CELL AND ITS COMPONENTS

Cells are the body’s smallest structural and functional units. They are grouped together to form tissues,
each of which has a specialized function, e.g. blood, muscle, bone. Different tissues are grouped together
to form organs, e.g. the heart, stomach and brain. Organs are grouped together to form systems, each of
which performs a particular function that maintains homeostasis and contributes to the health of the

A cell consists of a plasma membrane enclosing a number of organelles suspended in a watery fluid
called cytosol.


The cytosol (intracellular fluid) is the fluid portion of the cytoplasm that surrounds organelles and
constitutes about 55% of total cell volume. Although it varies in composition and consistency from one
part of a cell to another, cytosol is 75–90% water plus various dissolved and suspended components.
Among these are different types of ions, glucose, amino acids, fatty acids, proteins, lipids, ATP, and waste

The cytosol is the site of many chemical reactions required for a cell’s existence


Organelles are specialized structures within the cell that have characteristic shapes; they perform specific
functions in cellular growth, maintenance, and reproduction.

They include: the nucleus, mitochondria, ribosomes, endoplasmic reticulum, Golgi apparatus, lysosomes
and the cytoskeleton. The cell content excluding the nucleus is the cytoplasm, i.e. the cytosol and other


 The cytoskeleton is a network of protein filaments that extends throughout the cytosol. Three types of
filamentous proteins contribute to the cytoskeleton’s structure, as well as the structure of other


1. Serves as a scaffold that helps to determine a cell’s shape and to organize the cellular contents.

2. Aids movement of organelles within the cell, of chromosomes during cell division, and of whole cells
such as phagocytes.

In the order of their increasing diameter, these structures are:

 Microfilaments: These are the thinnest elements of the cytoskeleton. They are composed of the
   protein actin, and are most prevalent at the edge of a cell. Microfilaments have two general functions:
   They help generate movement and provide mechanical support that is responsible for the basic
   strength and shapes of cells.

 Intermediate filaments: these filaments are thicker than microfilaments but thinner than
   microtubules. Several different proteins can compose intermediate filaments, which are exceptionally
   strong. They are found in parts of cells subject to mechanical stress, help stabilize the position of
   organelles such as the nucleus, and help attach cells to one another.
 Microtubules: These are the largest of the cytoskeletal components and are long, unbranched hollow
   tubes composed mainly of the protein tubulin. The assembly of microtubules begins in an organelle
   called the centrosome. Microtubules help determine cell shape. They also function in the movement of
   organelles such as secretory vesicles, of chromosomes during cell division, and of specialized cell
   projections, such as cilia and flagella.


 The centrosome, located near the nucleus, consists of two components: a pair of centrioles and
pericentriolar material. The two centrioles are cylindrical structures, each composed of nine clusters

of three microtubules (triplets) arranged in a circular pattern. The long axis of one centriole is at a
right angle to the long axis of the other. Surrounding the centrioles is pericentriolar material, which
contains hundreds of ring-shaped complexes composed of the protein tubulin.

These tubulin complexes are the organizing centers for growth of the mitotic spindle, which plays a
critical role in cell division, and for microtubule formation in nondividing cells. During cell division,
centrosomes replicate so that succeeding generations of cells have the capacity for cell division.

Function: The pericentriolar material of the centrosome contains tubulins that build microtubules in
nondividing cells and form the mitotic spindle during cell division.


Microtubules are the dominant components of cilia and flagella, which are motile projections of the cell

Cilia are numerous, short, hair like projections that extend from the surface of the cell. Each cilium
contains a core of 20 microtubules surrounded by plasma membrane.

Flagella are similar in structure to cilia but are typically much longer. Flagella usually move an entire cell.
A flagellum generates forward motion along its axis by rapidly wiggling in a wavelike pattern.

The only example of a flagellum in the human body is a sperm cell’s tail, which propels the sperm toward
the oocyte in the uterine tube.


1. Cilia move fluids along a cell’s surface.

2. A flagellum moves an entire cell.


Ribosomes are the sites of protein synthesis. The name of these tiny organelles reflects their high content
of one type of ribonucleic acid.

Structurally, a ribosome consists of two subunits, one about half the size of the other. The large and small
subunits are made separately in the nucleolus, a spherical body inside the nucleus. Once produced, the
large and small subunits exit the nucleus separately, then come together in the cytoplasm.

Some ribosomes are attached to the outer surface of the nuclear membrane and to an extensively folded
membrane called the endoplasmic reticulum. These ribosomes synthesize proteins destined for specific
organelles, for insertion in the plasma membrane, or for export from the cell. Other ribosomes are “free”
or unattached to other cytoplasmic structures. Free ribosomes synthesize proteins used in the cytosol.
Ribosomes are also located within mitochondria, where they synthesize mitochondrial proteins.


1. Ribosomes associated with endoplasmic reticulum synthesize proteins destined for insertion in the
plasma membrane or secretion from the cell.

2. Free ribosomes synthesize proteins used in the cytosol.


The endoplasmic reticulum or ER is a network of membranes in the form of flattened sacs or tubules. The
ER extends from the nuclear envelope (membrane around the nucleus), to which it is connected,
throughout the cytoplasm.

Cells contain two distinct forms of ER, which differ in structure and function. Rough ER is continuous
with the nuclear membrane and usually is folded into a series of flattened sacs. The outer surface of
rough ER is studded with ribosomes, the sites of protein synthesis. Rough ER produces secretory
proteins, membrane proteins, and many organellar proteins.

Smooth ER extends from the rough ER to form a network of membrane tubules. Unlike rough ER,
smooth ER does not have ribosomes on the outer surfaces of its membrane. Smooth ER does not
synthesize proteins, but it does synthesize fatty acids and steroids, such as estrogens and testosterone.

In liver cells, enzymes of the smooth ER help release glucose into the bloodstream and inactivate or
detoxify lipid-soluble drugs or potentially harmful substances.


1. Rough ER synthesizes glycoproteins and phospholipids that are transferred into cellular organelles,
inserted into the plasma membrane, or secreted during exocytosis.

2. Smooth ER synthesizes fatty acids and steroids, such as estrogens and testosterone; inactivates or
detoxifies drugs and other potentially harmful substances; removes the phosphate group from glucose-6-
phosphate; and stores and releases calcium ions that trigger contraction in muscle cells.


Most of the proteins synthesized by ribosomes attached to rough ER are ultimately transported to other
regions of the cell.

The first step in the transport pathway is through an organelle called the Golgi complex. It consists of 3 to
20 cisternae, small, flattened membranous sacs with bulging edges that resemble a stack of pita bread.
The cisternae are often curved, giving the Golgi complex a cuplike shape.

The convex entry or cis face is a cisterna that faces the rough ER. The concave exit or trans face is a
cisterna that faces the plasma membrane. Sacs between the entry and exit faces are called medial

1. Modifies, sorts, packages, and transports proteins received from the rough ER.

2. Forms secretory vesicles that discharge processed proteins via exocytosis into extracellular fluid;
forms membrane vesicles that ferry new molecules to the plasma membrane; forms transport vesicles
that carry molecules to other organelles, such as lysosomes.


Lysosomes are membrane-enclosed vesicles that form from the Golgi
complex. A lysosome can engulf another organelle, digest it, and return
the digested components to the cytosol for reuse. In this way, old
organelles are continually replaced. The process by which entire worn-
out organelles are digested is called autophagy.


1. Digest substances that enter a cell via endocytosis and transport final products of digestion into

2. Carry out autophagy, the digestion of worn-out organelles.

3. Carry out autolysis, the digestion of entire cell.

4. Carry out extracellular digestion.


Another group of organelles similar in structure to lysosomes, but smaller, are the peroxisomes
Peroxisomes, also called microbodies, contain several oxidases, enzymes that can oxidize (remove
hydrogen atoms from) various organic substances.


Lysosomes degrade proteins delivered to them in vesicles. Cytosolic proteins also
require disposal at certain times in the life of a cell. Continuous destruction of
unneeded, damaged, or faulty proteins is the function of tiny barrel-shaped structures
consisting of four stacked rings of proteins around a central core called proteasomes.

Proteasomes were so named because they contain myriad proteases, enzymes that cut proteins into
small peptides.


Mitochondria are referred to as the “powerhouses” of the cell.

A mitochondrion consists of an outer mitochondrial membrane and an inner mitochondrial membrane
with a small fluid-filled space between them. Both membranes are similar in structure to the plasma
membrane. The inner mitochondrial membrane contains a series of folds called cristae. The central fluid-
filled cavity of a mitochondrion, enclosed by the inner mitochondrial membrane, is the matrix.

The elaborate folds of the cristae provide an enormous surface area for the chemical reactions that are
part of the aerobic phase of cellular respiration, the reactions that produce most of a cell’s ATP. The
enzymes that catalyze these reactions are located on the cristae and in the matrix of the mitochondria.


The nucleus is a spherical or oval-shaped structure that usually is the most prominent feature of a cell.

A double membrane called the nuclear envelope separates the nucleus from the cytoplasm. Both layers of
the nuclear envelope are lipid bilayers similar to the plasma membrane.

The outer membrane of the nuclear envelope is continuous with rough ER and resembles it in structure.
Many openings called nuclear pores extend through the nuclear envelope. Each nuclear pore consists of a
circular arrangement of proteins surrounding a large central opening that is about 10 times wider than
the pore of a channel protein in the plasma membrane. Nuclear pores control the movement of
substances between the nucleus and the cytoplasm.

Inside the nucleus are one or more spherical bodies called nucleoli that function in producing
ribosomes. Each nucleolus is simply a cluster of protein, DNA, and RNA; it is not enclosed by a
membrane. Nucleoli are the sites of synthesis of rRNA and assembly of rRNA and proteins into
ribosomal subunits. Nucleoli are quite prominent in cells that synthesize large amounts of protein,
such as muscle and liver cells. Nucleoli disperse and disappear during cell division and reorganize
once new cells are formed.

Within the nucleus are most of the cell’s hereditary units, called genes, which control cellular structure
and direct cellular activities. Genes are arranged along chromosomes. Human somatic (body) cells
have 46 chromosomes, 23 inherited from each parent. Each chromosome is a long molecule of DNA
that is coiled together with several proteins. This complex of DNA, proteins, and some RNA is called
chromatin. The total genetic information carried in a cell or an organism is its genome.


The plasma membrane, a flexible yet sturdy barrier that surrounds and contains the cytoplasm of a
cell, is described by using a structural model called the fluid mosaic model.

                                                                                     Hydrophilic Head

                                                                                     Hydrophobic tail

                            Transmembrane Protein
Structure of the Plasma Membrane:

The Lipid Bilayer: The basic structural framework of the plasma membrane is the lipid bilayer, two
back-to-back layers made up of three types of lipid molecules—phospholipids, cholesterol, and
glycolipids. About 75% of the membrane lipids are phospholipids (lipids that contain phosphorus).
Present in smaller amounts are cholesterol (about 20%), a steroid with an attached -OH (hydroxyl)
group, and various glycolipids (about 5%), lipids with attached carbohydrate groups.

The bilayer arrangement occurs because the lipids are amphipathic molecules, which means that they
have both polar and nonpolar parts. In phospholipids, the polar part is the phosphate containing
“head,” which is hydrophilic (water loving). The nonpolar parts are the two long fatty acid “tails,”
which are hydrophobic (water fearing) hydrocarbon chains. Because “like seeks like,” the
phospholipid molecules orient themselves in the bilayer with their hydrophilic heads facing outward.

In this way, the heads face a watery fluid on either side—cytosol on the inside and extracellular fluid
on the outside. The hydrophobic fatty acid tails in each half of the bilayer point toward one another,
forming a nonpolar, hydrophobic region in the membrane’s interior.

Arrangement of Membrane Proteins:

Membrane proteins are classified as integral or peripheral according to whether they are firmly
embedded in the membrane.

Integral proteins extend into or through the lipid bilayer among the fatty acid tails and are firmly
embedded in it. Most integral proteins are transmembrane proteins, which means that they span the
entire lipid bilayer and protrude into both the cytosol and extracellular fluid.

Peripheral proteins are not as firmly embedded in the membrane. They associate more loosely with
the polar heads of membrane lipids or with integral proteins at the inner or outer surface of the

Many membrane proteins are glycoproteins, proteins with carbohydrate groups attached to the ends
that protrude into the extracellular fluid. The carbohydrate portions of glycolipids and glycoproteins
form an extensive sugary coat called the glycocalyx. The pattern of carbohydrates in the glycocalyx
varies from one cell to another. Therefore, the glycocalyx acts like a molecular “signature” that
enables cells to recognize one another.

Functions of Membrane Proteins:

    Some integral membrane proteins form ion channels, pores or holes through which specific
       ions, such as potassium ions, can flow to get into or out of the cell.

    Other integral proteins act as carriers, selectively moving a polar substance or ion from one
       side of the membrane to the other. Carriers are also known as transporters.

 Integral proteins called receptors serve as cellular recognition sites. Each type of receptor
   recognizes and binds a specific type of molecule. For instance, insulin receptors bind the
   hormone insulin. A specific molecule that binds to a receptor is called a ligand of that receptor.

 Some integral proteins are enzymes that catalyze specific chemical reactions at the inside or
   outside surface of the cell.

 Integral proteins may also serve as linkers, which anchor proteins in the plasma membranes of
   neighboring cells to one another or to protein filaments inside and outside the cell. Peripheral
   proteins also serve as enzymes and linkers.

 Membrane glycoproteins and glycolipids often serve as cell identity markers. They may enable
   a cell to recognize other cells of the same kind during tissue formation or to recognize and
   respond to potentially dangerous foreign cells. The ABO blood type markers are one example of
   cell identity markers.

 Peripheral proteins help support the plasma membrane, anchor integral proteins, and
   participate in mechanical activities such as moving materials and organelles within cells,
   changing cell shape in dividing and muscle cells, and attaching cells to one another.


Structure of the cell membrane is well suited for the transport of substances in and out of the cell.
Lipids and proteins of cell membrane play an important role in the transport of various substances
between extra cellular fluid (ECF) and intracellular fluid (ICF).
Two types of basic mechanisms are involved in the transport of substances across the cell membrane:
1. Passive transport mechanism
2. Active transport mechanism.

1. Passive transport mechanism:

In passive processes, a substance moves down its concentration or electrical gradient to cross the
membrane using only its own kinetic energy (energy of motion). Kinetic energy is intrinsic to the
particles that are moving. There is no input of energy from the cell.

It is also known as diffusion or downhill movement.

Diffusion is of two types, namely simple diffusion and facilitated diffusion.

Simple diffusion of substances occurs either through lipid layer or protein layer of the cell membrane.
Facilitated diffusion occurs with the help of the carrier proteins of the cell membrane. Thus, the
diffusion can be discussed under three headings:

    Simple diffusion through lipid layer.
    Facilitated or carrier-mediated diffusion.

Simple diffusion through lipid layer:
Simple diffusion is a passive process in which substances move freely through the lipid bilayer of the
plasma membranes of cells without the help of membrane transport proteins. Nonpolar, hydrophobic
molecules move across the lipid bilayer through the process of simple diffusion.
Such molecules include oxygen, carbon dioxide, and nitrogen gases; fatty acids; steroids; and fat-
soluble vitamins (A, D, E, and K). Small, uncharged polar molecules such as water, urea, and small
alcohols also pass through the lipid bilayer by simple diffusion.

Facilitated Diffusion or carrier-mediated diffusion:
Solutes that are too polar or highly charged to move through the lipid bilayer by simple diffusion can
cross the plasma membrane by a passive process called facilitated diffusion. In this process, an integral
membrane protein assists a specific substance across the membrane. The integral membrane protein
can be either a membrane channel or a carrier. e.g. glucose, amino acids.

In addition to diffusion, there are some special types of passive transport, viz.
1. Bulk flow
2. Filtration
3. Osmosis

Movement of water and solutes from an area of high hydrostatic pressure to an area of low hydrostatic
pressure is called filtration. Hydrostatic pressure is developed by the weight of the fluid. Filtration
process is seen at arterial end of the capillaries, where movement of fluid occurs along with dissolved
substances from blood into the interstitial fluid. It also occurs in glomeruli of kidneys.

Osmosis is the special type of diffusion. It is defined as the movement of water or any other solvent
from an area of lower concentration to an area of higher concentration of a solute, through a
semipermeable membrane.
The semipermeable membrane permits the passage of only water or other solvents but not the solutes.
Osmosis can occur whenever there is a difference in the solute concentration on either side of the
membrane. Osmosis depends upon osmotic pressure.
Osmotic pressure is the pressure created by the solutes in a fluid.

Osmosis across the cell membrane is of two types:
1. Endosmosis: Movement of water into the cell
2. Exosmosis: Movement of water out of the cell.

Active transport mechanism:

Active transport is the movement of substances against the chemical or electrical or electrochemical
gradient. It is like swimming against the water tide in a river. It is also called uphill transport. Active
transport requires energy, which is obtained mainly by breakdown of high energy compounds like
adenosine triphosphate (ATP).

Active transport is of two types:

1. Primary active transport

2. Secondary active transport

Primary active transport is the type of transport mechanism in which the energy is liberated directly
from the breakdown of ATP. By this method, the substances like sodium, potassium, calcium, hydrogen
and chloride are transported across the cell membrane. The most prevalent primary active transport
mechanism expels sodium ions from cells and brings potassium ions in. Because of the specific ions it
moves, this carrier is called the sodium-potassium pump.

Secondary active transport is the transport of a substance with sodium ion, by means of a common
carrier protein. When sodium is transported by a carrier protein, another substance is also
transported by the same protein simultaneously, either in the same direction (of sodium movement)
known as symporters or in the opposite direction (known as antiporters). Thus, the transport of
sodium is coupled with transport of another substance.

In addition to primary and secondary active transport systems, there are some special categories of
active transport which are generally called the vesicular transport.

Special categories of active transport:

1. Endocytosis

2. Exocytosis

3. Transcytosis


Endocytosis is defined as a transport mechanism by which the macromolecules enter the cell.
Macromolecules (substances with larger molecules) cannot pass through the cell membrane either by
active or by passive transport mechanism. Such substances are transported into the cell by endocytosis.

Endocytosis is of three types:

Pinocytosis- Cell Drinking

Phagocytosis- Cell Eating

Receptor-mediated endocytosis


Pinocytosis is a process by which macromolecules like bacteria and antigens are taken into the cells. It is
otherwise called the cell drinking.

Pinocytosis involves following events:

      Macromolecules (in the form of droplets of fluid) bind to the outer surface of the cell membrane
      Now, the cell membrane evaginates around the droplets
      Droplets are engulfed by the membrane
      Engulfed droplets are converted into vesicles and vacuoles, which are called endosomes
      Endosome travels into the interior of the cell
      Primary lysosome in the cytoplasm fuses with endosome and forms secondary lysosome
      Now, hydrolytic enzymes present in the secondary lysosome are activated resulting in digestion
       and degradation of the endosomal contents.

Phagocytosis is the process by which particles larger than the macromolecules are engulfed into the cells.
It is also called cell eating.

Larger bacteria, larger antigens and other larger foreign bodies are taken inside the cell by means of
phagocytosis. Only few cells in the body like neutrophils, monocytes and the tissue macrophages show
phagocytosis. Among these cells, the macrophages are the largest phagocytic cells.

Mechanism of phagocytosis:
    When bacteria or foreign body enters the body, first the phagocytic cell sends cytoplasmic
      extension (pseudopodium) around bacteria or foreign body
    Then, these particles are engulfed and are converted into endosome like vacuole. Vacuole is very
      large and it is usually called the phagosome
    Phagosome travels into the interior of cell
    Primary lysosome fuses with this phagosome and forms secondary lysosome
    Hydrolytic enzymes present in the secondary lysosome are activated resulting in digestion and
      degradation of the phagosomal contents

Receptor-mediated endocytosis:

Receptor-mediated endocytosis is the transport of macromolecules with the help of a receptor protein.
Surface of cell membrane has some pits which contain a receptor protein called clathrin. Together with a
receptor protein (clathrin), each pit is called receptor-coated pit. These receptor-coated pits are involved
in the receptormediated endocytosis

Mechanism of receptor-mediated endocytosis:

    Receptor-mediated endocytosis is induced by substances like ligands
    Ligand molecules approach the cell and bind to receptors in the coated pits and form
     ligandreceptor complex
    Ligand-receptor complex gets aggregated in the coated pits. Then, the pit is detached from cell
     membrane and becomes the coated vesicle. This coated vesicle forms the endosome
    Endosome travels into the interior of the cell. Primary lysosome in the cytoplasm fuses with
     endosome and forms secondary lysosome
    Now, the hydrolytic enzymes present in secondary lysosome are activated resulting in release of
     ligands into the cytoplasm
    Receptor may move to a new pit of the cell membrane.

Receptor-mediated endocytosis play an important role in the transport of several types of
macromolecules into the cells, viz hormones, growth factors, lipids, antibodies.


Exocytosis is the process by which the substances are expelled from the cell. In this process, the
substances are extruded from cell without passing through the cell membrane. This is the reverse of


Transcytosis is a transport mechanism in which an extracellular macromolecule enters through one side
of a cell, migrates across cytoplasm of the cell and exits through the other side.

                                            PROTEIN SYNTHESIS

Proteins determine the physical and chemical characteristics of the cell and therefore of the organism
formed from them. Some proteins help assemble cellular structures such as the plasma membrane, the
cytoskeleton, and other organelles. Others serve as hormones, enzymes, antibodies, and contractile
elements in muscular tissue.

Proteome: all the protein of an organism

Gene expression: Gene’s DNA is used as a template for the synthesis of specific protein

Protein synthesis happens in 2 steps

A) Transcription
B) Translation

   Transcription: process through which the information encoded in a specific region of DNA is copied
   to produce a specific molecule of RNA.
   Transcription happens in Nucleus. During transcription process the genetic information represented
   by the sequence of base triplets in DNA serves as a template for copying the information into
   complementary sequence of codons.

       The enzyme RNA polymerase catalyzes the transcription of DNA.

       Promoter: the specific nucleotide where the transcription begins.

       Terminator: specific nucleotide where the transcription ends.

   Transcription leads to 3 types of RNA molecules
   Ribosomal RNA (rRNA): Helps form the ribosomes
   Messenger RNA (mRNA): Carries the instructions from the nucleus to the ribosome for building a
   Transfer RNA (tRNA): Transfers appropriate amino acids to the ribosome for building the protein

   Translation: process through which the information encoded by mRNA is translated to amino acid
   sequence of protein

Translation is carried in the Cytosol of the cell.

Translation occurs in many steps as following

   a) Binding of mRNA to the small ribosomal unit, Binding of tRNA to the specific
   b) Attachment of large-ribosomal unit to small unit- mRNA complex
   c) The anticodon on the tRNA pairs with the codon on mRNA
   d) Large ribosomal subunit catalyzes the formation peptide bond which detaches
       the tRNA and the process of anticoding the codon is continued.
   e) Protein synthesis ends when the ribosome reaches the terminator codon.

Formed protein detaches from the tRNA. tRNA vacates the ribosomal unit and
ribosome splits into its large and small subunits.

                               CELL DIVISION & CELL DIVERSITY

The process by which the cells reproduce themselves is called cell division.
Somatic cell: Cell of the body other than germ cell
Reproductive cell/Gamete/Germ cell: Precursor cell destined to become gamete Ex: Sperm or
Mitosis: Process by which nuclear division in case of somatic cell takes place.
Cytokinesis: process by which the cytoplasm of somatic cell divides.
Meiosis: process by which the reproductive cells undergo division to produce gametes.
Cell cycle: an orderly sequence of events by which a cell duplicates its contents and divides.

Cell division can be classified into 2 types
1) Somatic cell division
2) Reproductive cell division

Somatic cell division: A cell undergoes a nuclear division called mitosis and a
cytoplasmic division called cytokinesis to produce two identical cells, each with the
same number and kind of chromosomes as the original cell. Somatic cell division
replaces dead or injured cells and adds new ones during tissue growth.

Reproductive cell division: This type of cell division produces gametes. This type of
cell division consists of 2 step division called meiosis in which the number of
chromosomes in the nucleus is reduced by half. The cells thus produced will be utilized
in sexual reproduction.

A cell reproduces itself by duplicating all its contents to pass to the next generation of
cells. The cell cycle consists of two steps
1) Interphase: When the cell is preparing its self for division.
2) Mitotic phase: when the cell is dividing.
Interphase: during this phase the cell replicates its DNA, produces additional cell
organelles and cytosolic components and prepares itself for cell division.

Interphase consists of 3 steps/phases

   a) G1 phase: is the interval between mitotic phase and the S phase. During G1
      the cell is metabolically active, it replicates most of its organelles and cytosolic
   b) S phase: is the interval between G1 and G2 phase during S phase the
      replication of DNA occurs
   c) G2 phase: is the interval between S phase and mitotic phase, during G2 cell
      growth continues and enzymes, proteins are synthesized for the cell division.

Once a cell completes its activities during the G1, S, and G2 phases of interphase, the
mitotic phase begins.

                                    Mitotic phase
The mitotic (M) phase of the cell cycle consists of a nuclear division (mitosis) and a
cytoplasmic division (cytokinesis) to form two identical cells.

Mitosis can be divided into 4 phases

   A)   Prophase
   B)   Metaphase
   C)   Anaphase
   D)   Telophase

Prophase: During this phase chromatin fibers condense and shorten into chromosomes,
each prophase chromosome consists of a pair of identical strands called chromatids. A
constricted region called a centromere holds the chromatid pair together. At the outside of
each centromere is a protein complex known as the kinetochore. The mitotic spindle is
responsible for the separation of chromatids to opposite poles of the cell. Then, the
nucleolus disappears and the nuclear envelope breaks down.

Metaphase: During metaphase, the microtubules of the mitotic spindle align the
centromeres of the chromatid pairs at the exact center of the mitotic spindle. This midpoint
region is called the metaphase plate.

Anaphase: During anaphase, the centromeres split, separating the two members of each
chromatid pair, which move toward opposite poles of the cell

Telophase: This is the final stage of mitosis, telophase, begins after chromosomal
movement stops. The identical sets of chromosomes, now at opposite poles of the cell,
uncoil and revert to the threadlike chromatin form. A nuclear envelope forms around each
chromatin mass, nucleoli reappear in the identical nuclei, and the mitotic spindle breaks

Cytokinesis process of division of a cell’s cytoplasm and organelles into two identical cells.
This process usually begins in late anaphase with the formation of a cleavage furrow, a
slight indentation of the plasma membrane, and is completed after telophase.

                                 Reproductive cell division

Meiosis, the reproductive cell division that occurs in the gonads (ovaries and testes),
produces gametes in which the number of chromosomes is reduced by half. As a result,
gametes contain a single set of 23 chromosomes and thus are haploid (n) cells. Fertilization
restores the diploid number of chromosomes.

Meiosis occurs in two successive stages: Meiosis I and Meiosis II.

Meiosis I: consist of 4 phases

   A)   Prophase I
   B)   Metaphase I
   C)   Anaphase I
   D)   Telophase I

Prophase I: Is an extended phase in which the chromosomes shorten and thicken, the
nuclear envelope and nucleoli disappear, and the mitotic spindle forms.

Two events that are not seen in mitotic prophase occur during prophase I of meiosis.

First, the two sister chromatids of each pair of homologous chromosomes pair off, an event
called synapsis. The resulting four chromatids form a structure called a tetrad.

Second, parts of the chromatids of two homologous chromosomes may be exchanged with
one another. Such an exchange between parts of nonsister (genetically different)
chromatids is termed crossing-over.

crossing-over process permits an exchange of genes between chromatids of homologous
chromosomes. Due to crossing-over, the resulting cells are genetically unlike each other
and genetically unlike the starting cell that produced them. Crossing-over accounts for part
of the great genetic variation among humans and other organisms that form gametes via

Metaphase I: the tetrads formed by the homologous pairs of chromosomes line up along
the metaphase plate of the cell, with homologous chromosomes side by side.

Anaphase I: the members of each homologous pair of chromosomes separate as they are
pulled to opposite poles of the cell by the microtubules attached to the centromeres. The
paired chromatids, held by a centromere, remain together.

Telophase I and cytokinesis of meiosis are similar to telophase and cytokinesis of

The net effect of meiosis I is that each resulting cell contains the haploid number of
chromosomes because it contains only one member of each pair of the homologous
chromosomes present in the starting cell.

MEIOSIS II : consists of four phases

   A)   Prophase II
   B)   Metaphase II
   C)   Anaphase II
   D)   Telophase II

These phases are similar to those that occur during mitosis

Meiosis I begins with a diploid starting cell and ends with two cells, each with the
haploid number of chromosomes.

During meiosis II, each of the two haploid cells formed during meiosis I divides; the
net result is four haploid gametes that are genetically different from the original
diploid starting cell.

                                      CELL DIVERSITY

The body of an average human adult is composed of nearly 100 trillion cells. All of these
cells can be classified into about 200 different cell types. Cells vary considerably in size and

Cells may be round, oval, flat, cube-shaped, column-shaped, elongated, star-shaped,
cylindrical, or disc-shaped. A cell’s shape is related to its function in the body.

For example, a sperm cell has a long whip like tail (flagellum) that it uses for locomotion.
The disc shape of a red blood cell gives it a large surface area that enhances its ability to
pass oxygen to other cells.

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