Cell biology (cytology)

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.

Cell theory

Watson & Crick (1953): developed the model of DNA which is the hereditary material.

2- The cell is the smallest living thing that can perform all the functions of life.

3- All cells must come from preexisting cells.

Cell theory:It states:1- All living organisms are made of cells.

Types of cells

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.

There are two basic types of cells (according to internal complexity) which are:

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

Bacteria

General plan of prokaryotic cell

* Single strand* Circular* Attached to cell membrane* Attached with small amount of protein e.g. Bacteria

2- Eukaryotes

An eukaryotic animal cell

The main characters of

eukaryotic cells are:

* Include complex forms.

* The presence of nuclear membrane (nucleus).

•The presence of

membranous

organelles.

e.g. Animal and plant cells

An animal cell

Another shape of

eukaryotic animal cell

Another shape of eukaryotic animal cell

Another shape of eukaryotic animal cell

mitochondrion

microtubules

microfilaments

chloroplast

plasmodesmata

ribosomes

Golgi apparatusplasma membrane

cell wall

peroxisome

smooth ER

rough endoplasmic reticulum (ER)

nucleolus

chromatin

nuclear envelope

Central vacuole

nucleus

An eukaryotic plant cell

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).

Cell shapeVariable: oval, spindle, amoeboid, flat, polyhedral, spherical, square, columnar….etc.

Cell sizeVariable: related to the function.

3- Plant cells have cell wall (composed cellulose, pectin or both of them). ???!!!!!! 4- Plant cells have large central vacuole. ???!!!!

* 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 Nucleus

Organelles (organoids) Inclusions

Non-membranous org.Membranous org.

1- Cell membrane2- Mitochondria3- Endoplasmic reticulum4- Golgi apparatus5- Lysosomes6- Microbodies (peroxisomes)

1- Ribosomes 2- Microtubules3- Centrioles 1- stored food

2- secretory granules

3- colored pigments 4- Crystals

1- Nuclear membrane 2- Nuclear sap3- Nucleoli 4- Chromatin network

Cytoplasmic organellesCytoplasmic organelles

It is very difficult to seen by light microscope (80-100 Angstrom). It is very difficult to seen by light microscope (80-100 Angstrom).

By using electron microscope, it shows By using electron microscope, it shows three layers modelthree layers model

Three layers (trilamellar) model

Dark layer

Light layer

Dark layer

A- Membranous organellesA- Membranous organelles

1- The plasma (cell) membrane (plasmalemma):1- The plasma (cell) membrane (plasmalemma):

Molecular structure of cell membrane:

1) Lipid component:

It is made of

i- Phosopholipid molecules:

a- Heads: (phosphate groups) (hydrophilic) (polar) (charged).

b- Tails: (fatty acids) (hydrophobic) (non-polar) (non-charged).

Dark layer

Dark layer

Cytoplasm

Exracellular (intercellular) fluid

Light layer

Phosphate polar heads

Fatty acids non-polar tails

Cytoplasm

Extracellular fluid

Phospholipid molecule

Bilipid

layer

Hydrop

hobic tails(fatty acid

tails)(non-polar)

Hydrophillic heads (phosphate groups)(polar)

Hydrophillic heads

Phospholipid bilayer (Trilamellar membrane)

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).

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.

Molecular structure of cell membrane (continue):

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

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.

It is polysaccharides. It may be attached to:

Extracellular fluid

cytoplasm

phospholipidcholesterol

proteincarbohydrateglycoprotein

glycolipid

filaments of cytoskeleton

Plasma (cell) membrane)fluid mosaic model(

lipids

Gly

coca

lyx

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( )2( )3(

)4( )5( )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).

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.

1) It does not require energy. It is achieved by the kinetic energy of the molecules.

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.

Inside the cell

Outside the cell

A transport protein

Highconcentration

of solutes

Inside the cell

Outside the cell

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.

Lowconcentration

of solutes

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

2) A hypotonic solution

3) An isotonic solution

Water diffuses out of the cell till equilibrium is reached. It will shrink and die. This condition is called plasmolysis.

Water diffuses into the cell till equilibrium is reached.

It causes it to swell and often burst.This condition is called cytolysis.

2- Active transport:

It uses energy (in the form of ATP).It takes place against the concentration gradient.

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.

+H2O Osmosis

Simple

Summary

3- Bulk transport (vesicle-mediated transport) (endocytosis & exocytosis):

It moves large molecules into the cell.

It needs energy like active transport. It transports large molecules through vesicles.It comprises:

a) Endocytosis:

It includes three different processes which are:

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

NoteEndocytosis removes membranes from cell surface to form vesicles.

ii- Pinocytosis

iii- Receptor-mediated endocytosis:

When specific molecules - such as microbes - in the extracellular fluid bind to sites on the plasma membrane.

iii- Receptor-mediated endocytosis

SummaryEndocytosis

b) Exocytosis

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.

2- Mitochondria

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.

It is found in all nucleated cells, (absent in RBCs).

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.

: فى النواة ثشبه الميتوكوندريا أن الخالصةبغشائين ( 1) تحاط .أنها

الــ (( 2 ) على تحتوى الجينات) DNAأنها يكون والذى النواة فى الموجود. للكرموزومات المكونة

الريبوسومات** ( السيتوبالزم) Ribosomesأما فى .فتوجد

NotesThe 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.

2- Mitochondria (continue)

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)

Functions of ER:1- Helps molecules to transport through the cell and from one cell to another (both rough & mooth ER).

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.

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.

c) Intermediate vesicles which are found in large number close to the periphery of the medial region of sacs 9 cisternae).

b) Outgoing transport vesicles (large vesicles) which are detached from the trans face of cisternae. These vesicles are filled with protein.

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

Their number is affected by different physiological and pathological changes. Decrease their number during fasting and ageing.

They are saclike structure surrounded by a single membrane. It contains powerful digesting enzymes such as acid phosphatase, deoxyribonuclease, ribonuclease, … etc.

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:

Heterophagy Autophagy

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.

NoteIf 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.

??!!!

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 contain enzymes such as that involved in the degradation of fatty acids and amino acids and catalase.

They are about the same size, or slightly larger than lysosomes.

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.

H2O2catalase O2 + 2(H2O)

B- Non-membranous organelles1- 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.

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.

The mechanism

2- MicrotubulesThe 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- CentriolesCentrioles are short hollow cylindrical tubules that found near the nucleus.

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.

There are two centrioles at right angles to each other . Centrosome

The 9+2 arrangement of microtubules in

a flagellum or cilium.

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.

Microvilli

The upper respiratory tract have cilia while sperms use flagella to move.

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

Structure of the nuclear envelope and nuclear pores

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.

Functions: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 are involved in protein synthesis. They appear as thin threads. They found in active cells.

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:

The nitrogenous base may be:

A nucleotide

1- A nitrogenous base, 2- A pentose sugar and

3- A phosphate group.

i- Pyrimidines: They include: Cytosine (C), Thymine (T) and Uracil (U).

ii- Purines: They comprise:

Adenine (A) and Guanine (G).

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.

3 Nucleotid

es

A single strand of DNA

A single strand of RNA

A double strand of DNA

Notes

ProteinTranslationRNATranscriptionDNA

Enzymes MetabolismGenes

DNA sequence RNA sequence amino acid sequence

Triplet sequence in DNA(TAC)

Codon in mRNA(AUG)

Amino acid in protein(Met.)

RNA is formed in the nucleus and pass to the cytoplasm carrying informations about the structure of protein which will synthesized in the ribosomes.

DNA is found mainly in the nucleus. Very small amount is found in the mitochodria.

There are different types of RNA; the most famous of them are messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA).

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). 

CYTOGENETICSCell division

Cell division in prokaryotesProkaryotes 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 eukaryotesThere are two types of cell divisions which are mitosis and meiosis.

The cell cycleThere 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, and2) The spindle fibers which go toward the equator of the cell.

2- MetaphaseSpindle 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.

Remember that the chromosomes are still duplicated during metaphase.

3- AnaphaseCentromeres 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.

Chromosomes are found in the most condensed state.

4- TelophaseNuclear 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):

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.

It is the division of the cytoplasm.

PrometaphaseProphaseInterphase

Metaphase Anaphase 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.

Telophase II Telophase II

Meiosis IIProphase II Binary fissionMetaphase II

Prophase I

Meiosis I

Metaphase I

Anaphase I Telophase I

Comparison between mitosis & meiosis

Mitosis MeiosisIt is an indirect division. It is a reduction division.

It occurs in somatic cells. It occurs in germ cells of gonads (testes or ovaries).

Two daughter cells are produced with diploid number of chromosomes (2n).

Four daughter cells are produced with haploid number of chromosomes (n).

No crossing over takes place. 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 andIII- Maturation phase.

Spermatogenesis Oogenesis

I- Multiplication phaseBy repeated mitotic cell division

(i.e. by mitosis)

II- Growth phaseBy growing

III- Maturation phaseBy meiosis

nn

2n

2n2n

2n2n 2n2n

2n

n n

2n

2n2n

2n2n2n2n

2n

nn

n n

nnnn

Primordial germ cell

Spermatogonium Oogonium

1ry spermatocyte 1ry oocyte 2ry spermatocyte 2ry oocyte 1st polar

body

2nd polar bodyMature ovum

SpermatidSpermatozoon

2 polar bodies

1st meiotic division

2nd meiotic division 2nd meiotic division

1st meiotic division

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).

Each ejaculation should contain 200-300 million sperms. When the sperms are formed, they are moved into the epididymis where they become mature then stored. From puberty of a female to menopause, one egg is normally formed per month.

FertilizationIt the fusion of two haploid gametes (sperm and egg) to produce a diploid zygote.

Mature human sperm

Tail

Neck

At puberty, a male will produce approximately 1000 sperm per second .

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.

Sequence of fertilization1- 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.

1) Cleavage:After fertilization, the zygote divides repeatedly by a series of mitotic divisions.

Embryonic developmentThe 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).

Zygote 2-blastomere stage

4-blastomere stage

8-blastomere stage 16-blastomere stage

vertical

at right angle to the1st division

horizontal

double vertical

32-blastomere stage (morula) double horizontal

64-blastomere stage 128-blastomere stage A blastula

@ 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.

The blastula

@ It is a hollow structure formed at the end of cleavage process.

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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