)
Yes
Chloroplasts
No
Yes
No
Cell sizes and shapes
Cells can be remarkably different in size, shape and function. Cells come in many different sizes. Some
cells are visible to the naked eye (such as eggs of birds), however, most cells are microscopic and cannot
be seen by the naked eye. Microscopes were developed to visualize cells. The size of the cell is
measured in micrometers (µm) in diameter. For example bacterial cells range 0.2 to 0.3 µm, liver cell 20
µm, plant cell 30 to 40 µm.
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬- ‫ كلية العلوم‬-‫حقوق الطبع و النسخ محفوظة لقسم علم الحيوان‬
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Cells also come in many different shapes (e.g. spindle, aster, and oval, spherical, cylindrical). Some cells
change their shape (e.g. amoebae, macrophage) others have typical shape (e.g. spermatozoa, epithelial
cells). The shapes of cells have evolved to help them carry out their specific function in the body.
Cell Structure
Despite the fact that there are many different cell types, sizes, and shapes, for descriptive purposes, the
concept of a "generalized animal cell" is introduced. It includes features from all cell types. A cell
consists of three parts:
I)
The cell membrane: a selective barrier which encloses a cell
II)
The cytoplasm: all the cellular contents between the plasma membrane and the nucleus. It
includes: the cytosol (a jelly-like fluid ), all the organelles other than the nucleus, and the
cytoskeleton
III)
The nucleus: contain genetic material (a fine network of treads called chromatin which is DNA
(deoxyribonucleic acid) associated with particular proteins. At the time of cell division,
chromatin becomes condensed to form rod-like bodies known as chromosomes)
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬- ‫ كلية العلوم‬-‫حقوق الطبع و النسخ محفوظة لقسم علم الحيوان‬
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Cell membrane = plasma membrane = plasmalemma
Every cell is enclosed by a cell membrane. The cell membrane separates the material outside the cell
(extracellular) from the material inside the cell (intracellular). It defines cell boundaries. It maintains the
integrity of a cell. It regulates the exchange of materials between cytoplasm and extra cellular fluid due
to its selective permeability. It is also important in intercellular communication and cell identity.
STRUCTURE OF THE CELL MEMBRANE
The cell membrane is a “phospholipid bilayer” made of membrane lipids (phospholipids, glycolipids,
cholesterol) and membrane proteins (integral, peripheral).
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬- ‫ كلية العلوم‬-‫حقوق الطبع و النسخ محفوظة لقسم علم الحيوان‬
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The membrane lipids
Phospholipids are the most abundant lipids in the membrane (75%), they form the lipid bilayer.
Phospholipids are amphipathic molecules with polar heads (glycerol attached to phosphate) and
nonpolar tails (two long fatty acid hydrocarbon chains). The phosphate heads are polar molecules and so
are water-soluble (hydrophilic = water loving). They are oriented toward the extracellular space or
cytoplasm (intracellular).The lipid tails are non-polar and therefore are not water-soluble (hydrophobic
= water hating). They are oriented toward the interior of the membrane.
Cholesterol (steroid lipid soluable) is also present in the membrane. Found in both leaflets of lipid
bilayer in between the phospholipd tail molecules. It constitutes 20% of membrane lipids. It maintains
the fluidity and increases the stability of the membrane.
Glycolipids are phospholipids with short oligosaccharide chains. Always found on extra cellular side of
cell membrane. Least common of the membrane lipids (2 to 5 %)
Membrane proteins
Much of the membrane is made up of a 'sea' of phospholipids with protein molecules 'floating' in
between the phospholipids. Some of these proteins span the whole width of the cell membrane. They
are called transmembrane or integral proteins. Peripheral proteins which do not protroude into the
phospholipid layer, they always adhere to the intracellular face of the membrane. There are also short
polysaccharide chains that are attached to the outer surface of the membrane. Most of these
carbohydrates are attached to proteins and are called Glycoproteins. Proteins in the cell membrane
provide structural support, form channels for passage of materials, act as receptor sites, function as
carrier molecules, and provide cell identify markers (glycocalyx).
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬- ‫ كلية العلوم‬-‫حقوق الطبع و النسخ محفوظة لقسم علم الحيوان‬
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The plasma membrane structure is called the Fluid Mosaic Model, because the membrane is fluid
(phospholipids and proteins are not fixed in one place = they float) and because of the mosaic (patterns)
arrangement of the protein molecules created on the membrane’s surface.
Mehods of Membrane Transport
Plasma membrane has selective permeability so some substances pass quickly, some with difficulty, and
some not at all.
It is important that the cell is supplied with all the substances it needs (e.g. oxygen) and that waste
substances (e.g. carbon dioxide), or substances for export, leave the cell. There are various ways by
which substances can be moved or transported through the cell membrane. There are three methods of
membrane transport: Passive transport, Carrier mediated transport, and Bulk transport.
Passive transport
Movement of substances across a cell membrane without any direct energy expenditure by the cell. It
includes diffusion and osmosis.
Diffusion
Diffusion is a movement of particles from an area of high concentration to an area of lower
concentration of the particles or molecules, thus tending to equalize the concentration throughout.
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬- ‫ كلية العلوم‬-‫حقوق الطبع و النسخ محفوظة لقسم علم الحيوان‬
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This is the process that is used in oxygen entering a cell, and carbon dioxide leaving (respiratory gases)
and any lipid soluble molecule (e.g. alcohol, steroids).
Osmosis
The movement of water molecules through a semi-permeable membrane in response to a concentration
and or pressure gradient.
Carrier mediated transport
When substances are allowed to cross the cell membrane with the help of integral proteins (carrier or
channel proteins). Two types: facilitated transport and active transport.
Facilitated transport
Facilitated transport: diffusion of molecules from high concentration to low concentration. Substances
are moving down the concentration gradient so no energy is required. Example glucose molecules.
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬- ‫ كلية العلوم‬-‫حقوق الطبع و النسخ محفوظة لقسم علم الحيوان‬
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Active transport
Movement of individual ions and molecules across cell membrane, against. (up) concentration gradient
(from low to high) by ATP expenditure. Example Na – K pump.
Bulk transport
Movement of particles or fluid through cell membrane by means of vesicles needs ATP. Two types:
endocytosis and exocytosis.
Endocytosis
The ingestion of solid or fluid material by cells, materials entering the cell can do so when the plasma
membrane invaginates to surround the material. The membrane seals off to form a vesicle, which can
then move into the cell. Two types: phagocytosis and pinocytosis.
Phagocytosis = “cell eating”
The material is relatively large or solid, and is digested by enzymes after fusion of the vesicle with a
lysosome. This occurs in white blood cells that ingest bacteria and other foreign bodies.
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬- ‫ كلية العلوم‬-‫حقوق الطبع و النسخ محفوظة لقسم علم الحيوان‬
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Pinocytosis = “cell drinking”
The material is fluid, minute vesicles are formed.
Exocytosis
The material to be transported out of the cell is surrounded by membrane. The vesicle will fuse with the
cell surface membrane and the contents leave.
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬- ‫ كلية العلوم‬-‫حقوق الطبع و النسخ محفوظة لقسم علم الحيوان‬
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Plasma membrane surface extensions
On the apical surface of cell membrane 3 types of extensions could be found:
1) Cilia (cilium) short, numerous, hair-like projections that move in a wave like motion. In Human it
is found in:
I) upper part of respiratory tract (remove microbes and debris from lungs).
II) Fallopian tubes (movement of ovum)
2) Flagella (flagellum) are long projections that move in a whip-like motion. In humans it is found
only in sperm.
Both cilia and flagella are cylindrical structures which arise from centrioles and anchored by basal
body. They are made up of micrototubules. There are two microtubules at the center, surrounded
by a pinwheel-like ring of nine microtubule pairs. An arrangement called “9 + 2” structure.
Flagella and cilia are the major means of locomotion in unicellular organisms, such as paramecium
and euglena.
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬- ‫ كلية العلوم‬-‫حقوق الطبع و النسخ محفوظة لقسم علم الحيوان‬
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3)
Microvilli (microvillus) are folds of plasma membrane to increase the surface area for greater
absorption. Found at brush border of the epithelial cells of small intestine and kidney tubules.
Intercellular junctions
Intercellular junctions are important for cell communication and cell adhering.
5 types of intercellular junctions could be found on the lateral surface of plasma membrane:
1) Tight junction (zonula occludens) completely encircles an epithelial cell near its apex and joins it
tightly to the neighbouring cells. This seals off the intercellular space forming a virtually
impermeable barrier to fluid and substances between the cells. Found in stomach, intestine,
bladder, and kidney.
2) Gap junction (communication junctions) is clusters of tightly packed channels that allow small
molecules (metabolites, second messengers, and ions) to travel between adjoining cells. This
molecular exchange is essential during embryonic development. Found in the intercalated discs
of heart muscle.
3) Adherens junction (zonula adherens) are protein complexes that hold cells together, usually
more basal than tight junctions.
4) Desmosome junction (macula adherens) is like a spot weld between two cells, a patch that holds
cells together and enable a tissue to resist stress. Found in epidermis, cardiac muscle, and cervix
of the uterus.
5) Hemidesmosome junction a structure representing half of a desmosome, that contribute to the
attachment of epithelial cells to the underlying basement membrane (basal lamina).
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬- ‫ كلية العلوم‬-‫حقوق الطبع و النسخ محفوظة لقسم علم الحيوان‬
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II) Cytoplasm
The cytoplasm is the gel-like fluid inside the cell. It is the medium for chemical reaction. It provides a
platform upon which other organelles can operate within the cell. All of the functions for cell expansion,
growth and replication are carried out in the cytoplasm of a cell. It includes the cytosol and all the
organelles other than the nucleus and the cytoskeleton.
Cytosol this is the gel like solution within the cell membrane. It contains enzymes for glycolysis (part of
respiration) and other metabolic reactions together with sugars, salts, amino acids, nucleotides and
everything else needed for the cell to function.
Cytoplasmic organelles are "little organs" that are suspended in the cytoplasm of the cell. Each type of
organelle has a definite structure and a specific metabolic task. Some are surrounded by one or two
layers of unit membrane (membranous organelles which include: Nucleus, mitochondria, endoplasmic
reticulm, Golgi apparatus, lysosomes, and perioxisomes). Organelles that are not surrounded by
membranes (nonmembraneous organelles Include: ribosomes, centrioles, centrosome,
1) Mitochondrion (pl. Mitochondria). This may be cylindrical, rod-shaped or spherical shaped
organelle (8µm long), where aerobic respiration takes place in all eukaryotic cells. Mitochondria are
surrounded by a double membrane: the outer membrane is smooth and quite permeable, while the
inner membrane is highly folded into cristae, which give it a large surface area. The space enclosed
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬- ‫ كلية العلوم‬-‫حقوق الطبع و النسخ محفوظة لقسم علم الحيوان‬
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by the inner membrane is called the mitochondrial matrix, and contains small circular DNA (mtDNA).
The inner membrane is the site of ATP synthesis.
2) Endoplasmic reticulum (ER), a system of flattened cavities (cisternae) lined by a thin membrane. It is
the site of the synthesis of many substances in the cell and so provides a compartmentalised area in
which this takes place. The cavities also function as a transporting system whereby substances can
move through them from one part of the cell to another. There are 2 types of ER - rough (RER) and
smooth (SER).
Rough Endoplasmic Reticulum (RER) Contain numerous ribosomes on its outer surface, which give it
its rough appearance. The ribosomes synthesize proteins, especially proteins secreted (exported)
outside cell or packaged in other organelles such as lysosomes
Smooth Endoplasmic Reticulum (SER) no ribosomes on its surface. Cisternae are more tubular and
branch extensively. it is the site for phospholipids and steroid synthesis. It is also involved in
detoxification (liver and kidney cells) and calcium storage and release (skeletal muscle).
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬- ‫ كلية العلوم‬-‫حقوق الطبع و النسخ محفوظة لقسم علم الحيوان‬
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3) Golgi Body (or Golgi Apparatus) another series of flattened stacked cisternae. The primary function
of
this
organelle
is
to
process
(modify)
and
then
package
proteins
and
lipids
after they are synthesized from ER but before they continue on to their destination. It Receives
newly synthesized proteins (from RER) and lipids (SER) adds a functional group ( by enzymatically
modifying the polypeptide chain, or adding carbohydrates). It package protein or lipids into a
secretory vesicle. Some vesicles become lysosomes or peroxisomes. Some vesicles migrate to the
plasma membrane and fuse with it (releasing their contents by exocytosis), contributing fresh
protein and phospholipid to the membrane.
4) Lysosomes are tiny, spherical, sac-like structures scattered all over the cytoplasm. Their main
function is intracellular digestion. They contain powerful hydrolytic enzymes (hydrolases) capable of
digesting all organic material (proteins, nucleic acids, complex carbohydrates, phospholipids, and
other substrates) They are also capable of digesting worn out cell organelles (autophagy), or
even digesting the entire damaged cell containing them (autolysis).
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬- ‫ كلية العلوم‬-‫حقوق الطبع و النسخ محفوظة لقسم علم الحيوان‬
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5) Peroxisomes are small, membrane-bound sacs that resemble lysosomes but contain different
enzymes. They contain powerful oxidative (oxidase) enzymes. Their chief function is to remove toxic
substances (detoxification). Hence these organelles are abundant in liver and kidney cells.
Peroxisomes oxidize various materials and then break down the hydrogen peroxide (H2O2) that
results into water and oxygen using an enzyme called catalase.
H2O2--------------
H2O + O2
6) Ribosomes are the smallest and most numerous of the cell organelles, and are the sites of protein
synthesis. They are composed of protein and ribosomal ribonucleic acid (rRNA), and are
manufactured in the nucleolus of the nucleus. Ribosomes are spherical, granular particles which
occur free in the cytoplasm, where they make proteins for the cell's own use, or they are found
attached to the rough endoplasmic reticulum, where they make proteins for export from the cell.
Ribosomes are the machinery that provides a surface location for the assembly of amino acids into
proteins. In living organisms, DNA produces RNA, which then makes protein for the body. Ribosomes
are responsible for reading the information (genetic code) on the messenger RNA (mRNA) and using
that information to produce the right proteins. Ribosomes are often found in groups called
polysomes. Polysomes are variable in length and are strings of ribosomes joined by messenger RNA.
7) Centrioles are a pair of short hallow cylindrical structures, usually found near the nucleus. They are
composed of microtubules organized as “9 + 0” pattern of microtubules triplets. Two centrioles lie
perpendicular to each other within a small clear area of cytoplasm called the centrosome. Involved
in cell division. Before each division the centriole replicates itself and the two centrioles move to
opposite ends of the cell, where they initiate the formation of mitotic spindle fibers that organizes
and separates the chromosomes.
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬- ‫ كلية العلوم‬-‫حقوق الطبع و النسخ محفوظة لقسم علم الحيوان‬
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8) Vacuoles These are membrane-bound sacs (vesicles) containing water or dilute solutions of salts and
other solutes. Most cells can have small vacuoles that are formed as required. Some unicellular
protoctists have feeding vacuoles for digesting food, or contractile vacuoles for expelling water.
Cytoskeleton this is a network of protein fibers extending throughout all eukaryotic cells, used for
support, transport and motility. The cytoskeleton is attached to the cell membrane and gives the cell its
shape, as well as holding all the organelles in position. There are three types of protein fibers
(microfilaments, intermediate filaments and microtubules).
Microtubules: hollow rod-like structures with walls of tubulin protein. Provide the structural support of
cells and can aid transport through the cell. Form components of cilia, flagella, centerioles, basal bodies,
and mitotic spindle
Microfilaments: rod-like structures made of contractile protein actin. Like microtubules, provide support
and aid movement.
Intermediate filaments: Network of fibrous protein (many different types depending on cell type ex.
Keratin in hair, nail, epidermis). Provide physical support, resist stress, and hold cells together.
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬- ‫ كلية العلوم‬-‫حقوق الطبع و النسخ محفوظة لقسم علم الحيوان‬
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III)
Nucleus
This is a prominent, spherical or oval structure found at the centre of the cell. The nucleus is a highly
specialized and serves as the information and administrative centre of the cell. The nucleus has two
major functions: It stores the cell's genetic material, or DNA, and it controls and coordinates the cell's
activities, which include intermediary metabolism, growth, protein synthesis, and reproduction (cell
division). The nucleus is surrounded by a nuclear envelope, which is a double membrane with nuclear
pores which allow the selective transfer of material between the nucleoplasm and the cytoplasm. The
outer layer of nuclear envelope is continuous with the membrane of the rough endoplasmic reticulum.
The interior is called the nucleoplasm, which is full of chromatin (DNA (deoxyribonucleic acid) associated
with histone proteins) a DNA/protein complex containing the genes. During cell division the chromatin
becomes condensed into discrete observable rod-like bodies known as chromosomes. The
chromosomes contain genes, which are responsible for storing and transmitting hereditary
characteristics from one generation to the next. Another structure within the nucleus is the nucleolus.
The nucleolus is a dense, spherical granule responsible for the synthesis of ribosomal RNA (rRNA). In the
cytoplasm rRNA will assemble into ribosomes (the organelles on which protein synthesis takes place).
The genetic material is composed of the nucleic acid DNA = Deoxyribonucleic Acid
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬- ‫ كلية العلوم‬-‫حقوق الطبع و النسخ محفوظة لقسم علم الحيوان‬
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DNA
is
wrapped
tightly
around
protein
histones
(nucleosomes)
to
form
chromatin
and coiled tightly to form chromosomes. Chromatin is a term designating the structure in which DNA
exists within cells at interphase (non-dividing stage of cell), it is in uncoiled and dispersed.
While a chromosome is a relative term, usually it is a term designating the structure in which DNA exists
within cells at metaphase of cell division (mitosis or meiosis). Chromosomes are made up of duplicated
DNA. At metaphase of cell division DNA condenses and super coils by a factor around x16000.
Chromatin structure
The structure of chromatin is determined and stabilized through the interaction of the DNA with DNAbinding proteins: the histone proteins and non-histone proteins. The histone proteins are the major
class of binding proteins involved in maintaining the compacted structure of chromatin. There are five
different histone proteins, identified as H1, H2A, H2B, H3, and H4. The binding of DNA by the histones
generates a structure called nucleosome.
The nucleosome core contains an octamere protein structure consisting of 2 subunits each of H2A, H2B,
H3, and H4. Histone H1 occupies the internucleosomal DNA and is identifies as the linker histone.
These nucleosomal core structures would appear as “beads-on-a-string” if the DNA were pulled into
linear structure and observed under an electron microscope.
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬- ‫ كلية العلوم‬-‫حقوق الطبع و النسخ محفوظة لقسم علم الحيوان‬
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The nucleosome cores themselves coil into a solenoid shape which itself coils to further compact the
DNA (30nm fiber). These final coils are compacted further into the characteristic chromatin seen in a
metaphase chromosome (200 nm fiber).
There are two types of chromatin.
1) Euchromatin is the genetically active portion and is involved in transcription of mRNA to produce
proteins used in cell function and growth. It stains lightly.
2) Heterochromatin contains inactive DNA and is the portion of chromatin that is most condensed, since
it not being used. It is found around centromer and telomere regions of chromosome, as well as the
inactive X chromosome in females. It stains darkly.
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬- ‫ كلية العلوم‬-‫حقوق الطبع و النسخ محفوظة لقسم علم الحيوان‬
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Chromosome structure
Each chromosome is made up of two longitudinal strands called chromatids. Each chromatid has a
double helical DNA molecule. The two chromatids are held together by a structure called the
centromere. Usually chromosomes have two arms: p arm (short arm) and q arm (long arm). Each
chromatid has two telomere regions at its end. During cell division, the spindle fibers are attached to the
centromeres. During anaphase, when chromosomes or chromatids separate, they form different shapes
based on the position of the centromere.
4 types of chromosomes according to centromere position:
1) Metacentric – the centromere ia at the centre and the chromosome is V shaped during anaphase.
2) Submetacentric – the centromere ia a little away from centre and the chromosome gets an L shaped
during anaphase.
3) Acrocentric- the centromere is nearer to one end and the chromosome is J shaped during anaphase.
4) Telocentric- the centromere is at the end and the chromosome is l shaped during anaphase.
‫ سيتم مساءلة و مقاضاة كل من يقوم بالنسخ من اجل المتاجرة‬.‫ جامعة طرابلس‬- ‫ كلية العلوم‬-‫حقوق الطبع و النسخ محفوظة لقسم علم الحيوان‬
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Chromosomes are classified into two types:
1) Autosomal chromosomes- autosomes, chromosomes other than sex chromosomes. In humans
1-22. Chromosomes exist in homologous pairs in diploid cells.
2)
Sex chromosomes- X and Y. Females have two X chromosomes (XX) and males have one X
chromosome (XY).
Chromosome number
Chromosome number is constant for each species. It is species specific. It is not correlated to the
complexity of the organism. Somatic cells contain diploid chromosome number (2n). Gametes (sperm
and ova) contain haploid chromosome number (n).
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Karyotype
Karyotype is the general appearance of individual’s chromosomes taking from somatic cell. Aligned
according to: size, number, and shape. It shows structural or numerical chromosome abnormalities. The
karyotype of human male is 46 XY, for human female is 46 XX, for Down’s syndrome female is 47 XX +
21, for Turner syndrome is 45 X0.
Chromosome territory
Recent research has identified that each interphase chromosome occupies a specific domain
(chromosome territory) within the nucleus and is not scattered widely in a disordered manner.
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Nucleic acids
Two types of nucleic acids:
1) DNA = deoxyribonucleic acid
2) RNA = ribonucleic acid
There are 3 types of RNA in the cell. They have different structures and functions.
1) Messenger RNA = mRNA
2) Ribosomal RNA = rRNA
3) Transfer RNA = tRNA
Composition of nucleic acids
All nucleic acids are made up of repeating molecules called nucleotides. A nucleotide is composed of 3
components:
1) Sugar
2) Nitrogen base
3) Phosphate group
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Nitrogen bases
Phosphodiester bond formation
Nucleotides are linked in a chain of polynucleotides through phosphodiester bonds. Between 5’
phosphate of one nucleotide and the 3’ hydroxyl group of the previous nucleotide.
The DNA chain is said to grow from 5’ to 3’, which means that the first DNA base has a free 5’ end,
with attached phosphates. The last nucleotide has a free 3’ OH group on it.
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All other bases have their 5’ carbons attached to a phosphate, which is attached to the 3’ OH group
of the previous nucleotide.
DNA structure
In 1953, James Watson and Francis Crick discovered the DNA structure. They described it as double
helix (DNA is double stranded helical structure). The strands have two important characteristics:
complementary and antiparallel.
Complementary means
A will only base pair with T by 2 hydrogen bonds
C will only base pair with G by 3 hydrogen bonds
Antiparallel means they are oriented in different directions.
5’----------------- 3’
3’ ----------------- 5’
DNA replication
DNA replication is the process where an entire double-stranded DNA is copied to produce a second,
identical DNA double helix. It occurs in the nucleus. DNA is replicated by a process called
semiconservative replication. This means one-half of each new molecule of DNA is old (template
strand) one-half of new molecule of DNA is new (complementary strand).
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Enzymes needed for DNA replication
DNA helicase: separates DNA strands
DNA polymerase: synthesize complementary strand.
Primase: synthesize RNA primer
DNA ligase: ligates Okazaki fragments
DNA gyrase (DNA topoisomerase): loose supercoils
Single-strand binding proteins bind to the separated DNA strands to keep them apart.
DNA replication begins with a partial separation of the double helix at an area called replication fork.
DNA helicase separates the two DNA strands by breaking the hydrogen bonds between them.
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DNA synthesis can only proceed in one direction. This is because new nucleotides can only be added to a
growing DNA polymer by addition onto the free hydroxyl group at the 3’ end. The other end, the 5’ end,
has no free hydroxyl group. Hence, the direction of DNA synthesis is always from 5’ to 3’.
Mechanics of DNA synthesis
Eukaryotic gene structure
Eukaryotic genes are monocistronic (produce only one mRNA) and contain regions called exons which
code for amino acids; and intron regions which don’t code for amino acids. The introns must be
removed from the mRNA in order to be translated correctly.
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Gene expression
Gene expression is the process by which information from a gene is used in the synthesis of a functional
gene product. These products are often proteins. It is done in two steps: DNA transcription and mRNA
translation.
DNA transcription
Transcription is the process of making an RNA copy of a single gene. The enzyme used in transcription is
RNA polymerase.
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The template strand is the DNA strand being copied = antisense strand (3’ – 5’). The mRNA strand is the
same as the DNA strand 5’ to 3’ except U have replaced T. the mRNA leaves nucleus and goes to
ribosomes for translation to make protein.
Genetic code
The genetic code in mRNA is translated into proteins by ribosomes. Each 3 nucleotides in mRNA form a
codon. Each codon specifies only ONE amino acid. There are 64 possible codons:
1 start codon (AUG) (which specifies methionine)
3 stop codons: UAA, UAG, UGA
61 code for amino acids
Only 20 amino acids!!!
61 codons!!!
Some amino acids can be specified by many codons. For example amino acid glycine has 4
different codons: GGU, GGC, GGA and GGG.
Genetic code table
Translation = protein synthesis
Information in mRNA is translated into primary sequence of a protein in 3 steps:
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1) Initiation
2) Elongation
3) Termination
Need 3 types of RNA:
1) mRNA : carries genetic code
2) tRNA: carries amino acids to ribosome
3) rRNA: forms ribosomes (site of protein synthesis)
Functional sites of tRNA
The tRNA has two important sites: one for amino acid attachment and the other for binding with mRNA
through complementary base pairing (anticodon).
Functional sites of ribosome
Ribosomes contain three discrete sites: Peptidyl site (P site), Aminoacyl site (A site), Exit site (E site).
Initiation = Start: means the ribosome binds to mRNA at start codon (AUG)
Elongation: means Amino acids are added one by one. How?
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1) tRNA complexes bind to mRNA codon by forming complementary base pairs with the tRNA anticodon
2) The ribosome moves from codon to codon along the mRNA.
Termination: means stop synthesizing protein. It occurs when a stop codon is reached in the mRNA.
Three stop codons: UAG, UAA, and UGA, which is recognized by proteins called release factors
not tRNAs.
Cell division
Cell division is a process by which cells reproduce.
In prokaryotic organisms, cells divide by binary division.
In eukaryotic organisms reproduce by either I) Asexual reproduction or
II) Sexual reproduction
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Sexual organisms have 2 types of cell division: Mitosis and Meiosis.
Mitosis is the normal cell division, which goes on throughout life in all parts of the body. It is important
for embryonic development, growth, replacement of dead cells, and for repair of damaged tissues. The
chromosome number in mitosis is always diploid (2n). Diploid means two copies (pair) of each
chromosome designated as “2n”. For example, human body cells are diploid, they contain 46
chromosomes or 23 pairs of chromosomes.
Meiosis is the special cell division that occurs in the gonads. It creates the sperm and ova (eggs), the
gametes. It is important for sexual reproduction. The chromosome number in mitosis is always haploid
(n). Haploid – one copy of each chromosome designated as “n”. Human germ (sex) cells are haploid.
Gametes (sperm & ovum) are haploid = 23 chromosomes.
When body cells divide (enter mitosis), they need to copy (duplicate) their chromosomes, separate
duplicated chromosomes to produce 2 new genetically identical body cells that have the same DNA and
Same number of chromosomes as the original body cell.
Cell cycle : Cell cycle is the sequence of events (phases: G1, S, G2, M) during the life of the cell.
Cell cycle has two parts:
1- interphase growth and preparation. DNA uncondensed (chromatin). Longest part of the cell cycle, has
3 stages: G1, S, G2
2- mitosis
(karyokinesis) separation of replicated chromosomes. DNA condensed (chromosomes).
Mitosis is a nuclear division that preserves diploid number of chromosomes. It is the shortest part of the
cell cycle, has 4 stages: prophase, metaphase, anaphase, telophase followed by cytokinesis.
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Interphase
Chromosomes are decondensed, chromosomes replicate, the centrosome (centrioles) divides.
Prophase
Chromosomes condense. Nuclear envelope dissociates. Centrioles move to opposite poles
Mitotic spindle apparatus forms.
Metaphase
Pairs of sister chromatids align themselves at the metaphase plate. Centrioles at opposite ends of cell.
Anaphase
Centromeres divide: each 2-chromatid chromosome becomes two 1-chromatid chromosomes.
Chromosomes pulled to opposite poles by the spindle fibers.
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Telophase
Chromosomes de-condense. Nuclear envelope reappears. Cytokinesis: the cytoplasm is divided into 2
cells.
Cytokinesis
Cytokinesis means cytoplasmic division. It is the actual division of the cell into two new cells.
Meiosis is a reductional division. 46 chromosomes 23 chromosomes. 2n (diploid) n (haploid)
Somatic cell - sex cell.
Meiosis is always followed by fertilization. Fertilization means the fusion of male (sperm) and female
(ovum) gametes. The resulting cell is called a zygote. Upon fertilization, a 1n sperm meets a 1n ovum
and a zygote (2n) is formed.
Meiosis is very important because it keeps the number of chromosomes constant generation after
generation. Without meiosis, the number of chromosomes would continue to increase each generation.
Each generation: 1 sperm (23 chromosomes) + 1 egg (23 chromosomes) = 1 zygote (46 chromosomes).
The new zygote grows by mitosis. Each new cell has 46 chromosomes.
Meiosis: is the process of nuclear division in gametogenesis in which one replication of chromosomes is
followed by two successive divisions of the nucleus to produce four haploid nuclei.
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Gametogenesis is the process of gamete formation. Two types:
1) Spermatogenesis: formation of male gametes (sperms). Occurs in testes.
2) Oogenesis: formation of female gametes (ova). Occurs in ovaries.
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When gamete (sperm or egg) cells reproduce themselves the process is called MEIOSIS.
During meiosis, a single (1) diploid cell divides and produces FOUR (4) haploid reproductive cells.
In Meiosis there is one chromosome duplication followed by two cellular divisions (into four cells) so
Meiosis is broken down into Meiosis I and Meiosis II. In meiosis I the homologous chromosomes
separate. In meiosis II the sister chromatids separate.
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Key differences between meiosis and mitosis
Meiosis reduces the number of chromosomes by half. Daughter cells differ from parent, and each other.
Meiosis involves two divisions, Mitosis only one.
Meiosis I involves:
Synapsis – homologous chromosomes pair up. Chiasmata form (crossing over of non-sister
chromatids).
In Metaphase I, homologous pairs line up at metaphase plate. In Anaphase I, sister chromatids
do NOT separate. Overall, separation of homologous pairs of chromosomes, rather than sister
chromatids of individual chromosome.
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Cellular respiration
Cellular respiration is a process in which the energy in glucose is transferred to ATP. Cellular respiration
allows organisms to use (release) energy stored in the chemical bonds of glucose (C6H12O6). The energy
in glucose is used to produce ATP. Cells use ATP to supply their energy needs.
ATP = Adenosine Tri Phosphate
How Cells Acquire ATP? By cellular respiration.
Cellular respiration includes the various metabolic pathways that break down glucose to produce ATP.
C6H12O6 + 6O2→6CO2 + 6H2O + Energy (36 ATP)
Two types of cellular respiration: 1) Aerobic = O2 present
2) Anaerobic = O2 absent (fermentation)
Aerobic cellular respiration
The complete breakdown of glucose to carbon dioxide and water involves four phases:
a. Glycolysis
b. Formation of Acetyl CoA
c. Citric Acid Cycle = Krebs cycle
d. Electron Transport Chain
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Glygolysis
The break down of glucose (6-carbon monosaccharides) into two molecules of pyruvic acid (3-carbon)
Occurs in the cytoplasm (cytosol). There are ten steps in glycolysis and each one is catalyzed by a specific
enzyme (which will be studied in Biochemisty course).
Formation of acetyl CoA
Pyruvic acid is decarboxylated—CO2 is removed and pyruvic acid, a C3 compound, becomes a C2
compound.
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NAD removes hydrogen atoms from the C2 compound (an oxidation reaction) and converts it to an
acetyl group (acetic acid). The acetyl group binds to coenzyme A. The result is acetyl-coenzyme A
(acetyl-CoA), which is ready to enter the citric acid cycle.
Citric acid cycle
Electron transport chain
In the electron transport system, NADH and FADH2 are oxidized and the energy is used to produce ATP.
The system contains membrane-bound electron carriers that pass electrons from one to another. When
a carrier reduces other, some of the energy that is released as a result of that reduction is used to pump
hydrogen ions across the membrane into the intermembrane space. The remaining energy is used to
reduce the next carrier. As a result of the electron transport system, hydrogen ions become
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concentrated in the intermembrane space. These concentrated ions contain energy much like a dam.
The enzyme ATP synthase is able to use the energy of this osmotic gradient to produce ATP as the
hydrogen ions move under osmotic pressure through the enzyme back into the matrix of the
mitochondrion.
Oxygen is the final electron acceptor. The low-energy electrons that emerge from the electron transport
system are taken up by O2. The negatively charged oxygen molecules take up protons from the medium
and form water (2H+ + 2e- + 1/2 O2 H2O).
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