TOPIC 6: Cells

TOPIC 6: Cells

Much of the diversity of forms and functions in living organisms results from small atoms being combined

in different ways to form a number of molecules and molecules form macromolecules. Eventually, these

macromolecules build cells, tissues, organs and finally, an entire organism.

Cells

A cell is the smallest unit of life that can survive and reproduce on its own, given

information in DNA, energy, and raw materials. Some cells live and reproduce

independently. Others do so as part of a multicelled organism.

Discovery of cells

In the middle of the 17th century, one of the pioneers of microscopy, Robert

Hooke (1635–1703), decided to examine a piece of cork tissue with his home-built

microscope. He saw numerous box shaped structures that he thought resembled

row of empty boxes or rooms, so he called them ‘cells’.

Cell theory

Matthias Schleiden and Theodor Schwann, hypothesized that a plant cell is an independent living unit even

when it is part of a plant and both concluded that the tissues of animals as well as plants are composed of

cells and their products. Together, the two scientists recognized that cells have a life of their own even

when they are part of a multicelled body.

Later, physiologist Rudolf Virchow realized that all cells he studied descended from another living cell.

These and many other observations yielded three generalizations that today constitute the cell theory:

1) Every organism is composed of one or more cells

2) Cell is smallest unit having properties of life

3) Continuity of life arises from growth and division of single cells

Thus, Cell theory is that all organisms consist of one or more cells, which are the basic unit of life.

Cell

A cell is the smallest unit that shows the properties of life.

These properties include -

• Can survive on its own or has potential to do so

• Is highly organized for metabolism

• Senses and responds to environment

• Has potential to reproduce

Structure of Cells

Despite their differences, however, all cells share certain organizational and functional features. Every cell

has a plasma membrane. A plasma membrane is selectively permeable, allows only certain materials to

cross. All cell membranes, including the plasma membrane, consist mainly of lipids. The plasma

membrane encloses a fluid or jellylike mixture of water, sugars, ions, and proteins called cytoplasm. Some

or all of a cell’s metabolism occurs in the cytoplasm, and the cell’s internal components are suspended in

it. All cells start out life with DNA, although a few types of cells lose it as they mature.

Cell type

Biologists have categorized cells into two general types: eukaryotic and prokaryotic cells.

The cells of plants, animals, fungi, protozoa, and algae are eukaryotic, and are placed in a category called

Eucarya . All eukaryotic cells have their genetic material surrounded by a nuclear membrane forming the

cellular nucleus. They also have a large number and variety of complex organelles, each specialized in the

metabolic function it performs. In general, they are large in comparison to Prokaryotic cells. These cell

types do not have a nuclear membrane; therefore they lack a cellular nucleus. In addition, they display

unique chemical and metabolic characteristics but do not have the variety and number of organelles seen

in eukaryotes.

Lipid Bilayer

Lipids—mainly phospholipids—make up the bulk

of a cell membrane. A phospholipid consists of a

phosphate containing head and two fatty acid

tails. The polar head is hydrophilic, which means

that it interacts with water molecules. The

nonpolar tails are hydrophobic, so they do not

interact with water molecules, but they do

interact with the tails of other phospholipids.

Lipid bilayers are the basic structural and

functional framework of all cell membranes, gives

membrane it's fluidity.

Fluid mosaic

Other molecules, including steroids and proteins,

are embedded in or associated with the lipid

bilayer of every cell membrane. Most of these

molecules move around the membrane more or

less freely. A cell membrane behaves like a two-

dimensional liquid of mixed composition, so we

describe it as a fluid mosaic. The “mosaic” part of

the name comes from a cell membrane’s mixed composition of lipids and proteins. The fluidity occurs

because the phospholipids in a cell membrane are not bonded to one another. They stay organized as a

bilayer as a result of collective hydrophobic and hydrophilic attractions.

Membrane proteins Separate

Many types of proteins are associated with a cell membrane, and each type adds a specific function to it,

different cell membranes can have different characteristics depending on which proteins are associated

with them. For example, a plasma membrane has certain proteins that no internal cell membrane has.

Many plasma membrane proteins are enzymes. Others are adhesion proteins, which fasten cells together

in animal tissues. Recognition proteins function as identity tags for a cell type, individual, or species. Being

able to recognize “self” means that foreign cells (harmful ones, in particular) can also be recognized.

Receptor proteins bind to a particular substance outside of the cell, such as a hormone or toxin (Figure

4.8C). Binding triggers a change in the cell’s activities that may involve metabolism, movement, division,

or even cell death. Receptors for different types of substances occur on different cells, but all are critical

for homeostasis. Additional proteins occur on all cell membranes. Transport proteins move specific

substances across a membrane, typically by forming a channel through it. These proteins are important

because lipid bilayers are impermeable to most substances, including ions and polar molecules. Some

transport proteins are open channels through which a substance moves on its own across a membrane.

Movement of Molecules Across the Membrane

Cells must continuously receive

nutrients and rid themselves of waste

products—one of the characteristics of

life. Many of the proteins that are

associated with the plasma membrane

are involved in moving molecules across

the membrane. Some proteins are

capable of moving from one side of the

plasma membrane to the other and

shuttle certain molecules across the

membrane. Others extend from one side

of the membrane to the other and form

channels through which substances can

travel. Some of these channels operate

like border checkpoints, which open and

close when circumstances dictate. Some

molecules pass through the membrane

passively, whereas others are assisted

by metabolic activities within the membrane.

Microscopes

Microscopes allow us to study cells in detail. The ones that use visible light to illuminate objects are called

light microscopes. There are two types: Simple and Compound. A more powerful microscope is the

Electron microscopes use electrons instead of visible light to illuminate samples. Because electrons travel

in wavelengths that are much shorter than those of visible light, electron microscopes can resolve details

that are much smaller than you can see with light microscopes. Electron microscopes use magnetic fields

to focus beams of electrons onto a sample.

Limitations of Light

• Wavelengths of light are 400-750 nm

• If a structure is less than one-half of a wavelength long, it will not be visible

• Light microscopes can resolve objects down to about 200 nm in size

Electron Microscopy

• Uses streams of accelerated electrons rather than light

• Electrons are focused by magnets rather than glass lenses

• Can resolve structures down to 0.5 nm

Cell size

Almost all cells are too small to see with the naked eye. Why? The answer begins with the processes that

keep a cell alive. A living cell must exchange substances with its environment at a rate that keeps pace

with its metabolism. These exchanges occur across the plasma membrane, which can handle only so many

exchanges at a time. Thus, cell size is limited by a physical relationship called the surface-to-volume ratio.

By this ratio, an object’s volume increases with the cube of its diameter, but its surface area increases

only with the square. If the cell gets too big, the inward flow of nutrients and the outward flow of wastes

across that membrane will not be fast enough to keep the cell alive.

Two Major Cell Types

According to their structure, cells can be of two types:

• Prokaryotes eg. Bacteria

• Eukaryotes eg. Fungi, Plants, Animals

Prokaryotic cells are so called because they have no nucleus (‘prokaryote’ comes from the Greek,

meaning ‘before the nucleus’). They also have no organelles (internal structures), so there is little

compartmentalization of function within them. From the mid-20th century, when the electron microscope

was developed, it became possible to study the internal detail of cells.

• The cell wall surrounds the cell. It protects the cell from bursting and is composed of peptidoglycan,

which is a mixture of carbohydrate and amino acids.

• The plasma membrane controls the movement of materials into and out of the cell. Some substances

are pumped in and out using active transport.

• Cytoplasm inside the membrane

contains all the enzymes for the

chemical reactions of the cell. It also

contains the genetic material.

• The chromosome is found in a

region of the cytoplasm called the

nucleoid. The DNA is not contained

in a nuclear envelope and also it is

‘naked’ – that is, not associated with

any proteins. Bacteria also contain

additional small circles of DNA called

plasmids. Plasmids replicate

independently and may be passed

from one cell to another.

• Ribosomes are found in all prokaryotic cells, where they synthesize proteins. They can be seen in very

large numbers in cells that are actively producing protein.

• A fagellum is present in some prokaryotic cells. A flagellum, which projects from the cell wall, enables a

cell to move.

• Some bacteria have pili (singular pilus). These structures, found on the cell wall, can connect to other

bacterial cells, drawing them together so that genetic material can be exchanged between them.

Prokaryotic cells are usually much smaller in volume than more complex cells because they have no

nucleus. Their means of division is also simple. As they grow, their DNA replicates and separates into two

different areas of the cytoplasm, which then divides into two. This is called binary fission. It differs slightly

from mitosis (a type of cell division) in eukaryotic cells.

Eukaryotes

The cells of plants, animals, fungi, protozoa, and algae are eukaryotic, and are placed in a category called

Eucarya . All eukaryotic cells have their genetic material surrounded by a nuclear membrane forming the

cellular nucleus. They also have a large number and variety of complex organelles, each specialized in the

metabolic function it performs. In general, they are large in comparison to prokaryotic cells.

Animal cells

• Plasma membrane

• Nucleus

• Ribosomes

• Endoplasmic reticulum

• Golgi body

• Vesicles

• Mitochondria

• Cytoskeleton

Plant cells

• Plasma membrane

• Nucleus

• Ribosomes

• Endoplasmic reticulum

• Golgi body

• Vesicles

• Mitochondria

• Cytoskeleton

• Cell wall

• Central vacuole

• Chloroplast

Functions of Nucleus

• Keeps the DNA molecules of eukaryotic cells separated from

metabolic machinery of cytoplasm

• Makes it easier to organize DNA and to copy it before parent

cells divide into daughter cells

Components of Nucleus

– Nuclear envelope

– Nucleoplasm

– Nucleolus

– Chromosome

– Chromatin

Nucleus:

The nucleus is the defining organelle of eukaryotic cells. The nucleus is separated from the cytoplasm by a

double membrane (two phospholipid bilayers); known as the nuclear envelope. The nuclear envelope

controls the passage of molecules between the nucleus and cytoplasm. The nucleus contains the DNA, the

stored genetic instructions of each cell. In addition, important reactions for interpreting the genetic

instructions occur in the nucleus.

In the nucleus, DNA is organized into discrete units called chromosomes

Each chromosome is composed of a single DNA molecule associated with proteins

The nucleolus is located within the nucleus and is the site of ribosomal RNA (rRNA) synthesis

Nucleolus

Dense mass of material in nucleus

May be one or more

Cluster of DNA and proteins

Materials (mostly rRNA) from which ribosomal subunits are built

Subunits must pass through nuclear pores to reach cytoplasm

Chromatin

Chromatin is composed of long molecules of DNA, along with proteins. Most of the time, the

chromatin is arranged as a long, tangled mass of threads in the nucleus. However, during cell

division, the chromatin becomes tightly coiled into short, dense structures called

chromosomes (chromo=color; some=body). Chromatin and chromosomes are really the

same molecules, but they differ in structural arrangement. In addition to chromosomes, the

nucleus may also contain one, two, or several nucleoli. A nucleolus is the site of ribosome

manufacture. Specific parts of the DNA become organized within the nucleus to produce

ribosomes. A nucleolus is composed of this DNA,

specific granules, and partially completed

ribosomes.

The DNA and proteins of chromosomes are together called chromatin

Chromatin condenses to form discrete chromosomes as a cell prepares to divide

Chromosome is one DNA molecule and its associated proteins

Appearance changes as cell divides

Mitochondria

The mitochondrion (plural, mitochondria) is a type of organelle that specializes in making ATP (molecule

used by cells as main energy source). They have various enzymes to catalyze cellular respiration. Bacteria

have no mitochondria; they make ATP in their cell walls and cytoplasm. Cells that have a very high

demand for energy tend to have many mitochondria e.g. liver needs more because needs more energy.

Mitochondria, like most organelles, can move within the cell and they grow and divide independently.

Each has two membranes, one highly folded inside the other. Double-membrane system: Smooth outer

membrane (lipid bilayer) faces cytoplasm and permeable to small solutes; blocks macromolecules wheras

Inner Membrane (cristae) folds back on itself to enlarge surface area for chemical reactions to take

place. Membranes form two distinct compartments. ATP-making machinery is embedded in the inner

mitochondrial membrane.

Mitochondria and chloroplasts have

similarities with bacteria,

Enveloped by a double membrane

Contain free ribosomes and circular DNA

molecules

Grow and reproduce somewhat

independently in cells

They may have evolved from ancient bacteria that were

engulfed but not digested. Mitochondria and chloroplasts

developed because as a prokaryote it gained protection by

living inside the eukaryote and in turn produced energy for

the eukaryote (symbiotic relationship).

Chloroplasts: Capture of Light Energy

Plastids are a category of membrane-enclosed organelles that function in photosynthesis or storage in

plant and algal cells. Plastids called chloroplasts are organelles specialized for photosynthesis. Chloroplasts

contain the green pigment chlorophyll, as well as enzymes and other molecules that function in

photosynthesis. Chloroplasts are found in leaves and other green organs of plants and in algae.

• Chloroplast structure includes

Stroma: Each has two outer

membranes enclosing a semifluid

interior, the stroma, that contains

enzymes and the chloroplast’s own

DNA.

Thylakoids: Inside the stroma, a

third, highly folded membrane forms

a single, continuous compartment.

The folded membrane resembles

stacks of flattened disks. The stacks

are called grana (singular, granum).

Photosynthesis takes place at this

membrane, which is called the

thylakoid membrane. The abundance

of chlorophylls in thylakoids is the reason most plants are green. By the process of

photosynthesis, chlorophylls and other molecules in the thylakoid membrane harness the

energy in sunlight to drive the synthesis of ATP. The ATP is then used inside the stroma to

build carbohydrates from carbon dioxide and water.

Ribosomes: Protein Factories

Ribosomes are nonmembranous organelles responsible for the synthesis of proteins from amino acids.

They are composed of RNA and protein. Each ribosome is composed of two subunits—a large one and a

small one. As mentioned before, they are constructed in the Nucleolus. Ribosomes carry out protein

synthesis in two locations

– bound ribosomes: Many ribosomes are

attached to the endoplasmic reticulum.

Because ER that has attached ribosomes

appears rough when viewed through an

electron microscope it is called rough ER.

Areas of rough ER are active sites of

protein production.

– free ribosomes: Many ribosomes are also

found floating freely in the cytoplasm

wherever proteins are being assembled.

Cells that are actively producing protein

(e.g., liver cells) have great numbers of free and attached ribosomes.

Ribosomes are not surrounded by membrane (found in prokaryotic cells too)

Cytomembrane System

The cytomembrane system is a series of interacting organelles between the nucleus and the plasma

membrane. Its main function is to make lipids, enzymes, and proteins for secretion, or for insertion into

cell membranes. It also destroys toxins, recycles wastes, and has other specialized functions. The

system’s components vary among different types of cells, but here we present the most common ones:

Components of Cytomembrane System

– Endoplasmic reticulum

– Golgi bodies

– Vesicles

Endoplasmic Reticulum

Part of the cytomembrane system is an

extension of the nuclear envelope called

endoplasmic reticulum, or ER. ER forms a

continuous compartment that folds into

flattened sacs and tubes. The space inside

the compartment is the site where many new

polypeptide chains are modified. Two kinds of

ER, rough and smooth, are named for their

appearance in electron micrographs.

Thousands of ribosomes are attached to the

outer surface of rough ER.

Rough ER

Arranged into flattened sacs

Ribosomes on surface give it a rough

appearance

Some polypeptide chains enter rough ER and are modified

Cells that specialize in secreting proteins have lots of rough ER

Smooth ER

A series of interconnected tubules

No ribosomes on surface

Lipids assembled inside tubules

Smooth ER of liver inactivates wastes, drugs

Sarcoplasmic reticulum of muscle is a specialized form that stores calcium

Functions of Smooth & Rough ER

• The smooth ER

1. Synthesizes lipids

2. Metabolizes carbohydrates

3. Detoxifies drugs and poisons

4. Stores calcium ions

• The rough ER

1. Has bound ribosomes

2. Distributes transport vesicles,

proteins surrounded by membranes

3. Is a membrane factory for the cell

Golgi Bodies

Golgi : The Golgi is a series of flattened membrane compartments, whose purpose is to process and

package proteins produced in-the rough endoplasmic reticulum. The processed molecules are packaged

into membrane vesicles, then targeted and transported to-their final destinations.

Functions of the Golgi apparatus

• Modifies products of the ER

• Manufactures certain macromolecules

• Sorts and packages materials into transport vesicles

Vesicles

Small, membrane-enclosed, saclike vesicles form in great numbers, in a variety of types, either on their

own or by budding. There are many types but two main are:

Lysosomes: Digestion & recycling centers

Lysosomes that bud from Golgi bodies take part in intracellular digestion. They contain powerful enzymes

that can break down carbohydrates, proteins, nucleic acids, and lipids. Vesicles inside white blood cells or

amoebas deliver ingested bacteria, cell parts, and other debris to lysosomes for destruction. The enzymes

work best in the acidic environment inside the lysosome. Lysosomes break down worn out cell parts or

molecules so they can be used to build new cellular structures. Some types of cell can engulf another cell

by phagocytosis; this forms a food vacuole. A lysosome fuses with the food vacuole and digests the

molecules Lysosomes also use enzymes to recycle the cell’s own organelles and macromolecules, a

process called autophagy

Peroxisomes: In plants and animals, vesicles called peroxisomes

form and divide on their own, so they are not part of the endomembrane system. Peroxisomes contain

enzymes that digest fatty acids and amino acids. They also break down hydrogen peroxide, a toxic by-

product of fatty acid metabolism. Peroxisome enzymes convert hydrogen peroxide to water and oxygen,

or use it in reactions that break down alcohol and other toxins.

The Nucleus, Endoplasmic Reticulum and Golgi Work Together to Produce and Transport

Proteins

For some organelles, including the mitochondria, chloroplasts, and the interior of the nucleus, proteins are

delivered directly from the cytosol. For others, including the Golgi apparatus, lysosomes, endosomes, and

the nuclear membranes, proteins and lipids are delivered indirectly via the ER, which is itself a major site

of lipid and protein synthesis. Proteins enter the ER directly from the cytosol: some are retained there, but

most are transported by vesicles to the Golgi apparatus and then onward to other organelles or the

plasma membrane.

The cytoskeleton is a network of fibers that organizes structures and activities in the cell

• Between the nucleus and plasma membrane of all eukaryotic cells is a system of interconnected

protein filaments collectively called the cytoskeleton. The cytoskeleton is a network of fibers

extending throughout the cytoplasm. Elements of the cytoskeleton reinforce, organize, and move

cell structures, anchoring many organelles.

– Microtubules

Microtubules are long, hollow

cylinders that consist of subunits of

the protein tubulin. They form a

dynamic scaffolding for many cellular

processes, rapidly assembling when

they are needed and then

disassembling when they are not. For

example, before a eukaryotic cell

divides, microtubules assemble,

separate the cell’s duplicated

chromosomes, then disassemble. As

another example,

microtubules that form in the growing

end of a young nerve cell support and

guide its lengthening in a particular

direction.

– Microfilaments

Microfilaments are fibers that consist

primarily of subunits of the globular

protein actin. They strengthen or

change the shape of eukaryotic cells.

Crosslinked, bundled, or gel-like

arrays of them make up the cell

cortex, which is a reinforcing mesh

under the plasma membrane. Actin

microfilaments that form at the edge

of a cell drag or extend it in a certain

direction. Myosin and Actin

microfilaments interact to bring about

contraction of muscle cells.

– Intermediate filaments

Intermediate filaments that support

cells and tissues are the most stable

elements of the cytoskeleton. These

filaments form a framework that lends

structure and resilience to cells and

tissues. Some kinds underlie and

reinforce membranes. The nuclear

envelope, for example, is supported by

an inner layer of intermediate

filaments called lamins. Other kinds

connect to structures that lock cell

membranes together in tissues

Microtubules control the beating of cilia and

flagella, locomotor appendages of some cells.

Cilia and flagella differ in their beating patterns

Cilia – Cilia (singular, cilium) are short, hairlike

structures that project from the surface of some

cells. Cilia are usually more profuse than

flagella. The coordinated waving of many cilia

propels cells through fluid, and stirs fluid around

stationary cells.

Flagella – Eukaryotic flagella are structures that

whip back and forth to propel cells such as

sperm through fluid. They have a different

internal structure and type of motion than

flagella of bacteria.

Plant Cells

Plants are eukaryotes and have the typical eukaryotic cell organization, consisting of nucleus and

cytoplasm. The cytoplasm is enclosed by a plasma membrane and contains numerous membrane-enclosed

organelles, including plastids, mitochondria, microbodies, oleosomes, and a large central vacuole.

Chloroplasts and mitochondria are semiautonomous organelles that contain their own DNA.

The main Characteristics are given below:

Cell wall – rigid, support & protect plant,

Cellulose fiber embedded

Vacuoles – fluid-filled; store enzymes & metabolic wastes

Plastids – contain DNA surrounded by 2 membranes

Store starch/fats

Absorb visible light – pigments

Chloroplast – site where photosynthesis takes place

Thylakoids – membranous sacs contains chlorophyll

Plants have & we don’t:

Cell wall

Vacuoles

Plastids (where photosynthesis takes place)

Cell Walls of Plants

The cell wall is an extracellular

structure that distinguishes plant

cells from animal cells, made of

cellulose fibers embedded in other

polysaccharides and protein

Prokaryotes, fungi, and some

protists also have cell walls

The cell wall protects the plant

cell, maintains its shape, and

prevents excessive uptake of

water

The Extracellular Matrix (ECM) of

Animal Cells

Most cells of multicelled organisms are

surrounded and organized by a nonliving,

complex mixture of fibrous proteins and

polysaccharides called extracellular matrix,

or ECM. Secreted by the cells it surrounds,

ECM supports and anchors cells, separates

tissues, and functions in cell signaling.

Different types of cells secrete different

kinds of ECM. The cell wall around the

plasma membrane of plant cells is a type of

ECM that is structurally different from the

cell wall of bacteria and archaeans. Both

types of wall

protect, support, and impart shape to a

cell. Both are also porous: Water and

solutes easily cross it on the way to and from the plasma membrane. Cells could not live without

exchanging these substances with their environment. Plant and animals secrete substances such as

collagen, proteoglycans, lignin and fibronectin with their ECM.

Cells send and receive ions, molecules, or signals through some junctions. ECM proteins bind to receptor

proteins in the plasma membrane called integrins

Membrane structure results in selective permeability

• A cell must exchange materials with its surroundings, a process controlled by the plasma

membrane

• Plasma membranes are selectively permeable, regulating the cell’s molecular traffic

• Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass

through the membrane rapidly

• Polar molecules, such as sugars, do not cross the membrane easily

Transport proteins

Transport proteins allow passage of hydrophilic substances across the membrane Some transport

proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a

tunnel. Channel proteins called aquaporins facilitate the passage of water. Other transport proteins,

called carrier proteins, bind to molecules and change shape to shuttle them across the membrane. A

transport protein is specific for the substance it moves

Passive transport is diffusion of a substance across a membrane

• Diffusion is the tendency for

molecules to spread out evenly into

the available space

• Although each molecule moves

randomly, diffusion of a population of

molecules may be directional

• At dynamic equilibrium, as many

molecules cross the membrane in one

direction as in the other

Substances diffuse down their

concentration gradient, the region along

which the density of a chemical substance increases or decreases. No work must be done to move

substances down the concentration gradient.The diffusion of a substance across a biological membrane is

passive transport because no energy is expended by the cell to make it happen.

Osmosis

Osmosis is the diffusion of water across a selectively permeable membrane

Water diffuses across a membrane from the region of lower solute concentration to the region of

higher solute concentration until the solute concentration is equal on both sides

H2O balance of Cells

• Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water

• Isotonic solution: Solute concentration is the

same as that inside the cell; no net water

movement across the plasma membrane

• Hypertonic solution: Solute concentration is

greater than that inside the cell; cell loses water

• Hypotonic solution: Solute concentration is less

than that inside the cell; cell gains water

• Hypertonic or hypotonic environments create

osmotic problems for organisms

PPrrookkaarryyoottiicc CCeellll EEuukkaarryyoottiicc CCeellll

1. Generally small (1-10 µm) in size

and volume

1. Generally large (5-100 µm). Eukaryotic cells

are about 15 times the size of a typical

prokaryote and can be as much as 1000 times

greater in volume.

2. Cell wall is present 2. Cell walls may or may not be present.

3. Nucleus is absent 3. Nucleus is present

4. Prokaryotic cell division occurs

through fission or budding, no

mitosis occurs.

4. Mitosis, including mitotic spindle, centrioles

in many species.

5. Prokaryotes generally lack

membrane-bound cell

compartments: such as mitochondria

and chloroplasts.

5. Mitochondria and chloroplasts are present in

Eukaryotes.

6. Single circular chromosome 6. Multiple linear chromosomes

7. Chromosome found in a

cytoplasmic region called the

nucleoid.

7. Chromosomes found in a membrane-bound

nucleus.

8. No internal membranes

Some infolded plasma membrane,

No Cytoskeleton

8. Extensive network of internal membranes,

Complex, with microtubules, intermediate

filaments and actin filaments

9. Intracellular movement is absent 9. Cytoplasmic streaming, endocytosis,

phagocytosis, mitosis, vesicle transport.

The Cell Cycle Mitosis & Beyond…

The Key Roles of Cell Division

The ability of organisms to produce more of their own kind best distinguishes living things from nonliving

matter. The continuity of life is based on the reproduction of cells, or cell division. In unicellular

organisms, division of one cell reproduces the entire organism. Multicellular organisms depend on cell

division for

Development from a fertilized cell

Growth

Repair

Cell division is an integral part of the cell cycle, the life of a cell from formation to its own division. Most

cell division results in daughter cells with identical genetic information, DNA. The exception is meiosis, a

special type of division that can produce sperm and egg cells

Somatic cells (nonreproductive cells) have two sets of chromosomes

Gametes (reproductive cells: sperm and eggs) have half as many chromosomes as somatic cells

When is cell division occurring?

GROWTH -increase number of cells

REPAIR -replace lost cells due to injury, disease

CANCER – Abnormally high rates of cell division due to mutation

Different kinds of cells divide at different rates:

E. coli – 20 minutes

Yeast cell – 2 hours

Amoeba – a few days

Human embryo cell – 15-20 minutes

Human adult cell – 8 hours to 100 days

Aging

All cells die after a certain number of divisions (programmed cell death-”apoptosis”). At any

given time some cells are dividing and some cells are dying

Childhood Cell division > cell death

Adulthood Cell division = cell death

Aging Cell division < cell death

Most cell division results in genetically identical daughter cells

When eukaryotic cells divide, two events occur. (1) The replicated genetic information of a cell is equally

distributed in mitosis. (2) After mitosis, the cytoplasm of the cell also divides into two new cells. This

division of the cell’s cytoplasm is called cytokinesis —cell splitting.

Gametes are produced by a variation of cell division called meiosis. It is a method of eukaryotic

cell division that results in daughter cells that have half the genetic information of the parent cell. These

daughter cells contain half the genetic information of the parent cell, are not genetically identical to the

parent cell from which they were produced, and can be used in sexual reproduction.

Organization of the Genetic Material

• All the DNA in a cell constitutes the cell’s genome

• A genome consists of a single DNA molecule (in prokaryotic cells) or a number of DNA molecules

(in eukaryotic cells)

• DNA molecules in a cell are packaged into chromosomes

• Eukaryotic chromosomes consist of chromatin, a complex of DNA and protein that condenses

during cell division

• Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus

Overview of the Cell Cycle

Actively dividing eukaryote cells pass through a series of stages known collectively as the cell cycle: two

gap phases (G1 and G2); an S (for synthesis) phase, in which the genetic material is duplicated; and an M

phase, in which mitosis partitions the genetic material and the cell divides.

G1 phase. Metabolic changes prepare the cell for division. At a certain point - the restriction point - the

cell is committed to division and moves into the S phase.

S phase. DNA synthesis replicates the genetic material. Each chromosome now consists of two sister

chromatids.

G2 phase. Metabolic changes assemble the cytoplasmic materials necessary for mitosis and cytokinesis.

M phase. A nuclear division (mitosis) followed by a cell division (cytokinesis).

The period between mitotic divisions - that is, G1, S and G2 - is known as interphase.

The cell grows during all three phases, but chromosomes are duplicated only during the S phase

Interphase

Interphase is a stage of the cell cycle during which the cell engages in normal metabolic activities and

prepares for the next cell division. Most cells spend the greater part of their life in the interphase stage.

After the required preparatory steps the cell proceeds into the stages of mitosis. Mitosis is the portion of

the cell cycle in which the cell divides its genetic information. Scientists split interphase and mitosis into

smaller steps in order to describe how the cell divides in more detail. Interphase contains three distinct

phases of cell activity—G 1 , S, and G 2 . During each of these parts of interphase, the cell is engaged in

specific activities needed to prepare for cell division.

• G1: Little new cell absorbs nutrients and grows larger. Does protein synthesis, its job.

• S phase: Synthesis of new DNA (DNA replication) for daughter cells in preparation for

mitosis.

• G2: Cell continues to grow, do protein synthesis, do its job. Gets too large, needs to

divide.

Structure of a eukaryotic chromosome

• Unreplicated chromosome

Prior to cell division: chromosomes (DNA) are

replicated (duplicated)

Duplicated chromosome

attached at their centromeres

as long as attached, known as sister

chromatids

Chromosomes exist in 2 different states, before and after

they replicate their DNA. Before replication, chromosomes have one chromatid. After replication,

chromosomes have 2 sister chromatids, held together at the centromere. Each chromatid is one piece of

DNA with its supporting proteins. Remember that diploid cells have two copies of each chromosome, one

from each parent. These pairs of chromosomes are NOT attached together.

What is Mitotic Cell Division?

Division of somatic cells (body cells)

(non reproductive cells) in eukaryotic organisms

A single cell divides into two identical daughter cells (cellular reproduction)

Maintains chromosome ploidy of cell

Ploidy – refers to the number of pairs of chromosomes in cells

Haploid – one copy of each chromosome – designated as “n”

Diploid – two copies (pair) of each chromosome – designated as “2n”

Mitosis

Mitosis is a form of eukaryotic cell division that produces two daughter cells with the same genetic

component as the parent cell. Chromosomes replicated during the S phase are divided in such a way as to

ensure that each daughter cell receives a

copy of every chromosome. In actively

dividing animal cells, the whole process

takes about one hour.

The replicated chromosomes are attached to

a 'mitotic apparatus' that aligns them and

then separates the sister chromatids to

produce an even partitioning of the genetic

material. This separation of the genetic

material in a mitotic nuclear division

(or karyokinesis) is followed by a

separation of the cell cytoplasm in a cellular

division (or cytokinesis) to produce two

daughter cells.

In some single-celled organisms mitosis

forms the basis of asexual reproduction. In

diploid multicellular organisms sexual

reproduction involves the fusion of two

haploid gametes to produce a diploid zygote.

Mitotic divisions of the zygote and daughter cells are then responsible for the subsequent growth and

development of the organism. In the adult organism, mitosis plays a role in cell replacement, wound

healing and tumour formation.

Mitosis, although a continuous process, is conventionally divided into five stages: prophase,

prometaphase, metaphase, anaphase and telophase.

As a cell enters mitosis from interphase it

has 2 complete sets of chromosomes

because of replication in the S phase.

Each set must be re-arranged and

distributed into the 2 new daughter

nuclei. This is mitosis

Prophase

Chromatin condenses (coils) into chromosomes. Sister chromatids

joined by centromere.

Nuclear membrane dissolves.

Centrioles divide and move to opposite poles forming spindle

between them

Prophase occupies over half of mitosis. The nuclear membrane breaks

down to form a number of small vesicles and the nucleolus disintegrates.

A structure known as the centrosome duplicates itself to form two

daughter centrosomes that migrate to opposite ends of the cell. The

centrosomes organise the production of microtubules that form the spindle

fibres that constitute the mitotic spindle. The chromosomes condense into compact structures. Each

replicated chromosome can now be seen to consist of two identical chromatids (or sister chromatids)

held together by a structure known as the centromere.

Metaphase…

Sister chromatids line up on metaphase plate.

Centromeres lock on to spindle fibre

Prometaphase

The chromosomes, led by their centromeres, migrate to the equatorial

plane in the mid-line of the cell - at right-angles to the axis formed by the

centrosomes. This region of the mitotic spindle is known as

the metaphase plate. The spindle fibres bind to a structure associated

with the centromere of each chromosome called a kinetochore. Individual

spindle fibres bind to a kinetochore structure on each side of the

centromere. The chromosomes continue to condense.

Metaphase

The chromosomes align themselves along the metaphase plate of the

spindle apparatus.

Anaphase…

Centromeres divide.

Spindle fibres contract pulling sister chromatids apart to

poles

The shortest stage of mitosis. The centromeres divide, and the sister

chromatids of each chromosome are pulled apart - or 'disjoin' - and move

to the opposite ends of the cell, pulled by spindle fibres attached to the

kinetochore regions. The separated sister chromatids are now referred to

as daughter chromosomes. (It is the alignment and separation in

metaphase and anaphase that is important in ensuring that each

daughter cell receives a copy of every chromosome.)

Telophase…

New nuclear membranes form around new

nuclei

The final stage of mitosis, and a reversal of many of the

processes observed during prophase. The nuclear

membrane reforms around the chromosomes grouped at

either pole of the cell, the chromosomes uncoil and become

diffuse, and the spindle fibres disappear.

CYTO KINESIS – Cytoplasm splits into 2 cells.

Different cell types use different strategies for achieving cytokinesis.

Animal cells: Cleavage furrow forms from outside to inside.

In animal cells, cytokinesis results

from the formation of a cleavage

furrow. The cleavage furrow is an

indentation of the plasma membrane

that pinches in toward the center of

the cell, thus splitting the cytoplasm

in two. In an animal cell, cytokinesis

begins at the plasma membrane and proceeds to the center.

Plant cells: Division/cell plate forms from

inside out. In plant cells, a cell plate begins

to form at the center of the cell and grows out

to the plasma membrane. The cell plate is

made of normal plasma membrane

components. It is formed by both daughter cells, so that, when complete, the two cells have separate

membranes. The cell wall is then formed between the newly formed cells.

Cytokinesis

The final cellular division to form two new cells. In plants a cell plate forms along the line of the

metaphase plate; in animals there is a constriction of the cytoplasm. The cell then enters interphase - the

interval between mitotic divisions.

Cell now returns to interphase. The chromosomes uncoil back into chromatin. The whole cell

cycle starts over again…..